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Human Anatomy and Physiology - The Muscoskeletal System


In the human limb skeleton can you identify a bone which is rather feeble and does not have much role in supporting body weight or maintain posture, and doctors use pieces of it to transplant to other broken bones, where necessary?


For bone grafts, they usually use parts of the bones, so their removal should not result in problems with the "body weight maintenance" or "posture."

Parts of the fibula (calf bone) can be used, for example, as a graft to repair avascular necrosis of the femoral neck or congenital pseudoarthrosis of the tibia (shinbone) (International Orthopaedics).

For spinal fusion (joining the vertebra in the spine), they typically use the bone from the iliac crest (the top part of the pelvis) (OrthoInfo).

Other bones that can be used as grafts:

  • Parts of the ribs and certain skull bones for reconstruction of the facial bones
  • Parts of the mandible for tooth implants
  • Upper part of the tibia for various purposes
  • Distal part of the ulna for correction of the wrist in rheumatoid arthritis

Book Title: Anatomy & Physiology

Book Description: An adapted version of the OpenStax Anatomy & Physiology (https://openstax.org/details/books/anatomy-and-physiology) with revised content and artwork, Open Oregon State, Oregon State University.
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Muscular system

The muscular system is an organ system composed of specialized contractile tissue called the muscle tissue. There are three types of muscle tissue, based on which all the muscles are classified into three groups:

  • Cardiac muscle, which forms the muscular layer of the heart (myocardium)
  • Smooth muscle, which comprises the walls of blood vessels and hollow organs
  • Skeletal muscle, which attaches to the bones and provides voluntary movement.

Based on their histological appearance, these types are classified into striated and non-striated muscles with the skeletal and cardiac muscles being grouped as striated, while the smooth muscle is non-striated. The skeletal muscles are the only ones that we can control by the power of our will, as they are innervated by the somatic part of the nervous system. In contrast to this, the cardiac and smooth muscles are innervated by the autonomic nervous system, thus being controlled involuntarily by the autonomic centers in our brain.


Musculoskeletal System

The musculoskeletal system consists of bones of the skeleton, the joints and the skeletal muscles. It provides form, support, stability, and movement to the body.

The musculoskeletal system’s functions include supporting the body, allowing motion, and protecting vital organs. The skeletal also acts as the main storage system for calcium and phosphorus. Further, it contains important components of the hematopoietic system.

Bones are connected to other bones and muscle by tendons and ligaments. Bones provide stability to the body. Muscles hold the bones in place and also help in their movement. Different bones are connected by joints for producing motion. Cartilage prevents bones from rubbing directly onto each other. Muscles contract to move the bone attached at the joint.

Many diseases and disorders adversely affect the functioning of the musculoskeletal system. Some of these diseases may be difficult to diagnose due to the close relation of the musculoskeletal system with other organ systems.


Skeletal System Anatomy and Physiology

Besides contributing to body shape and form, our bones perform several important body functions.

  1. Support. Bones, the “steel girders” and “reinforced concrete” of the body, form the internal framework that supports the body and cradle its soft organs the bones of the legs act as pillars to support the body trunk when we stand, and the rib cage supports the thoracic wall.
  2. Protection. Bones protect soft body organs for example, the fused bones of the skull provide a snug enclosure for the brain, the vertebrae surround the spinal cord, and the rib cage helps protect the vital organs of the thorax.
  3. Movement. Skeletal muscles, attached to bones by tendons, use the bones as levers to move the body and its parts.
  4. Storage. Fat is stored in the internal cavities of bones bone itself serves as a storehouse for minerals, the most important of which are calcium and phosphorus because most of the body’s calcium is deposited in the bones as calcium salts, the bones are a convenient place to get more calcium ions for the blood as they are used up.
  5. Blood cell formation. Blood cell formation, or hematopoiesis, occurs within the marrow cavities of certain bones.

Anatomy of the Skeletal System

The skeleton is subdivided into two divisions: the axial skeleton, the bones that form the longitudinal axis of the body, and the appendicular skeleton, the bones of the limbs and girdles.

Classification of Bones

The adult skeleton is composed of 206 bones and there are two basic types of osseous, or bone, tissue: compact bone and spongy bone, and are classified into four groups according to shape: long, short, flat, and irregular.

  • Compact bone. Compact bone is dense and looks smooth and homogeneous.
  • Spongy bone. Spongy bone is composed of long, needle-like pieces of bone and lots of open space.
  • Long bones. Long bones are typically longer than they are wide as a rule, they have a shaft with heads at both ends, and are mostly compact bone.
  • Short bones. Short bones are generally cube-shaped and mostly contains spongy bone sesamoid bones, which form within tendons, are a special type of short bone.
  • Flat bones. Flat bones are thin, flattened, and usually curved they have two thin layers of compact bone sandwiching a layer of spongy bone between them.
  • Irregular bones. Bones that do not fit one of the preceding categories are called irregular bones.

Long Bone

The structure of a long bone is shown both through gross anatomy and microscopic anatomy.

Gross Anatomy

The gross structure of a long bone consists of the following:

  • Diaphysis. The diaphysis, or shaft, makes up most of the bone’s length and is composed of compact bone it is covered and protected by a fibrous connective tissue membrane, the periosteum.
  • Sharpey’s fibers. Hundreds of connective tissue fibers called perforating or Sharpey’s, fibers secure the periosteum to the underlying bone.
  • Epiphyses. The epiphyses are the ends of the long bone each epiphysis consists of a thin layer of compact bone enclosing an area filled with spongy bone.
  • Articular cartilage. Articular cartilage, instead of a periosteum, covers its external surface because the articular cartilage is glassy hyaline cartilage, it provides a smooth, slippery surface that decreases friction at joint surfaces.
  • Epiphyseal line. In adult bones, there is a thin line of bony tissue spanning the epiphysis that looks a bit different from the rest of the bone in the area this is the epiphyseal line.
  • Epiphyseal plate. The epiphyseal line is a remnant of the epiphyseal plate (a flat plate of hyaline cartilage) seen in young, growing bone epiphyseal plates can cause the lengthwise growth of a long bone by the end of puberty, when hormones inhibit long bone growth, epiphyseal plates have been completely replaced by bones, leaving only the epiphyseal lines to mark their previous location.
  • Yellow marrow. In adults, the cavity of the shaft is primarily a storage area for adipose (fat) tissue called the yellow marrow, or medullary, cavity.
  • Red marrow. However, in infants, this area forms blood cells and red marrow is found there in adult bones, red marrow is confined to cavities in the spongy bone of flat bones and epiphyses of some long bones.
  • Bone markings. Even when looking casually at bones, one can see that their surfaces are not smooth but scarred with bumps, holes, and ridges these bone markings reveal where muscles, tendons, and ligaments were attached and where blood vessels and nerves passed.
  • Categories of bone markings. There are two categories of bone markings: (a) projections, or processes, which grow out from the bone surface, and (b) depressions, or cavities which are indentations in the bone a little trick for remembering some of the bone markings are all the terms beginning with T are projections, while those beginning with F (except facet) are depressions.
Microscopic Anatomy

To the naked eye, spongy bone has a spiky, open appearance, whereas compact bone appears to be very dense.

  • Osteocytes. The mature bone cells, osteocytes, are found within the matrix in tiny cavities called lacunae.
  • Lamellae. The lacunae are arranged in concentric circles called lamellae around central (Haversian) canals.
  • Osteon. Each complex consisting of central canals and matrix rings is called an osteon, or Haversian system.
  • Canaliculi. Tiny canals, canaliculi, radiate outward from the central canals to all lacunae the canaliculi form a transportation system that connects all the bone cells to the nutrient supply through the hard bone matrix.
  • Perforating canals. The communication pathway from the outside of the bone to its interior (and the central canals) is completed by perforating (Volkmann’s) canals, which run into the compact bone at right angles to the shaft.

Axial Skeleton

The axial skeleton, which forms the longitudinal axis of the body, is divided into three parts: the skull, the vertebral column, and the bony thorax.

Skull

The skull is formed by two sets of bones: the cranium and the facial bones.

Cranium

The cranium encloses and protects the fragile brain tissue and is composed of eight large flat bones.

  • Frontal bone. The frontal bone forms the forehead, the bony projections under the eyebrows, and the superior part of each eye’s orbits.
  • Parietal bones. The paired parietal bones form most of the superior and lateral walls of the cranium they meet in the midline of the skull at the sagittal suture and form the coronal suture, where they meet the frontal bone.
  • Temporal bones. The temporal bones lie inferior to the parietal bones they join them at the squamous sutures.

There are several bone markings that appear at the temporal bone:

  1. External acoustic meatus. The external acoustic meatus is a canal that leads to the eardrum and middle ear it is the route by which sound enters the ear.
  2. Styloid process. The styloid process, a sharp, needlelike projection, is just inferior to the external auditory meatus.
  3. Zygomatic process. The zygomatic process is a thin bridge of bone that joins with the cheekbone (zygomatic bone) anteriorly.
  4. Mastoid process. The mastoid process, which full of air cavities (mastoid sinuses), is a rough projection posterior and inferior to the external acoustic meatus it provides an attachment site for some muscles of the neck.
  5. Jugular foramen. The jugular foramen, at the junction of the occipital and temporal bones, allows passage of the jugular vein, the largest vein of the head, which drains the brain just anterior to it in the cranial cavity is the internal acoustic meatus, which transmits cranial nerves VII and VIII.
  • Occipital bone. The occipital bone joins the parietal bones anteriorly at the lambdoid suture in the base of the occipital bone is a large opening, the foramen magnum, which surrounds the lower part of the brain allows the spinal cord to connect with the brain.
  • Sphenoid bone. The butterfly-shaped sphenoid bone spans the width of the skull and forms part of the floor of the cranial cavity in the midline of the sphenoid is a small depression, the sella turcica or Turk’s saddle, which forms a snug enclosure for the pituitary gland.
  • Foramen ovale. The foramen ovale, a large oval opening in line with the posterior end of the sella turcica, allows fibers of cranial nerve V to pass to the chewing muscles of the lower jaw.
  • Optic canal. The optic canal allows the optic nerve to pass to the eye.
  • Superior orbital fissure. The slitlike superior orbital fissure is where the cranial nerves controlling eye movements pass.
  • Sphenoid sinuses. The central part of the sphenoid bone is riddled with air cavities, the sphenoid sinuses.
  • Ethmoid bone. The ethmoid bone is very irregularly shaped and lies anterior to the sphenoid it forms the roof of the nasal cavity and part of the medial walls of the orbits.
  • Crista galli. Projecting from its superior surface is the crista galli the outermost covering of the brain attaches to this projection.
  • Cribriform plates. These holey areas, the cribriform plates, allow nerve fibers carrying impulses from the olfactory receptors of the nose to reach the brain.
  • Superior and middle nasal conchae. Extensions of the ethmoid bone, the superior and middle nasal conchae, form part of the lateral walls of the nasal cavity and increase the turbulence of air flowing through the nasal passages.
Facial Bones

Fourteen bones compose the face twelve are paired, only the mandible and vomer are single.

  • Maxillae. The two maxillae, or maxillary bones, fuse to form the upper jaw all facial bones except the mandible join the maxillae thus, they are the main or “keystone”, bones of the face the maxillae carry the upper teeth in the alveolar margin.
  • Palatine bones. The paired palatine bones lie posterior to the palatine processes of the maxillae they form the posterior part of the hard palate.
  • Zygomatic bones. The zygomatic bones are commonly referred to as the cheek bones they also form a good-sized portion of the lateral walls of the orbits, or eye sockets.
  • Lacrimal bones. The lacrimal bones are finger-sized bones forming part of the medial walls of each orbit each lacrimal bones has a groove that serves as a passageway for tears.
  • Nasal bones. The small rectangular bones forming the bridge of the nose are the nasal bones.
  • Vomer bone. The single bone in the medial line of the nasal cavity is the vomer the vomer forms most of the bony nasal septum.
  • Inferior nasal conchae. The interior nasal conchae are thin, curved bones projecting medially from the lateral walls of the nasal cavity.
  • Mandible. The mandible, or lower jaw, is the largest and strongest bone of the face it joins the temporal bones on each side of the face, forming the only freely movable joints in the skull the horizontal part of the mandible (the body) forms the chin two upright bars of bone (the rami) extend from the body to connect the mandible to the temporal bone.

The Hyoid Bone

Though not really part of the skull, the hyoid bone is closely related to the mandible and temporal bones.

  • Location. It is suspended in the midneck region about 2 cm (1 inch) above the larynx, where it is anchored by ligaments to the styloid processes of the temporal bones.
  • Parts. Horseshoe-shaped, with a body and two pairs of horns, or cornua, the hyoid bone serves as a movable base for the tongue and as an attachment point for neck muscles that raise and lower the larynx when we swallow and speak.

Fetal Skull

The skull of a fetus or newborn infant is different in many ways from an adult skull.

  • Size. The adult skull represents only one-eighth of the total body length, whereas that of a newborn infant is one-fourth as long as its entire body.
  • Fontanels. In the newborn, the skull also has a fibrous regions that have yet to be converted to bone these fibrous membranes connecting the cranial bones are called fontanels.
  • Anterior fontanel. The largest fontanel is the diamond-shaped anterior fontanel the fontanel allows the fetal skull to be compressed slightly during birth.

Vertebral Column (Spine)

Serving as the axial support of the body, the vertebral column, or spine, extends from the skull, which it supports, to the pelvis, where it transmits the weight of the body to the lower limbs.

  • Composition. The spine is formed from 26 irregular bones connected and reinforced by ligaments in such a way that a flexible, curved structure results.
  • Spinal cord. Running through the central cavity of the vertebral column is the delicate spinal cord, which the vertebral column surrounds and protects.
  • Vertebrae. Before birth, the spine consists of 33 separate bones called vertebrae, but 9 of these eventually fuse to form the two composite bones, the sacrum and the coccyx, that construct the inferior portion of the vertebral column.
  • Cervical vertebrae. Of the 24 single bones, the 7 vertebrae of the neck are cervical vertebrae.
  • Thoracic vertebrae. The next 12 are the thoracic vertebrae.
  • Lumbar vertebrae. The remaining 5 supporting the lower back are lumbar vertebrae.
  • Intervertebral discs. The individual vertebrae are separated by pads of flexible fibrocartilage-intervertebral discs- that cushion the vertebrae and absorb shock while allowing the spine flexibility.
  • Primary curvatures. The spinal curves in the thoracic and sacral regions are referred to as primary curvatures because they are present when we are born.
  • Secondary curvatures. The curvatures in the cervical and lumbar regions are referred to as secondary curvatures because they develop some time after birth.
  • Body or centrum. Disc-like, weight-bearing part of the vertebra facing anteriorly in the vertebral column.
  • Vertebral arch. Arch formed from the joining of all posterior extensions, the laminae and pedicles, from the vertebral body.
  • Vertebral foramen. Canal through which the spinal cord passes.
  • Transverse processes. Two lateral projections from the vertebral arch.
  • Spinous process. Single projection arising from the posterior aspect of the vertebral arch (actually the fused laminate).
  • Superior and inferior articular processes. Paired projections lateral to the vertebral foramen, allowing a vertebra to form joints with adjacent vertebrae.
Cervical Vertebrae

The seven cervical vertebrae (C1 to C7) form the neck region of the spine.

  • Atlas. The atlas (C1) has no body the superior surfaces of its transverse processes contain large depressions that receive the occipital condyles of the skull.
  • Axis. The axis (C2) acts as a pivot for the rotation of the atlas (and skull) above it has a large upright process, the dens, which acts as the pivot point.
  • Foramina. The transverse processes of the cervical vertebrae contain foramina (openings) through which the vertebral arteries pass on their way to the brain above.
Thoracic Vertebrae

The twelve thoracic vertebrae (T1 to T12) are all typical.

  • Size. They are larger than the cervical vertebrae and are distinguished by the fact that they are the only vertebrae to articulate with the ribs.
  • Shape. The body is somewhat heart-shaped and has two costal facets on each side, which receive the heads of the ribs.
  • Transverse processes. The two transverse processes of each thoracic vertebrae articulate with the nearby knoblike tubercles of the ribs.
  • Spinous process. The spinous process is long and hooks sharply downward, causing the vertebra to look like a giraffe’s head viewed from the side.
Lumbar Vertebrae

The five lumbar vertebrae (L1 to L5) have massive, blocklike bodies.

  • Spinous processes. Their short, hatchet-shaped spinous processes make them look like a moose head from the lateral aspect.
  • Strength. Because most of the stress on the vertebral column occurs in the lumbar region, these are the sturdiest of the vertebrae.
Sacrum

The sacrum is formed by the fusion of five vertebrae.

  • Alae. The winglike alae articulate laterally with the hip bones, forming the sacroiliac joints.
  • Median sacral crest. Its posterior midline surface is roughened by the median sacral crest, the fused spinous processes of the sacral vertebrae.
  • Posterior sacral foramina. This is flanked laterally by the posterior sacral foramina.
  • Sacral canal. The vertebral canal continues inside the sacrum as the sacral canal and terminates in a large inferior opening called the sacral hiatus.
Coccyx

The coccyx is formed from the fusion of three to five tiny, irregular shaped vertebrae.

  • Tailbone. It is the human “tailbone”, a remnant of the tail that other vertebrate animals have.

Thoracic Cage

The sternum, ribs, and thoracic vertebrae make up the bony thorax The bony thorax is routinely called the thoracic cage because it forms a protective, cone-shaped cage of slender bones around the organs of the thoracic cavity.

Sternum

The sternum (breastbone) is a typical flat bone and the result of the fusion of three bones- the manubrium, body, and xiphoid process.

  • Landmarks. The sternum has three important bony landmarks- the jugular notch, the sternal angle, and the xiphisternal joint.
  • Jugular notch. The jugular notch (concave upper border of the manubrium) can be palpated easily, generally it is at the level of the third thoracic vertebra.
  • Sternal angle. The sternal angle results where the manubrium and the body meet at a slight angle to each other, so that a transverse ridge is formed at the level of the second ribs.
  • Xiphisternal joint. The xiphisternal joint, the point where the sternal body and xiphoid process fuse, lies at the level of the ninth thoracic vertebra.
Ribs

Twelve pairs of ribs form the walls of the bony thorax.

  • True ribs. The true ribs, the first seven pairs, attach directly to the sternum by costal cartilages.
  • False ribs. False ribs, the next five pairs, either attach indirectly to the sternum or are not attached to the sternum at all.
  • Floating ribs. The last two pairs of false ribs lack the sternal attachments, so they are called the floating ribs.

Appendicular Skeleton

The appendicular skeleton is composed of 126 bones of the limbs and the pectoral and pelvic girdles, which attach the limbs to the axial skeleton.

Bones of the Shoulder Girdle

Each shoulder girdle, or pectoral girdle, consists of two bones – a clavicle and a scapula.

  • Clavicle. The clavicle, or collarbone, is a slender, doubly curved bone it attaches to the manubrium of the sternum medially and to the scapula laterally, where it helps to form the shoulder joint it acts as a brace to hold the arm away from the top of the thorax and helps prevent shoulder dislocation.
  • Scapulae. The scapulae, or shoulder blades, are triangular and commonly called “wings” because they flare when we move our arms posteriorly.
  • Parts of the scapula. Each scapula has a flattened body and two important processes- the acromion and the coracoid.
  • Acromion. The acromion is the enlarged end of the spine of the scapula and connects with the clavicle laterally at the acromioclavicular joint.
  • Coracoid. The beaklike coracoid process points over the top of the shoulder and anchors some of the muscles of the arm just medial to the coracoid process is the large suprascapular notch, which serves as a nerve passageway.
  • Borders of the scapula. The scapula has three borders- superior, medial (vertebral), and lateral (axillary).
  • Angles of the scapula. It also has three angles- superior, inferior, and lateral the glenoid cavity, a shallow socket that receives the head of the arm bone, is in the lateral angle.
  • Factors to free movement of the shoulder girdle. Each shoulder girdle attaches to the axial skeleton at only one point- the sternoclavicular joint the loose attachment of the scapula allows it to slide back and forth against the thorax as muscles act and, the glenoid cavity is shallow, and the shoulder joint is poorly reinforced by ligaments.

Bones of the Upper Limb

Thirty separate bones form the skeletal framework of each upper limb they form the foundations of the arm, forearm, and hand.

Arm

The arm is formed by a single bone, the humerus, which is a typical long bone.

  • Anatomical neck. Immediately inferior to the head is a slight constriction called anatomical neck.
  • Tubercles. Anterolateral to the head are two bony projections separated by the intertubercular sulcus– the greater and lesser tubercles, which are sites of muscle attachment.
  • Surgical neck. Just distal to the tubercles is the surgical neck, so named because it is the most frequently fractured part of the humerus.
  • Deltoid tuberosity. In the midpoint of the shaft is a roughened area called the deltoid tuberosity, where the large, fleshy deltoid muscle of the shoulder attaches.
  • Radial groove. Nearby, the radial groove runs obliquely down the posterior aspect of the shaft this groove marks the course of the radial nerve, an important nerve of the upper limb.
  • Trochlea and capitulum. At the distal end of the humerus is the medial trochlea, which looks somewhat like a spool, and the lateral ball-like capitulum both of these processes articulate with the bones of the forearm.
  • Fossa. Above the trochlea anteriorly is a depression, the coronoid fossa on the posterior surface is the olecranon fossa these two depressions, which are flanked by medial and lateralepicondyles, allow the corresponding processes of the ulna to move freely when the elbow is bent and extended.
Forearm

Two bones, the radius, and the ulna, form the skeleton of the forearm.

  • Radius. When the body is in the anatomical position, the radius is the lateral bone that is, it is on the thumb side of the forearm when the hand is rotated so that the palm faces backward, the distal end of the radius crosses over and ends up medial to the ulna.
  • Radioulnar Joints. Both proximally and distally the radius and ulna articulate at small radioulnar joints and the two bones are connected along their entire length by the flexible interosseous membrane.
  • Styloid process. Both the ulna and the radius have as styloid process at their distal end.
  • Radial tuberosity. The disc-shaped head of the radius also forms a joint with the capitulum of the humerus just below the head is the radial tuberosity, where the tendon of the biceps muscle attaches.
  • Ulna. When the upper limb is in the anatomical position, the ulna is the medial bone (on the little-finger side) of the forearm.
  • Trochlear notch. On its proximal end are the coronoid process and the posterior olecranon process, which are separated by the trochlear notch together, these two processes grip the trochlea of the humerus in a pliers-like joint.
Hand

The skeleton of the hand consists of carpals, the metacarpals, and the phalanges.

  • Carpal bones. The eight carpal bones, arranged in two irregular rows of four bones each, form the part of the hand called carpus, or, more commonly, the wrist the carpals are bound together by ligaments that restrict movements between them.
  • Metacarpals. The metacarpals are numbered 1 to 5 from the thumb side of the hand to the little finger when the fist is clenched, the heads of the metacarpals become obvious as the “knuckles“.
  • Phalanges. The phalanges are the bones of the fingers each hand contains 14 phalanges there are three in each finger (proximal, middle, and distal), except in the thumb, which has only two )proximal and distal.

Bones of the Pelvic Girdle

The pelvic girdle is formed by two coxal bones, or ossa coxae, commonly called hip bones.

  • Pelvic girdle. The bones of the pelvic girdle are large and heavy, and they are attached securely to the axial skeleton bearing weight is the most important function of this girdle because the total weight of the upper body rests on the bony pelvis.
  • Sockets. The sockets, which receives the thigh bones, are deep and heavily reinforced by ligaments that attach the limbs firmly to the girdle.
  • Bony pelvis. The reproductive organs, urinary bladder, and part of the large intestine lie within and are protected by the bony pelvis.
  • Ilium. The ilium, which connects posteriorly with the sacrum at the sacroiliac joint, is a large, flaring bone that forms most of the hip bone when you put your hands on your hips, they are resting over the alae, or winglike portions, of the ilia.
  • Iliac crest. The upper edge of an ala, the iliac crest, is an important anatomical landmark that is always kept in mind by those who give intramuscular injections the iliac crest ends anteriorly in the anterior superior iliac spine and posteriorly in the posterior superior iliac spine.
  • Ischium. The ischium is the “sit-down” bone, so called because it forms the most inferior part of the coxal bone.
  • Ischial tuberosity. The ischial tuberosity is a roughened area that receives weight when you are sitting.
  • Ischial spine. The ischial spine, superior to the tuberosity, is another important anatomical landmark, particularly in pregnant women, because it narrows the outlet of the pelvis through which the baby must pass during the birth process.
  • Greater sciatic notch. Another important structural feature of the ischium is the greater sciatic notch, which allows blood vessels and the large sciatic nerve to pass from the pelvis posteriorly into the thigh.
  • Pubis. The pubis, or pubic bone, is the most anterior part of the coxal bone.
  • Obturator foramen. An opening that allows blood vessels and nerves to pass into the anterior part of the thigh.
  • Pubic symphysis. The pubic bones of each hip bones fuse anteriorly to form a cartilaginous joint, the pubic symphysis.
  • Acetabulum. The ilium, ischium, and pubis fuse at a deep socket called the acetabulum, which means “vinegar cup” the acetabulum receives the head of the thigh bone.
  • False pelvis. The false pelvis is superior to the true pelvis it is the area medial to the flaring portions of the ilia.
  • True pelvis. The true pelvis is surrounded by bone and lies inferior to the flaring parts of the ilia and the pelvic brim the dimensions of the true pelvis of the woman are very important because they must be large enough to allow the infant’s head to pass during childbirth.
  • Outlet and inlet. The dimensions of the cavity, particularly the outlet (the inferior opening of the pelvis measured between the ischial spines, and the inlet (superior opening between the right and left sides of the pelvic brim) are critical, and thus they are carefully measured by the obstetrician.

Bones of the Lower Limbs

The lower limbs carry the total body weight when we are erect hence, it is not surprising that the bones forming the three segments of the lower limbs (thigh, leg, and foot) are much thicker and stronger than the comparable bones of the upper limb.

Thigh

The femur, or thigh bone, is the only bone in the thigh it is the heaviest, strongest bone in the body.

  • Parts. Its proximal end has a ball-like head, a neck, and greater and lesser trochanters (separated anteriorly by the intertrochanteric line and posteriorly by the intertrochanteric crest).
  • Gluteal tuberosity. These markings and the gluteal tuberosity, located on the shaft, all serve as sites for muscle attachment.
  • Head. The head of the femur articulates with the acetabulum of the hip bone in a deep, secure socket.
  • Neck. However, the neck of the femur is a common fracture site, especially in old age.
  • Lateral and medial condyles. Distally on the femur are the lateral and medial condyles, which articulate with the tibia below posteriorly these condyles are separated by the deep intercondylar fossa.
  • Patellar surface. Anteriorly on the distal femur is the smooth patellar surface, which forms a joint with the patella, or kneecap.
Leg

Connected along their length by an interosseous membrane, two bones, the tibia and fibula, form the skeleton of the leg.

  • Tibia. The tibia, or shinbone, is larger and more medial at the proximal end, the medial and lateral condyles articulate with the distal end of the femur to form the knee joint.
  • Tibial tuberosity. The patellar (kneecap) ligament attaches to the tibial tuberosity, a roughened area on the anterior tibial surface.
  • Medial malleolus. Distally, a process called medial malleolus forms the inner bulge of the ankle.
  • Anterior border. The anterior surface of the tibia is a sharp ridge, the anterior border, that is unprotected by the muscles thus, it is easily felt beneath the skin.
  • Fibula. The fibula, which lies along the tibia and forms joints with it both proximally and distally, is thin and sticklike the fibula has no part in forming the knee joint.
  • Lateral malleolus. Its distal end, the lateral malleolus, forms the outer part of the ankle.
Foot

The foot, composed of the tarsals, metatarsals, and phalanges, has two important functions. it supports our body weight and serves as a lever that allows us to propel our bodies forward when we walk and run.

  • Tarsus. the tarsus, forming the posterior half of the foot, is composed of seven tarsal bones.
  • Calcaneus and Talus. Body weight is carried mostly by the two largest tarsals, the calcaneus, or heel bone, and the talus (ankle), which lies between the tibia and the calcaneus.
  • Metatarsals.Five metatarsals form the sole.
  • Phalanges.14 phalanges form the toes each toe has three phalanges, except the great toe, which has two.
  • Arches. The bones in the foot are arranged to form three strong arches: two longitudinal (medial and lateral) and one transverse.

Joints

Joints, also called articulations, have two functions: they hold the bones together securely, but also give the rigid skeleton mobility.

  • Classification. Joints are classified in two ways- functionally and structurally.
  • Functional classification. The functional classification focuses on the amount of movement the joint allows.
  • Types of functional joints. There are synarthroses or immovable joints amphiarthroses, or slightly movable joints, and diarthrosis, or freely movable joints.
  • Diarthroses. Freely movable joints predominate in the limbs, where mobility is important.
  • Synarthroses and amphiarthroses. Immovable and slightly movable joints are restricted mainly to the axial skeleton, where firm attachments and protection of internal organs are priorities.
  • Structural classification. Structurally, there are fibrous, cartilaginous, and synovial joints these classifications are based on whether fibrous tissue, cartilage, or a joint cavity separates the bony regions at the joint.
Fibrous Joints

In fibrous joints, the bones are united by fibrous tissue.

  • Examples. The best examples of this type of joint are the sutures of the skull in sutures, the irregular edges of the bones interlock and are bound tightly together by connective tissue fibers, allowing essentially no movement.
  • Syndesmoses. In syndesmoses, the connecting fibers are longer than those of sutures thus the joint has more “give” the joint connecting the distal ends of the tibia and fibula is a syndesmosis.
Cartilaginous Joints

In cartilaginous joints, the bone ends are connected by cartilage.

  • Examples. Examples of this joint type that are slightly movable are the pubic symphysis of the pelvis and the intervertebral joints of the spinal column, where the articulating bone surfaces are connected by pads (discs) of fibrocartilage.
  • Synarthrotic cartilaginous joints. The hyaline cartilage epiphyseal plates of growing long bones and the cartilaginous joints between the first ribs and the sternum are immovable cartilaginous joints.
Synovial Joints

Synovial joints are joints in which the articulating bone ends are separated by a joint cavity containing a synovial fluid they account for all joints of the limbs.

  • Articular cartilage. Articular cartilage covers the ends of the bones forming the joints.
  • Fibrous articular capsule. The joint surfaces are enclosed by a sleeve or a capsule of fibrous connective tissue, and their capsule is lined with a smooth synovial membrane (the reason these joints are called synovial joints).
  • Joint cavity. The articular capsule encloses a cavity, called the joint cavity, which contains lubricating synovial fluid.
  • Reinforcing ligaments. The fibrous capsule is usually reinforced with ligaments.
  • Bursae. Bursae are flattened fibrous sacs lined with synovial membrane and containing a thin film of synovial fluid they are common where ligaments, muscles, skin, tendons, or bones rub together.
  • Tendon sheath. A tendon sheath is essentially an elongated bursa that wraps completely around a tendon subjected to friction, like a bun around a hotdog.
Types of Synovial Joints Based on Shape

The shapes of the articulating bone surfaces determine what movements are allowed at a joint based on such shapes, our synovial joints can be classified as plane, hinge, pivot, condyloid, saddle, and ball-and-socket joints.

  • Plane joint. In a plane joint, the articular surfaces are essentially flat, and only short slipping or gliding movements are allowed the movements of plane joints are nonaxial, that is, gliding does not involve rotation around any axis the intercarpal joints of the wrist are best examples of plane joints.
  • Hinge joint. In a hinge joint, the cylindrical end of one bone fits into a trough-shaped surface on another bone angular movement is allowed in just one plane, like a mechanical hinge hinge joints are classified as uniaxial they allow movement in only one axis, and examples are the elbow joint, ankle joint, and the joints between the phalanges of the fingers.
  • Pivot joint. In a pivot joint, the rounded end of one bone fits into a sleeve or ring of bone because the rotating bone can turn only around its long axis, pivot joints are also uniaxial joints the proximal radioulnar joint and the joint between the atlas and the dens of the axis are examples.
  • Condyloid joint. In a condyloid joint, the egg-shaped articular surface fits into an oval concavity in another condyloid joints allow the moving bone to travel (1) from side to side and (2) back and forth but the bone cannot rotate around its long axis movement occurs around two axes, hence these are biaxial joints.
  • Saddle joints. In saddle joints, each articular surface has both convex and concave areas, like a saddle these biaxial joints allow essentially the same movements as condyloid joints the best examples of saddle joints are the carpometacarpal joints in the thumb.
  • Ball-and-socket joint. In a ball-and-socket joint, the spherical head of one bone fits into a round socket in another these multiaxial joints allow movement in all axes, including rotation, and are the most freely moving synovial joints the shoulder and hip are examples.

Practice Quiz: Skeletal System Anatomy and Physiology

1. Which of the following is NOT considered a function of the skeletal system?

A. Support and protects body structures
B. Storage of minerals
C. Blood cell formation
D. Synthesize Vitamin D

1. Answer: D. Synthesize Vitamin D

D: This is a function of the integumentary system. The system synthesizes vitamin D3 which converts to calcitriol, for normal metabolism of calcium.
A: Bone is the major supporting tissue of the body and protects internal organs (e.g., ribcage protects the heart, lungs, and other internal organs).
B: Some minerals in the blood are taken into bone and stored. The principal minerals stored are calcium and phosphorus.
C: Many bones contain cavities filled with bone marrow that gives rise to blood cells.

2. Most of the mineral in bone is in the form of calcium phosphate crystals called _________

A. Synovial fluid
B. Marrow
C. Hydroxyapatite
D. Proteoglycans

2. Answer: C. Hydroxyapatite

  • C: Hydroxyapatiteis a calcium phosphate crystals contained in normal bone. The lattice-like structure of hydroxyapatite crystals accounts for the bones to withstand compression.
  • A: Synovial fluidforms a thin lubricating film covering the joint surfaces.
  • B:Marrowis the soft tissue in the medullary cavities of the bone.
  • D: The water-filledproteoglycansmakes cartilage smooth and resilient.

3. The following statements are true regarding red bone marrow, except.

A. Red marrow is the only site of blood formation in adults.
B. Adults have more red marrow than children.
C. In adults, it is found in the cancellous bone spaces found in flat bones.
D. In children, it is located in the medullary cavity of the long bones.

3. Answer: B. Adults have more red marrow than children.

  • B: Children’s bone have proportionately more red bone marrow than adults. As a person ages, red marrow is mostly replaced by yellow marrow.
  • A: Red bone marrow consists of a delicate, highly vascular fibrous tissue containing blood-forming cell calledhematopoietic stem cells.
  • C: In adults, red bone marrow is primarily found in theflat bones, such as the pelvic girdle and the sternum.
  • D: In children, it is found in the medullary cavity of thelong bones, such as the femur.

4. Most of the bones of the upper and lower limbs are long bones while, the sacrum and facial bones are categorized as ________.

A. Irregular bones
B. Flat bones
C. Short bones
D. Sesamoid bones

4. Answer: A. Irregular bones

  • A: Irregular bones vary in shape and structure and therefore do not fit into any other category (flat, short, long, or sesamoid). Examples are the irregular bones of the vertebral column, bones of the pelvis (pubis, ilium, and ischium)and facial bones.
  • B: Flat bones have relatively thin, flattened shape. Examples are the ribs, scapulae, and the sternum.
  • C: Short bonesare approximately as broad as they are long, such as the bones of the wrist and ankles.
  • D: Sesamoid bonesare bones embedded in tendons. These small, round bones are commonly found in the tendons of the hands, knees, and feet.

5. Arrange the following sequence of processes that occur during bone elongation within the epiphyseal plate.

  1. Calcification
  2. Proliferation
  3. Resting
  4. Hypertrophication

A. 3, 2, 4, 1
B. 2, 3, 1, 4
C. 4, 2, 3, 1
D. 1, 2, 3, 4

5. Answer: A. 3, 2, 4, 1

6. Which type of fracture where the bone bends and partially breaks?

A. Comminuted
B. Greenstick
C. Impacted
D. Slpira

6. Answer: B. Greenstick

  • B: Greenstick fracture. The fracture extends through a portion of the bone, causing it to bend on the other side.
  • A:Comminuted fracture is one in which the bone breaks into more than two fragments.
  • C: Impacted fractureoccurs when one of the fragments of one part of the bone is driven into the cancellous bone of another fragment.
  • D: Spiral fractures are complete fractures of long bones that result from a rotational force applied to the bone.

7. The joint between the metatarsal and carpal (trapezium) of the thumb is an example of

A. Gliding joint
B. Hinge joint
C. Saddle joint
D. Ball-and-socket joint

7. Answer: C. Saddle joint

  • C: An example of asaddle jointin the body is the carpometacarpal joint of the thumb that is formed between the trapezium bone and the first metacarpal.
  • A: An example of a gliding jointare the articular facets between vertebrae.
  • B: An example of a hinge joint knee, elbow, and finger joints.
  • D: An example of a ball-and-socket joint are the hip and shoulder joints.

8. Osteoclasts remove calcium from the bone, causing blood calcium levels to _______ Osteoblast deposit calcium into bone, causing blood calcium levels to______

A. Increase Increase
B. Decrease Increase
C. Decrease Decrease
D. Increase Decrease

8. Answer: D. Increase Decrease

9. This occurs when osteoblast begin to form bone in connective tissue membranes

A. Endochondral ossification
B. Bone growth
C. Intramembranous ossification
D. Bone remodeling

9. Answer: C. Intramembranous ossification

  • C: Intramembranous ossificationis the direct laying down of bone into the primitive connective tissue (mesenchyme).
  • A: Endochondral ossificationis the formation of long bones and other bones which include a hyaline cartilage precursor.
  • B: Bone growthoccurs by the deposition of new bone lamellae onto existing bone or other connective tissue.
  • D:Bone remodeling involves the removal of existing bone by osteoclasts and the deposition of new bone by osteoblast.

10. Frank is a 7-year-old boy who arrived at the emergency department with a history of numerous broken bones. The nurse observed that the client is short for his age, has abnormally curved vertebral column and his limbs are short and bowed. The client is most likely suffering from:

A. Rickets
B. Osteomalacia
C. Osteomyelitis
D. Osteogenesis imperfecta


Tissues, Organs, Organ Systems, and Organisms

Unicellular (single-celled) organisms can function independently, but the cells of multicellular organisms are dependent upon each other and are organized into five different levels in order to coordinate their specific functions and carry out all of life’s biological processes (see Figure 2.3 “Organization of Life”.

  • Cells are the basic structural and functional unit of all life. Examples include red blood cells and nerve cells. There are hundreds of types of cells. All cells in a person contain the same genetic information in DNA. However, each cell only expresses the genetic codes that relate to the cell’s specific structure and function. are groups of cells that share a common structure and function and work together. There are four basic types of human tissues: connective, which connects tissues epithelial, which lines and protects organs muscle, which contracts for movement and support and nerve, which responds and reacts to signals in the environment.
  • Organs are a group of tissues arranged in a specific manner to support a common physiological function. Examples include the brain, liver, and heart.
  • Organ systems are two or more organs that support a specific physiological function. Examples include the digestive system and central nervous system. There are eleven organ systems in the human body (see Table 2.1 “The Eleven Organ Systems in the Human Body and Their Major Functions”).
  • An organism is the complete living system capable of conducting all of life’s biological processes.

Figure 2.3 Organization of Life

“Organization Levels of Human Body” by Laia Martinez / CC BY-SA 4.0

Table 2.1 The Eleven Organ Systems in the Human Body and Their Major Functions

Organ System Organ Components Major Function
Cardiovascular heart, blood/lymph vessels, blood, lymph Transport nutrients and waste products
Digestive mouth, esophagus, stomach, intestines Digestion and absorption
Endocrine all glands (thyroid, ovaries, pancreas) Produce and release hormones
Immune white blood cells, lymphatic tissue, marrow Defend against foreign invaders
Integumentary skin, nails, hair, sweat glands Protective, body temperature regulation
Muscular skeletal, smooth, and cardiac muscle Body movement
Nervous brain, spinal cord, nerves Interprets and responds to stimuli
Reproductive gonads, genitals Reproduction and sexual characteristics
Respiratory lungs, nose, mouth, throat, trachea Gas exchange
Skeletal bones, tendons, ligaments, joints Structure and support
Urinary kidneys, bladder, ureters Waste excretion, water balance

The Anatomy and Physiology of the Musculoskeletal System

The musculoskeletal system has the function of supporting and protecting organs, the maintenance of the structure of the body, helping in the movement of organs, limbs and parts of the body, and nutrient storage (glycogen in muscles, calcium and phosphorus in bones).

Cartilaginous Tissue

3. Which types of tissue are cartilaginous and osseous tissue?

Cartilaginous and osseous tissues are considered connective tissues, since they are tissues in which the cells are relatively distant from others, with a large amount of extracellular matrix in the interstitial space.

4. What cells form cartilaginous tissue?

The main cells of cartilage are chondrocytes, which are produced from the chondroblasts that secrete interstitial matrix. It also contains chondroclasts, which are cells with a large number of lysosomesਊnd which are responsible for the digestion and remodeling of cartilaginous matrix. 

5. What is cartilaginous matrix made of?

Cartilaginous matrix is made of collagen fibers, mainly collagen type II, and of proteoglycans, proteins attached to glycosaminoglycans, chiefly hyaluronic acid. Proteoglycans are the reason for the typical rigidity of cartilage. 

6. What are some of the functions of cartilage in the human body?

Cartilage is responsible for the structural support of the nose and ears. The trachea and the bronchi are also organs with cartilaginous structures that prevent the closing of these tubes. Joints contain cartilage that covers the bones, providing a smooth surface to reduce the friction of joint movement. In the formation of bones, cartilage acts as a mold and is gradually substituted by osseous tissue.

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Osseous Tissue

7. What are the three main types of cells that form osseous tissue? What are their functions?

The three main types ofꃎlls of osseous tissue are osteoblasts, osteocytes and osteoclasts.

Osteoblasts are considered bone-forming cells since they are the cells that secrete the proteinaceous part of the bone matrix (collagen, glycoproteins and proteoglycans). Bone matrix is the intercellular space where the mineral substances of the bones are deposited.

Osteocytes are differentiated mature osteoblasts formed after these cells are completely surrounded by bone matrix. Osteocytes have the function of supporting the tissue.

Osteoclasts are the giant multinucleate cells that remodel osseous tissue. They are produced from monocytes and contain a large number of lysosomes. Osteoblasts secrete enzymes that digest osseous matrix, creating canals throughout the tissue. 

8. What is bone matrix? What are its main components?

Bone matrix is the content that fills the intercellular space of osseous tissue. Bone matrix is made of mineral substances (about 5%), mainly phosphorus and calcium salts, as well as organic substances (95%), mainly collagen, glycoproteins and proteoglycans. 

9. What are the Haversian canals and the Volkmann’s canals of the bones? Is osseous tissue vascularized?

The Haversian canals are longitudinal canals present in osseous tissue within which blood vessels and nerves pass. Osseous tissue distributes itself in a concentric manner around these canals. The Volkmann’s canals are communications between the Harvesian canals.

Osseous tissue is highly vascularized in its interior.

10. What are the functions of osseous tissue?

The main functions of osseous tissue are: to provide structural rigidity to the body and to delineate the spatial positioning of the other tissues and organs to support the weight of the body to serve as a site for mineral storage, mainly of calcium and phosphorus to form protective structures for important organs such as the brain, the spinal cord, the heart and the lungs to work as a lever and support for the muscles, providing movement and to contain the bone marrow where hematopoiesis occurs.

11. What are flat bones and long bones?

The main bones of the body can be classified as flat or long bones (some bones are not classified according to these categories). Examples of flat bones are the skull, the ribs, the hipbones, the scapulae and the sternum. Examples of long bones are the humerus, the radius, the ulna, the femur, the tibia and the fibula.

Muscle Tissues

12. What are the types of muscle tissues? What morphological features differentiate those types?

There are three types of muscle tissue: skeletal striated muscle tissue, cardiac striated muscle tissue and smooth muscle tissue.

Striated muscles present transversal stripes under microscopic viewਊnd their fibers (cells) are multinucleate (in skeletal) or may have more than one nucleus (in cardiac). Smooth muscle does not present transversal stripes and has spindle-shaped fibers, each of which has only one nucleus.

13. Which type of muscle tissue moves bones?

Bones are moved by the skeletal striated muscles. These muscles are voluntary (controlled by volition). 

14. Which type of muscle tissue contracts and relaxes the chambers of the heart?

The myocardium is made of cardiac striated muscle tissue. 

15. Which type of muscle tissue performs the peristaltic movements of the intestines?

Smooth muscle tissue is responsible for the peristaltic movements of the intestines. Smooth muscles are not controlled by volition.

16. Which type of muscle tissue helps to push the food down through the esophagus?

The esophageal wall in its superior portion is made of skeletal striated muscle. The inferior portion is made of smooth muscle. The intermediate portion contains both skeletal striated and smooth muscles. All of these muscles are important in pushing the food down towards the stomach.

Muscle Function Physiology

17. How is the striped pattern of striated muscle cells formed?

The functional units of muscle fibers are sarcomeres. Within sarcomeres, blocks of actin and myosin molecules are placed in an organized manner. The sarcomeres align in sequence to form myofibrils, which are longitudinally placed in the cytoplasm of muscle fibers (cells). The grouping of consecutive blocks of actin and myosin in parallel filaments creates the striped pattern of striated muscle tissue seen under a microscope. 

18. What are sarcomeres?

Sarcomeres are the contractile units of muscle tissue, formed of alternating actin blocks (thin filaments) and myosin blocks (thick filaments). Several sarcomeres placed in a linear sequence form a myofibril. Therefore, one muscle fiber (cell) has many myofibrils made of sarcomeres.

The compartments where myofibrils are inserted are delimited by an excitable membrane known as the sarcolemma. The sarcolemma is the plasma membrane of a muscle cell.

19. What are the main proteins that make up the sarcomere? What is the function of those molecules in muscle cells?

The sarcomere contains organized actin and myosin blocks. Troponin and tropomyosin also appear bound to actin.

When activated by calcium ions released in the proximities of the sarcomere, the actin molecules are pulled in by myosin molecules. This interaction between actin and myosin shortens the myofibrils, producing the phenomenon of muscle contraction.

20. What are the positions of actin and myosin molecules in the sarcomere before and during muscle contraction?

Schematically, actin filaments attached perpendicularly to both ends of the sarcomere (longitudinal sides) make contact with myosin filaments positioned in the middle of the sarcomere and in parallel to the actin filaments.

Before the contraction, the sarcomeres are extended (relaxed) since the contact between actin and myosin filaments is only made at their ends. During contraction, actin filaments slide along the myosin filaments and the sarcomeres shorten. 

21. How do calcium ions participate in muscle contraction? Why do both muscle contraction and muscle relaxation spend energy?

In muscle cells, calcium ions are stored within the sarcoplasmic reticulum. When a motor neuron emits a stimulus for muscle contraction, neurotransmitters called acetylcholine are released at the neuromuscular junction and the sarcolemma is excited. The excitation is transmitted to the sarcoplasmic reticulum, which then releasesꃊlcium ions into the sarcomeres.

In the sarcomeres, the calcium ions bind to troponin molecules attached to actin, thus activating the myosin binding sites of actin. The myosin, then able to bind to actin, pulls this protein and the sarcomere shortens. The combined simultaneous contraction of sarcomeres and myofibrils constitutes a muscle contraction. During muscle relaxation, the calcium ions return to the sarcoplasmic reticulum.

For myosin to bind to actin, and therefore for the contraction to occur, the hydrolysis of one ATP molecule is necessary. During relaxation, the return of calcium ions to the sarcoplasmic reticulum is also an active process that consumes ATP. Therefore, both muscle contraction and relaxation are energy-consuming processes.

22. What is myoglobin? What is the function of this molecule in muscle tissue?

Myoglobin is a pigment similar to hemoglobin which is present in muscle fibers. Myoglobin has a large affinity to oxygen. It keeps oxygen bound and releases the gas under strenuous muscle work. Therefore, myoglobin acts as an oxygen reserve for muscle cells.

23. How is phosphocreatine involved in muscle contraction and relaxation?

Phosphocreatine is the main means of energy storage of muscle cells.

During periods of relaxation, ATP molecules produced by aerobic cellular respiration transfer highly energized phosphate groups to creatine molecules, thus forming phosphocreatine. During periods of exercise, phosphocreatine and ADP resynthesize ATP to release energy for muscle contraction.

24. What happens when the oxygen supply is insufficient to maintain aerobic cellular respiration during muscle exercise?

If oxygen from hemoglobin or myoglobin is not enough to supply energy to the muscle cells, the cell begins to use lactic fermentation in an attempt to compensate for that deficiency.

Lactic fermentation releases lactic acid and this substance causes muscle fatigue and predisposes the muscles to cramps.

25. What is the neurotransmitter of the neuromuscular junction? How does the nervous system trigger muscle contraction?

The nervous cells that trigger muscle contraction are motor neurons. The neurotransmitter of the motor neurons is acetylcholine. When a motor neuron is excited, a depolarizing current flows along the membrane of its axon until reaching the synapse at the neuromuscular junction (the neural impulse passage zone between the axon extremity and the sarcolemma). Near the extremity of the axon, the depolarization allows calcium ions to enter the axon (note that calcium also has an important role here). The calcium ions stimulate the neuron into releasing acetylcholine in the synapse.

Acetylcholine then binds to special receptors on the outer surface of the sarcolemma the permeability of this membrane is altered and an action potential is created. The depolarization is then transmitted along the sarcolemma to the sarcoplasmic reticulum, which then releases calcium ions which cause the sarcomere to contract.

26. To increase the force of a muscle, is the intensity of muscle contraction increased? 

An increase in the strength of a muscle is not achieved by an increase in the intensity of the stimulation of each muscle fiber. The muscle fiber obeys an all-or-nothing rule, meaning that its contraction strength is only one and cannot be increased.

When the body needs to increase the strength of a muscle, a phenomenon known as spatial summation occurs: new muscle fibers are recruited in addition to the fibers already in action. Therefore, the strength of the muscle contraction increases only when the number of active muscle cells increases. 

27. What is the difference between the spatial summation and temporal summation of muscle fibers? What is tetany?

Spatial summation is the recruiting of new muscle fibers to increase muscle strength. Temporal summation occurs when a muscle fiber is continuously stimulated to contract without being able to go through relaxation.

When a muscle fiber remains in a continuous state of contraction via temporal summation, it is known as tetany (this is the clinical condition of patients contaminated by the toxin of tetanus bacteria). Tetany ends when all available energy for contraction is spent or when the stimulus ceases.

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Support, Movement, and Protection

Some functions of the skeletal system are more readily observable than others. When you move you can feel how your bones support you, facilitate your movement, and protect the soft organs of your body. Just as the steel beams of a building provide a scaffold to support its weight, the bones and cartilages of your skeletal system compose the scaffold that supports the rest of your body. Without the skeletal system, you would be a limp mass of organs, muscle, and skin. Bones facilitate movement by serving as points of attachment for your muscles. Bones also protect internal organs from injury by covering or surrounding them. For example, your ribs protect your lungs and heart, the bones of your vertebral column (spine) protect your spinal cord, and the bones of your cranium (skull) protect your brain (see Figure 6.1.1).


Skeletal System Anatomy

The skeletal system in an adult body is made up of 206 individual bones. These bones are arranged into two major divisions: the axial skeleton and the appendicular skeleton. The axial skeleton runs along the body’s midline axis and is made up of 80 bones in the following regions:

  • Skull
  • Hyoid
  • Auditory ossicles
  • Ribs
  • Sternum
  • Vertebral column

The appendicular skeleton is made up of 126 bones in the folowing regions:

Skull

The skull is composed of 22 bones that are fused together except for the mandible. These 21 fused bones are separate in children to allow the skull and brain to grow, but fuse to give added strength and protection as an adult. The mandible remains as a movable jaw bone and forms the only movable joint in the skull with the temporal bone.

The bones of the superior portion of the skull are known as the cranium and protect the brain from damage. The bones of the inferior and anterior portion of the skull are known as facial bones and support the eyes, nose, and mouth.

Hyoid and Auditory Ossicles

The hyoid is a small, U-shaped bone found just inferior to the mandible. The hyoid is the only bone in the body that does not form a joint with any other bone—it is a floating bone. The hyoid’s function is to help hold the trachea open and to form a bony connection for the tongue muscles.

The malleus, incus, and stapes—known collectively as the auditory ossicles—are the smallest bones in the body. Found in a small cavity inside of the temporal bone, they serve to transmit and amplify sound from the eardrum to the inner ear.

Vertebrae

Twenty-six vertebrae form the vertebral column of the human body. They are named by region:

  • Cervical (neck) - 7 vertebrae
  • Thoracic (chest) - 12 vertebrae
  • Lumbar (lower back) - 5 vertebrae
  • Sacrum - 1 vertebra
  • Coccyx (tailbone) - 1 vertebra

With the exception of the singular sacrum and coccyx, each vertebra is named for the first letter of its region and its position along the superior-inferior axis. For example, the most superior thoracic vertebra is called T1 and the most inferior is called T12.

Ribs and Sternum

The sternum, or breastbone, is a thin, knife-shaped bone located along the midline of the anterior side of the thoracic region of the skeleton. The sternum connects to the ribs by thin bands of cartilage called the costal cartilage.

There are 12 pairs of ribs that together with the sternum form the ribcage of the thoracic region. The first seven ribs are known as “true ribs” because they connect the thoracic vertebrae directly to the sternum through their own band of costal cartilage. Ribs 8, 9, and 10 all connect to the sternum through cartilage that is connected to the cartilage of the seventh rib, so we consider these to be “false ribs.” Ribs 11 and 12 are also false ribs, but are also considered to be “floating ribs” because they do not have any cartilage attachment to the sternum at all.

Pectoral Girdle and Upper Limb

The pectoral girdle connects the upper limb (arm) bones to the axial skeleton and consists of the left and right clavicles and left and right scapulae.

The humerus is the bone of the upper arm. It forms the ball and socket joint of the shoulder with the scapula and forms the elbow joint with the lower arm bones. The radius and ulna are the two bones of the forearm. The ulna is on the medial side of the forearm and forms a hinge joint with the humerus at the elbow. The radius allows the forearm and hand to turn over at the wrist joint.

The lower arm bones form the wrist joint with the carpals, a group of eight small bones that give added flexibility to the wrist. The carpals are connected to the five metacarpals that form the bones of the hand and connect to each of the fingers. Each finger has three bones known as phalanges, except for the thumb, which only has two phalanges.

Pelvic Girdle and Lower Limb

Formed by the left and right hip bones, the pelvic girdle connects the lower limb (leg) bones to the axial skeleton.

The femur is the largest bone in the body and the only bone of the thigh (femoral) region. The femur forms the ball and socket hip joint with the hip bone and forms the knee joint with the tibia and patella. Commonly called the kneecap, the patella is special because it is one of the few bones that are not present at birth. The patella forms in early childhood to support the knee for walking and crawling.

The tibia and fibula are the bones of the lower leg. The tibia is much larger than the fibula and bears almost all of the body’s weight. The fibula is mainly a muscle attachment point and is used to help maintain balance. The tibia and fibula form the ankle joint with the talus, one of the seven tarsal bones in the foot.

The tarsals are a group of seven small bones that form the posterior end of the foot and heel. The tarsals form joints with the five long metatarsals of the foot. Then each of the metatarsals forms a joint with one of the set of phalanges in the toes. Each toe has three phalanges, except for the big toe, which only has two phalanges.

Microscopic Structure of Bones

The skeleton makes up about 30-40% of an adult’s body mass. The skeleton’s mass is made up of nonliving bone matrix and many tiny bone cells. Roughly half of the bone matrix’s mass is water, while the other half is collagen protein and solid crystals of calcium carbonate and calcium phosphate.

Living bone cells are found on the edges of bones and in small cavities inside of the bone matrix. Although these cells make up very little of the total bone mass, they have several very important roles in the functions of the skeletal system. The bone cells allow bones to:

  • Grow and develop
  • Be repaired following an injury or daily wear
  • Be broken down to release their stored minerals

Types of Bones

All of the bones of the body can be broken down into five types: long, short, flat, irregular, and sesamoid.

  • Long. Long bones are longer than they are wide and are the major bones of the limbs. Long bones grow more than the other classes of bone throughout childhood and so are responsible for the bulk of our height as adults. A hollow medullary cavity is found in the center of long bones and serves as a storage area for bone marrow. Examples of long bones include the femur, tibia, fibula, metatarsals, and phalanges.
  • Short. Short bones are about as long as they are wide and are often cubed or round in shape. The carpal bones of the wrist and the tarsal bones of the foot are examples of short bones.
  • Flat. Flat bones vary greatly in size and shape, but have the common feature of being very thin in one direction. Because they are thin, flat bones do not have a medullary cavity like the long bones. The frontal, parietal, and occipital bones of the cranium—along with the ribs and hip bones—are all examples of flat bones.
  • Irregular. Irregular bones have a shape that does not fit the pattern of the long, short, or flat bones. The vertebrae, sacrum, and coccyx of the spine—as well as the sphenoid, ethmoid, and zygomatic bones of the skull—are all irregular bones.
  • Sesamoid. The sesamoid bones are formed after birth inside of tendons that run across joints. Sesamoid bones grow to protect the tendon from stresses and strains at the joint and can help to give a mechanical advantage to muscles pulling on the tendon. The patella and the pisiform bone of the carpals are the only sesamoid bones that are counted as part of the 206 bones of the body. Other sesamoid bones can form in the joints of the hands and feet, but are not present in all people.

Parts of Bones

The long bones of the body contain many distinct regions due to the way in which they develop. At birth, each long bone is made of three individual bones separated by hyaline cartilage. Each end bone is called an epiphysis (epi = on physis = to grow) while the middle bone is called a diaphysis (dia = passing through). The epiphyses and diaphysis grow towards one another and eventually fuse into one bone. The region of growth and eventual fusion in between the epiphysis and diaphysis is called the metaphysis (meta = after). Once the long bone parts have fused together, the only hyaline cartilage left in the bone is found as articular cartilage on the ends of the bone that form joints with other bones. The articular cartilage acts as a shock absorber and gliding surface between the bones to facilitate movement at the joint.

Looking at a bone in cross section, there are several distinct layered regions that make up a bone. The outside of a bone is covered in a thin layer of dense irregular connective tissue called the periosteum. The periosteum contains many strong collagen fibers that are used to firmly anchor tendons and muscles to the bone for movement. Stem cells and osteoblast cells in the periosteum are involved in the growth and repair of the outside of the bone due to stress and injury. Blood vessels present in the periosteum provide energy to the cells on the surface of the bone and penetrate into the bone itself to nourish the cells inside of the bone. The periosteum also contains nervous tissue and many nerve endings to give bone its sensitivity to pain when injured.

Deep to the periosteum is the compact bone that makes up the hard, mineralized portion of the bone. Compact bone is made of a matrix of hard mineral salts reinforced with tough collagen fibers. Many tiny cells called osteocytes live in small spaces in the matrix and help to maintain the strength and integrity of the compact bone.

Deep to the compact bone layer is a region of spongy bone where the bone tissue grows in thin columns called trabeculae with spaces for red bone marrow in between. The trabeculae grow in a specific pattern to resist outside stresses with the least amount of mass possible, keeping bones light but strong. Long bones have a spongy bone on their ends but have a hollow medullary cavity in the middle of the diaphysis. The medullary cavity contains red bone marrow during childhood, eventually turning into yellow bone marrow after puberty.

Articulations

An articulation, or joint, is a point of contact between bones, between a bone and cartilage, or between a bone and a tooth. Synovial joints are the most common type of articulation and feature a small gap between the bones. This gap allows a free range of motion and space for synovial fluid to lubricate the joint. Fibrous joints exist where bones are very tightly joined and offer little to no movement between the bones. Fibrous joints also hold teeth in their bony sockets. Finally, cartilaginous joints are formed where bone meets cartilage or where there is a layer of cartilage between two bones. These joints provide a small amount of flexibility in the joint due to the gel-like consistency of cartilage.


Skeletal system 1: the anatomy and physiology of bones

The skeletal system is formed of bones and cartilage, which are connected by ligaments to form a framework for the remainder of the body tissues. This article, the first in a two-part series on the structure and function of the skeletal system, reviews the anatomy and physiology of bone. Understanding the structure and purpose of the bone allows nurses to understand common pathophysiology and consider the most-appropriate steps to improve musculoskeletal health.

Citation: Walker J (2020) Skeletal system 1: the anatomy and physiology of bones. Nursing Times [online] 116: 2, 38-42.

Author: Jennie Walker is principal lecturer, Nottingham Trent University.

  • This article has been double-blind peer reviewed
  • Scroll down to read the article or download a print-friendly PDF here (if the PDF fails to fully download please try again using a different browser)
  • Read part 2 of this series here

Introduction

The skeletal system is composed of bones and cartilage connected by ligaments to form a framework for the rest of the body tissues. There are two parts to the skeleton:

  • Axial skeleton – bones along the axis of the body, including the skull, vertebral column and ribcage
  • Appendicular skeleton – appendages, such as the upper and lower limbs, pelvic girdle and shoulder girdle.

Function

As well as contributing to the body’s overall shape, the skeletal system has several key functions, including:

  • Support and movement
  • Protection
  • Mineral homeostasis
  • Blood-cell formation
  • Triglyceride storage.

Support and movement

Bones are a site of attachment for ligaments and tendons, providing a skeletal framework that can produce movement through the coordinated use of levers, muscles, tendons and ligaments. The bones act as levers, while the muscles generate the forces responsible for moving the bones.

Protection

Bones provide protective boundaries for soft organs: the cranium around the brain, the vertebral column surrounding the spinal cord, the ribcage containing the heart and lungs, and the pelvis protecting the urogenital organs.

Mineral homoeostasis

As the main reservoirs for minerals in the body, bones contain approximately 99% of the body’s calcium, 85% of its phosphate and 50% of its magnesium (Bartl and Bartl, 2017). They are essential in maintaining homoeostasis of minerals in the blood with minerals stored in the bone are released in response to the body’s demands, with levels maintained and regulated by hormones, such as parathyroid hormone.

Blood-cell formation (haemopoiesis)

Blood cells are formed from haemopoietic stem cells present in red bone marrow. Babies are born with only red bone marrow over time this is replaced by yellow marrow due to a decrease in erythropoietin, the hormone responsible for stimulating the production of erythrocytes (red blood cells) in the bone marrow. By adulthood, the amount of red marrow has halved, and this reduces further to around 30% in older age (Robson and Syndercombe Court, 2018).

Triglyceride storage

Yellow bone marrow (Fig 1) acts as a potential energy reserve for the body it consists largely of adipose cells, which store triglycerides (a type of lipid that occurs naturally in the blood) (Tortora and Derrickson, 2009).

Bone composition

Bone matrix has three main components:

  • 25% organic matrix (osteoid)
  • 50% inorganic mineral content (mineral salts)
  • 25% water (Robson and Syndercombe Court, 2018).

Organic matrix (osteoid) is made up of approximately 90% type-I collagen fibres and 10% other proteins, such as glycoprotein, osteocalcin, and proteoglycans (Bartl and Bartl, 2017). It forms the framework for bones, which are hardened through the deposit of the calcium and other minerals around the fibres (Robson and Syndercombe Court, 2018).

Mineral salts are first deposited between the gaps in the collagen layers with once these spaces are filled, minerals accumulate around the collagen fibres, crystallising and causing the tissue to harden this process is called ossification (Tortora and Derrickson, 2009). The hardness of the bone depends on the type and quantity of the minerals available for the body to use hydroxyapatite is one of the main minerals present in bones.

While bones need sufficient minerals to strengthen them, they also need to prevent being broken by maintaining sufficient flexibility to withstand the daily forces exerted on them. This flexibility and tensile strength of bone is derived from the collagen fibres. Over-mineralisation of the fibres or impaired collagen production can increase the brittleness of bones – as with the genetic disorder osteogenesis imperfecta – and increase bone fragility (Ralston and McInnes, 2014).

Structure

Bone architecture is made up of two types of bone tissue:

Cortical bone

Also known as compact bone, this dense outer layer provides support and protection for the inner cancellous structure. Cortical bone comprises three elements:

The periosteum is a tough, fibrous outer membrane. It is highly vascular and almost completely covers the bone, except for the surfaces that form joints these are covered by hyaline cartilage. Tendons and ligaments attach to the outer layer of the periosteum, whereas the inner layer contains osteoblasts (bone-forming cells) and osteoclasts (bone-resorbing cells) responsible for bone remodelling.

The function of the periosteum is to:

  • Protect the bone
  • Help with fracture repair
  • Nourish bone tissue (Robson and Syndercombe Court, 2018).

It also contains Volkmann’s canals, small channels running perpendicular to the diaphysis of the bone (Fig 1) these convey blood vessels, lymph vessels and nerves from the periosteal surface through to the intracortical layer. The periosteum has numerous sensory fibres, so bone injuries (such as fractures or tumours) can be extremely painful (Drake et al, 2019).

The intracortical bone is organised into structural units, referred to as osteons or Haversian systems (Fig 2). These are cylindrical structures, composed of concentric layers of bone called lamellae, whose structure contributes to the strength of the cortical bone. Osteocytes (mature bone cells) sit in the small spaces between the concentric layers of lamellae, which are known as lacunae. Canaliculi are microscopic canals between the lacunae, in which the osteocytes are networked to each other by filamentous extensions. In the centre of each osteon is a central (Haversian) canal through which the blood vessels, lymph vessels and nerves pass. These central canals tend to run parallel to the axis of the bone Volkmann’s canals connect adjacent osteons and the blood vessels of the central canals with the periosteum.

The endosteum consists of a thin layer of connective tissue that lines the inside of the cortical surface (Bartl and Bartl, 2017) (Fig 1).

Cancellous bone

Also known as spongy bone, cancellous bone is found in the outer cortical layer. It is formed of lamellae arranged in an irregular lattice structure of trabeculae, which gives a honeycomb appearance. The large gaps between the trabeculae help make the bones lighter, and so easier to mobilise.

Trabeculae are characteristically oriented along the lines of stress to help resist forces and reduce the risk of fracture (Tortora and Derrickson, 2009). The closer the trabecular structures are spaced, the greater the stability and structure of the bone (Bartl and Bartl, 2017). Red or yellow bone marrow exists in these spaces (Robson and Syndercombe Court, 2018). Red bone marrow in adults is found in the ribs, sternum, vertebrae and ends of long bones (Tortora and Derrickson, 2009) it is haemopoietic tissue, which produces erythrocytes, leucocytes (white blood cells) and platelets.

Blood supply

Bone and marrow are highly vascularised and account for approximately 10-20% of cardiac output (Bartl and Bartl, 2017). Blood vessels in bone are necessary for nearly all skeletal functions, including the delivery of oxygen and nutrients, homoeostasis and repair (Tomlinson and Silva, 2013). The blood supply in long bones is derived from the nutrient artery and the periosteal, epiphyseal and metaphyseal arteries (Iyer, 2019).

Each artery is also accompanied by nerve fibres, which branch into the marrow cavities. Arteries are the main source of blood and nutrients for long bones, entering through the nutrient foramen, then dividing into ascending and descending branches. The ends of long bones are supplied by the metaphyseal and epiphyseal arteries, which arise from the arteries from the associated joint (Bartl and Bartl, 2017).

If the blood supply to bone is disrupted, it can result in the death of bone tissue (osteonecrosis). A common example is following a fracture to the femoral neck, which disrupts the blood supply to the femoral head and causes the bone tissue to become necrotic. The femoral head structure then collapses, causing pain and dysfunction.

Growth

Bones begin to form in utero in the first eight weeks following fertilisation (Moini, 2019). The embryonic skeleton is first formed of mesenchyme (connective tissue) structures this primitive skeleton is referred to as the skeletal template. These structures are then developed into bone, either through intramembranous ossification or endochondral ossification (replacing cartilage with bone).

Bones are classified according to their shape (Box 1). Flat bones develop from membrane (membrane models) and sesamoid bones from tendon (tendon models) (Waugh and Grant, 2018). The term intra-membranous ossification describes the direct conversion of mesenchyme structures to bone, in which the fibrous tissues become ossified as the mesenchymal stem cells differentiate into osteoblasts. The osteoblasts then start to lay down bone matrix, which becomes ossified to form new bone.

Box 1. Types of bones

Long bones – typically longer than they are wide (such as humerus, radius, tibia, femur), they comprise a diaphysis (shaft) and epiphyses at the distal and proximal ends, joining at the metaphysis. In growing bone, this is the site where growth occurs and is known as the epiphyseal growth plate. Most long bones are located in the appendicular skeleton and function as levers to produce movement

Short bones – small and roughly cube-shaped, these contain mainly cancellous bone, with a thin outer layer of cortical bone (such as the bones in the hands and tarsal bones in the feet)

Flat bones – thin and usually slightly curved, typically containing a thin layer of cancellous bone surrounded by cortical bone (examples include the skull, ribs and scapula). Most are located in the axial skeleton and offer protection to underlying structures

Irregular bones – bones that do not fit in other categories because they have a range of different characteristics. They are formed of cancellous bone, with an outer layer of cortical bone (for example, the vertebrae and the pelvis)

Sesamoid bones – round or oval bones (such as the patella), which develop in tendons

Long, short and irregular bones develop from an initial model of hyaline cartilage (cartilage models). Once the cartilage model has been formed, the osteoblasts gradually replace the cartilage with bone matrix through endochondral ossification (Robson and Syndercombe Court, 2018). Mineralisation starts at the centre of the cartilage structure, which is known as the primary ossification centre. Secondary ossification centres also form at the epiphyses (epiphyseal growth plates) (Danning, 2019). The epiphyseal growth plate is composed of hyaline cartilage and has four regions (Fig 3):

Resting or quiescent zone – situated closest to the epiphysis, this is composed of small scattered chondrocytes with a low proliferation rate and anchors the growth plate to the epiphysis

Growth or proliferation zone – this area has larger chondrocytes, arranged like stacks of coins, which divide and are responsible for the longitudinal growth of the bone

Hypertrophic zone – this consists of large maturing chondrocytes, which migrate towards the metaphysis. There is no new growth at this layer

Calcification zone – this final zone of the growth plate is only a few cells thick. Through the process of endochondral ossification, the cells in this zone become ossified and form part of the ‘new diaphysis’ (Tortora and Derrickson, 2009).

Bones are not fully developed at birth, and continue to form until skeletal maturity is reached. By the end of adolescence around 90% of adult bone is formed and skeletal maturity occurs at around 20-25 years, although this can vary depending on geographical location and socio-economic conditions for example, malnutrition may delay bone maturity (Drake et al, 2019 Bartl and Bartl, 2017). In rare cases, a genetic mutation can disrupt cartilage development, and therefore the development of bone. This can result in reduced growth and short stature and is known as achondroplasia.

The human growth hormone (somatotropin) is the main stimulus for growth at the epiphyseal growth plates. During puberty, levels of sex hormones (oestrogen and testosterone) increase, which stops cell division within the growth plate. As the chondrocytes in the proliferation zone stop dividing, the growth plate thins and eventually calcifies, and longitudinal bone growth stops (Ralston and McInnes, 2014). Males are on average taller than females because male puberty tends to occur later, so male bones have more time to grow (Waugh and Grant, 2018). Over-secretion of human growth hormone during childhood can produce gigantism, whereby the person is taller and heavier than usually expected, while over-secretion in adults results in a condition called acromegaly.

If there is a fracture in the epiphyseal growth plate while bones are still growing, this can subsequently inhibit bone growth, resulting in reduced bone formation and the bone being shorter. It may also cause misalignment of the joint surfaces and cause a predisposition to developing secondary arthritis later in life. A discrepancy in leg length can lead to pelvic obliquity, with subsequent scoliosis caused by trying to compensate for the difference.

Remodelling

Once bone has formed and matured, it undergoes constant remodelling by osteoclasts and osteoblasts, whereby old bone tissue is replaced by new bone tissue (Fig 4). Bone remodelling has several functions, including mobilisation of calcium and other minerals from the skeletal tissue to maintain serum homoeostasis, replacing old tissue and repairing damaged bone, as well as helping the body adapt to different forces, loads and stress applied to the skeleton.

Calcium plays a significant role in the body and is required for muscle contraction, nerve conduction, cell division and blood coagulation. As only 1% of the body’s calcium is in the blood, the skeleton acts as storage facility, releasing calcium in response to the body’s demands. Serum calcium levels are tightly regulated by two hormones, which work antagonistically to maintain homoeostasis. Calcitonin facilitates the deposition of calcium to bone, lowering the serum levels, whereas the parathyroid hormone stimulates the release of calcium from bone, raising the serum calcium levels.

Osteoclasts are large multinucleated cells typically found at sites where there is active bone growth, repair or remodelling, such as around the periosteum, within the endosteum and in the removal of calluses formed during fracture healing (Waugh and Grant, 2018). The osteoclast cell membrane has numerous folds that face the surface of the bone and osteoclasts break down bone tissue by secreting lysosomal enzymes and acids into the space between the ruffled membrane (Robson and Syndercombe Court, 2018). These enzymes dissolve the minerals and some of the bone matrix. The minerals are released from the bone matrix into the extracellular space and the rest of the matrix is phagocytosed and metabolised in the cytoplasm of the osteoclasts (Bartl and Bartl, 2017). Once the area of bone has been resorbed, the osteoclasts move on, while the osteoblasts move in to rebuild the bone matrix.

Osteoblasts synthesise collagen fibres and other organic components that make up the bone matrix. They also secrete alkaline phosphatase, which initiates calcification through the deposit of calcium and other minerals around the matrix (Robson and Syndercombe Court, 2018). As the osteoblasts deposit new bone tissue around themselves, they become trapped in pockets of bone called lacunae. Once this happens, the cells differentiate into osteocytes, which are mature bone cells that no longer secrete bone matrix.

The remodelling process is achieved through the balanced activity of osteoclasts and osteoblasts. If bone is built without the appropriate balance of osteocytes, it results in abnormally thick bone or bony spurs. Conversely, too much tissue loss or calcium depletion can lead to fragile bone that is more susceptible to fracture. The larger surface area of cancellous bones is associated with a higher remodelling rate than cortical bone (Bartl and Bartl, 2017), which means osteoporosis is more evident in bones with a high proportion of cancellous bone, such as the head/neck of femur or vertebral bones (Robson and Syndercombe Court, 2018). Changes in the remodelling balance may also occur due to pathological conditions, such as Paget’s disease of bone, a condition characterised by focal areas of increased and disorganised bone remodelling affecting one or more bones. Typical features on X-ray include focal patches of lysis or sclerosis, cortical thickening, disorganised trabeculae and trabecular thickening.

As the body ages, bone may lose some of its strength and elasticity, making it more susceptible to fracture. This is due to the loss of mineral in the matrix and a reduction in the flexibility of the collagen.

Diet and lifestyle factors

Adequate intake of vitamins and minerals is essential for optimum bone formation and ongoing bone health. Two of the most important are calcium and vitamin D, but many others are needed to keep bones strong and healthy (Box 2).

Box 2. Vitamins and minerals needed for bone health

Key nutritional requirements for bone health include minerals such as calcium and phosphorus, as well as smaller qualities of fluoride, manganese, and iron (Robson and Syndercombe Court, 2018). Calcium, phosphorus and vitamin D are essential for effective bone mineralisation. Vitamin D promotes calcium absorption in the intestines, and deficiency in calcium or vitamin D can predispose an individual to ineffective mineralisation and increased risk of developing conditions such as osteoporosis and osteomalacia.

Other key vitamins for healthy bones include vitamin A for osteoblast function and vitamin C for collagen synthesis (Waugh and Grant, 2018).

Physical exercise, in particular weight-bearing exercise, is important in maintaining or increasing bone mineral density and the overall quality and strength of the bone. This is because osteoblasts are stimulated by load-bearing exercise and so bones subjected to mechanical stresses undergo a higher rate of bone remodelling. Reduced skeletal loading is associated with an increased risk of developing osteoporosis (Robson and Syndercombe Court, 2018).

Conclusion

Bones are an important part of the musculoskeletal system and serve many core functions, as well as supporting the body’s structure and facilitating movement. Bone is a dynamic structure, which is continually remodelled in response to stresses placed on the body. Changes to this remodelling process, or inadequate intake of nutrients, can result in changes to bone structure that may predispose the body to increased risk of fracture. Part 2 of this series will review the structure and function of the skeletal system.

Key points

  • Bones are key to providing the body with structural support and enabling movement
  • Most of the body’s minerals are stored in the bones
  • Diet and lifestyle can affect the quality of bone formation
  • After bones have formed they undergo constant remodelling
  • Changes in the remodelling process can result in pathology such as Paget’s disease of bone or osteoporosis

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Ralston SH, McInnes IB (2014) Rheumatology and bone disease. In: Walker BR et al (eds) Davidson’s Principles and Practice of Medicine. Edinburgh: Churchill Livingstone.

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Waugh A, Grant A (2018) The musculoskeletal system. In: Ross & Wilson Anatomy and Physiology in Health and Illness. London: Elsevier.


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