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Chapter 19: Musculoskeletal System

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Anatomy and Physiology of Bones

The bones provide attachment sites for muscles, enabling complex movement.

Bones also support and protect internal organs.

The organs of the skeletal system are largely composed of connective tissues, including bone and cartilage.

Connective tissue contains cells separated by matrix that contains fibers.

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Structure of BoneThe matrix of bone contains mineral salts. Bone cells are osteocytes and they lie in

tiny chambers called lacunae. Compact bone is highly organized into

tubular osteons, each with a central canal.

Spongy bone has an unorganized appearance but is designed for strength.

Spaces in spongy bone contain red bone marrow that produces blood cells.

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Anatomy of a bone

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Tissues Associated with BonesCartilage

Cartilage has a gel-like matrix with collagen and elastin fibers; it lacks blood vessels.

Hyaline cartilage is glassy and is found in the nose, ends of ribs, and in the larynx.

Fibrocartilage is stronger with thicker collagen fibers and is found in the disks between vertebrae.

Elastic cartilage has mainly elastin fibers and is in the ear flaps and epiglottis.

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Dense Fibrous Connective TissueDense fibrous connective tissue contains

fibroblasts are separated by bundles of collagen fibers.

This type of tissue is found at the flared sides of the nose, in ligaments that bind bone to bone, and in tendons that connect muscles to bone.

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Structure of a Long Bone

Bone is covered by fibrous connective tissue called the periosteum.

The diaphysis (shaft) of a long bone has a medullary cavity of yellow bone marrow containing fat.

Hyaline articular cartilage covers the ends of bones at the joint.

Epiphyses of bones have spongy bone.

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Bone Growth and RepairRemodeling of Bones

Bone is a living tissue that is constantly broken down and built up.

Osteoclasts are derived from monocytes and break down bone and deposit calcium in the blood.

Osteoblasts then rebuild the bone and some become osteocytes in lacunae.

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Bone Development and Growth

The embryonic human skeleton is at first hyaline cartilage, but it is later replaced by a bony skeleton in a process of endochondral ossification.

Osteoblasts form a primary ossification center.

A band of cartilage called a growth plate separates it from the secondary ossification center.

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Endochondral ossification of a long bone

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Bones of the Skeleton

The skeleton:

supports the body;

protects soft body parts;

permits flexible movement;

produces blood cells; and

serves as a storehouse for mineral salts, particularly calcium phosphate.

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Classification of the BonesThe 206 bones of the human may be

classified according to their shape or whether they are in the axial skeleton or appendicular skeleton.

Shapes include long bones, short cube-shaped bones, flat bones, round bones, and irregular bones such as vertebrae.

The bones are not smooth but have knobs and processes where muscles attach.

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The skeleton

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The Axial Skeleton

The axial skeleton lies in the midline of the body and consists of the skull, the hyoid bone, the vertebral column, and the rib cage.

The SkullThe skull contains the cranium, which

protects the brain, and also includes the facial bones.

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Newborns have incomplete skull bones with membranous fontanels that grow closed by 16 moths.

Some skull bones contain sinuses.

Infections in the mastoid sinuses can lead to mastoiditis, an inflammation that can lead to deafness.

The major bones of the cranium include the frontal, parietal, temporal, occipital, ethmoid, and sphenoid bones.

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Bones of the skull

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At the base of the occipital bone is the foramen magnum through which the spinal cord attaches to the brain.

The Facial BonesThe facial bones include the mandible

(lower jaw), maxillae (upper jaw and anterior hard palate), zygomatic bones (cheek bones), and the nasal bones.

Ears are only elastic cartilage. The nose is a mixture of bones, cartilage,

and fibrous connective tissue.

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Bones of the face

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The Hyoid Bone

The hyoid bone located above the larynx is the only bone in the body that does not articulate with another bone.

The hyoid bone anchors the tongue and serves as the site of attachment for the muscles associated with swallowing.

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The Vertebral ColumnThe vertebral column consists of 33

vertebrae, and supports the head and trunk, protects the spinal cord and roots of spinal nerves, and serves as a site for muscle attachment.

Scoliosis is a sideways curvature of the spine.

The first and second cervical vertebrae are the atlas and axis that allow the head to pivot.

Intervertebral discs act as padding.

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The vertebral column

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The Rib CageThe rib cage is composed of the thoracic

vertebrae, the ribs with their associated cartilages, and the sternum.

The rib cage protects the heart and lungs, and expands during inhalation.

The RibsThere are 12 pairs of ribs attached to the

thoracic vertebrae.

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The upper seven pairs of the ribs attach to the sternum (true ribs); the next three pairs connect indirectly to the sternum by means of common cartilage (false ribs), and the last two pairs are called floating ribs because they have no connection at all to the sternum.

The SternumThe sternum consists of the manubrium,

the body, and the xiphoid process that fuse during fetal development.

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Thoracic vertebrae and the rib cage

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The Appendicular SkeletonThe appendicular skeleton consists of the

bones of the pectoral girdle, arms, pelvic girdle, and legs.

The Pectoral Girdle and ArmThe pectoral girdle includes the clavicle

(collarbone) and scapula (shoulder blade).

The arm is made up of the humerus (upper arm), and ulna and radius (forearm).

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Tendons forming a socket for the humerus are the rotator cup.

Vigorous rotations of the arm can damage the rotator cuff.

The glenoid cavity of the scapula also articulates with the humerus.

The bones of the hand are: eight carpal bones, five metacarpal bones, and phalanges of the fingers and thumb.

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Bones of a pectoral girdle and arm

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The Pelvic Girdle and LegThe pelvic girdle is made of two coxal

bones; the pelvis is composed of the pelvic girdle, sacrum, and coccyx.

In the leg, the femur is the longest and strongest bone; the femur articulates with the coxal bones at the acetabulum.

The patella is the kneecap and the tibia and fibula form the lower leg.

Bones of the foot are: tarsal bones, calcaneus (heel), metatarsal bones, and phalanges.

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A coxal bone and the bones of a leg

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Articulations

There are three types of joints (articulations):

Fibrous joints such as the sutures of the cranium, are immovable.

Cartilaginous joints, like those between the ribs and sternum or the vertebral discs, are slightly movable.

Synovial joints consist of a membrane-lined synovial capsule that is freely movable.

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The knee, which is a synovial joint, also has pads of cartilage called menisci that add stability to uneven surfaces within the knee, along with fluid-filled sacs called bursae that ease friction between the tendons and ligaments.

There are different kinds of synovial joints based on the movements they permit.

Most movable are the ball-and-socket joints, such as the shoulder or hip joints.

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Knee joint

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Skeletal MusclesHumans have three types of muscle tissue:Smooth muscles lack striations and

comprise involuntary muscle in internal organs.

Cardiac muscle cells are striated, cylindrical and branched; fibers are intercalated to allow contractions to spread quickly.

Skeletal muscle fibers are striated, multinucleate, and voluntary.

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Skeletal Muscles Work in PairsSkeletal muscle is covered in layers of

fibrous connective tissue called fascia.

A skeletal muscle has an origin on the stationary bone; the end of the muscle that moves is the insertion.

Prime movers do most of the work but are assisted by synergists.

Whole muscles work in antagonistic pairs; for example, the biceps flexes the lower arm and the triceps extends it.

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Attachment of skeletal muscles

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Nomenclature

Skeletal muscles are named according to:

muscle size, muscle shape,

location, direction of fibers,

number of attachments, and action of the muscle.

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Human musculature, anterior

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Human musculature, posterior

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Mechanism of Muscle Fiber Contraction

Overview of Muscular Contraction The sarcolemma (plasma membrane) of a

muscle fiber forms transverse tubules (T tubules) that extend into the fiber and almost touch the sarcoplasmic reticulum which stores calcium ions.

The sarcoplasmic reticulum encases hundreds up to thousands of myofibrils, the contractile portions of muscle fibers.

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Contraction of a muscle

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Myofibrils and SarcomeresMyofibrils that run the length of a muscle

fiber are divided into contractile units called sarcomeres.

A sarcomere extends between two dark lines called Z lines.

The arrangement of myosin (thick) filaments and actin (thin) filaments in a sarcomere accounts for striations or banding patterns of myofibrils.

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Light micrograph of skeletal muscle

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Sliding FilamentsImpulses travel through T tubules to the

sarcoplasmic reticulum, which releases Ca2+, and the muscle fiber contracts.

When sarcomeres shorten, actin filaments slide past myosin filaments.

The movement of actin filaments in relation to myosin filaments is called the sliding filament theory of muscle contraction.

During the sliding process, the sarcomere shortens, but the filaments remain the same length.

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Muscle InnervationThe motor neuron axon bulb is separate

from the sarcolemma at a synaptic cleft within the neuromuscular junction.

Synaptic vesicles in the axon bulb release the neurotransmitter acetylcholine (Ach) that binds to protein receptors on the muscle fiber sarcolemma.

Next, impulses to travel down T tubules and calcium leaves the sarcoplasmic reticulum, resulting in myofibril contraction.

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Neuromuscular junction

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Two other proteins are associated with the actin filament: tropomyosin, that winds about the actin filament, and troponin that occurs at intervals along the tropomyosin threads.

Calcium ions bind to troponin, allowing tropomyosin to shift position to expose myosin binding sites.

A myosin filament is composed of many myosin molecules, each containing a head with an ATP binding site.

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Function of Ca2+ in muscle contraction

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Myosin heads function as ATPase enzymes, and once they break down ATP, the myosin heads are ready to attach to the next set of myosin binding sites on actin myofilaments.

The release of ADP + (P) causes the head to change its position; this is the power stroke that causes the actin filament to slide toward the center of a sarcomere.

When the myosin head catalyzes another ATP, the head detaches from actin, and the cycle begins again.

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Function of cross-bridges in muscle contraction

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Whole Muscle Contraction

Basic Laboratory Observations In the laboratory, muscle contraction can

be studied by using an excised frog muscle (gastrocnemius) and stimulating it with electricity.

Muscle contraction is recorded as a myogram and is described in terms of a single muscle twitch or sustained contraction called tetanus.

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A muscle twitch is divided into three stages: the latent period, or time between stimulation and when the contraction begins; the contraction period, during which the muscle shortens; and the relaxation period, when the muscle returns to its former length.

A muscle fiber contracts in an all-or-none fashion.

The contraction of a whole muscle varies in strength depending on the number of muscle fibers contracting.

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Physiology of skeletal muscle contraction

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Muscle Tone in the BodyIn the body, muscles exhibit tone, in

which some fibers within a muscle are always contracting.

Maintenance of muscle tone requires muscle spindles.

Recruitment and the Strength of Contraction

As the intensity of nervous stimulation increases, more and more motor units are activated; this is recruitment.

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Energy for Muscle ContractionA muscle fiber has three ways to acquire

ATP after muscle contraction begins:

(1) creatine phosphate, built up when a muscle is resting, donates phosphates to ADP, forming ATP;

(2) fermentation with the concomitant accumulation of lactic acid quickly produces ATP; and

(3)oxygen-dependent aerobic respiration that occurs within mitochondria.

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The three pathways for acquiring ATP work together during muscle contraction.

Myoglobin, an oxygen carrier similar to hemoglobin, is synthesized by muscle cells and accounts for the reddish-brown color of skeletal muscle.

Myoglobin serves as an extra source of oxygen during aerobic respiration in muscles.

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Oxygen DebtWhen a muscle uses up its available

supplies of oxygen, oxygen debt occurs, and the muscle cells switch to anaerobic means of supplying energy.

Fermentation results in oxygen debt because oxygen is needed to complete the metabolism of lactate; lactate builds up in muscle tissue in the absence of O2.

Repaying the oxygen debt requires replenishing creatinine phosphate and disposing of lactate.

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Energy and Muscle Contraction

Exercise and Size of Muscles

Lack of exercise causes atrophy or shortening of muscle fibers.

Frequent exercise can cause hypertrophy or increase in muscle size.

Regular exercise has many health benefits, including enhancing mood and relieving depression.

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Slow-Twitch and Fast-TwitchThe muscles of some individuals have

many slow-twitch fibers.These fibers are aerobic and have steady

power and endurance, enhancing performance at a sport such as cross-country running.

Muscles of others have many fast-twitch fibers.

These fibers are anaerobic, have explosive power but fatigue easily, enhancing sports like weight lifting.

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Slow- and fast-twitch muscle fibers

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Chapter Summary

Bone is an active living tissue that grows and undergoes repair.

The fetal skeleton is cartilaginous and is soon replaced by bone.

Bones are constantly being broken down and rebuilt by two specialized cells.

Skeletal bones are divided into those of the axial skeleton and those of the appendicular skeleton.

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Joints are classified according to anatomy; only one type is freely movable.

Skeletal muscles work in antagonistic pairs to move bones in opposite directions.

Muscles permit movement but have other functions as well.

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A chain of events lead from nervous stimulation to muscle fiber contraction.

At the neuromuscular junction, the nervous stimulus is passed from nerve fiber to muscle fiber.

In muscle fiber contraction, the protein myosin breaks down ATP.

In the body, muscles have tone, and vary in the strength of contraction.

Muscle fibers contract in an all-or-none fashion.

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The three sources of ATP for muscle contraction are aerobic respiration, creatine phosphate breakdown, and fermentation.

Muscle fibers differ in capabilities; some are better for one function or sport than others.

Exercise has many health benefits aside from increasing the strength and endurance of muscles.