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Chapter 2: The locomotor system

Skeleton

The skeleton consists of bones and cartilages. A bone is composed of several tissues, predominantly a specialized connective tissue that is, itself, called “bone.” Bones provide a framework of levers, they protect organs such as the brain and heart, their marrow forms certain blood cells, and they store and exchange calcium and phosphate ions.

The term osteology, meaning the study of bones, is derived from the Greek word osteon, meaning "bone." The Latin term os is used in names of specific bones, e.g., os coxae, or hip bone; the adjective is osseous.

Cartilage is a tough, resilient connective tissue composed of cells and fibers embedded in a firm, gel-like, intercellular matrix. Cartilage is an integral part of many bones, and some skeletal elements are entirely cartilaginous.

Bones

The skeleton includes the axial skeleton (bones of the head, neck, and trunk) and the appendicular skeleton (bones of the limbs). Bone may be present in locations other than in the bony skeleton. It often replaces the hyaline cartilage in parts of the laryngeal cartilages. Furthermore, it is sometimes formed in soft tissues, such as scars. Bone that forms where it is not normally present is called heterotopic bone.

Types:

Bones may be classified according to shape: long, short, flat, and irregular.

Long bones (fig. 2-1).

Long bones are those in which the length exceeds the breadth and thickness. They include the clavicle, humerus, radius, ulna, femur, tibia, and fibula, and also the metacarpals, metatarsals, and phalanges.

Each long bone has a shaft and two ends or extremities, which are usually articular. The shaft is also known as the diaphysis. The ends of a long bone are usually wider than the shaft, and are known as epiphyses. The epiphyses of a growing bone are either entirely cartilaginous or, if epiphysial ossification has begun, are separated from the shaft by cartilaginous epiphysial plates (discs). Clinically, the term epiphysis usually means bony epiphysis. The part of the shaft adjacent to an epiphysial disc contains the growth zone and newly formed bone and is called the metaphysis. The bony tissue of the metaphysis and of the epiphysis is continuous in the adult, with diappearance of the caritlagenous epiphyseal plate.

The shaft of a long bone (diaphysis) is a tube of compact bone ("compacta"), the cavity of which is known as a medullary (marrow) cavity. The cavity contains either red (blood-forming) or yellow (fatty) marrow, or combinations of both. The cavity of the epiphysis and metaphysis contains irregular, anastomosing bars or trabeculae, which form what is known as spongy or cancellous bone. The spaces between the trabeculae are filled with marrow. The bone on the articular surfaces of the ends is covered by cartilage, which is usually hyaline.

The shaft of a long bone is surrounded by a connective tissue sheath, the periosteum. Periosteum is composed of a tough, outer fibrous layer, which acts as a limiting membrane, and an inner, more cellular osteogenic layer. The inner surface of compact bone is lined by a thin, cellular layer, the endosteum. At the ends of the bone the periosteum is continuous with the joint capsule, but it does not cover the articular cartilage. Periosteum serves for the attachment of muscles and tendons to bone.

Short bones.

Short bones occur in the hands and feet and consist of spongy bone and marrow enclosed by a thin layer of compact bone. They are surrounded by periosteum, except on their articular surfaces.

Sesamoid bones.

Sesamoid bones are a type of short bone embedded within tendons or joint capsules. and These occur mainly in the hands and feet, although the patella represents a particularly large example of a sesamoid bone. They vary in size and number. Some clearly serve to alter the angle of pull of a tendon. Others, however, are so small that they are of scant functional importance.

Accessory bones.

Accessory, or supernumerary, bones are bones that are not regularly present. They occur chiefly in the hands and feet. They include some sesamoid bones and certain ununited epiphyses in the adult. They are of forensic importance in that, when seen in radiograms, they may be mistaken for fractures. Callus, however, is absent, the bones are smooth, and they are often present bilaterally.

Flat bones.

Flat bones include the ribs, sternum, scapulae, lateral part of the clavicle, and many bones of the skull. They consist of two layers of compact bone with intervening spongy bone and marrow. The intervening spongy layer in the bones of the vault of the skull is termed diploe: it contains many venous channels. Some bones, such as the lacrimal and parts of the scapula, are so thin that they consist of only a thin layer of compact bone.

Irregular Bones.

Irregular bones are those that do not readily fit into other groups. They include many of the skull bones, the vertebrae, and the hip bones.

Contours and markings

The shafts of long bones usually have three surfaces, separated from one another by three borders. The articular surfaces are smooth, even after articular cartilage is removed, as in a dried bone. A projecting articular process is often referred to as a head, its narrowed attachment to the rest of the bone as the neck. The remainder is the body or, in a long bone, the shaft. A condyle (knuckle) is a protruding mass that carries an articular surface. A ramus is a broad arm or process that projects from the main part or body of the bone.

Other prominences are called processes, trochanters, tuberosities, protuberances, tubercles, and spines. These are frequently the site of attachment of muscles and tendons. Linear prominences are ridges, crests, or lines, and linear depressions are grooves. Other depressions are fossae or foveae (pits). A large cavity in a bone is termed a sinus, a cell, or an antrum. A hole or opening in a bone is a foramen. If it has length, it is a canal, a hiatus, or an aqueduct. Many of these terms (e.g., canal, fossa, foramen, and aqueduct) are not, however, limited to bones and may be descriptors for other anatomical features.

The ends of bones, except for the articular surfaces, contain many foramina for blood vessels. Vascular foramina on the shaft of a long bone are much smaller, except for one or sometimes two large nutrient foramina that lead into oblique canals, which contain vessels that supply the bone marrow. The nutrient canals usually point away from the growing end of the bone and toward the epiphysis that unites first with the shaft. The directions of the vessels are indicated by the following mnemonic: To the elbow I go; from the knee I flee.

The surfaces of bones are commonly roughened and elevated where there are powerful fibrous attachments but smooth where muscle fibers are attached directly.

Blood and nerve supply (fig. 2-1).

The nutrient artery is shown in fig. 2-1, along with periosteal, metaphysial, and epiphysial vessels. In a growing bone, the metaphysial and epiphysial vessels are separated by the cartilaginous epiphysial plate. Both groups of vessels are important for the nutrition of the growth zone, and disturbances of blood supply may result in disturbances in growth. When growth stops and the epiphysial plate disappears, the metaphysial and epiphysial vessels anastomose within the bone.

Many nerve fibers accompany the blood vessels of bone. Most such fibers are vasomotor, but some are sensory, ending in periosteum and in the adventitia of blood vessels. Some of the sensory fibers are pain fibers. Periosteum is especially sensitive to tearing or tension. Fractures are painful, and an anesthetic injected between the broken ends of the bone may give relief. A tumor or infection that enlarges within a bone may be quite painful. Pain arising in a bone may be felt locally, or it may spread or be referred. For example, pain arising in the shaft of the femur may be felt diffusely in the thigh or knee.

Bone marrow

Before birth, the medullary cavities of bones, as well as the spaces between trabeculae, are filled with red marrow, which produces red blood corpuscles and to certain white blood cells (granulocytes). From infancy onward there is both a progressive diminution in the amount of blood cell-forming marrow and a progressive increase in the amount of fat (yellow marrow).

In the adult, red marrow is usually present in the ribs, vertebrae, sternum, and hip bones. This latter site is the common location of bone marrow biopsy, in which a small amount of red marrow is removed for cytopathologic analysis. The radius, ulna, tibia, and fibula contain fatty marrow in their shafts and epiphyses. The femur and humerus usually contain a small amount of red marrow in the upper parts of the shafts, and small patches may be present in their proximal epiphyses. The tarsal and carpal bones generally contain only fatty marrow. Loss of blood may be followed by an increase in the amount of red marrow as more blood cells are formed.

Development and growth

All bones begin as mesenchymal proliferations that appear early in the embryonic period. In membrane bones (comprising the clavicle, mandible, and certain skull bones), the cells differentiate into osteoblasts that lay down an organic matrix called osteoid. Bone salts are then deposited in this matrix. Some osteoblasts are trapped in the matrix and become osteocytes. Others continue to divide and form more osteoblasts on the surface of the bone. Bone grows only by apposition, that is, by the laying down of new bone on free surfaces.

Most bones, however, develop as cartilage bones. The mesenchymal proliferations become chondrified as the cells lay down cartilage matrix and form hyaline cartilages that have the shapes of the future bones. These cartilages are then replaced by bone, as illustrated in figure 2-2. There is usually more than one ossification center in each bone, and a cartilaginous plate (epiphyseal plate) is the site of lengthening of the bone. The centers of ossification at the lengthening portions of the long bone are termed the epiphyses, while the centers on various other processes of the bone are termed apophyses.

Skeletal maturation

Skeletal development involves three inter-related but dissociable components: increase in size (growth), increase in maturity, and aging. Skeletal maturation is "the metamorphosis of the cartilaginous and membranous skeleton of the foetus to the fully ossified bones of the adult". (Acheson). Skeletal status, however, does not necessarily correspond with height, weight, or age. In fact, the maturational changes in the skeleton are intimately related to those of the reproductive system. These in turn are directly responsible for most of the externally discernible changes on which the estimation of general bodily maturity is usually based. The skeleton of a healthy child develops as a unit, and the various bones tend to keep pace with one another. Hence, radiographic examination of a limited portion of the body is believed by some workers to suffice for an estimation of the entire skeleton. The hand is the portion most frequently examined: "As the hand grows, so grows the entire skeleton," it is sometimes stated.

The assessment of skeletal maturity is important in determining whether an individual child is advanced or retarded skeletally and, therefore, in diagnosing endocrine and nutritional disorders. * Skeletal status is frequently expressed in terms of skeletal age. This involves the comparison of radiograms of certain areas with standards for those areas; the skeletal age assigned is that of the standard that corresponds most closely. Detailed standards have been published for the normal postnatal development of the hand, knee, and foot. Tables showing the times of appearance of the postnatal ossific centers in the limbs are provided for the upper limb and the lower limb.

Skeletal Maturation Periods.

The following arbitrary periods are convenient:

  1. Embryonic period proper. This comprises the first eight postovulatory weeks of development. The clavicle, mandible, maxilla, humerus, radius, ulna, femur, and tibia commence to ossify during the last two weeks of this period.
  2. Fetal period. This begins at eight postovulatory weeks, when the crown-rump length has reached about 30 mm. The following elements commence to ossify early in the fetal period or sometimes late in the embryonic period: scapula, ilium, fibula, distal phalanges of the hand, and certain cranial bones (e.g., the frontal). The following begin to ossify during the first half of intra-uterine life: most cranial bones and most diaphyses (ribs, metacarpals, metatarsals, phalanges), calcaneus sometimes, ischium, pubis, some segments of the sternum, neural arches, and vertebral bodies. The following commence to ossify shortly before birth: calcaneus, talus, and cuboid; usually the distal end of the femur and the proximal end of the tibia; sometimes the coracoid process, the head of the humerus, and the capitate and hamate; rarely the head of the femur and the lateral cuneiform.
  3. Childhood. The period from birth to puberty includes infancy (i.e., the first one or two postnatal years). Most epiphyses in the limbs, together with the carpals, tarsals, and sesamoids, begin to ossify during childhood. Ossification centers generally appear one or two years earlier in girls than in boys. Furthermore, those epiphyses that appear first in a skeletal element usually are the last to unite with the diaphysis. They are located at the socalled growing ends (e.g., shoulder, wrist, knee).
  4. Adolescence. This includes puberty and the period from puberty to adulthood. Puberty usually occurs at 13+/-2 years of age in girls, and two years later in boys. Most of the secondary centers for the vertebrae, ribs, clavicle, scapula, and hip bone begin to ossify during adolescence. The fusions between epiphysial centers and diaphyses occur usually during the second and third decades. These fusions usually take place one or two years earlier in girls than in boys. The closure of epiphysial lines is under hormonal control.
  5. Adulthood. The humerus serves as a skeletal criterion for the transitions into adolescence and into adulthood, in that its distal epiphysis is the first of those of the long bones to unite, and its proximal epiphysis is the last (at age 19 or later). The center for the iliac crest fuses in early adulthood (age 21 to 23). The sutures of the vault of the skull commence to close at about the same time (from age 22 onward).

Cartilage

Cartilage is a tough, resilient connective tissue composed of cells and fibers embedded in a firm, gel-like intercellular matrix.

A skeletal element that is mainly or entirely cartilaginous is surrounded by a connective tissue membrane, the perichondrium, the structure of which is similar to periosteum. Cartilage grows by apposition, that is, by the laying down of new cartilage on the surface of the old. The new cartilage is formed by chondroblasts derived from the deeper cells of the perichondrium. Cartilage also grows interstitially, that is, by an increase in the size and number of its existing cells and by an increase in the amount of intercellular matrix. Adult cartilage grows slowly, and repair or regeneration after a severe injury is inadequate. Adult cartilage lacks nerves, and it usually lacks blood vessels.

Types

Cartilage is classified into three types: hyaline, fibrous, and elastic.

Additional reading

Enlow, D. H., Principles of Bone Remodeling, Thomas, Springfield, Illinois, 1963. An excellent review with original observations.

Frazer's Anatomy of the Human Skeleton, 6th ed., rev. by A. S. Breathnach, Churchill, London, 1965. A detailed synthesis of skeletal and muscular anatomy arranged regionally.

Vaughan, J. M., The Physiology of Bone, Clarendon Press, Oxford, 1970. An excellent account of bone as a tissue and of its role in mineral homeostasis.

Joints

A joint or articulation is "the connexion subsisting in the skeleton between any of its rigid component parts, whether bones or cartilages" (Bryce). Arthrology means the study of joints, and arthritis refers to their inflammation.

Joints may be classified into three main types: fibrous, cartilaginous, and synovial.

Fibrous joints

The bones of a fibrous joint (synarthrosis) are united by fibrous tissue. There are two types: sutures and syndesmoses. With few exceptions, little if any movement occurs at either type. The joint between a tooth and the bone of its socket is termed a gomphosis and is sometimes classed as a third type of fibrous joint.

Sutures. In the sutures of the skull, the bones are connected by several fibrous layers. The mechanisms of growth at these joints (still in dispute) are important in accommodating the growth of the brain.

Syndesmoses. A syndesmosis is a fibrous joint in which the intervening connective tis sue is considerably greater in amount than in a suture. Examples are the tibiofibular syndesmosis and the tympanostapedial syndesmosis.

Cartilagneous joints

The bones of cartilaginous joints are united either by hyaline cartilage or by fibrocartilage.

Hyaline Cartilage Joints. This type (synchondrosis) is a temporary union. The hyaline cartilage that joins the bones is a persistent part of the embryonic cartilaginous skeleton and as such serves as a growth zone for one or both of the bones that it joins. Most hyaline cartilage joints are obliterated, that is, replaced by bone, when growth ceases. Examples include epiphysial plates and the sphenooccipital synchondrosis.

Fibrocartilaginous Joints. In this type (amphiarthrosis), the skeletal elements are united by fibrocartilage during some phase of their existence. The fibrocartilage is usually separated from the bones by thin plates of hyaline cartilage. Fibrocartilaginous joints include the pubic symphysis and the intervertebral discs between the bodies of the vertebrae.

Synovial joints

Synovia is the fluid present in certain joints, which are consequently termed synovial. Similar fluid is present in bursae and in synovial tendon sheaths.

General characteristics

Synovial (diarthrodial) joints possess a cavity and are specialized to permit more or less free movement. Their chief characteristics (fig. 2-3) are as follows:

The articular surfaces of the bones are covered with cartilage, which is usually hyaline in type. The bones are united by a joint capsule and ligaments. The joint capsule consists of an outer, fibrous layer, with a vascular, connective tissue lining its inner surface. This is termed the synovial membrane, which produces the synovial fluid (synovia) that fills the joint cavity and lubricates the joint. The joint cavity is sometimes partially or completely subdivided by fibrous or fibrocartilaginous discs or menisci.

Types

Synovial joints may be classified according to axes of movement, assuming the existence of three mutually perpendicular axes. A joint that has but one axis of rotation, such as a hinge joint or pivot joint, is said to have one degree of freedom. Ellipsoidal and saddle joints have two degrees of freedom. Each can be flexed or extended, abducted or adducted, but not rotated (at least not independently). A ball-and-socket joint has three degrees of freedom.

Synovial joints may also be classified according to the shapes of the articular surfaces of the constituent bones. The types of synovial joints are plane, hinge and pivot (uniaxial), ellipsoidal and saddle (biaxial), condylar (modified biaxial), and ball-and-socket (triaxial). These shapes determine the type of movement and are partly responsible for determining the range of movement.

Movements

Active Movements. Usually one speaks of movement of a part or of movement at a joint; thus, flexion of the forearm or flexion at the elbow. Three types of active movements occur at synovial joints: (1) gliding or slipping movements, (2) angular movements about a horizontal or side-to-side axis (flexion and extension) or about an anteroposterior axis (abduction and adduction), and (3) rotary movements about a longitudinal axis (medial and lateral rotation). Whether one, several, or all types of movement occur at a particular joint depends upon the shape and ligamentous arrangement of that joint.

The range of movement at joints is limited by (1) the muscles, (2) the ligaments and capsule, (3) the shapes of the articular surfaces, and (4) the opposition of soft parts, such as the meeting of the front of the forearm and arm during full flexion at the elbow. The range of motion varies greatly in different individuals. In trained acrobats or gymnasts, the range of joint movement may be extraordinary.

Passive and Accessory Movements. Passive movements are produced by an external force, such as gravity or an examiner. For example, the examiner holds the subject's wrist so as to immobilize it and can then flex, extend, adduct, and abduct the subject's hand at the wrist, movements that the subject can normally carry out actively.

By careful manipulation, the examiner can also produce a slight degree of gliding and rotation at the wrist, movements that the subject cannot actively generate. These are called accessory movements (often classified with passive movements), and are defined as movements for which the muscular arrangements are not suitable, but which can be brought about by manipulation.

The production of passive and accessory movements is of value in testing and in diagnosing muscle and joint disorders.

Structure and function.

The mechanical analysis of joints is very complicated, and articular movements involve spherical as well as plane geometry.

The lubricating mechanisms of synovial joints are such that the effects of friction on articular cartilage are minimized. This is brought about by the nature of the lubricating fluid (viscous synovial fluid), by the nature of the cartilaginous bearing surfaces that adsorb and absorb synovial fluid, and by a variety of mechanisms that permit a replaceable fluid rather than an irreplaceable bearing to reduce friction.

The relationship of the epiphysial plate to the line of capsular attachment is important (see fig. 2-1). For example, the epiphysial plate is a barrier to the spread of infection between the metaphysis and the epiphysis. If the epiphysial plate is intra-articular, then part of the metaphysis is also intra-articular, and a metaphysial infection may involve the joint. In such instances, a metaphysial fracture becomes intra-articular, always serious because of possible damage to articular surfaces. If the capsule is attached directly to the periphery of the epiphysial plate, damage to the joint may involve the plate and thereby interfere with growth.

Intra-articular structures.

Menisci, intra-articular discs, fat pads, and synovial folds (fig. 2-3) aid in spreading synovial fluid throughout the joint, and thereby assist in lubrication.

Intra-articular discs and menisci, which are composed mostly of fibrous tissue but may contain some fibrocartilage, are attached at their periphery to the joint capsule. They are usually present in joints at which flexion and extension are coupled with gliding (e.g., in the knee), and that require a rounded combined with a relatively flattened surface.

Periarticular tissues.

The fascial investments around the joint blend with capsule and ligaments, with musculotendinous expansions, and with the looser connective tissue that invests the vessels and nerves approaching the joint.

Joints are often injured, and they are subject to many disorders, some of which involve the periarticular tissues as well as the joints themselves. Increased fibrosis (adhesions) of the periarticular tissues may limit movement almost as much as does fibrosis within a joint.

Absorption from joint cavity.

A capillary network and a lymphatic plexus lie in the synovial membrane, adjacent to the joint cavity. Diffusion takes place readily between these vessels and the cavity. Hence, traumatic infection of a joint may be followed by septicemia. Most substances in the blood stream, normal or pathological, easily enter the joint cavity.

Blood and nerve supply.

The pattern is illustrated in figure 2-4. Articular and epiphysial vessels arise more or less in common and form networks around the joint and in the synovial membrane, respectively.

The principles of distribution of nerves to joints were best expressed by Hilton in 1863: "The same trunks of nerves, whose branches supply the groups of muscles moving a joint, furnish also a distribution of nerves to the skin over the insertions of the same muscles; and what at this moment more especially merits our attention-the interior of the joint receives its nerves from the same source." Articular nerves contain sensory and autonomic fibers, the distribution of which is summarized in figure 2-4.

Some of the sensory fibers form proprioceptive endings in the capsule and ligaments. These endings are very sensitive to position and movement. Their central connections are such that they are concerned with the reflex control of posture and locomotion and the detection of position and movement.

Other sensory fibers form pain endings, which are most numerous in joint capsules and ligaments. Twisting or stretching of these structures is very painful. The fibrous capsule is highly sensitive; synovial membrane is relatively insensitive.

Use-Destruction (Wear and Tear, Attrition). With time, articular cartilage wears away, sometimes to the extent of exposing, eroding, and polishing or eburnating the underlying bone. Use-destruction may be hastened or exaggerated by trauma, disease, and biochemical changes in articular cartilage. The bone adjacent to such damaged joints may expand as "osteophytes."

Additional reading

Barnett, C. H., Davies, D. V., and MacConaill, M. A., Synovial Joints. Longmans, London, 1961. A good account of the biology, mechanics, and functions of joints.

Freeman, M. A. R. (ed.), Adult Articular Cartilage, Pitman, London, 1973. A good account of lubrication and synovial fluid.

Gardner, E., The Structure and Function of Joints, in Arthritis, 8th ed., ed. by J. L. Hollander and D. J. McCarty, Lea & Febiger, Philadelphia, 1972.

Muscles

Movement is carried out by specialized cells called muscle fibers, the latent energy of which can be controlled by the nervous system. Muscle fibers are classified as skeletal (or striated), cardiac, and smooth.

Skeletal muscle fibers are long, multinucleated cells having a characteristic crossstriated appearance under the microscope. These cells are supplied by motor fibers from cells in the central nervous system. The muscle of the heart is also composed of crossstriated fibers, but its activity is regulated by the autonomic nervous system. The walls of most organs and many blood vessels contain fusiform (spindle-shaped) muscle fibers that are arranged in sheets, layers, or bundles. These cells lack cross-striations and are therefore called smooth muscle fibers. Their activity is regulated by the autonomic nervous system and certain circulating hormones, as well as often reacting to local mechanical factors. They supply the motive power for various aspects of digestion, circulation, secretion, and excretion.

Skeletal muscles are sometimes called voluntary muscles, because they can usually be controlled voluntarily. However, many of the actions of skeletal muscles are automatic, and the actions of some of them are reflex and only to a limited extent under voluntary control. Smooth muscle and cardiac muscle are sometimes spoken of as involuntary muscle.

Skeletal muscles

General characteristics.

Most muscles are discrete structures that cross one or more joints and, by contracting, can cause movements at these joints. Exceptions are certain subcutaneous muscles (e.g., facial muscles) that move or wrinkle the skin or close orifices, the muscles that move the eyes, and other muscles associated with the respiratory and digestive systems.

Each muscle fiber is surrounded by a delicate connective tissue sheath, the endomysium. Muscle fibers are grouped into fasciculi, each of which is enclosed by a connective tissue sheath termed perimysium. A muscle as a whole is composed of many fasciculi and is surrounded by epimysium, which is closely associated with fascia and is sometimes fused with it.

The fibers of a muscle of rectangular or quadrate shape run parallel to the long axis of the muscle (fig. 2-5). The fibers of a muscle of pennate shape are parallel to one another, but lie at an angle with respect to the tendon. The fibers of a triangular or fusiform muscle converge upon a tendon.

The names of muscles usually indicate some structural or functional feature. A name may indicate shape, e.g., trapezius, rhomboid, or gracilis. A name may refer to location, e.g., tibialis posterior. The number of heads of origin is indicated by the terms biceps, triceps, and quadriceps. Action is reflected in terms such as levator scapulae and extensor digitorum.

Muscles are variable in their attachments: they may be absent, and many supernumerary muscles have been described. Variations of muscles are so numerous that detailed accounts of them are available only in special works.

Individual muscles are described according to their origin, insertion, nerve supply, and action. Certain features of blood supply are also important.

Origin and insertion.

Most muscles are attached either directly or by means of their tendons or aponeuroses to bones, cartilages, ligaments, or fasciae, or to some combination of these. Other muscles are attached to organs, such as the eyeball, and still others are attached to skin. When a muscle contracts and shortens, one of its attachments usually remains fixed and the other moves. The fixed attachment is called the origin, the movable one the insertion. In the limbs, the more distal parts are generally more mobile. Therefore the distal attachment is usually called the insertion. However, the anatomical insertion may remain fixed and the origin may move. Sometimes both ends remain fixed: the muscle then stabilizes a joint. The belly of a muscle is the part between the origin and the insertion.

Blood and nerve supply.

Muscles are supplied by adjacent vessels, but the pattern varies. Some muscles receive vessels that arise from a single stem, which enters either the belly or one of the ends, whereas others are supplied by a succession of anastomosing vessels.

Each muscle is supplied by one or more nerves, containing motor and sensory fibers that are usually derived from several spinal nerves. Some groups of muscles, however, are supplied mainly if not entirely by one segment of the spinal cord. For example, the motor fibers that supply the intrinsic muscles of the hand arise from the first thoracic segment of the spinal cord. Not infrequently, muscles having similar functions are supplied by the same peripheral nerve.

Nerves usually enter the deep surface of a muscle. The point of entrance is known as the "motor point" of a muscle, because electrical stimulation here is more effective in producing muscular contraction than it is elsewhere on the muscle, nerve fibers being more sensitive to electrical stimulation than are muscle fibers.

Each motor nerve fiber that enters a muscle supplies many muscle fibers. The parent nerve cell and its motor fiber, together with all of the muscle fibers that it supplies, make up a motor unit. Motor units range from a few muscle fibers in muscles of fine control (such as eye muscles) to several thousand fibers (such as in large, postural muscles like the gluteus maximus).

Denervation of muscle.

Skeletal muscle cannot function without a nerve supply. A denervated muscle becomes flabby and atrophic. The process of atrophy consists of a decrease in size of individual muscle fibers. Each fiber shows occasional spontaneous contractions termed fibrillations. In spite of the atrophy, the muscle fibers retain their histological characteristics for a year or more, eventually being replaced by fat and connective tissue. Provided that nerve regeneration occurs, human muscles may regain fairly normal function up to a year after denervation.

Actions and Functions.

In a muscle as a whole, gradation of activity is made possible by the number of motor units recruited to a movement. Force is built up by first increasing the frequency of activation of a single motor nerve fiber and then by adding activity of more motor nerve fibers that will then increase their frequency of activation. If all the motor units are activated simultaneously, the muscle will contract once. But if motor units are activated out of phase or asynchronously (nerve impulses reaching motor units at different times), tension is maintained in the muscle.

Long and rectangular muscles produce a greater range of movement, whereas pennate muscles exert more force. Power is greatest when the insertion is far removed from the axis of movement, whereas speed is usually greatest when the insertion is near the axis.

The actions of muscles that cross two or more joints are particularly complicated. For instance, the hamstring muscles that cross the hip and knee joints cannot shorten enough to extend the hip and flex the knee completely at the same time. If the hips are flexed fully, as in bending forward to touch the floor, the hamstrings may not be able to lengthen enough to allow one to touch the floor without bending the knees. This is also known as the ligamentous action of muscles: it restricts movement at a joint. It is due in part to relative inextensibility of connective tissue and tendons, and can be modified greatly by training. The term contracture means a more or less permanent shortening of the connective tissue components of a muscle.

The pattern of muscular activity is controlled by the central nervous system. Most movements, even so-called simple ones, are complex and in many respects automatic. The overall pattern of movement may be voluntary, but the functions of individual muscles are complex, variable, and often not under voluntary control. For example, if one reaches out and picks something off a table, the use of the fingers is the chief movement. But in order to get the fingers to the object, the forearm is extended (the elbow flexors relaxing), other muscles stabilize the shoulder, and still others stabilize the trunk and lower limbs so as to ensure maintenance of posture.

Functional classifications.

Muscles may be classified according to the functions they serve in such patterns, namely as prime movers, antagonists, fixation muscles, and synergists. A special category includes those that have a paradoxical or eccentric action, in which muscles lengthen while contracting (fig. 2-6). In so doing, they perform negative work. A muscle may be a prime mover in one pattern, an antagonist in another, or a synergist in a third.

Testing of muscles.

Five chief methods are available to determine the action of a muscle. These are the anatomical method, palpation, electrical stimulation, electromyography, and the clinical method. No one of these methods alone is sufficient to provide full and accurate information.

  1. Anatomical Method. Actions are deduced from the origin and insertion and are verified by pulling upon the muscle, for example, during an operation or ina cadaver specimen. The anatomical method may be the only way of determining the actions of muscles too deep to be examined during life. This method shows what a muscle can do, but not necessarily what it actually does.
  2. Palpation. The subject is asked to perform a certain movement, and the examiner inspects and palpates the participating muscles. The movement may be carried out without loading or extra weight and with gravity minimized so far as possible by support or by the recumbent position. Alternatively, the movement may be carried out against gravity, as when flexing the forearm from the anatomical position, with or without extra load. Finally it may be tested with a heavy load, most simply by fixing the limb by an opposing force. For example, the examiner requests the subject to flex the forearm and at the same time holds the forearm so as to prevent flexion. Palpation of muscles that are contracting against resistance provides the best and simplest way of learning the locations and actions of muscles in the living body. Palpation is also the simplest and most direct method of testing weak or paralyzed muscles, and it is widely used clinically. However, when several muscles take part, it may not be possible to determine the functions of each muscle by palpation alone.
  3. Electrical Stimulation. The electrical stimulation of a muscle over its motor point causes the muscle to contract and to remain contracted if repetitive stimulation is used. Like the anatomical method, electrical stimulation shows what a muscle can do, but not necessarily what its functions are.
  4. Electromyography. The mechanical twitch of a muscle fiber is preceded by a conducted electrical impulse that can be detected and recorded with appropriate instruments. When an entire muscle is active, the electrical activity of its fibers can be detected by electrodes placed within the muscle or on the overlying skin. The recorded response constitutes an electromyogram (EMG). Records can be obtained from several muscles simultaneously and during physiological activities. This makes electromyography valuable for studying patterns of activity. Finally, the electromyographic pattern may be altered by nervous or muscular disease. Electromyography, therefore, can be used in diagnosis. The disadvantage, as in palpation, is the difficulty in assessing the precise function of a muscle that is taking part in a movement pattern.
  5. Clinical Method. A study of patients who have paralyzed muscles or muscle groups provides valuable information about muscle function, primarily by determining which functions are lost. But great caution must be exercised. In some central nervous system disorders, a muscle may be paralyzed in one movement yet take part in another. Even in the presence of peripheral nerve injuries or direct muscle involvement, patients may learn trick movements with other muscles that compensate for or mask the weakness or paralysis.

Reflexes and muscle tone

Many muscular actions are reflex in nature, that is, they are brought about by sensory impulses that reach the spinal cord and activate motor cells. The quick withdrawal of a burned finger and the blinking of the eyelids when something touches the cornea are examples of reflexes. It is generally held that muscles that support the body against gravity possess tone, owing to the operation of stretch reflexes initiated by the action of gravity in stretching the muscles. Whether this is strictly or always true in the human is open to question. There is evidence that when a subject is in an easy standing position, little if any muscular contraction or tone can be detected in human antigravity muscles.

The available evidence indicates that the only "tone" possessed by a completely relaxed muscle is that provided by its passive elastic tension. No impulses reach a completely relaxed muscle, and no conducted electrical activity can be detected.

Structure and function

Each skeletal muscle fiber is a long, multinucleated cell that consists of a mass of myofibrils. Most muscle fibers are less than 10 to 15 cm long, but some may be more than 30 cm long.

Resting muscle is soft, freely extensible, and elastic. Active muscle is hard, develops tension, resists stretching, and lifts loads. Muscles may thus be compared with machines for converting chemically stored energy into mechanical work. Muscles are also important in the maintenance of body temperature. Resting muscle under constant conditions liberates heat, which forms a considerable fraction of the basic metabolic rate.

One of the most characteristic changes after death is the stiffening of muscles, known as rigor mortis. Its time of onset and its duration are variable. It is due chiefly to the loss of adenosine triphosphate (ATP) from the muscles.

Tendons and aponeuroses

The attachment of muscle to bone (or other tissue) is usually by a long, cord-like tendon or sinew or by a broad, relatively thin aponeurosis. Tendons and aponeuroses are both composed of more or less parallel bundles of collagenous fibers. Tendons and aponeuroses are surrounded by a thin sheath of looser connective tissue. Where tendons are attached to bone, the bundles of collagenous fibers fan out in the periosteum.

Tendons are supplied by sensory fibers that reach them from nerves to muscles. They also receive sensory fibers from nearby superficial or deep nerves.

Synovial Tendon Sheaths. Where tendons run in osseofibrous tunnels, for example, in the hand and foot, they are covered by double-layered synovial sheaths (fig. 2-7). The mesotendineum, which is the tissue that forms the continuity between the synovial layers, carries blood vessels to the tendon. The fluid in the cavity of the sheath is similar to synovial fluid and facilitates movement by minimizing friction.

The lining of the sheath, like synovial membrane, is extremely cellular and vascular. It reacts to infection or to trauma by forming more fluid and by cellular proliferation. Such reactions may result in adhesions between the two layers and a consequent restriction of movement of the tendon.

Bursae

Bursae (from L. bursa, a purse), like synovial tendon sheaths, are connective tissue sacs with a slippery inner surface and are filled with synovial fluid. Bursae are present where tendons rub against bone, ligaments, or other tendons, or where skin moves over a bony prominence. They may develop in response to friction. Bursae facilitate movement by minimizing friction.

Bursae are of clinical importance. Some communicate with joint cavities, and to open such a bursa is to enter the joint cavity, always a potentially dangerous procedure from the standpoint of infection. Some bursae are prone to fill with fluid when injured, for example, the bursae in front of or below the patella (housemaid's knee).

Fascia

Fascia is a packing material, a connective tissue that remains between areas of more specialized tissue, such as muscle. The superficial fascia is the majority of the subcutaneous tissue immediately deep to the dermis, with which it blends. The superficial fascial may appear in layers in some parts of the body, with the more portions containing a lot of fat and the deeper layers being more fibrous. This transmits the cutaneouls nerves and blood vessels.

Fascia forms fibrous membranes that separate muscles from one another and invest them, and as such it is often called deep fascia. Its functions include providing origins and insertions for muscles, serving as an elastic sheath for muscles, and forming specialized retaining bands (retinacula) and fibrous sheaths for tendons. It provides pathways for the passage of vessels and nerves and surrounds these structures as neurovascular sheaths. It permits the gliding of one structure on another. The mobility, elasticity, and slipperiness of living fascia can never be appreciated by dissecting embalmed material.

The main fascial investment of some muscles is indistinguishable from epimysium. Other muscles are more' clearly separated from fascia, and are freer to move against adjacent muscles. In either instance, muscles or groups of muscles are generally separated by intermuscular septa, which are deep prolongations of fascia.

In the lower limb, the return of blood to the heart is impeded by gravity and aided by muscular action. However, muscles would swell with blood were it not for the tough fascial investment of these muscles, which serves as an elastic stocking. The investment also prevents bulging during contraction and thus makes muscular contraction more efficient in pumping blood upward.

Fascia is more or less continuous over the entire body, but it is commonly named accord ing to region, for example, pectoral fascia. It is attached to the superficial bony prominences that it covers, blending with periosteum, and, by way of intermuscular septa, is more deeply attached to bone.

Fascia may limit or control the spread of pus. When shortened because of injury or disease, fascia may limit movement. Strips of fascia are sometimes used for the repair of tendinous or aponeurotic defects.

Proprioceptive endings in aponeuroses and retinacula probably have a kinesthetic as well as a mechanical function.

Additional reading

Basmajian, J. V., Muscles Alive, 4th ed., Williams & Wilkins, Baltimore, 1978. An excellent study of muscle functions as revealed by electromyography.

Lockhart, R. D., Living Anatomy, 6th ed., Faber & Faber, London, 1963. Photographs showing muscles in action and methods of testing.

Rosse, C., and Clawson, D. K., The Musculoskeletal System in Health and Disease, Harper & Row, Hagerstown, Maryland, 1980. An attractive account offunctional anatomy, clinical applications, and diseases.

Royce, J., Surface Anatomy, Davis, Philadelphia, 1965. Photographs and key drawings of the living body.

Questions

2-1 Is there a difference between membrane bones and cartilage bones in the adult?

2-1 Cartilage and membrane bones are similar in the adult.

2-2 Where is red marrow found in the adult?

2-2 Red marrow in the adult is found chiefly in the ribs, vertebrae, sternum, hip bones, and upper ends of the humeri and femora. The usual sites for bone marrow biopsy are the sternum and ilium.

2-3 Which portion of the body is examined most frequently in the assessment of skeletal maturation?

2-3 The hand, for which special radiographic atlases are available for reference as to norms, is commonly used in assessing skeletal maturation.

2-4 Are epiphysial centers visible radiographically in the knee at birth?

2-4 The epiphysial center for the distal end of the femur is almost always visible at birth; that for the proximal end is frequently to be seen at full term. This can be important foensically when an infant's body is found, for example.

2-5 Which parts of the limb bones are cartilaginous in the adult?

2-5 The articular surfaces of the limb bones remain cartilaginous.

2-6 What result would be expected from premature closure of epiphysial plates?

2-6 Shortening of the bones of the limbs, as seen, for example, in certain dwarfs (achondroplasia), would be expected from premature closure of epiphysial plates.

2-7 Provide examples of (a) plane, (b) hinge, (c) pivot, (d) ellipsoidal, (e) saddle, (f) condylar, and (g) ball-and-socket joints.

2-7 (a) Most carpal and tarsal joints are plane. (b) The elbow is a hinge. (c) The proximal radio-ulnar joint is a pivot. (d) The wrist is ellipsoidal. (e) The carpometacarpal joint of the thumb is a saddle joint. (f) The knee is condylar. (g) The shoulder and hip are ball-and-socket joints.

2-8 What are (a) the origin and (b) the functions of synovial fluid?

2-8 Synovial fluid arises in synovial membrane and functions chiefly in the lubrication and nourishment of articular cartilage.

2-9 What is the importance of the relationship between the epiphysial plate and the line of capsular attachment?

2-9 Where an epiphysial line is intra-articular, as in the hip joint, infection can spread from the epiphysis (e.g., head of the femur) into the joint, and disease in the diaphysis can reach the joint before being stopped by the epiphysial plate. See figure 12-18.

2-10 What are the advantages of pennate muscles?

2-10 Pennate muscles have relatively more fibers and hence are more powerful, because the power of a muscle is directly proportional to the number of its constituent fibers.

2-11 What is the total number of (a) bones and (b) muscles in the body?

2-11 The total number of bones and muscles in the body is a rather meaningless question. The number of bones, sometimes given as 206 or 208, depends on what is included (e.g., auditory ossicles may be included but sesamoid bones may be excluded) and on age (the number of skeletal pieces is much greater in children and becomes reduced by fusion in later life). Similarly, the number of skeletal muscles, given variously as 346 to 501, depends on what is included (e.g., muscular strips that are not fully separated). Moreover, variations (absent and supernumerary structures) are common in the musculoskeletal system.

Figure legends

Figure 2-1 Diagram of a long bone and its blood supply. The inset shows the lamellae of the compacta arranged in osteons, i.e., vascular canals surrounded by concentric layers of bone.

Figure 2-2 Diagrams of the development of a long bone. A, Cartilaginous model. B, Bone collar. C, Vascular invasion of bone collar and cartilage. D, Endochondral ossification begins. E, Cartilaginous epiphyses begin to be vascularized (arrows). F, An epiphysial center of ossification appears. G, An epiphysial center begins in the other end. H, Two epiphysial plates are evident. I, The last (second) epiphysial center to appear fuses first with the shaft. J, The first epiphysial center to appear (where most growth in length occurs) fuses last with the shaft.

Figure 2-3 Synovial joints. The joint cavity is exaggerated. Articular cartilage, menisci, and intra-articular discs are not covered by synovial membrane, but intraarticular ligaments are.

Figure 2-4 The blood and nerve supply of a synovial joint. An artery is shown supplying the epiphysis, joint capsule, and synovial membrane. The nerve contains (1) sensory (mostly pain) fibers from the capsule and synovial membrane, (2) autonomic (postganglionic sympathetic) fibers to blood vessels, (3) sensory (pain) fibers from the adventitia of blood vessels, and (4) proprioceptive fibers. Arrowheads indicate direction of conduction.

Figure 2-5 The arrangement of fibers in muscles. The fibers are basically parallel (upper row) or pennate (or penniform), i.e., arranged as in a feather (lower row). A, Quadrilateral, e.g., pronator quadratus. B, Straplike, e.g., sartorius. C, Fusiform, e.g., flexor carpi radialis. D, Unipennate, e.g., flexor pollicis longus. E, Bipennate, e.g., rectus femoris. F, Multipennate, e.g., deltoid. Pennate muscles usually contain a larger number of fibers and hence provide greater power.

Figure 2-6 Muscular actions. When the arm is abducted against the examiner's resistance, the deltoid becomes tense. On adduction against resistance, the deltoid relaxes and the weight sinks into it. On adduction produced by lowering a pail from a horizontal position, the pectoralis major is relaxed. The contracted deltoid controls the descent by lengthening. The deltoid is now an antagonist to gravity, which is the prime mover, and is doing negative work (paradoxical action).

Figure 2-7 Synovial and fibrous sheaths of a tendon, and a section of the synovial sheath.

* Additionally, certain conditions, such as scoliosis (lateral curvature of the spine) progress up to the point of skeletal maturation. In this case, the fusion of the apophysis at the iliac crest (“Risser’s sign”) is used as an index of maturation.

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