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Muscular system

Contributed by: Warren F. Walker, Iain S. Young, John D. Altringham, Charles R. Noback

Publication year: 2014

The muscular system consists of muscle cells, the contractile elements with the specialized property

of exerting tension during contraction, and associated connective tissues. The three morphologic types

of muscles are voluntary muscle, involuntary muscle, and cardiac muscle. The voluntary, striated, or skeletal

muscles are involved with general posture and movements of the head, body, and limbs. The involuntary,

nonstriated, or smooth muscles are the muscles of the walls of hollow organs of the digestive, circulatory,

respiratory, and reproductive systems, and other visceral structures. Cardiac muscle is the intrinsic muscle tissue

of the heart. See also: MUSCLE .

In this article, the comparative embryology of the voluntary and involuntary muscles of the vertebrates will be

outlined, followed by the comparative anatomy of the muscular system.

Comparative Anatomy

Phylogenetically speaking, muscles are very plastic organs. They have undergone considerable change during the

evolution of vertebrates, correlated in large part with changes in the organisms’ environments and in their

methods of support and locomotion. Establishment of homologies among muscles is not easy. Adult relationships

can be misleading because muscles have subdivided during their evolution, and parts have migrated far from

their original positions. Nerve supply is a more reliable criterion, because nerves have tended to follow the

muscles through their evolutionary gymnastics, but often homologies cannot be established without recourse to

embryonic development. Determination of the development of the thousands of individual muscles among the

vertebrate classes is a monumental task. Comparison of muscles among vertebrates is greatly facilitated if the

muscular system is subdivided into groups whose homology can be more easily established in the various classes.

Muscle groups are particularly distinct in elasmobranchs and other primitive fishes, and they are generally defined

on the basis of their embryonic origin in these animals. Two major groups of skeletal muscles are recognized,

somatic (parietal) muscles, which develop from the myotomes, and branchiomeric muscles, which develop in the

pharyngeal wall from lateral plate mesoderm. The somatic musculature is subdivided into axial muscles, which

develop directly from the myotomes and lie along the longitudinal axis of the body, and appendicular muscles,

which develop within the limb bud from mesoderm derived phylogenetically as buds from the myotomes.

The vertebrate muscular system is the largest of the organ systems, making up 35–40% of the body weight in

humans. The movement of vertebrates is accomplished exclusively by muscular action, and muscles play the

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Image of 1 Fig. 1 Superficial muscles. ( a ) Cyclostome ( Petromyzon ). ( b ) Elasmobranch ( Squalus ). ( After H. W. Rand, The Chordates, Blakiston, 1950 )

major role in transporting materials within the body. Muscles also help to tie the bones of the skeleton together

and supplement the skeleton in supporting the body against gravity. See also: SKELETAL SYSTEM .

Axial musculature

Most of the axial musculature is located along the back and flanks of the body, and this part is referred to as trunk

musculature. But anteriorly the axial musculature is modified and assigned to other subgroups. Certain of the

occipital and neck myotomes form the hypobranchial muscles, and the most anterior myotomes form the

extrinsic ocular muscles.

The trunk musculature of cyclostomes consists of a long series of segmental myomeres, each consisting of many

longitudinal fibers attaching onto the myosepta ( Fig. 1 ). Each is folded in such a way as to appear approximately

zigzag-shaped on the surface. The arrangement in jawed fishes is essentially the same, but the folding of the

myomeres is more complex, and each is divided by a horizontal connective-tissue septum into dorsal (epaxial)

and ventral (hypaxial) portions. A spinal nerve passes to each myomere, the dorsal ramus going to the epaxial

portion and the ventral ramus to the hypaxial portion. This pattern of innervation persists in all higher

vertebrates.

Epaxial musculature. The epaxial musculature remains powerful in most cases. In amphibians, it consists of a group

of medial and deep fibers that interlace the vertebrae, and a larger group of superficial fibers (dorsalis trunci).

Segmentation is retained and undulations of the trunk and tail still play a role in the locomotion of many

amphibians ( Fig. 2 ).



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Image of 2 Fig. 2 Superficial muscles of three vertebrates, showing segmentation. ( a ) Amphibian ( Necturus ). ( b ) Reptile ( Sphenodon ). ( c ) Mammal ( Felis ). ( After H. W. Rand, The Chordates, Blakiston, 1950 )

In typical reptiles, the epaxial musculature is more complex. A medial and deep group of small, largely segmental

muscles bind the vertebrae together and constitute the transversospinalis system; more laterally the musculature

is arranged in two more extensive longitudinal groups, the longissimus dorsi, which lies dorsal to the transverse

processes, and the iliocostalis, which is attached to the ribs.

These three main divisions persist in mammals, but posteriorly there is a union of the iliocostalis, longissimus,

and sometimes the more superficial part of the transversospinalis system to form a powerful erector spinae

(sacropinalis) complex that helps to support the vertebral column. In mammals, the body is held off the ground

by the limbs; thus the backbone is sometimes compared to a girder supported anteriorly and posteriorly by



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pillars. Much of the epaxial musculature functions as tie members resisting tension stresses along this girder.

Anteriorly there is a cleavage of the epaxial divisions into a host of muscles associated with the complex head and

neck movements.

In birds, the epaxial musculature in the trunk is greatly reduced, correlated with a fusion of many of the trunk

vertebrae.

Hypaxial musculature. The hypaxial musculature of tetrapods can be subdivided into three groups: (1) a

subvertebral (hyposkeletal) group located ventral to the transverse processes and lateral to the centra of the

vertebrae, (2) the flank muscles forming the lateral part of the body wall, and (3) the ventral abdominal muscles

located on each side of the midventral line.

The subvertebral musculature assists the epaxial muscles in the support and movement of the vertebral column.

In mammals, it consists of longitudinal bundles—the longus colli in the neck and the anterior thorax, the

quadratus lumborum, and psoas minor more posteriorly.

Most of the flank musculature takes the form of broad, thin sheets of muscle that form much of the body wall and

support the viscera. The ancestral, segmental nature of this musculature is retained throughout the trunk in

salamanders, but is lost in higher tetrapods except in those parts of the trunk where ribs are well developed (Fig.

2). Three layers can be distinguished in the abdominal region of most tetrapods: a superficial external oblique,

whose fibers extend caudally and ventrally; an internal oblique with fibers at right angles to the preceding; and a

deep transversus abdominis. This pattern is much the same in the costal region, external intercostals, internal

intercostals, and a reduced transversus thoracis being present in mammals. In reptiles, the pattern is more

complex; the external layer is represented by supracostals, external intercostals, and sometimes a subcutaneous

muscle.

Respiratory movements of reptiles and birds are accomplished by the costal and abdominal muscles described

above, but in mammals, which have a higher metabolic rate, additional respiratory muscles have evolved from

the hypaxial muscles: the diaphragm (a derivative of cervical myotomes), serratus dorsalis, scalenes, and

transversus costarum. See also: RESPIRATORY SYSTEM .

Other parts of the hypaxial flank musculature have gained an attachment to the pectoral girdle where they help

to transfer body weight from the vertebral column to the girdle and appendage and help to regulate the

movement of the girdle. Only a few muscles of this type, the thoracoscapularis and levator scapulae, for example,

are present in primitive tetrapods such as salamanders, and the body is not held far off the ground. In mammals,

however, this group includes such large and powerful muscles as the serratus ventralis, rhomboideus, and levator

scapulae ventralis. In the pelvic region of tetrapods, weight is transferred to the appendage directly across the

sacroiliac joint and not by muscles.



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The midventral hypaxial musculature in all tetrapods consists of the rectus abdominis, a longitudinal muscle on

each side of the midline that extends from the pelvic region to the anterior part of the trunk (Fig. 2). It has

evolved from the oblique flank muscles and in some salamanders remains closely associated with them.

Transverse tendinous inscriptions are often present and are believed to represent persistent myosepta.

Hypobranchial musculature. The hypobranchial musculature extends from the pectoral girdle forward along the

ventral surface of the neck and pharynx to the hyoid arch, chin, and into the tongue. It is regarded as a

continuation of part of the hypaxial trunk musculature, because it develops ontogenetically in most vertebrates

from the ventral portion of several occipital and neck myotomes that grow around the back of the gill region and

then into the neck and tongue. The innervation of the hypobranchial muscles in amniotes by the cervical nerves

and the hypoglossal nerve, which itself has evolved from certain of the spinal and occipital nerves of fishes and

amphibians, further indicates the myotomic origin of this group.

The hypobranchial musculature of cyclostomes (Fig. 1) retains its primitive myomeric character, but the

myomeres fuse to form longitudinal muscles in higher vertebrates. Traces of myosepta are evident in fishes and

some amphibians, but these disappear in amniotes.

In all gnathostomes, the hypobranchial musculature can be divided at the level of the hyoid arch into prehyoid

(infrahyoid) and posthyoid (suprahyoid) groups ( Fig. 3 ). The prehyoid group of elasmobranchs consists of a

single pair of muscles, the coracomandibulars, extending caudally from the jaw symphysis to attach to the

posthyoid group slightly anterior to the pectoral girdle. The posthyoid group consists of a superficial mass, which

can be subdivided into a coracoarcural and coracohyoid, extending between the pectoral girdle and hyoid arch,

and a deeper mass, the coracobranchials, extending from the pectoral girdle to the ventral surface of the

branchial arches. The coracobranchials act to expand the pharynx and gill pouches; the others help to support

the floor of the pharynx and help to move the hyoid arch and open the mouth.

During the evolution of terrestrial vertebrates, loss of most of the coracobranchials occurs, along with the

reduction and loss of many parts of the branchial arches, but the rest of the hypobranchial musculature remains

and becomes modified in correlation with the evolution of a muscular tongue and the more complex problem of

deglutition in a terrestrial environment. The prehyoid group of amphibians consists primarily of a geniohyoid

extending from the chin to the hyoid, but a few muscle fibers have separated from it and enter the tongue. These

represent the beginning of a complex group of muscles that manipulates the tongue of amniotes: genioglossus,

hyoglossus, styloglossus, and probably the intrinsic lingualis. The posthyoid group of primitive terrestrial

vertebrates consists primarily of a rectus cervicis extending between the ventral part of the pectoral girdle and

hyoid arch, but several slips separate from it to go to other parts of the girdle or to the remnants of the branchial

arches. In higher vertebrates, the rectus cervicis has split into several muscles acting upon the larynx and hyoid:

sternohyoid, sternothyroid, thyrohyoid, and omohyoid.



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Image of 3 Fig. 3 Diagram of the major hypobranchial muscles. ( a ) Shark, in ventral view. ( b ) Mammal, in lateral view. ( After H. W. Rand, The Chordates, Blakiston, 1950 )

Extrinsic ocular muscles. The extrinsic ocular muscles develop from the prootic somites (head cavities). All

vertebrates have six in common ( Fig. 4 ). The median (internal), superior, and inferior recti, together with the

inferior oblique, develop from the first somite and are innervated by the oculomotor nerve. The superior oblique

and lateral (external) rectus develop from the second and third somites, respectively, and are supplied,

respectively, by the trochlear and abducens nerves. All of these muscles insert on the eyeball and move it. In

addition, most terrestrial vertebrates, with the exception of birds, have a retractor oculi, a cone-shaped muscle

lying deep to the recti, that pulls the eyeball deeper into its socket. The retractor oculi has evolved from the

lateral rectus and continues to be innervated by the abducens. In many reptiles and in birds, one or more small

muscles have also separated from the lateral rectus, or from the retractor oculi, and act upon the nictitating

membrane. A levator palpebrae superioris, present in mammals, completes the ocular group. This muscle

elevates the upper eyelid in opposition to the action of certain facial muscles. It has evolved from the superior

rectus and is innervated by the oculomotor nerve. See also: EYE (VERTEBRATE) .

Appendicular musculature

Limb muscles are often classified as intrinsic if they lie entirely within the confines of the appendage and girdle,

and extrinsic if they extend from the girdle or appendage to other parts of the body. This scheme has certain

merits, but is misleading from the phylogenetic point of view, because the extrinsic muscles are not all

appendicular in the sense in which this group has been defined. Some are appendicular muscles that have

developed directly from mesoderm in the limb bud, but others are trunk, hypobranchial, and branchiomeric

muscles that have secondarily become associated with the girdle. These muscles are considered with the group

to which they phylogenetically belong.



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Image of 4 Fig. 4 Human extrinsic ocular muscles, which develop from prootic somites. ( After H. W. Rand, The Chordates, Blakiston, 1950 )

Fishes. The paired fins of fishes are primarily horizontal stabilizing keels, but they are also used in deceleration

and steering, and to help control the depth at which the fish swims. Fin movements are not complex or powerful

and the appendicular muscles in the strictest sense or morphologically simple. A single dorsal muscle (abductor)

and a comparable ventral muscle (adductor) extend from the girdle into the fin (Fig. 1). Certain fibers of these

muscles are so arranged that they can protract and retract the fin. The appendicular muscles are supplied by the

ventral rami of spinal nerves.

Terrestrial vertebrates. In terrestrial vertebrates, the limbs become the main organs for support and locomotion, and

the appendicular muscles become correspondingly powerful and complex. The muscles are too numerous to

describe individually, but they can be sorted into dorsal and ventral groups, because tetrapod muscles originate

embryonically in piscine fashion from a dorsal and a ventral premuscular mass within the limb bud (see table ). In

general, the ventral muscles, which also spread onto the anterior surface of the girdle and appendage, act to

protract and adduct the limb and to flex its distal segments; the dorsal muscles, which also extend onto the

posterior surface of the girdle and appendage, have the opposite effects (retraction, abduction, and extension).

The limb muscles also serve as flexible ties or braces that can fix the bones at a joint and support the body.

Amphibians and reptiles. When at rest, the belly of most amphibians and reptiles is on the ground, and the proximal

segment of each limb extends laterally and slightly dorsally from its articulation with the girdle. Locomotion

involves the partial adduction of the humerus and femur to raise the body off the ground, as well as their

protraction and retraction. Ventral adductor muscles, such as the pectoralis and supracoracoideus in the pectoral

region and the puboischiofemoralis externus in the pelvic region, are relatively large and powerful. During the

evolution of mammals, the limbs have rotated under the body and extend ventrally from the girdle. The body is

held off the ground by bony columns braced by muscles. Adductor muscles are less powerful and certain ones of

them have migrated to other positions and have assumed other functions. The supracoracoideus, for example,

has extended dorsally onto the scapula to form the mammalian supraspinatus and infraspinatus ( Fig. 5 ). These

muscles now act as braces for the limb in its new position and play a role in its protraction and retraction.



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Unlabelled image

Birds. Flight in birds has entailed a considerable modification of the musculature of the pectoral region. As one

example, the ventral adductor muscles are exceedingly large and powerful, and the area from which they arise is

increased by the enlargement of the sternum and the evolution of a large sternal keel. Not only does a ventral

muscle, the pectoralis, play a major role in the downstroke of the humerus, but a ventral muscle, the



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Image of 5 Fig. 5 Lateral views of the shoulder and upper arm muscle structure of two animals. ( a, b ) Lizard. ( c, d ) Opossum. Superficial muscles are shown in a and c ; deep muscles are shown in b and d . ( After A. S. Romer, The Vertebrate Body, 3d ed., Saunders, 1962 )

supracoracoideus, is active in the upstroke as well. The tendon of insertion of the supracoracoideus has shifted

so that it passes through a canal between the clavicle, coracoid, and scapula to attach to the upper surface of the

humerus. Its action is analogous to pulling down on a rope that passes over a pulley and down onto a weight.

Branchiomeric musculature

The branchiomeric (branchial) muscles of fishes form a conspicuous part of the muscular system and are rather

complex. In jawed fishes, they can be subdivided according to the visceral arch with which they are associated.

Mandibular muscles act upon the first, or mandibular arch, and are supplied by the trigeminal nerve. The group

includes such muscles as the levator palatoquadrati, which in the dogfish helps to support the palatoquadrate

cartilage; the adductor mandibulae, the powerful muscles closing the jaws; and the intermandibularis, which

together with certain hypobranchial and hyoid muscles, opens the jaws (Fig. 1).

Hyoid muscles act on the second or hyoid arch and are supplied by the facial nerve. The hyoid arch is modified in

sharks and many other fishes to help support the palatoquadrate, and its musculature is correspondingly



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modified. The gill slits of bony fishes are covered by an operculum, which has developed from the gill septum of

the hyoid arch, and part of the hyoid musculature controls its movement.

The remaining visceral arches (branchial arches) and their muscles are associated with the gills. The muscles of

the third arch are supplied by the glossopharyngeal nerve; those of the fourth through seventh arches by the

vagus. Muscles of a typical branchial arch include constrictors, interbranchials, adductors, and interarcuals, all of

which act to compress the gill pouches and force water out through the gill slits. The gill pouches are opened

primarily by the action of the coracobranchials—a part of the hypobranchial musculature. In addition, each

branchial arch has a levator, but the levators have united to form a single muscle, the cucullaris, and their

insertion has shifted from all but the last branchial arch onto the pectoral girdle (Fig. 1).

During the evolution of terrestrial vertebrates, gills are lost and the visceral arches become reduced and greatly

modified. Most of the mandibular muscles remain associated with the jaws and form the various muscles of

mastication. In a mammal, these are temporalis, masseter, pterygoids, anterior belly of the digastric, and the

mylohyoid (Fig. 2). All but the last two close the jaws. The tensori palati, in the soft palate, and the tensor

tympani, which attaches to the malleus (a derivative of the mandibular arch), also belong to this group. Only a

few hyoid muscles remain associated with the hyoid arch or its derivatives: stylohyoid, posterior belly of the

digastric, and stapedius. Most of the hyoid musculature has spread out beneath the skin of the face and neck to

form the platysma and the numerous facial muscles ( Fig. 6 ). Most of the musculature of the branchial arches is

lost, but parts of it form the intrinsic muscles of the larynx and certain pharyngeal muscles. The cucullaris, in

contrast, enlarges and subdivides to form the trapezius and sternocleidomastoid, muscles that act on the pectoral

girdle and head (Fig. 2). The mammalian motor nerve to these muscles, the spinal accessory, is homologous to

part of the vagus of fishes.

Integumentary musculature

In a number of terrestrial vertebrates, particularly amniotes, certain of the more superficial skeletal muscles of the

body have spread out beneath the skin and inserted into it. These may be described as integumentary muscles,

but it should be emphasized that they are not a natural phylogenetic group but are derived from several different

groups.

Integumentary muscles are particularly well developed in mammals and include the facial muscles and platysma,

derived from the hyoid musculature, and often a large cutaneous trunci. The last is derived from the pectoralis

and latissimus dorsi and fans out beneath the skin of the trunk. The twitching of the skin of an ungulate is caused

by this muscle.

Birds and reptiles have a sphincter colli (Fig. 2), a superficial neck muscle derived from the hyoid musculature

and hence homologous to the platysma, but they lack facial muscles. Other integumentary muscles, derived from

appendicular and trunk muscles, attach to the feathers, especially the large flight feathers on the wings and tails.



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Image of 6 Fig. 6 Facial muscles. ( a ) Monkey. ( b ) Human. ( After H . W . Rand, The Chordates, Blakiston, 1950 )

In snakes, costocutaneous muscles extend from the ribs to the large ventral scales and play an important role in

locomotion.

Histology

Three histological types of muscle are recognized: smooth, striated (skeletal), and cardiac. Smooth and cardiac

muscle fibers generally occur in layers in the walls of organs, but striated muscle fibers are usually grouped into

distinct entities, the skeletal muscles of gross anatomy. In some vertebrates, such as the rabbit, red and white

skeletal muscles can be distinguished, but in most, an individual muscle contains a variable mixture of red and

white fibers. Red fibers contain more sarcoplasm and myoglobin than white fibers. Myoglobin has a greater

affinity for oxygen than hemoglobin, hence oxygen can be taken from the blood and stored by red muscle cells.

The contraction of red fibers is more sustained, less subject to fatigue, and often slower than the contraction of

white fibers. Red muscles, or muscles containing a preponderance of red fibers, tend to be found in situations



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where the muscles are particularly active in either moving the body or maintaining posture. Some examples are

the diaphragm and other respiratory muscles, and the gluteus maximus. White muscles, or muscles with a

preponderance of white fibers, are associated with a more intermittent and often a faster and more powerful

movement. The biceps and digital muscles are examples.

Each skeletal muscle is individualized by a connective tissue sheath, the epimysium, which is a part of the deep

fascia, and by its distinctive attachments onto skeletal elements. The epimysium is continuous with the

connective tissue that invests the bundles of fibers within a muscle and with the connective tissue investing the

individual fibers. Attachments to the skeleton are made by a continuation of this connective tissue into the

periosteum surrounding the bone, and sometimes by connective tissue fibers that penetrate the bone. The

connective tissue connection between muscle and bone may take the form of a cord-shaped tendon or a broad

sheet known as an aponeurosis; or it may be relatively inconspicuous, in which case the muscle is said to have a

fleshy attachment. The attachment of the muscle that tends to be stationary as the muscle contracts is its origin;

the opposite attachment, which pulls on a structure that can be moved, is its insertion, but it must be recognized

that the force exerted by a muscle during contraction is the same at each end.

Skeletal muscles vary greatly with respect to the number of fibers they contain and in the length and arrangement

of their fibers. Important determinants of muscle architecture are the extent and nature of the movement that is

to be brought about, the force that must be exerted, and the space available for the muscle in a particular area.

The extent to which a muscle shortens is a function of the length of its fibers. Experimentally, muscle fibers can

shorten to about one-half of their resting length, but they attach to bones so close to the joints that they seldom

shorten this much. The strength of a muscle, on the other hand, is approximately proportional to the number of

fibers it contains.

Many muscles, such as the sartorius on the thigh of mammals, have long fibers arranged parallel to each other

( Fig. 7 ). An advantage of a strap muscle of this type is that it provides maximum excursion. Quite a different

arrangement is seen in a muscle such as the deltoid on the shoulder, which has a central tendon onto which

many short, diagonal fibers attach at rather acute angles (Fig. 7). Pinnate muscles have many more fibers than a

strap muscle of comparable mass, hence they can exert a greater force, but they shorten over a shorter distance.

The full force of the contraction is not realized, however, because the muscle fibers pull on the tendon at an

angle. The contractile force is resolved into an effective force along the axis of the tendon and a lost component

at right angles to the tendon. This slight disadvantage is outweighed by major advantages. In this way much of the

force of many muscle fibers can act through a common tendon upon a restricted area. This is possible because

the strength of a tendon is 30–120 times that of a muscle of equal cross section. Another advantage is that the

change in angle of the muscle fibers that occurs during contraction (Fig. 7) keeps bulging to a minimum. Some

pinnate muscles, such as the tensor tympani in the middle ear of mammals, can act within a confined space.

The examples selected of muscles with fibers arranged completely parallel to one another or acutely pinnate are

extremes of a continuum of muscle architecture. Some degree of pinnation is very common. The architecture of



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Image of 7 Fig. 7 Diagram showing two types of muscle ar chitectur e. ( a ) Parallel muscle. ( b ) Pinnate muscle. ( c ) Pinnate fiber before and after contraction.

any particular skeletal muscle is dependent upon the requirements of packing within a given area, and the

amount of force and extent of excursion needed.

Muscles are usually arranged so that one muscle or group of muscles will pull a structure in a certain direction,

and an opposing muscle or group will pull the structure in the opposite direction. Several sets of terms describe

these antagonistic actions. Flexion is the movement of a distal part of an appendage toward a more proximal part;

this occurs at the elbow and knee. It also describes the bending of the head or trunk toward the ventral surface.

Extension is the opposite movement. Flexion and extension are sometimes also applied to forward and backward

movements of the appendage at the shoulder and hip, but because they have been used in conflicting senses by

different authors, the terms protraction for a forward movement and retraction for a backward movement are

more appropriate. Abduction is the movement of a part away from some point of reference, and adduction is

movement toward it. For the appendages, the reference point is the midventral line of the body. Various types of

rotary movement occur. For example, rotation of the bones of the forearm so that the palm of the hand faces up

is supination; the opposite movement is pronation.

Muscle Mechanics

Many of the bones serve as lever arms, and the contractions of muscles are forces acting on these arms. The

relationship between most muscles and bones is such that the lever systems are classified as third order ( Fig. 8 ).

The joint, of course, is the fulcrum and it is at one end of the lever. The length of the force arm is the

perpendicular distance from the fulcrum to the line of action of the muscle; the length of the work arm is the

perpendicular distance from the fulcrum to the point of application of the power generated in the lever.

Compactness of the body and physiological properties of the muscle necessitates that a muscle attach close to

the fulcrum; therefore, the force arm is considerably shorter than the work arm. Most muscles are at a

mechanical disadvantage, for they must generate forces greater than the work to be done, but an advantage of

this is that a small muscular excursion can induce a much greater movement at the end of the lever.



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Image of 8 Fig. 8 Typical vertebrate lever system.

Slight shifts in the attachments of a muscle that bring it toward or away from the fulcrum, and changes in the

length of the work arm, can alter the relationship between force and amount of speed of movement.

In general, the force of a muscle is inversely related to the amount and speed of movement that it can cause.

Certain patterns of the skeleton and muscles are adapted for extensive, fast movement at the expense of force,

whereas others are adapted for force at the expense of speed. In the limb of a horse, which is adapted for long

strides and speed, the muscles that move the limb insert close to the fulcrum and the appendage is long. This

provides a short force arm but a very long work arm to the lever system (Fig. 8). In the front leg of a mole, which

is adapted for powerful digging, the distance from the fulcrum to the insertion of the muscles is relatively greater

and the length of the appendage is less, with the result that the length of the force arm is increased relative to the

length of the work arm. See also: BIOMECHANICS .

Locomotion

Different groups of muscles have different mechanical properties. Investigation of these properties in relation to

their performance reveals the functional adaptation of the muscular system.

Running mammals. With increased running speed, the fraction of the stride for which the foot is on the ground

decreases, so that the feet must exert larger forces while they are on the ground to make the average force over a

complete stride match the body weight. Therefore, with increasing speed, a greater muscle mass would need to

be active to produce the necessary force.

The properties of extensor digitorum longus muscle and soleus muscles, which are components of the calf of the

leg, are representative of fast and slow muscle ( Fig. 9 ). Extensor digitorum longus muscle consists wholly of fast

glycolytic and fast-oxidative-glycolytic fibers in roughly equal proportions. In contrast, the soleus consists of only



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Image of 9 Fig. 9 Superficial muscles of the human body. ( a ) Anterior view. ( b ) Posterior view. ( After C. A. Villee et al., Biology, 2d ed., Saunders, 1989 )

slow-oxidative and fast-oxidative-glycolytic fibers. These different fiber populations impart different properties to

these two muscles. Extensor digitorum longus muscle has a faster contraction time, lower fatigue resistance, and

higher power output than soleus. During normal function, these locomotory muscles undergo changes in length,

doing work as they shorten and absorbing energy during reextension.

During walking and trotting, the soleus muscle produces 85–100% of its maximal power output. The frequency

of maximum power output of extensor digitorum longus muscle coincides with the higher stride frequencies

employed during fast galloping. This match between locomotion mechanics and muscle properties has been

observed in other mammals and in reptiles—examples of the evolutionary optimization of design.

Warren F. Walker

Fish swimming. As tailbeat frequency and swimming speed increase, there is a sequential recruitment of myotomal

muscle (that is, skeletal muscle produced from a somatic cell) from superficial to deep muscle fibers. At slow,

sustainable swimming speeds, only the slow muscle fibers close to the lateral line are active. The cost of

locomotion is low at these speeds and, consequently, this fiber type typically makes up just a few percent of the



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Image of 10 Fig. 10 Function of the myotomal muscle. ( a ) In anguilliform swimmers, waves of muscle contraction pass alternately down the sides of the body, throwing it into a series of large-amplitude undulations pushing against the water, generating power (p) thrust. ( b ) In carangiform swimming, one undulatory wave passes down the body at a given instant; thrust is generated alternately on the two sides and all the thrust is generated at the tailblade. The myotomal muscle generates power (p), which is transmitted down the fish toward the tail by muscle which acts temporarily as a tendon (t).

total myotomal muscle mass. Slow muscle fibers generate maximum power at low tailbeat frequencies and can

sustain activity for long periods of time. The power required for swimming increases rapidly with increasing

speed and requires the recruitment of fast muscle fibers, which form the bulk of the myotomal muscle. Fast

muscle fiber has a higher intrinsic power output that generates maximum power at high tailbeat frequencies, but

it fatigues rapidly; therefore, fish can swim at these high speeds of only a brief period of time.

Most fish swim primarily by lateral oscillations of the body. The nature of these oscillations changes among

species, and is related to body form ( Fig. 10 ). For example, long, slender, round-bodied fish (such as the eel)

have a swimming pattern described as anguilliform, that is waves of muscle contraction pass alternately down

both sides of the body, causing a series of large-amplitude undulations. As the undulations pass down the body,

they push against the water, generating the thrust which moves the fish forward. The power generated by the

muscle is converted to hydrodynamic thrust all along the body. This thrust is generated on both sides of the body

at any instant, because there is more than one undulatory wave passing down the body at a given time. Fish with

a tailblade swim in a different way. In carangiform swimming, as seen in the mackerel and other fish with similar

body forms, all of the thrust is generated at the tailblade.

To swim, the anguilliform and carangiform fish must use their myotomal muscle in different ways. In the eel,

myotomal muscle functions in the same way all along the body. As the wave of contraction passes down the

body, the muscle generates power, which is passed directly to the water to generate hydrodynamic thrust. In



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contrast, the function of the active muscle in carangiform swimmers varies along the body. As the wave of

contraction starts near the head, myotomal muscle generated power. However, this is not transmitted directly to

the water, but down the fish toward the tail by muscle which acts temporarily as a tendon. The muscle

transmitting this power is active, but is being stretched because of the complex interaction between the motions

of the fish and the water. Under these conditions, muscle tissue is very stiff and is therefore a good power

transmission element. As the active region passes down the body, muscle which previously transmitted power

may generate power, which is transmitted to the tail by muscle located still further down the body. When wave

of power is generated, it passes down the body, and it is transmitted by more posterior, stiffened muscle to the

tail, which generates the thrust.

Given the enormous diversity of fish body forms and swimming modes, there are many variations on these basic

patterns of muscle use. The scup, for example, is a fish with a short, deep body with high dorsal and ventral fins.

When it swims, there is only 0.65 of an undulatory wave on the body at a given instant. Since it has a large

tailblade, it might be expected to swim like a mackerel, generating all of its thrust at the tailblade. However, it

appears to have some of the characteristics of the eel, and may pass power directly to the water all along its

flattened body and through its high fins. A species of sculpin, a bottom-dwelling predator, shows that muscle

recruitment patterns vary with the mode of swimming. Fast starts and slow turns, for example, require uses of

the myotomal muscle.

Insect flight muscle

Insect flight muscle is divided on structural and physiological grounds into synchronous and asynchronous types,

and on the basis of its mechanical operation within the insect into direct and indirect flight muscles. There is a

large degree of structural diversity between types of synchronous muscle, but the functional distinction between

it and asynchronous muscle is far more fundamental. The fibers of all insect flight muscles are rich in

mitochondria and rely on aerobic metabolism despite the high energy demands of flight.

Direct flight muscle. This type of muscle is present in what are usually regarded as primitive insects, for example,

the dragonflies ( Fig. 11 a ). One end of each direct flight muscle is attached to the base of the wing, and the other

end to the inside of the thorax. Contraction of the flight muscles, therefore, drives the wings directly. The

wingbeat frequencies of insects with direct flight muscles are typically less than 100 Hz. All insects with direct

flight muscles have synchronous muscle.

Indirect flight muscle. Indirect muscles attach to the inside of the thorax rather than the wing. Muscle contraction

deforms the thorax and, through a complex hinge, moves the wings up and down. The indirect elevator muscles

run between the roof and floor, and contraction pulls the roof downward and raises the wings. Contraction of

the depressors, which run from front to back, buckles the thorax, raises the roof, and lowers the wings (Fig. 11

b ). The mass of wings, the aerodynamic forces acting upon them, and the elasticity of the thorax act as a resonant

system that enables some insects to operate with wingbeat frequencies up to 1000 Hz. In these insects, both



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Image of 11 Fig. 11 Two types of insect musculature. ( a ) Direct flight muscle. ( b ) Indirect flight muscle. ( After R. Eckert, D. Randall, and G. Augustine, Animal Physiology Mechanisms and Adaptations, 3d ed., W. H. Freeman, 1988 ).

synchronous and asynchronous indirect flight muscles are found, but only those with asynchronous muscle can

achieve the high wingbeat frequencies.

Synchronous muscle. Synchronous muscle is characterized by an equal and simultaneous neural input and muscular

contraction, each mechanical contraction being caused by a burst of neural activity. The rhythm of the wingbeat

is thus neurogenic in origin. Insects with this type of muscle rarely have wingbeat frequencies that exceed 100

Hz. This is a limitation imposed by the neurogenic nature, and the time required to activate and relax the muscle

in each wingbeat. Some highly modified synchronous flight muscles are found, for example, in the singing

muscles of cicadas.

Asynchronous muscle. Very high wingbeat frequencies are found in some species of insects (for example, the

wingbeat frequency of some midges is of the order of 1000 Hz). These high frequencies are found in insects that

possess asynchronous flight muscles, that is, muscles where the number of nerve impulses to the muscle is much

lower than the high wingbeat frequency. The wingbeat frequency of blowfly is approximately 120 Hz, but the

frequency of neural input is only 3 Hz. The nervous input to these muscles facilitates rather than controls the

frequency of contraction.



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The rhythmic contraction of asynchronous muscle that drives the wings results from a unique resonant coupling

between the flight muscles and the elastic thorax. When the active muscle is stretched by the recoil of the elastic

thorax, it develops force after a short delay. However, this delay is long enough to allow a complete recoil of the

thorax, before delayed force development deforms it. This properly of a stretch-induced delayed force

development is due to a unique myosinfilament structure. The depressors and elevators work in opposition to

oscillate the thorax and beat the wings. The time course of delayed force development enables the muscles and

thorax to resonate at the same natural frequency. Because the system is operating at its resonant frequency, a

minimum amount of energy is required to maintain the oscillations once the wingbeating has started. Because the

muscle does not need to be repeatedly switched on and off, and because the thorax is relatively stiff, the system

can operate at high frequencies.

Body size. If evolution has resulted in optimization of muscle function, size-dependent changes in these processes

should be reflected in the properties of the muscles which drive them. The main function of muscle is to

generate power, normally cyclically and repetitively, for example, contracting and relaxating of the diaphragm

during breathing, or of the leg extensor muscles in walking. The body demands the greatest effort from the

muscles at particular frequencies, for example, the stride frequency of a sprint. Therefore, under normal

circumstances, the muscles should perform best over those frequencies. See also: RESPIRATORY SYSTEM .

Iain S. Young, John D. Altringham

Embryology

The muscles are derived from mesoderm, the middle germ layer. The exceptions are the sphincter and dilator

muscles of the iris and the myoepithelial cells of the sweat and mammary glands, which are derived from

ectoderm. The embryonic mesoderm that differentiates into muscle tissues includes the dorsal mesoderm, head

mesenchyme, intermediate mesoderm, and lateral mesoderm ( Fig. 12 ). The dorsal mesoderm that condenses into

bilateral columns adjacent to the neural tube forms the segmentally arranged myotomes. Most intrinsic voluntary

muscles of the neck and trunk are differentiated from these myotomes. Some voluntary head musculature

(muscles of eye and tongue) and the limb musculature are derived from myotomes in the lower vertebrates; the

limb muscles in higher vertebrates are mainly derived from lateral plate mesoderm. In tetrapods, the myoblast

component of all truncal and appendicular muscles originate from somites (somitic origin). The voluntary

muscles of the branchial (visceral) arches of the head and neck are derived directly from head mesenchyme.

Cardiac muscle is derived from the splanchnic mesoderm. The involuntary musculature is differentiated from the

intermediate mesoderm (mesomere, urogenital mesoderm, or nephrotomic mesoderm) and the lateral mesoderm

(splanchnic mesoderm and hypomere). The intermediate mesoderm differentiates into much of the urogenital

system, and the lateral mesoderm differentiates into the vascular, digestive, and respiratory systems and related

structures. See also: EMBRYONIC DIFFERENTIATION ; EMBRYONIC INDUCTION .



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Image of 12 Fig. 12 Dif fer entiation of myomer es and the establishment of the primor dia of various muscle gr oups. ( a ) Early separation of the myomere into an epimere, supplied by the dorsal ramus of the spinal nerve of the level, and a hypomere, supplied by the ventral ramus of the nerve. ( b ) Later stage in the differentiation of muscle primordia at abdominal levels. ( c ) Primordial muscle masses at the level of the arm buds. ( After C. E. Corliss, Patten’s Human Embryology, McGraw-Hill, 1976 )

Dif fer entiation of striated muscles

Striated muscles differentiate from the myotomes of the somites and from mesenchyme of nonmyotomic origin.

Muscle cells, fascial cells, tendon cells, and aponeurotic cells are derived from these structures. The premuscle

cells (myoblasts) migrate as the organism develops. When the myotome is adjacent to the neural tube, it receives

its initial innervation which is retained during subsequent migration of the developing muscles. The innervation

of the primordial muscle masses and the retention of this innervation during development are significant as a

means of determining the homology of muscles of different species. The nerves are probably not concerned with

organizing these muscle masses, because muscles will develop without any innervation in certain monsters. See

also: NERVOUS SYSTEM (VERTEBRATE) .

The segmental pattern of the myotomes may be retained in the adult, as in the intercostal muscles of the thorax

of mammals and the trunk muscles of fishes. The pattern may be modified as in the flat muscles of the mammalian

abdominal wall. The segmental derivation of the muscle masses may be masked by migration as in the eye and

tongue muscles, by fusion of muscles from several segments as in the rectus abdominis muscle (the product of



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the fusion of muscle masses from successive myotomes), by the splitting of muscles into layers as in the flat

abdominal muscles, and by fusion and realignment of muscles as in the back musculature of higher vertebrates

where muscle fascicles extend through as many as six segments. During metamorphosis in frogs, the segmental

patterns of many muscles in the tadpole are altered by the migration, fusion, and splitting of muscle masses to

form many muscles in the adult frog.

Axial musculature

In all vertebrates, the muscles of the neck, trunk, and tail are derived from myotomes, although claims have been

made that some myoblasts differentiate in dermatomes. The myotomes develop and migrate laterally and

ventrally as the epimere to form the dorsal or epaxial muscle mass (dorsal and lateral back muscles), and as the

hypomere to form the ventral or hypaxial muscle mass (lateral and ventral muscles) [Fig. 12]. The subsequent

fate of this migration is said to differ in different animals. In the aquatic vertebrates, whose primary means of

locomotion is swimming, the musculature is oriented to produce undulatory motion. Each myotome

differentiates into a muscle mass which forms a band from the back to the belly. The muscle cells within each

band are directed cephalocaudally. The epaxial musculature (muscle mass innervated by the dorsal ramus of a

spinal nerve and located dorsal to the vertebral column) is approximately equal in size to the hypaxial

musculature (muscle mass innervated by the ventral ramus of a spinal nerve and located ventral to the vertebral

column). In most terrestrial animals, the bilateral appendages assume primacy in locomotion. In these animals,

the myotomes differentiate into the body-wall musculature pattern in which the epaxial musculature is restricted

to the back and the hypaxial musculature is present on the lateral and ventral aspect of the body wall. In animals

adapted for aerial locomotion, the primitive segmental pattern is modified. The epaxial musculature is greatly

reduced in the trunk but is well developed in the nuchal region. The hypaxial musculature is mainly

concentrated in the pectoral musculature.

Fate of myotomes. The fate of the myotomes during their differentiation into the axial muscles differs in the various

vertebrates ( Fig. 13 ). In the fishes, the embryonic pattern is retained because the epaxial and hypaxial muscles

are relatively equal in size. In the amphibians, two adult patterns are developed. In aquatic amphibians, such as

Necturus , the embryonic segmental pattern is retained. In terrestrial amphibians, such as the frog, the myotomic

derivatives are modified during metamorphosis by migration, fusion, and splitting. As a result, the embryonic

pattern is modified as a functional adaptation to life on land. In the other terrestrial animals, reptiles and

mammals, the embryonic myotomic segmentation is altered further than in the terrestrial amphibians. The epaxial

muscles form the erector spinae back muscles. Some hypaxial muscles retain vestiges of the segmental patterns

(intercostal muscles of abdomen), whereas other muscles are the products of migration, fusion, and splitting

during development (flat abdominal muscles). The muscle patterns are altered more drastically in birds.

The tail bud mesoderm differentiates into myotomes in tailed animals that exhibit lateral movements. In these

forms—fish, tailed amphibians, crocodiles, and whales—the myotomes differentiate into well-developed epaxial

muscles and hypaxial muscles that retain their embryonic metamerism. The musculature of animals with



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Image of 13 Fig. 13 Development of muscles from the visceral arches and from the myotomes in various vertebrates. ( a ) Basic areas of the embryo from which voluntary muscles develop. ( b ) Shark. ( c ) Necturus . ( d ) Frog. ( e –g ) Cat. ( h, i ) Goose. ( After O. E. Nelsen, Comparative Embryology of the Vertebrates, Blakiston, 1953 )

prehensile tails and tails adapted for grasping and wagging movements is partially the result of the migration of

myotomes from the hindlimb area. Of significance in mammals is the derivation of the diaphragm from portions



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of the neck myotomes and their subsequent migration through the neck and thorax to the thoracoabdominal

boundary.

Axial muscles. Each axial muscle in the vertebrates is derived from one or more specific myotomes which are

numbered according to the vertebral level (cervical, thoracic, lumbar, sacral, caudal, or coccygeal regions) at

which the myotome first differentiated. Because the innervation of each myotome occurs during early

development, the number in a region of each spinal nerve is identical with that associated with each myotome. In

the different classes of animals, the regional number of a myotome and the spinal nerve varies because the

number of vertebrae in any region is not similar in the vertebrate classes. Because of this variability, only the

segmental numbers of the myotomes in relation to the adult muscles in humans, as an example, will be presented

below.

Each myotome expands by growing ventrally to form two myomeres: the epimere and hypomere. The epimere,

innervated by the dorsal ramus of a spinal nerve, subdivides into a dorsal segment, which gives rise to the deep

intervertebral muscles of the neck and back, and a more ventral segment, which gives rise to the long muscles of

the neck and back (Fig. 12). The hypomere, innervated by the ventral ramus of a spinal nerve, differentiates into

the flexor muscles of the trunk (hypaxial muscles); intercostal muscles the thorax; muscles of the abdominal

wall; and rectus thoracic and abdominal muscles of the ventral body wall (Fig. 12).

Epaxial muscles. The epaxial muscles of humans, the intrinsic extensor muscles of the back, are derived from the

dorsal portions of 29 myotomes—the first cervical segment through the fourth sacral segments inclusive. These

include 8 cervical, 12 thoracic, 5 lumbar, and 4 sacral segments. The hypaxial cervical musculature is derived

from the ventral portions of the 8 cervical myotomes. The myotomes differentiate into a prevertebral portion

(immediately in front of the vertebral column), a lateral sheet, and a ventral or rectus column. As a result, the

prevertebral portion forms the prevertebral muscles; the lateral sheet forms the scalene muscles; and the rectus

column forms the geniohyoid and infrahyoid muscles. The diaphragm is derived from the hypaxial division of the

third through fifth cervical myotomes from whence it migrates.

Hypaxial muscles. In humans, the muscles of the ventral and lateral thoracolumbar wall are derived from the

hypaxial divisions of the first thoracic through the first lumbar myotomes inclusive. These myotomes differentiate

into a main lateral sheet and a rectus column (ventral edge). The intercostal muscles, the oblique abdominal

muscles, and the transversus abdominis muscle develop from the main lateral sheet, whereas the rectus

abdominis muscle develops from the rectus column. The quadratus lumborum muscle of the posterior abdominal

region is derived from the hypaxial divisions of the first through fifth lumbar myotomes. Although direct

evidence is difficult to observe, it is probable that the muscles of the pelvic diaphragm (the muscular floor of

pelvis, including the coccygeus and levator ani muscles) are derived from the hypaxial divisions of the last four

sacral and all coccygeal myotomes.



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In the chick, the skeletal muscles of the dorsal and dorsolateral trunk are derived from myotomic mesoderm,

whereas the hypaxial muscles differentiate from the somatic mesoderm of the lateral plate.

Paired appendages

Theoretically the paired appendages were derived originally either from gill arches (gill-arch theory) or from fin

folds (lateral-fold theory). On the basis of the embryonic development of the fin musculature, the lateral-fold

theory is favored because, in fish, the fin musculature is differentiated from the myotomes of somites. Muscle

buds of potential muscle cells differentiate at the lower edges of the myotomes and, as slips of tissue, invade the

mesenchyme of each limb bud as the dorsal and the ventral premuscle masses of myoblasts. The dorsal

premuscle mass differentiates into the dorsal, elevator, and extensor muscles, and the ventral premuscle mass

into the ventral, depressor, and adductor muscles of the fins. These muscles are innervated by spinal nerves.

Apparently the lateral plate mesoderm is not capable of forming the fin musculature.

In the tetrapods, including the amphibians, reptiles, birds, and mammals, the muscles of the limbs assume a

relatively large bulk and a complexity of organization to cope with locomotion on land. The embryonic

derivation of most of the myoblasts, which form this musculature in the tetrapods, differs from that in the fishes:

much of the limb musculature of tetrapods is said to be derived in place from lateral plate mesoderm and only

some from the myotomic mesoderm of somites and pharyngeal mesoderm. This mesenchyme is initially found

surrounding the developing skeletal elements (future bones). The standard version is that this mesenchyme does

not originate from the myotomic regions of the somites.

On the basis of their mesodermal embryonic precursors and their innervation, the appendicular musculature may

be classified into three groups: (1) Mammalian muscles, derived from the mesoderm of the posterodorsal

pharyngeal region, include the trapezius and the sternomastoid muscles, which differentiate from the mesoderm

of the last branchial arch. These muscles have attachments which extend from the neurocranium and cervical

vertebrae to the proximal bones of the skeleton of the forelimb. They are innervated by the spinal accessory

cranial nerve. (2) Muscles derived from myotomic mesoderm of somites include, among others, the rhomboids,

pectorals, and serratus muscles of the forelimb and the quadratus lumborum and psoas muscles of the hindlimb.

They are the true metameric derivatives of the embryonic myotomes. These muscles have attachments which

extend from the vertebrae and ribs to the appendages; they are innervated by the anterior rami of the segmental

spinal nerves proximal to where these rami form the brachial plexus of the forelimb and the lumbosacral plexus

of the hindlimb. (3) Muscles derived from the nonsegmental core of the limb bud include all the intrinsic limb

muscles, muscles with attachments located wholly within the limb ( Fig. 14 ). They differentiate within each limb

bud and are innervated by the nerves of the brachial plexus and of the lumbosacral plexus. In general, the

muscles differentiating from the dorsal premuscle mass of the limb bud become the extensor muscles of the

limb, those from the ventral premuscle mass become the flexor muscles, those from the dorsal premuscle mass

near the trunk become the abductor muscles, and those from the ventral premuscle mass near the trunk become

the adductor muscles.



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Image of 14 Fig. 14 Diagrams of the origin of a limb. ( a, b ) The development of a limb bud of an amphibian. ( c ) Limb bud of a chick embryo. ( After T. Torrey, Morphogenesis of the Vertebrates, 2d ed ., 1967 )

Head and visceral arch musculature

The muscles of the head and visceral arches are derived from myotomic mesoderm and from nonmyotomic

mesoderm ( Fig. 15 ). The extraocular muscles of the eye and the tongue muscles are differentiated from

myotomes or from a mesoderm that phylogenetically was originally derived from myotomes. The visceral arch

musculature in all vertebrates is derived from mesenchyme of nonmyotomic origin. Some facial muscles are said

to originate from neural crest cells.

The extraocular muscles of sharks are derived from the preotic somites of the premandibular myotomes (cranial

nerve III), the mandibular myotome (cranial nerve IV), and the hyoid myotome (cranial nerve VI). The extrinsic

muscles of the eye include six which are found in all vertebrates: the superior rectus, internal (anterior) rectus,

inferior rectus, and inferior oblique muscles, which are innervated by cranial nerve III; the superior oblique

muscle, which is innervated by cranial nerve IV, and the external (posterior or lateral) rectus muscle, which is

innervated by cranial nerve VI. In addition, the retractor oculi of many mammals and the quadratus muscle and

pyramidalis muscle of birds are in this category. The tongue musculature in the sharks develops from six postotic

myotomes that migrate ventrally to the hypobranchial region. In higher vertebrates, three postotic (occipital)

myotomes appear to provide the mesodermal source of this musculature which is innervated in all vertebrates by

cranial nerve XII (hypoglossal). Because direct myotomic origin of the extraocular muscles and the tongue

muscles is difficult to demonstrate in the higher vertebrates, the literature disagrees.

The mesoderm of the branchial (gill) arches is derived from head mesoderm which develops in place and not

from any myotome. The first branchial (mandibular) arch mesoderm differentiates into the muscles of mastication

that are innervated by cranial nerve V (trigeminal). The muscles derived from this mesoderm in the fishes are the

mandibular adductor muscle and the first ventral constrictor muscle; in the amphibians the temporal, masseter,

pterygoid, and mylohyoid muscles; in birds the pterygotemporal, temporal, and digastric muscles; and in



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Image of 15 Fig. 15 Basic plan of vertebrate head. ( After O . E . Nelsen, Comparative Embryology of the Vertebrates, Blakiston, 1953 )

mammals the muscles of mastication (temporal, masseter, and pterygoid muscles), mylohyoid muscle, anterior

belly of the digastric muscle, tensor palatini muscle, and tensor tympani muscle. The second branchial (hyoid)

arch mesoderm differentiates into those muscles innervated by cranial nerve VII (facial). The muscles derived

from this mesoderm in fishes are the hyoid gill arch muscles; in amphibians the subhyoid and mandibular

depressor muscles; in birds the sphincters of the neck and the mandibular depressor muscles; and in mammals

the muscles of facial expression and other muscles such as the stylohyoid muscle, stapedius muscle, and the

posterior belly of the digastric muscle. The mesodermal derivatives of this arch in mammals migrate to the scalp

(occipitofrontalis muscle), ear region (auricular muscle), neck (platysma), and the face (orbicularis oculi,

orbicularis oris, and others), collectively called the muscles of facial expression. The third visceral (first branchial

arch) arch mesoderm differentiates into those muscles innervated by cranial nerve IX (glossopharyngeal). The

muscles derived from this mesoderm in fishes are the gill constrictor muscles of this arch and in the higher

vertebrates (mammals) the stylopharyngeus muscle and the upper constrictors of the pharynx. The mesoderm of

the last three visceral arches (second, third, and fourth branchial arches) differentiates into the muscles

innervated by cranial nerve X (vagus). These are the gill constrictor muscles in the fishes and the lower

pharyngeal constrictor and laryngeal muscles in the higher vertebrates.



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The sternocleidomastoid muscles and trapezius muscles of mammals are innervated by the spinal cord division of

cranial nerve XI (spinal accessory). These muscles are derived either from the mesoderm of the last visceral arch

or from postotic myotomes.

Voluntary skin muscles

The skin muscles, the voluntary muscles that move the skin, are divided into two groups, the muscles of facial

expression innervated by cranial nerve VII, and the panniculus carnosus of the body-wall skin innervated by the

anterior (ventral) thoracic nerves. The panniculus carnosus is derived from mesodermal cells that normally form

the pectoral muscles. This muscle, found in such animals as the porcupine, dog, cat, horse, and guinea pig, may

have both its origin and insertion in the skin of some species or its origin on the greater tuberosity of the

humerus and its insertion in the fascia of the skin of the back and thigh of other species.

Involuntary muscles

Involuntary muscles arise independently of the segmental myotomes and visceral arches. The visceral mesoderm

differentiates into the mesenchyme that forms the smooth muscles of the digestive system, respiratory system,

and many blood vessels. The heart also differentiates from this mesoderm. The mesenchyme of the somatopleuric

mesoderm of the body wall, head, and limb buds differentiates into the smooth muscles of the blood vessels of

these regions. Mesenchyme, whatever its origin, is a potential source of smooth muscle. The smooth muscles of

many organs develop in place from mesenchymal cells. The smooth muscles of the structures of the urinary

system and the genital systems are derived from mesenchymal cells of the intermediate mesoderm (nephrotomic

mesoderm). The smooth muscles of ectodermal origin are the dilator muscle of the iris and the myoepithelial

cells of the ducts of sweat glands.

Histogenesis of Muscle

Embryonic muscle cells (myoblasts) are derived from mesenchymal cells of mesodermal origin.

Voluntary muscle

The myoblasts of voluntary muscles of either myotomic or nonmyotomic origin are mononucleated

spindle-shaped cells with clear cytoplasm. Embryonic stem cells from the mesoderm of the myotomes divide to

form bipolar cells. These mononucleated myoblasts cease dividing and develop specialized cell surfaces that

render them capable of fusing to form multinucleated elongated myotubes. Myosin and messenger RNA required

for its synthesis are present in the myoblasts. After fusion, the synthesis of myosin rapidly increases. This is

followed by the appearance of contractile filaments, receptor sites for the neurotransmitter (acetylcholine),

T-tubular system, and sarcoplasmic reticulum.



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The first myofibrils are sparse and coarse. Cross striations are formed almost as soon as the myofibrils are visible.

The elongated myotubes have centrally located nuclei and peripherally located myofibrils in the early stages of

differentiation. Later, when the myofibrils fill the cells, the nuclei become peripherally located adjacent to the

sarcolemma. In humans, at the time of birth, the muscle cells resemble those of the adult except that the nuclei

are rounder, the myofibrils are more slender, and the cross striations are less prominent. The multinuclear

striated muscle cells are the result of nuclear mitosis which is unaccompanied by cytoplasmic divisions. The

postnatal growth of muscle cells is by hypertrophy.

Involuntary muscle

Smooth muscles arise from mesenchymal cells. During embryonic life, the cells migrate and concentrate in the

vicinity of the epithelial linings of the hollow organs. These myoblasts elongate and orient themselves as in the

adult organ. Myofibrils (contractile elements) are visible in the early stages and become more numerous during

later development. The iridic muscles (sphincter and dilator pupillae) and myoepithelial cells of sweat and

mammary glands are derived from ectodermal cells.

Cardiac muscle

Cardiac muscle develops from the splanchnic mesoderm of the heart tube. The myoblasts adhere to each other,

but unlike skeletal muscle, the plasma membranes do not disintegrate—rather the sites of adhesion give rise to

the intercalated discs (gap junctions). As in skeletal muscles, myofibrils and the other structural muscle elements

differentiate early.

Charles R. Noback

Bibliography

N. D. Agnish, Possible role of somites in developing mouse limbs, in vitro, Anat. Rec. , 184:340–341, 1976

R. McNeill Alexander, Exploring Biomechanics: Animals in Motion , 1992

B. I. Balinsky, An Introduction to Embryology , 5th ed., 1981

A. Chevalier, Role of the somitic mesoderm in the development of the thorax in bird embryos, II: Origin of

thoracic and appendicular musculature, J. Embryol. Exp. Morphol. , 49:73–88, 1979

K. L. Moore, The Developing Human , 6th ed., 1998

R. O’Rahilly and F. Muller, Developing Stages in Human Embryos , 1987



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A. S. Romer, The Vertebrate Body , 6th ed., 1986

T. W. Torrey and A. Feduccia, Morphogenesis of the Vertebrates , 4th ed., 1979

W. F. Walker, Jr., Vertebrate Dissection , 9th ed., 2000

Additional Readings

J. E. Garc ́ıa-Arrar ́as and I. Y. Dolmatov, Echinoderms: Potential model systems for studies on muscle regeneration,

Curr. Pharmaceut. Design , 16(8):942, 2010

L. Sherwood, Human Physiology: From Cells to Systems , 8th ed., Brooks ∕ Cole, Belmont, CA, 2013

S. Webster and R. deWreede, Embryology at a Glance , John Wiley & Sons, Chichester, West Sussex, UK, 2012


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