THE MUSCULAR SYSTEM

The muscular tissue of the body constitutes from one-third to one-half of the body mass of the average vertebrate.

Muscular tissue functions in:

• movement and locomotion, through its direct connection with the skeletal system
• more subtle movements associated with maintaining posture/vertical position
• help to generate heat due to catabolic reactions that are associated with muscular activity (such that animals shiver or increase overall movement when body temperature drops)
• can be modified into other structures, such as electric organs in some fish
The general structure of a muscle fiber include (Fig. 10.2, p. 347) myofibrils (chains of repeating subunits) composed of two kinds of filaments: • thin filaments (composed of myosin) and thick filaments (composed of actin, tropomyosin and troponin) that interact by binding to produce a sliding movement between the filaments, and that creates tension in the muscle fiber leading to muscle contraction. There are three generally recognized muscle tissue types: smooth, cardiac and skeletal, each tissue type with a distinct location in the body, cellular organization (histology), and general action of the muscle fibers (physiology)

Because of the multiple functions of muscles, criteria for classifying muscles include:

1. Color • red - highly vascularized and rich in myoglobin; resist fatigue
• white - low vascularization and lower in myoglobin; quicker to fatigue
2. Location • somatic - move bone or cartilage
• visceral - control activities of organs, vessels, or ducts
3. Nervous system control • voluntary - under immediate conscious control
• involuntary - are not
4. Embryonic origin
5. General microscopic appearance • skeletal - Fig. 10.2, p. 347
• cardiac - Fig. 10.3, p. 348
• smooth - Fig. 10.4, p. 348
Smooth muscle • found lining the walls of blood vessels, visceral organs (such as the digestive tract and uterus) and are also found attached to hairs in the integument.
• two general types: - unitary smooth muscle has self-initiated or myogenic contraction to aid in sustaining the rhythmic movement of the organ with which it is associated
- multiunit smooth muscle has neurogenic contraction, which requires action potentials sent by neurons to regulate its action.
Cardiac muscle • are found solely in the musculature of the heart wall
• in cardiac muscle the branching of the cells increase its overall connectivity and the cells are firmly united with each other through the intercalated disks
• cardiac muscle does not fatigue readily, which is a desirable trait in the muscles that maintain circulation of blood
• action of the cardiac muscle fibers shows mixed control, such that the myogenic rhythm of the heart is maintained by neurogenic control and the entire unit of the cardiac muscle acts as a syncytium, or single functional unit
Skeletal muscle • skeletal muscles are closely associated with the skeleton and are used in locomotion
• each skeletal muscle fiber is also a syncytium due to the close connection between cellular units
• fibers are closely associated with connective tissues and are under voluntary control by the nervous system.

Histology
    Striated?
    Shape
    Branched?
    Nucleus location
    Disks?

Physiology
    Neurological control
    Speed of action

Smooth

Not striated
Spindle-shaped
Not branched
Nucleus central
No disks
 

Involuntary
Slow

Cardiac

Striated
Cylindrical
Branched
Nucleus central
Intercalated disks
 

Involuntary
Fast

Skeletal

Striated
Cylindrical
Not branched
Nucleus peripheral
No disks
 

Voluntary
Fast

General Muscle terminology

As you are familiar with from lab, many unique terms are associated with the muscular system, ranging from describing how a muscle works to the general shape of the muscle itself.

The term "muscle" has at least two meanings:

• muscle cell or fiber - the active contractile component: muscle cells and their endomysium
• muscle organ - the whole organ: muscle cells plus associated connective tissues, nerves, blood supply
Action • takes place by contraction, which creates tension in the muscle so that it shortens and thus moves what it is attached to (whether it is a bone, hair or the epithelium of an organ)
• for skeletal muscles, each muscular unit may be described based on a number of factors, such as where the main body of the muscle (belly) is located, such as shoulder muscles, pectoral muscles, gluteal muscles, etc.
Muscle is not attached directly to bone by the contractile muscle fibers - various wrappings of connective tissues extend beyond the ends of the muscle fibers to connect with the periosteum of the bone: • tendon - cordlike attachment
• aponeurosis - thin flat sheet
• fascia - thin flat sheets of connective tissues that wrap and bind parts of the body together
• raphe - junction of two muscles at a band of connective tissue to form a line of fusion, such as the linea alba
Basis for muscle contraction: • a muscle receiving no nervous stimulus is relaxed or in a resting state - soft shape retained by surrounding collagen fibers
• when nervous stimuli applied beyond muscle’s threshold level, contraction results and tensile force is generated, constituting the active state
• the attached bone and/or mass that must be moved represents the load - whether a muscle actually contracts depends on the relative balance between the tensile force of contraction and the load to be moved (Fig. 10.6, p. 351)
Major contractile characteristics of a muscle include how rapidly it reaches maximum tension and how long it can sustain this tension

Tension and strength are directly related to the number of cross-bridges between muscle filaments

• in the shortest position, filament overlap interferes with cross-bridge formation and tension is low (Fig. 10.6a)
• in the longest position, filaments overlap very little with few cross-bridge formations and tension low (Fig. 10.6b)
• intermediate lengths generate maximum cross-bridging (Fig. 10.6c)

Tonic fibers:

• relatively slow contracting and produce low force
• can sustain contraction for prolonged periods of time
• comprise most of the axial and appendicular skeleton
Twitch (phasic) fibers: • generally produce fast contractions so they often make up muscles used for rapid movement
• slow twitch vs. fast twitch relative, but slow take ~2X longer to reach maximum forces
Origin: the end of a muscle that attaches to the more fixed part of the skeleton, which is the proximal end in limb muscles

Insertion: the point of attachment of a muscle that moves the most when the muscle shortens, and is the most distal end of limb muscles

For the biceps, the belly lies anterior to the humerus, the origin the coracoid process of the scapula, and the insertion is the radial tuberosity. For the triceps, the origins are the posterior surface of the humerus and the infraglenoid tubercle of the scapula, and the insertion is the olecranon of the ulna.

The action of skeletal muscles can be:

• antagonistic - oppose or resist the action of another muscle (such as is the case of the biceps and the triceps)
• synergistic - work together to produce a common effect (such as in the action of making a fist, in which the muscles of the forearm and fingers work together)
Other actions of muscles include: Flexor - decreases the angle at a joint
Extensor - increases the angle at a joint
Abductor - moves a bone away from the midline
Adductor - moves a bone closer to the midline
Levator - produces an upward movement
Depressor - produces a downward movement
Supinator - turns the palm upward or anteriorly
Pronator - turns the palm downward
Sphincter - decreases the size of an opening
Tensor - makes a body part more rigid
Rotator - moves a bone around its longitudinal axis
We can also describe muscles based on shape, such as in the arrangement of the muscle fibers: Strap-shaped muscles - have parallel fibers and broad attachments (teres major)
Fusiform muscles - parallel fibers, but narrow tendons for attachments (biceps)
Pinnate muscles - diagonally arranged fibers that insert on the side of the muscle into a tendon. (subscapularis)
Or Size: Maximus = largest
Minimus = smallest
Longus = longest
Brevis = shortest
Number of origins: Biceps = two origins
Triceps = three origins
Quadriceps = four origins
Relative shape Deltoid = triangular
Trapezius = trapezoid
Serratus = saw-toothed
Rhomboideus = rhomboid or diamond-shaped
The main muscle groups correspond with the divisions used for the skeletal system: Axial muscles - trunk and tail muscles of fishes and tetrapods
Branchiometric muscles - also called visceral muscles, such as those associated with the gills, jaws and hyoid apparatus
Appendicular muscles - fin muscles of fishes and limb muscles of tetrapods
Muscles arise from three embryonic sources: mesenchyme - dispersed throughout the body giving rise to smooth muscles within the walls of blood vessels and some viscera
splanchnic layer of the lateral plate mesoderm - develops into the smooth muscle layers of the digestive tract and into the walls of the heart
paraxial mesoderm, or somites, and specifically the myotome layer of the somite - the primary source of skeletal muscles during development - within the head region, the myotome does not become completely segmented, and instead forms seven pairs of somitomeres that will produce the musculature of the head region (Fig. 10.22a, p. 365)
- the remainder of the somites in the body develop into the trunk and appendicular muscles
Homologies

During muscle evolution, some muscles have fused with one another, others have split into distinct new muscles, some have become reduced in prominence, and others have changed their points of attachment and hence their evolution

Muscle homology can be determined in three ways:

- attachment similarity
- functional similarity
- nervous innervation, due to conserved relationships between a muscle and its nerve supply
Establishing similarity can help compare the different groups of muscles (cranial, axial and appendicular) among the different vertebrate classes

Cranial muscles

External ocular muscles - six extrinsic ocular muscles which attach to the surface of the eye and are responsible for moving the eye within the orbit (Fig. 10.23, p. 366):

Dorsal (superior) oblique     Ventral (inferior) oblique

Dorsal (superior) rectus       Ventral (inferior) rectus

Medial rectus                       Lateral rectus

These muscles are innervated by the oculomotor nerve

Some tetrapods also have a retractor bulbi, which pulls the eyeball further into the orbit to allow for coverage by the nictitating membrane (lacking in humans)

Branchiometric muscles - develop from the myotomes caudal to those that produce the ocular muscles

• closely associated with the visceral skeleton so they are used in both breathing and feeding.
• perform the function of operating the jaw, opening and closing the spiracle (which functions in water intake into the gills when the fish is eating)
• may be subdivided based on what visceral arch they are associated with:
Fishes' gill arch
First arch

Second arch

Third to seventh arch
 

Muscle
Adductor mandibulae
Intermandibularis
Constrictor
Levator
Constrictors
Levators
Action
closes jaw
compresses throat
compresses gills/pharynx
lifts gill bars
compresses gills/pharynx
lifts gill bars

The cucullaris is attached to the last branchial arch but is associated with the pectoral girdle

In tetrapods the branchiometric musculature changed in tandem with changes in the visceral skeleton to make the animals more adapted to a terrestrial environment - resulted in a loss of many branchiometric muscles
 
 
Tetrapod gill arch
First arch
 
 
 

Second arch
Other arches
 

Muscle
Masseter
Temporalis
Pterygoids
Digastric
Mylohyoid
Platysma
Sternomastoid
Cleidomastoid
Action
closes jaw
closes jaw
function in jaw movement
opens the jaw
opens the jaw
moves skin of face and neck
turn head
turn head

Epibranchial and hypobranchial muscles - dorsal and ventral muscles associated with the head and trunk region that perform functions associated with jaw and tongue movement

• muscles of fishes associated with feeding and breathing include: - Coracoarcuals - opens mouth
- Coracomandibular - opens mouth
- Coracohyoid - helps in feeding
- Coracobranchial - helps in swallowing
• muscles in tetrapods are associated with the hyoid apparatus and the tongue: - Tongue muscles - hyoglossus, styloglossus, genioglossus
- Geniohyoid - draws hyoid cranially
- Sternohyoid - draws hyoid posteriorly
- Sternothyroid - draws larynx caudally
- these muscles are also used in speech and sound production in tetrapods
Homologies between the branchial and hypobranchial muscles of several different vertebrate taxa are shown in Table 10.3.

Trunk/axial muscles

The axial musculature associated with the trunk can function either in locomotion or breathing
Axial musculature begins as myotomes separated by myosepta which are then further divided into two regions:

• epaxial muscles - muscles on the dorsal part of the body
• hypaxial muscles - muscles on the ventral part of the body that are separated by the lateral septum (Fig. 10.26, p. 368)
Fishes

In fishes, the trunk muscles remain divided into folded muscle segments or myomeres, that are divided into myosepta

• these muscles contract alternately to produce an undulating motion that propels the fish through the water
• internally, these muscles remain divided into dorsal (epaxial) and ventral (hypaxial) sections by the lateral septum
Tetrapods

In tetrapods, the trunk muscles function more in maintenance of posture, head movement, and respiration rather than in locomotion, which has shifted to the appendicular muscles

The epaxial muscles of the tetrapod trunk skeleton include:

Longissimus dorsi - extends vertebral column
Iliocostalis - draws ribs together
Multifidus spinae - extends vertebral column
Spinalis dorsi - extends vertebral column
The hypaxial muscles of the tetrapod trunk skeleton include: Abdominal muscles: Rectus abdominis - compresses abdomen
Internal oblique - compresses abdomen
External oblique - constricts abdomen
Internal oblique - constricts abdomen
Respiratory muscles: Serratus - draw ribs cranially)
Scalenus - flexes the neck)
Diaphragm - separates the thoracic/abdominal cavities, functions in breathing
Intercostals - protract/retract ribs
Appendicular muscles

Appendicular muscle development originates from the somites as ougrowths of the somite myotome into the limb bud - myotomic buds to the appendages

As the limb bud grows, the appendicular musculature subdivides into the muscle mass that lies above the appendicular skeleton (dorsal muscles) and the mass that lies below the appendicular skeleton (ventral muscles)

These muscle masses later differentiate into multiple muscle groups depending on the type of organism

Fishes

In general, most of the locomotion of fishes is dependent on the action of the axial musculature, which undergoes alternate contraction and relaxation to produce undulating movements of the body

Fins (appendicular appendages) function more in maintaining stability, braking and maneuvering - thus, the range of movement of fins is much more limited than that of tetrapod limbs

• ventral muscles in fishes go to the formation of the abductor muscle, which pulls the fins ventrally and cranially
• dorsal muscles go to the formation of the adductor muscle found on the posterodorsal part of the fin and moves the fin dorsally and caudally
Tetrapods

The tetrapod appendicular musculature is more complex than that of fishes because the limbs function in both support and locomotion

In tetrapods the function of the dorsal and ventral muscle groups is reversed from that seen in fishes

• the dorsal muscles, which in fishes were responsible for adduction will instead abduct or extend the appendages
• the ventral muscles formerly used for abduction are instead used for adduction or flexion
Pectoral region

Muscles of the back

Muscles of the chest
Muscles of the shoulder
 

Muscles of the arm
 
 

Pelvic region










 

Dorsal muscles
(extensors)
Latissimus dorsi
Cutaneous maximus

Deltoids
Subscapularis
Teres major
Triceps
Supinator
Extensors of the digits

Dorsal muscles
(extensors)

Gluteal muscles
Quadriceps
    rectus femoris
    vastus medius
    vastus intermedius
    vastus lateralis
Sartorius
Iliopsoas
Extensors of the digits
Ventral muscles
(flexors)



Pectoralis
Supraspinatus
Infraspinatus

Biceps
Pronator
Flexors of the digits

Ventral muscles
(flexors)

Adductor femoris
Semimembranosus
Semitendinosus
Gracilis
Biceps femoris
Gastrocnemius
Caudofemoralis

Flexors of the digits 

Locomotion

The study of locomotion completes our understanding of the skeletal and muscular systems, because it examines the functional relationship between the two systems as well as between the organism and its environment

We will discuss three important categories of locomotion: swimming, terrestrial locomotion, and flight.

Swimming

We can first distinguish vertebrates that swim by whether they are primary swimmers (species for which swimming is the sole pattern of locomotion) or secondary swimmers (species which have readapted completely or partially to an aquatic mode of life).

Some general requirements of swimmers are that they must:

1) reduce the resistance that water offers to motions of the moving body
2) propel themselves in a relatively dense medium
3) control vertical position in the water
4) maintain orientation and steer the body
In addition, secondary swimmers must also undergo secondary adaptations to their circulatory, respiratory and sensory systems to tolerate the high pressures and exposure to water that swimming entails

Primary swimmers are generally undulatory swimmers that use the musculature of the fins only, or the fins in combination with the trunk and tail to propel themselves through the water

Characteristics of the primary swimmers are:

• a fusiform body that is held rigid by strong articulation of the vertebral column
• segmented myomeres that allow individual muscle units to exert forces over the entire side of the body
• integument that is attached strongly the underlying musculature by connective tissue to increase the compactness of the body
The musculature, skeletal system and integument form an integrated unit that helps to streamline the animal and reduce drag as it moves through the water

Secondary swimmers are generally oscillatory swimmers that propel themselves through the water with paddle-like movements of the appendages

• appendages may also be modified into webbing or flippers to assist in propulsion
• generally have well-developed appendicular musculature
• can reduce pressure drag around the body by temporarily streamlining themselves when moving through water
Terrestrial locomotion

In terms of terrestrial locomotion, there are many different modes that an animal may use:

Cursorial - tetrapods that travel far or fast on the land. Cursorial animals possess a relatively elongate body, in which the vertebral column acts to increase running stride by stretching out to increase forward propulsion

Saltatorial - tetrapods that jump or hop. Saltatorial animals have bodies in which the weight is shifted to the hind legs, the legs are powerful and strongly constructed, and the center of mass is aligned with the sacrum

Scansorial - tetrapods adept at climbing. Scansorial animals have strengthened pectoral musculature and appendages, and modified phalanges for clinging to vertical surfaces

Fossorial - tetrapods that are adept at digging, and live a somewhat subterranean existence. Fossorial animals have highly flexible vertebral columns, strong pectoral musculature, and modified phalanges for digging

Each of these different modes of life requires dramatic modifications in the skeletal system (in terms of the parts of the body that receive the most stress) and the associated musculature

Animals with bipedal locomotion or that are scansorial have a foot posture that is more plantigrade, in which the soles of the feet are placed flat on the ground. In contrast, most cursorial animals have a more digitigrade posture, in which the wrist and ankle are carried off the ground and the animal walks on its digits. Or, the animal may be extremely long legged, and walk only on the tips of the digits such that the terminal end of the digit is modified to form a hoof, and other digits are lost, a posture called unguligrade.

Flight

Tetrapods that fly can do so in three different ways.

  • Parachuting - use of limbs and body to increase overall surface area to break an inadvertent fall
  • Gliding - use of broad membranes attached to limbs to increase surface area and travel a greater horizontal distance through the air
  • True flight - use of wings to actively sustain movement through the air
  • Tetrapods that use active flight, such as birds, have pectoral appendages that are reduced to a single digit and highly developed pectoral musculature necessary to sustain active flight

    In birds, the primary flight muscles originate on the ventral surface of an expanded, keeled sternum:

    • pectoralis - depressor of the wing
    • supracoracoideus - levator of the wing
    In bats, the flight muscles are associated with movement of the humerus and scapula and are located on the side of the thorax: • primary wing depressors: - pectoralis
    - subscapularis
    - serratus anterior
    • primary wing levators: - deltoideus
    - trapezius
    - spinatus  
    Definitions Abductor - moves a bone away from the midline

    Adductor - moves a bone closer to the midline

    Antagonistic - condition in which a muscle opposes or resists the action of another muscle

    Aponeurosis - sheetlike tendon of a muscle

    Cursorial - tetrapods that travel far or fast on the land

    Depressor - produces a downward movement

    Digitigrade - posture in which the wrist and ankle are carried off the ground and the animal walks on its digits

    Epaxial - pertaining to structures that lie above or beside the vertebral axis

    Extensor - increases the angle at a joint

    Fascia-sheets of connective tissue that lie beneath the skin or ensheathe groups of muscles

    Flexor - decreases the angle at a joint

    Flight - use of wings to actively sustain movement through the air

    Fossorial - tetrapods that are adept at digging, and live a somewhat subterranean existence

    Gliding - use of broad membranes attached to limbs to increase surface area and travel a greater horizontal distance through the air

    Hypaxial - pertaining to structures that lie ventral to the vertebral axis

    Insertion - the point of attachment of a muscle that moves the most when the muscle shortens, or the most distal end of limb muscles

    Levator - produces an upward movement

    Origin - the end of a muscle that attaches to the more fixed part of the skeleton, which is the proximal end in limb muscles

    Oscillatory swimmers - propel themselves through the water with paddle-like movements of the appendages

    Parachuting - use of limbs and body to increase overall surface area to break an inadvertent fall

    Plantigrade - posture in which the soles of the feet are placed flat on the ground during locomotion

    Primary swimmers - species for which swimming is the sole pattern of locomotion

    Pronator - turns the palm downward

    Raphe - junction of two muscles at a band of connective tissue to form a line of fusion, such as linea alba

    Rotator - moves a bone around its longitudinal axis

    Saltatorial - tetrapods that jump or hop

    Scansorial - tetrapods adept at climbing

    Secondary swimmers - species which have readapted completely or partially to an aquatic mode of life from a terrestrial life

    Sphincter - decreases the size of an opening

    Supinator - turns the palm upward or anteriorly

    Synergistic - condition in which the muscles work together to produce a common effect

    Tensor - makes a body part more rigid

    Undulatory swimmers - use the musculature of the fins only, or the fins in combination with the trunk and tail, to propel themselves through the water

    Unguligrade - a locomotory posture used by long legged tetrapods, which walk only on the tips of the digits such that the terminal end of the digit is modified to form a hoof, and other digits are lost