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 muscles 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