The Muscular System
Chapter 11
Introduction
• Muscles make things happen:
– Supply force for motion
– Hold animals in position
– Act upon the viscera
– Act in heat regulation
– Produce electrical currents
Types of Muscle Tissues
• The criteria for the classification of muscles is
based on the reason for classifying them at
the time:
1. According to color (Red & White)
2. According to position (Somatic & Visceral)
3. Nervous control (Voluntary & Involuntary)
4. Microscopic appearance (Cardiac, Skeletal, &
Smooth)
Skeletal Muscle
• Skeletal muscle tissue is the most common type
of muscle in the body.
• The cells, or muscle fibers, of skeletal muscles are
very large, multinucleated cells.
– Each muscle fiber is known as a syncytium, a single
cell with multiple nuclei.
• The actin and myosin filaments of skeletal muscle
partially overlap.
– Makes the muscle appear striated.
• Contraction is initiated by nervous impulses and
is described as neurogenic.
• The partial overlap of actin and myosin
filaments increases the ability of the
myofilaments to interact with each other
– Therefore, skeletal muscles can contract with
considerable force.
• When a muscle fiber is stimulated,
1. A redistribution of ions across the plasma
membrane leads to the development of an
electrical action potential,
2. Which travels rapidly to all parts of the muscle
fiber,
3. And the muscle contracts.
• The activity of skeletal muscles is under the
voluntary control of the animal,
– The complex sequence of events to activate the
muscle occurs subconsciously.
Cardiac Muscle
• Cardiac muscle, is composed of moderately
elongated cells that frequently branch
• Each cell originates from a single myoblast, so it
contains a single nucleus, located near the center
of the cell.
• Like skeletal muscles, the actin and myosin
filaments partially overlap appear striated.
• The individual cells are interconnected with each
other end to end, at specialized intercalated
disks.
• Unlike skeletal muscles the action of cardiac
muscle is not voluntary and it does not fatigue.
• Contraction is myogenic, originating within
the muscle, and is triggered within a region
called the pacemaker, or sinoatrial node.
• Nerve fibers terminate on the pacemaker, and
many other cells, so the inherent muscle
rhythm is modulated by neurogenic control.
• The design of the cardiac muscles allows them
to contract with nearly as much force as
skeletal muscles.
Smooth Muscles
• Smooth muscle fibers are elongated, spindle
shaped cells.
• Because each arises from a single myoblast
the cell has a single nucleus located near its
center.
• Actin and myosin filaments are present in the
cytoplasm of the cell
– They do not line up in a regular manner.
– Thus, smooth muscle appears to have a
homogenous texture.
• Smooth muscle fibers are part of the walls of
blood vessels and visceral organs, and they
also attach to the hairs in mammal skin.
• Their actions are involuntary, and the
contractions tend to be slow and sustained.
• Smooth muscles do not fatigue.
Muscle Organization and Connective
Tissue
• In all types of muscle tissues the individual muscle
fibers are enveloped by a thin layer of connective
tissue, known as endomysium, through which the
blood vessels and nerves that supply the fibers travel.
• Groups of skeletal muscle fibers surrounded by their
endomysium form small bundles, or fasciculi, held
together by a layer of connective tissue called
perimysium.
• Many fascicule, in turn, are surrounded by an
epimysium and aggregate into units that we recognize
as individual muscles.
Tendons
• Skeletal muscles have distinct attachments to
skeletal elements or the well defined
connective tissue septa by tendons.
• Tendons consist of an extension of the
connective tissue within the muscle into the
connective tissue that surrounds the bone.
– They are cord-like bands that frequently
penetrates the bone.
• Skeletal muscles, or their tendons, extend across
one or more joints and either;
– Move skeletal elements relative to one another, or
– Stabilize the skeletal elements at the joint s they don’t
move.
• It is convenient to describe the opposite
attachment points of skeletal muscle as the origin
and insertion points.
– The origin is thought of as the point of attachment
that remains fixed during contraction,
– The insertion point is the point that moves;
– However, the point that moves may change with
circumstance.
• By convention the origin is the proximal end of he
muscle, and the insertion is the distal end.
Muscle Function
• The muscle fibers within a skeletal muscle are
organized into motor units
– Consisting of a motor neuron and the muscle
fibers it supplies.
• All muscle fibers in a motor unit are activated
when the neuron supplying them is activated.
• Muscles differ with respect to the number of
muscle fibers within motor units.
• During normal muscle activity, an everchanging rotation of active, relaxing, and
quiescent motor units occurs.
• As functional demands change the proportion
of active motor units increases or decreases.
• When a very fine regulation of contraction is
needed each motor unit contains only a dozen
or fewer muscle fibers.
– Activating only one, or a few, motor units can
cause delicate movements.
– When a strong force is required each unit may
contain as many as 2000 or more muscle fibers.
Muscle Contraction
• Muscle contraction results from the interaction of
actin and myosin myofilaments.
• An action potential initiated on the membrane of
a muscle fiber by a nerve impulse or by other
means initiates a series of biochemical changes
that lead to the formation of cross bridges
between actin and myosin myofilaments.
• The amount of tension or force that a muscle
fiber can generate is a function of the number of
actin-myosin attachments that can be made at
one time.
Modes of Contraction
• The tension that results from the interaction
of myofilaments within a muscle is called a
muscle contraction.
• Depending on the circumstances the muscle
may or may not shorten.
– If the muscle shortens it causes bones, or other
structures to which it attaches to move.
• A muscle contraction that initiates a shortening of the
muscle fiber is called an isotonic contraction.
• Movements of the body are caused by
isotonic contractions.
• In isometric contractions, tension develops,
but little if any shortening of the muscle takes
place.
– Muscles that hold an animal or hold a part in a
fixed place contract isometrically.
Types of Muscle Action
• Muscles perform their functions by developing
tension and often shortening.
• They are restored to their resting length upon
relaxation by an antagonistic force that operates
in a direction opposite to the direction of
contraction.
• Usually, muscles are arranged into antagonistic
groups such that one pulls a structure in one
direction, and its antagonist pulls in the opposite
direction.
• Sets of terms may define many antagonistic
actions.
– The movement of a distal limb segment toward a
more proximal one is called flexion.
– Extension is in the opposite direction and straitens
the limb.
– Adduction describes the movement of a part
toward some point of reference,
– Abduction is away from the point of reference.
– Rotation is the movement of the bone around
some fixed point.
Fiber Orientation
• Muscles also differ in the length and arrangement
of the muscle fibers within them:
– Strap-Shaped muscles contain long, parallel fibers and
have relatively broad attachments.
– Fusiform muscles are similar, except that their muscle
fibers lead to narrow tendons at the end of the
muscle, so the force of contraction is on a smaller
area.
– Pennate muscles contain short, diagonally arranged
fibers that insert into tendons on one side,
unipennate, of the muscle or on both sides of a
tendon, bipennate.
• These variables in muscle architecture affect
– The degree of muscle contraction,
– The velocity of contraction,
– The force of contraction, and
– The power a muscle can develop.
• Muscle power is equal to the amount of force
a muscle can generate multiplied by the
velocity of the contraction.
– The speed degree of contraction depends on
muscle length.
– The strength, force, of contraction depends on the
number of myofilaments in a fiber and the
number of fibers within a muscle.
Embryonic Origins
• A regional grouping of muscles or a functional
grouping is useful in dissection and muscle
studies,
– Such groupings often include muscles of difference
evolutionary, or phylogenetic development.
• During the course of evolution muscles sometimes change
their position or points of attachment.
• For this reason, the group to which a muscle
belongs is best seen by looking at its embryonic
development and nerve supply.
• Following the division of the body into a somatic
and visceral half, we can differentiate muscles
into somatic and visceral groups.
• Most somatic muscles lie in the “outer” tube of
the body and are develop from myotomes that
derive from embryonic somites.
– They form most of the skeletal muscles of the body.
• Visceral muscles develop in the inner tube of the
body and form the inner, splanchnic, layer of the
body.
– They contribute to the walls of the visceral organs and
the heart.
Somatic Muscle Groups
• Somatic muscles can be subclassified into axial muscles
located along the longitudinal axis of the body or
appendicular muscles that develops and migrates into
the limb buds.
• Axial muscles can be further subclassified according to
the group of body segments from which they arise.
– Extrinsic ocular muscles form the muscles of the eye.
– Branchiomeric muscles form the mandibular and branchial
muscles.
– The first part of the trunk musculature are the epibranchial
and hypobranchial muscles.
– The remaining myomeres form the muscles of the trunk.
• Appendicular muscles are defined as those
muscles that begin their differentiation within
the limb buds.
– Appendicular muscles always insert on the girdles
or bones of the paired appendages.
– They can usually be sorted into dorsal and ventral
groups based on their position relative to the
skeleton and girdles.
Axial Muscles
• Vertebrates have many individual muscles,
and they vary greatly as the methods of
support, locomotion, feeding, gas exchange,
and other activities of vertebrates change
during their adaptation to their many habitats
and modes of life.
Branchiomeric Muscles
• The branchiomeric muscles lie in the lateral
wall of the pharynx
• The branchiomeric and hypobranchial muscles
work together in breathing movements,
capturing food, and swallowing.
• In some cases they are assisted by the
epibranchial muscles
Extrinsic Ocular Muscles
• The most rostral axial muscles belong to the
extrinsic ocular group.
• These small, strap-shaped muscles arise from the
wall of the orbit and insert on the surface of the
eyeball.
• They rotate the eyeball as needed and the ability
to rotate the eye is common to all vertebrates
with well-developed eyes.
• Nearly all vertebrates have the same 6 muscles:
– The ventral oblique, ventral rectus, medial rectus,
dorsal rectus, dorsal oblique, and dorsal rectus.
Trunk and Tail Muscles
• Fishes: The embryonic myotomes of the trunk
and tail develop into a series of folded muscle
segments, the myomeres, of adult fishes.
– The sequential contraction of myomeres, acting
with the vert column, causes a series of lateral
undulations by which the fish swims.
– Individual myomeres are separated by myosepta,
a horizontal skeletogenous septum divides the
myomeres into dorsal epaxial and ventral hypaxial
halves.
• Tetrapods: Major changes occur in the trunk
and tail muscles because their role in
locomotion decreases in the transition to the
terrestrial environment.
– Although trunk muscles become less important in
locomotion, they play an important role in
mediating flexion and extension of the spine and
in supporting the body against gravity.
– The epaxial muscles are especially important in
supporting the body and moving the vertebral
column and head.
• The hypaxial muscles of vertebrates can be
divided into three groups:
– A subventral group that act on the vertebral axis
and assist the epaxial muscles in supporting the
body.
– A ventral group that includes the abdominal
muscles that support the abdomen and assist in
lateral and ventral trunk flexion.
– A lateral group that lies on the flank and forms 3-4
layers within the abdomen.
Evolution of the Appendicular Muscles
• The paired appendages of most fishes do not deliver
major propulsive thrust.
• The fin may provide lift, but they are primarily used in
maintaining stability, breaking, and maneuvering.
– Often a single dorsal, extensor, muscles is located dorsally
on the fin and pulls it dorsally;
– While a ventral flexor muscle pulls the fin ventrally.
• The structure and movements of the paired
appendages of terrestrial vertebrates are far more
complex because the limbs support the body and
provide propulsive thrust for locomotion.
• Appendicular muscles constitute the bulk of
the muscles found in terrestrial vertebrates.
• Despite their complexity they can be divided
into dorsal and ventral groups.
– Collectively the dorsal muscles are homologous to
the fish extensor,
• The dorsal muscles of both tetrapod pectoral and pelvic
girdles are responsible for abducting and extending the
limbs as a whole and these actions occur during the
swing phase of the step.
– And, Those of the ventral group to the fish’s flexor
muscles.
• The pectoral and pelvic limbs as a whole are
advanced or protracted during the swing
phase.
• A force is developed that could retract, or
draw the limb back during the stance phase;
– Because the feet remain firmly on the ground, this
force advances the trunk relative to the feet.
• Both the dorsal and ventral muscles
participate in these actions, depending on
whether the limbs lie anterior or posterior to
the shoulder and hip joints.