CHAPTER 6: Muscular System
• Muscular Tissue
o Contributes to homeostasis by producing body movements
o Producing heat to maintain normal body temperature
o Stabilize body position
o Regulate organ volume
o Propel fluids and food matter through various body systems
• Introduction
o Motion results from the alternating contraction and relaxation of muscles, which make up
40–50% of total adult body weight (depending on the percentage of body fat, gender, and
exercise regimen).
o The muscular strength reflects the primary function of muscle—the transformation of
chemical energy into mechanical energy to generate force, perform work, and produce
movement.
o The scientific study of muscles is known as myology.
• Overview of Muscle Tissues
o Skeletal and smooth muscles cells are elongated. They are called muscle fibers.
o The ability of muscle to shorten or contract depends on myofilaments
o Prefixes:
▪ myo- and mys- (muscle) or sarco- (flesh)
• Muscle Types
o Skeletal Muscle
▪ Package into organs called skeletal muscles that attach to body’s skeleton
▪ Cover bony underpinnings
▪ Help form smoother contours of body
▪ Muscle fibers are huge, cigar-shaped, multinucleate cells
▪ Largest of the muscle fiber types (up t30 cm in length)
▪ Ex: antigravity muscles of hip—big enough to be seen with naked eye
▪ Striated muscles—fibers have obvious stripes; alternating light and dark protein bands
(striations)
▪ Voluntary muscles—subject to conscious control
• Its activity can be consciously controlled by neurons (nerve cells) that are part
of the somatic (voluntary) division of the nervous system.
• However, some skeletal muscles are activated by reflexes
▪ Skeletal muscle—can contract rapidly and with great force. Tires easily and must rest
after period of activity.
▪ Fibers are soft and fragile
▪ Thousands of fibers are bundled together by connective tissue—provides strength
and support
▪ The subcutaneous layer or hypodermis
• Separates muscle from skin
• Composed of areolar connective tissue and adipose tissue
o The adipose tissue of the subcutaneous layer stores most of the body’s
triglycerides, serves as an insulating layer that reduces heat loss, and
protects muscles from physical trauma.
• Provides a pathway for nerves, blood vessels, and lymphatic vessels to enter
and exit muscles.
▪ Fascia: a dense sheet or broad band of irregular connective tissue that lines the body
wall and limbs and supports and surrounds muscles and other organs of the body.
• Fascia holds muscles with similar functions together. Fascia allows free
movement of muscles; carries nerves, blood vessels, and lymphatic vessels;
and fills spaces between muscles.
▪ Connective tissue wrappings of skeletal muscle
• Endomysium: encloses each muscle fiber
• Sheathed muscle fibers are enclosed by coarser fibrous membrane called
perimysium to form bundle of fibers called fascicle.
1
, • Fascicles are bound together by tougher overcoat of connective tissue called
epimysium, covers entire muscle
• Epimysia bend into cord like tendons or sheet like aponeuroses— attach
muscles indirectly to bones, cartilages, connective tissue coverings
o An example is the epicranial aponeurosis on top of the skull between
the frontal and occipital bellies of the occipitofrontalis muscle.
▪ Tendons: provides durability—mostly tough collagenic fibers, can cross rough bony
projections. Conserves space—more tendons than fleshy muscles can pass over a
joint.
o Smooth Muscle
▪ Nonstriated, Involuntary, Visceral
▪ Found in walls of hollow visceral organs (e.g., stomach, urinary bladder, respiratory
passages)
▪ Smooth muscle propels substances along a definite tract within body
▪ Smooth muscle cells are spindle shaped, unnucleated, and surrounded by scant
endomysium
▪ Arranged in layers—inner circular layer, outer longitudinal layer
▪ Two layers alternately contract and relax, change size and shape of organ
▪ Contraction is slow and sustained
▪ Found in the skin, attached to hair follicles.
▪ Both cardiac muscle and smooth muscle are regulated by neurons that are part of
the autonomic (involuntary) division of the nervous system and by hormones released
by endocrine glands.
o Cardiac Muscle
▪ Found in the heart where is forms the bulk of the walls
▪ Striated, Involuntary
▪ Cardiac cells are cushioned by small amounts of soft connective tissue
(endomysium), arranged in spiral or figure-8 bundles
2
, ▪ When heart contracts, its internal layers become smaller, forcing blood into larger
arteries leaving heart
▪ Intercalated discs: special junctions that join branching cells of cardiac muscle fibers
▪ Contracts at fairly steady rate set by pacemaker
▪ Autorhythmicity: term used for the built-in rhythm of the heart. The heart beats
because it has a natural pacemaker that initiates each contraction. Several
hormones and neurotransmitters can adjust heart rate by speeding or slowing the
pacemaker.
▪ Heart can also be stimulated by nervous system
• Muscle Functions
o Producing Movement
▪ From muscle contraction
▪ Skeletal muscles are responsible for mobility of body
• Locomotion and manipulating things with upper limb
• Enable us to respond quickly to changes in external environment
▪ Smooth (lining blood vessel walls) and cardiac muscle work together to circulate
blood and maintain BP
▪ Smooth muscle of hollow organs forces fluids and other substances through internal
body channels
o Maintaining Posture and Body Position
▪ Maintain an erect/seated posture despite gravity
o Stabilizing Joints
▪ Skeletal muscles pull on bones to stabilize joints
▪ Muscle tendons are important in reinforcing and stabilizing joints that have poorly
fitting articulating surfaces (e.g., shoulder joint)
o Generating Heat (thermogenesis)
▪ As by product of muscle activity
3
, ▪ 3/4 ATP used to power muscle contraction escapes as heat, which is vital maintaining
normal body temperature
▪ Skeletal muscle accounts for 40% of body’s mass—most responsible muscle type for
generating heat
o Additional Functions
▪ Skeletal muscles protect fragile internal organs by enclosure
▪ Smooth muscles form valves to regulate passage of substances through internal body
openings, dilate and constrict pupils, activate arrector pili muscles (cause hairs to
stand)
• Nerve and Blood Supply
o Skeletal muscles are well supplied with nerves and blood vessels. Generally, an artery and
one or two veins accompany each nerve that penetrates a skeletal muscle. The neurons
that stimulate skeletal muscle to contract are somatic motor neurons.
o Each somatic motor neuron has a threadlike axon that extends from the brain or spinal cord
to a group of skeletal muscle fibers. The axon of a somatic motor neuron typically branches
many times, each branch extending to a different skeletal muscle fiber.
o Microscopic blood vessels called capillaries are plentiful in muscular tissue; each muscle fiber
is in close contact with one or more capillaries.
o The blood capillaries bring in oxygen and nutrients and remove heat and the waste products
of muscle metabolism. Especially during contraction, a muscle fiber synthesizes and uses
considerable ATP (adenosine triphosphate). These reactions, which you will learn more about
later on, require oxygen, glucose, fatty acids, and other substances that are delivered to the
muscle fiber in the blood.
• Microscopic Anatomy of Skeletal Muscle
o The most important components of a skeletal muscle are the muscle fibers themselves. The
diameter of a mature skeletal muscle fiber ranges from 10 to 100 um.* The typical length of
a mature skeletal muscle fiber is about 10 cm (4 in.), although some are as long as 30 cm (12
in.).
o *One micrometer (um) is 10u6 meter (1/25,000 in.).
o Because each skeletal muscle fiber arises during embryonic development from the fusion of
a hundred or more small mesodermal cells called myoblasts, each mature skeletal muscle
fiber has a hundred or more nuclei.
o Once fusion has occurred, the muscle fiber loses its ability to undergo cell division. Thus, the
number of skeletal muscle fibers is set before you are born, and most of these cells last a
lifetime.
▪ Sarcolemma, Transverse Tubules, and Sarcoplasm
• The multiple nuclei of a skeletal muscle fiber are located just beneath the
sarcolemma, the plasma membrane of a muscle cell.
• Thousands of tiny invaginations of the sarcolemma, called transverse (T)
tubules, tunnel in from the surface toward the center of each muscle fiber.
Because T tubules are open to the outside of the fiber, they are filled with
interstitial fluid.
• Muscle action potentials travel along the sarcolemma and through the T
tubules, quickly spreading throughout the muscle fiber. This arrangement
ensures that an action potential excites all parts of the muscle fiber at
essentially the same instant.
• Within the sarcolemma is the sarcoplasm, the cytoplasm of a muscle fiber.
Sarcoplasm includes a substantial amount of glycogen, which is a large
molecule composed of many glucose molecules. Glycogen can be used for
synthesis of ATP.
• In addition, the sarcoplasm contains a red-colored protein called myoglobin.
This protein, found only in muscle, binds oxygen molecules that diffuse into
muscle fibers from interstitial fluid. Myoglobin releases oxygen when it is
needed by the mitochondria for ATP production.
4
• Muscular Tissue
o Contributes to homeostasis by producing body movements
o Producing heat to maintain normal body temperature
o Stabilize body position
o Regulate organ volume
o Propel fluids and food matter through various body systems
• Introduction
o Motion results from the alternating contraction and relaxation of muscles, which make up
40–50% of total adult body weight (depending on the percentage of body fat, gender, and
exercise regimen).
o The muscular strength reflects the primary function of muscle—the transformation of
chemical energy into mechanical energy to generate force, perform work, and produce
movement.
o The scientific study of muscles is known as myology.
• Overview of Muscle Tissues
o Skeletal and smooth muscles cells are elongated. They are called muscle fibers.
o The ability of muscle to shorten or contract depends on myofilaments
o Prefixes:
▪ myo- and mys- (muscle) or sarco- (flesh)
• Muscle Types
o Skeletal Muscle
▪ Package into organs called skeletal muscles that attach to body’s skeleton
▪ Cover bony underpinnings
▪ Help form smoother contours of body
▪ Muscle fibers are huge, cigar-shaped, multinucleate cells
▪ Largest of the muscle fiber types (up t30 cm in length)
▪ Ex: antigravity muscles of hip—big enough to be seen with naked eye
▪ Striated muscles—fibers have obvious stripes; alternating light and dark protein bands
(striations)
▪ Voluntary muscles—subject to conscious control
• Its activity can be consciously controlled by neurons (nerve cells) that are part
of the somatic (voluntary) division of the nervous system.
• However, some skeletal muscles are activated by reflexes
▪ Skeletal muscle—can contract rapidly and with great force. Tires easily and must rest
after period of activity.
▪ Fibers are soft and fragile
▪ Thousands of fibers are bundled together by connective tissue—provides strength
and support
▪ The subcutaneous layer or hypodermis
• Separates muscle from skin
• Composed of areolar connective tissue and adipose tissue
o The adipose tissue of the subcutaneous layer stores most of the body’s
triglycerides, serves as an insulating layer that reduces heat loss, and
protects muscles from physical trauma.
• Provides a pathway for nerves, blood vessels, and lymphatic vessels to enter
and exit muscles.
▪ Fascia: a dense sheet or broad band of irregular connective tissue that lines the body
wall and limbs and supports and surrounds muscles and other organs of the body.
• Fascia holds muscles with similar functions together. Fascia allows free
movement of muscles; carries nerves, blood vessels, and lymphatic vessels;
and fills spaces between muscles.
▪ Connective tissue wrappings of skeletal muscle
• Endomysium: encloses each muscle fiber
• Sheathed muscle fibers are enclosed by coarser fibrous membrane called
perimysium to form bundle of fibers called fascicle.
1
, • Fascicles are bound together by tougher overcoat of connective tissue called
epimysium, covers entire muscle
• Epimysia bend into cord like tendons or sheet like aponeuroses— attach
muscles indirectly to bones, cartilages, connective tissue coverings
o An example is the epicranial aponeurosis on top of the skull between
the frontal and occipital bellies of the occipitofrontalis muscle.
▪ Tendons: provides durability—mostly tough collagenic fibers, can cross rough bony
projections. Conserves space—more tendons than fleshy muscles can pass over a
joint.
o Smooth Muscle
▪ Nonstriated, Involuntary, Visceral
▪ Found in walls of hollow visceral organs (e.g., stomach, urinary bladder, respiratory
passages)
▪ Smooth muscle propels substances along a definite tract within body
▪ Smooth muscle cells are spindle shaped, unnucleated, and surrounded by scant
endomysium
▪ Arranged in layers—inner circular layer, outer longitudinal layer
▪ Two layers alternately contract and relax, change size and shape of organ
▪ Contraction is slow and sustained
▪ Found in the skin, attached to hair follicles.
▪ Both cardiac muscle and smooth muscle are regulated by neurons that are part of
the autonomic (involuntary) division of the nervous system and by hormones released
by endocrine glands.
o Cardiac Muscle
▪ Found in the heart where is forms the bulk of the walls
▪ Striated, Involuntary
▪ Cardiac cells are cushioned by small amounts of soft connective tissue
(endomysium), arranged in spiral or figure-8 bundles
2
, ▪ When heart contracts, its internal layers become smaller, forcing blood into larger
arteries leaving heart
▪ Intercalated discs: special junctions that join branching cells of cardiac muscle fibers
▪ Contracts at fairly steady rate set by pacemaker
▪ Autorhythmicity: term used for the built-in rhythm of the heart. The heart beats
because it has a natural pacemaker that initiates each contraction. Several
hormones and neurotransmitters can adjust heart rate by speeding or slowing the
pacemaker.
▪ Heart can also be stimulated by nervous system
• Muscle Functions
o Producing Movement
▪ From muscle contraction
▪ Skeletal muscles are responsible for mobility of body
• Locomotion and manipulating things with upper limb
• Enable us to respond quickly to changes in external environment
▪ Smooth (lining blood vessel walls) and cardiac muscle work together to circulate
blood and maintain BP
▪ Smooth muscle of hollow organs forces fluids and other substances through internal
body channels
o Maintaining Posture and Body Position
▪ Maintain an erect/seated posture despite gravity
o Stabilizing Joints
▪ Skeletal muscles pull on bones to stabilize joints
▪ Muscle tendons are important in reinforcing and stabilizing joints that have poorly
fitting articulating surfaces (e.g., shoulder joint)
o Generating Heat (thermogenesis)
▪ As by product of muscle activity
3
, ▪ 3/4 ATP used to power muscle contraction escapes as heat, which is vital maintaining
normal body temperature
▪ Skeletal muscle accounts for 40% of body’s mass—most responsible muscle type for
generating heat
o Additional Functions
▪ Skeletal muscles protect fragile internal organs by enclosure
▪ Smooth muscles form valves to regulate passage of substances through internal body
openings, dilate and constrict pupils, activate arrector pili muscles (cause hairs to
stand)
• Nerve and Blood Supply
o Skeletal muscles are well supplied with nerves and blood vessels. Generally, an artery and
one or two veins accompany each nerve that penetrates a skeletal muscle. The neurons
that stimulate skeletal muscle to contract are somatic motor neurons.
o Each somatic motor neuron has a threadlike axon that extends from the brain or spinal cord
to a group of skeletal muscle fibers. The axon of a somatic motor neuron typically branches
many times, each branch extending to a different skeletal muscle fiber.
o Microscopic blood vessels called capillaries are plentiful in muscular tissue; each muscle fiber
is in close contact with one or more capillaries.
o The blood capillaries bring in oxygen and nutrients and remove heat and the waste products
of muscle metabolism. Especially during contraction, a muscle fiber synthesizes and uses
considerable ATP (adenosine triphosphate). These reactions, which you will learn more about
later on, require oxygen, glucose, fatty acids, and other substances that are delivered to the
muscle fiber in the blood.
• Microscopic Anatomy of Skeletal Muscle
o The most important components of a skeletal muscle are the muscle fibers themselves. The
diameter of a mature skeletal muscle fiber ranges from 10 to 100 um.* The typical length of
a mature skeletal muscle fiber is about 10 cm (4 in.), although some are as long as 30 cm (12
in.).
o *One micrometer (um) is 10u6 meter (1/25,000 in.).
o Because each skeletal muscle fiber arises during embryonic development from the fusion of
a hundred or more small mesodermal cells called myoblasts, each mature skeletal muscle
fiber has a hundred or more nuclei.
o Once fusion has occurred, the muscle fiber loses its ability to undergo cell division. Thus, the
number of skeletal muscle fibers is set before you are born, and most of these cells last a
lifetime.
▪ Sarcolemma, Transverse Tubules, and Sarcoplasm
• The multiple nuclei of a skeletal muscle fiber are located just beneath the
sarcolemma, the plasma membrane of a muscle cell.
• Thousands of tiny invaginations of the sarcolemma, called transverse (T)
tubules, tunnel in from the surface toward the center of each muscle fiber.
Because T tubules are open to the outside of the fiber, they are filled with
interstitial fluid.
• Muscle action potentials travel along the sarcolemma and through the T
tubules, quickly spreading throughout the muscle fiber. This arrangement
ensures that an action potential excites all parts of the muscle fiber at
essentially the same instant.
• Within the sarcolemma is the sarcoplasm, the cytoplasm of a muscle fiber.
Sarcoplasm includes a substantial amount of glycogen, which is a large
molecule composed of many glucose molecules. Glycogen can be used for
synthesis of ATP.
• In addition, the sarcoplasm contains a red-colored protein called myoglobin.
This protein, found only in muscle, binds oxygen molecules that diffuse into
muscle fibers from interstitial fluid. Myoglobin releases oxygen when it is
needed by the mitochondria for ATP production.
4
