Chapter 9
Muscular System:
Histology and Physiology
9-1
Functions of the Muscular System
• Body movement (skeletal muscles attached to bones)
• Maintenance of posture
• Respiration (skeletal muscles of thorax are responsible
for the movement necessary for respiration)
• Production of body heat (when skeletal muscles
contact, heat is given off as a by-product)
• Communication (speaking, writing)
• Constriction of organs and vessels (contraction
of smooth muscle)
• Heart beat (contraction of cardiac muscle)
9-2
General Functional
Characteristics of Muscle
• Contractility: ability of a muscle to shorten
with force
• Excitability: capacity of muscle to respond
to a stimulus (by nerve or hormone)
• Extensibility: muscle can be stretched to its
normal resting length and beyond to a
limited degree
• Elasticity: ability of muscle to recoil to
original resting length after stretched
9-3
Types of Muscle Tissue
• Skeletal
– Responsible for locomotion, facial expressions, posture, respiratory
movements, other types of body movement
– Voluntary
• Smooth
– Walls of hollow organs, blood vessels, eye, glands, skin
– Some functions: propel urine, mix food in digestive tract,
dilating/constricting pupils, regulating blood flow
– In some locations, autorhythmic
– Controlled involuntarily by endocrine and autonomic nervous systems
• Cardiac
– Heart: major source of movement of blood
– Autorhythmic
– Controlled involuntarily by endocrine and autonomic nervous systems
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9-5
Skeletal Muscle Structure
• Composed of muscle cells
(fibers), connective tissue, blood
vessels, nerves
• Fibers are long, cylindrical,
multinucleated
• Tend to be smaller diameter in
small muscles and larger in large
muscles. 1 mm- 4 cm in length
• Develop from myoblasts (they are
converted to muscle fibers as contractile
proteins accumulate within their cytoplasm);
numbers remain constant (# of
muscle fibers remain constant after birth---so, enlargement of muscles is an increase in
size rather than #)
• Striated appearance due to light
9-6
and dark banding
Connective Tissue Coverings of Muscle
• Layers
– External lamina. Delicate, reticular
fibers. Surrounds sarcolemma (P.M.)
– Endomysium. Loose C.T. with
reticular fibers.
– Perimysium. Denser C.T. surrounding
a group of muscle fibers. Each group
called a fasciculus
– Epimysium. C.T. that surrounds a
whole muscle (many fascicles)
• Fascia: connective tissue sheet
– Forms layer under the skin
– Holds muscles together and separates
them into functional groups.
– Allows free movements of muscles.
– Carries nerves (motor neurons, sensory
neurons), blood vessels, and
lymphatics.
– Continuous with connective tissue of
tendons and periosteum.
9-7
Nerves and Blood Vessel
Supply
• Motor neurons: stimulate
muscle fibers to contract.
Nerve cells with cell
bodies in brain or spinal
cord; axons extend to
skeletal muscle fibers
through nerves
• Axons branch so that each
muscle fiber is innervated
• Capillary beds surround
muscle fibers
9-8
Muscle Fibers
• Nuclei just inside sarcolemma
• Cell packed with myofibrils within cytoplasm
(sarcoplasm = cytoplasm without myofibrils)
– Threadlike (extends from one end of muscle fiber to the other)
– Composed of protein threads called myofilaments:
thin (actin 8nm) and thick (myosin 12nm)
– Sarcomeres: actin & myosin myofilaments form
highly ordered units called sarcomeres. They are
joined end to end to form the myofibrils.
9-9
Parts of a Muscle
9-10
Actin and Myosin Myofilaments
9-11
Actin (Thin) Myofilaments
• Two strands of fibrous (F) actin form a
double helix extending the length of the
myofilament; attached at either end at
sarcomere.
– Composed of G actin monomers
each of which has an active site
– Actin site can bind myosin during
muscle contraction.
• Tropomyosin: an elongated protein
winds along the groove of the F actin
double helix.
•Troponin is composed of three subunits: one that binds to actin, a second that
binds to tropomyosin, and a third that binds to calcium ions. Spaced between
the ends of the tropomyosin molecules in the groove between the F actin
strands.
•The tropomyosin/troponin complex regulates the interaction between active
sites on G actin and myosin.
9-12
Myosin (Thick) Myofilament
•
•
•
Many elongated myosin
molecules shaped like golf
clubs.
Molecule consists of two heavy
myosin molecules wound
together to form a rod portion
lying parallel to the myosin
myofilament and two heads
that extend laterally.
Myosin heads
1. Can bind to active sites on the actin molecules to form cross-bridges.
2. Attached to the rod portion by a hinge region that can bend and straighten
during contraction.
3. Have ATPase activity: activity that breaks down adenosine triphosphate
(ATP), releasing energy. Part of the energy is used to bend the hinge
region of the myosin molecule during contraction
9-13
Sarcomeres: Z Disk to Z Disk
• Z disk: filamentous network of
protein. Serves as attachment
for actin myofilaments
• Striated appearance
– I bands: from Z disks to ends of
thick filaments
– A bands: length of thick
filaments
– H zone: region in A band where
actin and myosin do not overlap
– M line: middle of H zone;
delicate filaments holding myosin
in place
•In muscle fibers, A and I bands of parallel myofibrils are aligned.
•Titin filaments: elastic chains of amino acids; make muscles
extensible and elastic
9-14
Sliding Filament Model
• Actin myofilaments sliding over
myosin to shorten sarcomeres
– Actin and myosin do not change
length
– Shortening sarcomeres
responsible for skeletal muscle
contraction
• During relaxation, sarcomeres
lengthen because of some external
force, like forces produced by other
muscles (contraction of antagonistic
muscles) or by gravity.
- agonist = muscle that accomplishes
a certain movement,
such as flexion.
- antagonist = muscle acting in
opposition to agonist.
9-15
Sarcomere Shortening
9-16
Physiology of Skeletal Muscle Fibers
• Nervous system controls muscle contractions through action potentials
• Resting membrane potentials
– Membrane voltage difference across membranes (polarized)
• Inside cell more negative due to accumulation of large protein molecules.
More K+ on inside than outside. K+ leaks out (through leak channels) but not
completely because negative molecules hold some back.
• Outside cell more positive and more Na+ on outside than inside.
• Na+ /K+ pump maintains this situation.
9-17
– Must exist for action potential to occur
Ion Channels
• Types
– Ligand-gated. Ligands are molecules that bind to
receptors. Receptor: protein or glycoprotein with a
receptor site
• Example: neurotransmitters
• Gate is closed until neurotransmitter attaches to
receptor molecule. When Ach (acetylcholine) attaches
to receptor on muscle cell, Na gate opens. Na moves
into cell due to concentration gradient
– Voltage-gated
• Open and close in response to small voltage changes
across plasma membrane
• Each is specific for one type of ion
9-18
Action Potentials
• Phases
– Depolarization: Inside of plasma membrane
becomes less negative. If change reaches
threshold, depolarization occurs
– Repolarization: return of resting membrane
potential. Note that during repolarization, the
membrane potential drops lower than its
original resting potential, then rebounds. This
is because Na plus K together are higher, but
then Na/K pump restores the resting potential
• All-or-none principle: like camera flash system
• Propagate: Spread from one location to another.
Action potential does not move along the
membrane: new action potential at each
successive location.
• Frequency: number of action potential produced
per unit of time
9-19
Gated Ion Channels and
the Action Potential
9-20
Action Potential Propagation
9-21
Neuromuscular Junction
• Synapse: axon terminal
resting in an
invagination of the
sarcolemma
• Neuromuscular
junction (NMJ):
– Presynaptic terminal:
axon terminal with
synaptic vesicles
– Synaptic cleft: space
– Postsynaptic membrane
or motor end-plate
9-22
Function of Neuromuscular Junction
• Synaptic vesicles
– Neurotransmitter: substance
released from a presynaptic
membrane that diffuses across
the synaptic cleft and
stimulates (or inhibits) the
production of an action
potential in the postsynaptic
membrane.
• Acetylcholine
– Acetylcholinesterase: A
degrading enzyme in synaptic
cleft. Prevents accumulation of
ACh
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9-24
Excitation-Contraction Coupling
• Mechanism by which an action
potential causes muscle fiber
contraction
• Involves
– Sarcolemma
– Transverse (T) tubules: invaginations
of sarcolemma
– Terminal cisternae
– Sarcoplasmic reticulum: smooth ER
– Triad: T tubule, two adjacent
terminal cisternae
– Ca2+
– Troponin
9-25
Action Potentials and Muscle
Contraction
9-26
Cross-Bridge Movement
9-27
Relaxation
• Ca2+ moves back into sarcoplasmic
reticulum by active transport. Requires
energy
• Ca2+ moves away from troponintropomyosin complex
• Complex re-establishes its position and
blocks binding sites.
9-28
Muscle Twitch
• Muscle contraction in
response to a stimulus that
causes action potential in
one or more muscle fibers
• Muscle contraction
measures as force, also
called tension. Requires
up to a second to occur.
• Phases
– Lag or latent
(neuromuscular junction & step
#1 of cross-bridge movement)
– Contraction (step #2 - #6
of cross-bridge movement)
– Relaxation (powerpoint
slide # 28)
9-29
9-30
Stimulus Strength and Muscle
Contraction
• All-or-none law for muscle fibers
– Contraction of equal force in response to
each action potential
• Sub-threshold stimulus: no
action potential; no
contraction
• Threshold stimulus: action
potential; contraction
• Stronger than threshold; action
potential; contraction equal to
that with threshold stimulus
• Motor units: a single motor neuron
and all muscle fibers innervated by it
9-31
Contraction of the Whole Muscle
• Whole muscles exhibit characteristics that are more complex than
those of individual muscle fibers or motor units. Instead of
responding in an all-or-none fashion, whole muscles respond to
stimuli in a graded fashion, which means that the strength of the
contractions can range from weak to strong.
• Remember: There are many muscle fibers in one fasciculi and many fasciculi in
one whole muscle.
• Strength of contraction in whole muscle is graded: ranges from weak
to strong depending on stimulus strength
• Multiple motor unit summation: the force in which a whole muscle
contracts depends on the number of motor units stimulated to contract.
(force of contraction increases as more & more motor units are stimulated). A
muscle has many motor units
– Submaximal stimuli
– Maximal stimulus
– Supramaximal stimuli
9-32
Contraction of the Whole Muscle
9-33
Stimulus Frequency and Muscle Contraction
• Relaxation of a muscle fiber is not required before a second action potential
can stimulate a second contraction.
• As the frequency of action potentials increase, the frequency of contraction
increases
– Incomplete tetanus: muscle fibers partially relax between contraction
– Complete tetanus: no relaxation between contractions
– Multiple-wave summation: muscle tension increases as contraction
frequencies increase
9-34
Types of Muscle Contractions
• Isometric: no change in length of muscle but
tension increases during contraction
– Postural muscles of body ex: muscles hold spine erect while
person is sitting or standing
• Isotonic: change in length but tension constant
ex: waving using computer keyboard
– Concentric: tension is so great it overcomes opposing
resistance and muscle shortens
ex: raising of a weight during a bicep curl.
– Eccentric: tension maintained but muscle lengthens
ex: person slowly lowers a heavy weight
• Muscle tone: constant tension by muscles for long
periods of time
9-35
Fatigue
• Decreased capacity to work and reduced efficiency of
performance
• Types
– Psychological: depends on emotional state of
individual ex: burst of activity in tired athlete in response to
encouragement from spectators shows how psychological fatigue
can be overcome
– Muscular: results from ATP depletion ex: fatigue in lower
limbs of marathon runners or in upper & lower limbs of swimmers
– Synaptic: occurs in NMJ due to lack of acetylcholine
ex: rare-----only under extreme exertion
9-36
Physiological Contracture and
Rigor Mortis
• Physiological contracture: state of extreme
fatigue (extreme exercise) where due to lack of
ATP neither contraction nor relaxation can occur
• Rigor mortis: development of rigid muscles
several hours after death. Ca2+ leaks into
sarcoplasm and attaches to myosin heads and
crossbridges form but no ATP available to bind to
myosin---------so the cross-bridges are unable to
release. Rigor ends as tissues start to deteriorate.
9-37
Energy Sources
• ATP provides immediate energy for muscle contractions.
Produced from three sources
– Creatine phosphate
• During resting conditions stores energy to synthesize ATP
• ADP + Creatine phosphate------------------ Creatine + 1ATP
(Creatine Kinase)
– Anaerobic respiration
• Occurs in absence of oxygen and results in breakdown of
glucose to yield ATP and lactic acid
– Aerobic respiration
• Requires oxygen and breaks down glucose to produce ATP,
carbon dioxide and water
• More efficient than anaerobic
9-38
Slow and Fast Fibers
• Slow-twitch oxidative
– Contract more slowly, smaller in diameter, better blood supply, more mitochondria
(also called oxidative because carry out aerobic respiration), more fatigue-resistant than
fast-twitch, large amount of myoglobin (dark pigment which binds oxygen & acts as a
muscle reservoir for oxygen when blood does not supply adequate amount) .
– Postural muscles, more in lower than upper limbs. Dark meat of chicken.
– Functions: Maintenance of posture & performance in endurance activities.
• Fast-twitch
– Respond rapidly to nervous stimulation, contain myosin that can break down ATP
more rapidly than that in Type I, less blood supply, fewer and smaller mitochondria
than slow-twitch (adapted to perform anaerobic respiration)
– Lower limbs in sprinter, upper limbs of most people. White meat in chicken.
– Comes in oxidative and glycolytic forms
– Functions: Rapid, intense movements of short duration
• Distribution of fast-twitch and slow-twitch
•
– Most muscles have both but varies for each muscle
Exercise: weight lifting enlarges fast-twitch & aerobic training enlarges slow-twitch
• Effects of exercise: change in size of muscle fibers
– Hypertrophy: increase in muscle size
• Increase in myofibrils
• Increase in nuclei due to fusion of satellite cells
• Increase in strength
– Atrophy: decrease in muscle size
• Reverse except in severe situations where cells die
9-39
9-40
Smooth Muscle
•
•
•
•
•
Not striated, fibers smaller than those in skeletal muscle
Spindle-shaped; single, central nucleus
More actin than myosin
Caveolae: indentations in sarcolemma; may act like T tubules
Dense bodies instead of Z disks as in skeletal muscle; have noncontractile
intermediate filaments
• Ca2+ required to initiate contractions; binds to calmodulin (protein). Calmodulin
molecules with Ca++ bound to them activate an enzyme called myosin kinase,
which transfers a phosphate group from ATP to heads of myosin molecules.
Cross-bridging occurs
• Relaxation: caused by enzyme myosin phosphatase
9-41
9-42
Electrical Properties of Smooth
Muscle
• Slow waves of
depolarization and
repolarization transferred
from cell to cell
• Depolarization caused by
spontaneous diffusion of
Na+ and Ca2+ into cell
• Does not follow all-ornone law
• Contraction regulated by
nervous system and by
hormones (ex: epinephrine)
9-43
Regulation of Smooth Muscle
• Innervated by autonomic nervous system
(composed of nerve fibers that send impulses from CNS to smooth
muscle, cardiac muscle, glands)
• Neurotransmitters are acetylcholine and
norepinephrine (increases cardiac output, blood glucose
levels)
• Hormones important as epinephrine and
oxytocin
• Receptors present on plasma membrane;
which neurotransmitters or hormones bind
determines response
9-44
Cardiac Muscle
•
•
•
•
•
•
•
Found only in heart
Striated
Each cell usually has one nucleus
Has intercalated disks and gap junctions
Autorhythmic cells
Action potentials of longer duration
The depolarization of cardiac muscle results from
influx of Na+ and Ca2+ across the plasma
membrane
9-45
Effects of Aging on Skeletal
Muscle
• Reduced muscle mass
• Increased time for muscle to contract in
response to nervous stimuli
• Reduced stamina
• Increased recovery time
• Loss of muscle fibers
9-46