Title: Muscular System: Histology and Physiology
1Muscular SystemHistology and Physiology
2Functions 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)
3 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
4Types 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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6Skeletal 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 and dark banding
7Connective 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.
8Nerves 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
9Muscle 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.
10Parts of a Muscle
11Actin and Myosin Myofilaments
12Actin (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.
13Myosin (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
- Can bind to active sites on the actin molecules
to form cross-bridges. - Attached to the rod portion by a hinge region
that can bend and straighten during contraction. - 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
14Sarcomeres 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
15Sliding 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.
16Sarcomere Shortening
17Physiology 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.
- Must exist for action potential to occur
18Ion 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
19Action 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
20Gated Ion Channels and the Action Potential
21Action Potential Propagation
22Neuromuscular 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
23Function 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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25Excitation-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
26Action Potentials and Muscle Contraction
27Cross-Bridge Movement
28Relaxation
- Ca2 moves back into sarcoplasmic reticulum by
active transport. Requires energy - Ca2 moves away from troponin-tropomyosin complex
- Complex re-establishes its position and blocks
binding sites.
29Muscle 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)
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31Stimulus 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
32Contraction 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
33Contraction of the Whole Muscle
34Stimulus 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
35Types 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
36Fatigue
- 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
37Physiological 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.
38Energy 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
39Slow 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
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41Smooth 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
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43Electrical 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-or-none law
- Contraction regulated by nervous system and by
hormones (ex epinephrine)
44Regulation 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
45Cardiac 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
46Effects 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