Anatomy & Physiology 34A Lecture
Muscular Tissue & Physiology
I. Overview
A. Characteristics & Types of Muscle Tissue
B. Skeletal Muscle
C. Microscopic Anatomy of Skeletal Muscle
D. Nerve-Muscle Relationship
E. Behavior of Skeletal Muscle Fibers
F. Behavior of Whole Muscles
G. Muscle Metabolism
H. Cardiac Muscle
I. Smooth Muscle
J. Disorders of Muscle Tissue
II. Characteristics & Types of Muscle Tissue
A. Characteristics of all muscle tissues:
1. Excitability – neurotransmitters from nerves stimulate electrical
changes in muscle cells’ plasma membranes
2. Conductivity – electrical impulses initiate cellular processes
leading to muscle contraction
3. Contractility - muscle cells shorten & generate pulling force
4. Extensibility - after contraction, muscles can be stretched by
contraction of an opposing muscle
5. Elasticity - after stretching, muscles can recoil to their resting
length
B. Types of Muscle Tissues
1. Skeletal muscle – straited, multinucleate cells. Functions:
a. Voluntary Movement via skeletal muscles attached to bones
b. Maintenance of posture via contractions of skeletal muscles
c. Joint stabilization via muscle tone - a constant, low level of
force generated - keeps tension on tendons that cross joints
d. Heat generation - helps to maintain a constant body temp.
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2. Cardiac muscle – involuntary, straited, uninucleate cells separated
by intercalated discs. Functions:
a. Involuntary, composes heart myocardium
b. Cells contract automatically in synchronized rhythm
c. Regulated by the Autonomic Nervous System
3. Smooth muscle - involuntary nonstraited, uninucleate cells.
Functions:
a. Found in the walls of body tubes (e.g.: GI, respiratory,
urogenital tracts, blood vessels)
b. Promote wavelike peristaltic contractions in the GI tract
c. Responsible for vasodilation and vasoconstriction of blood
vessels
III. Skeletal Muscle
A. Blood & Nerve Supply to Skeletal Muscle
1. Each skeletal muscle is supplied by at least one nerve, one
artery, and one or more veins
2. The nerves and vessels branch off in the muscle CT, so each
muscle fiber has its own neuron axon & capillaries
3. A neuromuscular junction is the point at which a neuron
signals a muscle fiber to contract
IV. Microscopic Anatomy of Skeletal Muscle Tissue
A. Skeletal Muscles are composed of muscle fibers (cells). Muscle
structures from large to small include: whole muscle → fascicle
groups → muscle fibers → myofibrils → myofilaments
B. Skeletal muscle cells are
1. Long cells consisting of many myofibrils composed of
a. Myofilaments of actin (thin) and myosin (thick) protein
fibers
b. Sarcomeres - contractile units in myofibrils, between Z lines.
1) I bands - are light bands of actin only
2) A bands - are dark bands of myosin and actin
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3) H zone - central part of A band with myosin, no actin
4) M line - center of H zone with rods that hold myosins
together
5) Z discs - protein discs where actin filaments attach;
distance between Z lines shortens during contraction.
Each cell is multinucleate, with peripheral nuclei.
Cells have many mitochondria
Sarcoplasm – muscle cell cytoplasm
Sarcolemma (plasma membrane) surrounds each muscle cell
Transverse (T) tubules - infoldings of the sarcolemma through
the muscle cell; carries electrical impulse into the cell
Sarcoplasmic reticulum (S.R.) - smooth E. R. that surrounds
each myofibril within the sarcolemma; contains Ca2+ ions
necessary for muscle contraction
Terminal cisternae - sac-like portions of the S. R. on both sides
of the T-tubules
Triad - complex of a T tubule between two terminal cisternae
B. Three types of Myofilaments (protein microfilaments) compose a
myofibril
1. Thick filaments - “golf club” like myosin proteins composed of
a. 2 intertwined polypeptides forming the shaftlike tail and
b. A double globular protein head projecting from the tail at an
angle.
c. A thick filament is a bundle of 200-500 myosin “golf clubs”
with their heads spiraling outward from the bundle
2. Thin filaments – contain actin, tropomyosin, and troponin
proteins.
a. Actin proteins are composed of
1) Fibrous (F) actin – 2 intertwined strands of protein
“beads” called
2) Globular (G) actin, each of which has an active (myosin
binding) site that can bind a myosin head
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b. Tropomyosin is a fibrous protein that blocks actin’s myosin
binding sites when the muscle cell is relaxed
c. Troponin is a small calcium-binding protein attached to the
tropomyosin molecule at regular intervals
3. Elastic filaments – springy titin proteins that connect the thick
filament to the Z disc and keep the thick and thin filaments
aligned, to resist muscle overstretching
V. Nerve – Muscle Relationship
A. Motor neurons – somatic motor neuron axons extend from cell
bodies in brain stem and spinal cord to innervate skeletal muscle
cells
B. Motor unit - a neuron and all the muscle cells it stimulates
1. Muscles that require fine motor control, such as eye muscles,
finger muscles, etc., have few muscle fibers per neuron (e.g., 25/1)
2. Muscles involved in strength, such as in the back, legs, etc.,
have more muscle fibers per neuron (e.g., 1,000/1)
C. Neuromuscular Junction (synapse) - area where a neuron axon
meets a muscle cell
1. The axon end (synaptic bulb) fits into a depression in the muscle
sarcolemma called the motor end plate, with a minute gap
between the bulb and end plate
2. Synaptic cleft – microscopic fluid-filled gap between axon
synaptic bulb and muscle cell motor end plate
3. Synaptic vesicles filled with a neurotransmitter (acetylcholine
= ACh) are found within the axon synaptic bulbs
4. ACh Receptors found in the muscle motor end plate bind ACh
when it is released by the axon synaptic bulb, which stimulates
the muscle cell
5. Acetylcholinesterase (AChE) is released from the sarcolemma
to degrade ACh in the cleft and end muscle stimulation
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VI. Behavior of Skeletal Muscle Fibers
A. Muscle contraction and relaxation occurs in 4 major phases:
excitation, excitation-contraction coupling, contraction, and
relaxation
1. Excitation – the process by which the electrical nerve impulse is
transmitted to the muscle cell
a. A nerve impulse stimulates the uptake of Ca2+ ions into the
axon synaptic bulb
b. Ca2+ ions stimulate the exocytosis of ACh from synaptic
vesicles into the synaptic cleft
c. ACh diffuses across the cleft and binds to ACh receptors
(sodium ion channels) in the sarcolemma motor end plate
d. An electrical impulse is initiated in the motor end plate as
sodium enters the muscle cell through the ion channels
2. Excitation-contraction coupling – events that link the electrical
impulse on the sarcolemma to the activation of the myofilaments,
preparing them to contract
a. The electrical impulse ripples across the motor end plate, then
down into the sarcoplasm via the T-tubules
b. The impulse causes the sarcoplasmic reticulum cisternae to
release Ca2+ ions into the cytosol
c. Ca2+ ions bind to the troponin of the thin filaments, causing a
shape change that removes tropomyosin from the myosin binding
sites of actin
3. Contraction – the sliding filament model describes how thin
filaments slide over thick filaments to cause muscle contraction
a. ATP bound to myosin heads is hydrolyzed to ADP + Pi,
“cocking” the heads into an extended, high energy position
b. Energized myosin heads bind to exposed myosin-binding
sites on actin, forming cross-bridges
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c. Power stroke - myosin heads release the ADP + Pi and flex
into a bent, low energy position, pulling the thin filaments
toward the center of the sarcomere
d. Recovery stroke – ATP binds to myosin heads, which causes
the myosin to detach from the actin
4. Relaxation – muscle fiber relaxes and returns to its resting
length
a. Nerve impulses cease, and ACh is no longer released by the
synaptic bulb
b. ACh separates from motor end plate receptors, and is broken
down by AChE, which ends muscle stimulation
c. Ca2+ ions separate from troponin and are actively transported
from the cytosol back into the SR terminal cisternae
d. Tropomyosin returns to its position blocking myosin-binding
sites on actin
B. Tonus (muscle tone) - the normal state of skeletal muscle, in which
a muscle rests in a state of partial contraction.
1. Keeps the muscle ready to react to a stimulus
2. Helps to maintain posture
3. Aids the return of blood to the heart
VII. Behavior of Whole Muscles
A. Threshold, Latent Period, & Twitch
1. Threshold is the minimum voltage required to produce a muscle
contraction; lower voltages do not cause muscle contraction
2. A twitch is one cycle of muscle contraction and relaxation
3. The brief time between the stimulus and the twitch is the latent
period (about 2 millisec)
4. For a few millisec after contraction, the muscle cannot contract
again, no matter how much stimulus is received, this is the
refractory period
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B. Contraction Strength of Twitches is achieved by three methods:
optimal sarcomere length, recruitment of motor units, and high
frequency stimulation
1. The tension of a muscle twitch is determined by the length of
individual sarcomeres
a. Each sarcomere will contract with optimum force if the
myosins and actins are at optimum length before contraction
b. Warm up exercises before strenuous activity help sarcomeres
to be at optimal length, and prevent muscle tears
2. Recruitment (multiple motor unit summation)– as more strength
is needed, more neurons “fire,” activating more motor units
3. High frequency stimulation
a. Less than 10 stimuli/sec allows muscle to fully recover between
stimuli, generates weak muscle tension
b. 10-20 stimuli/sec causes each twitch to develop more tension
than the twitch before it (the staircase phenomenon = treppe)
c. 20-40 stimuli/sec allows twitchs to “piggyback” on each other,
generating higher tension (temporal summation), produces
sustained partial contraction called incomplete tetanus
d. At 40-50 stimuli/sec, the muscle does not relax between stimuli,
so twitches blend into a smooth, prolonged contraction called
complete tetanus (not the same as lockjaw tetanus)
C. Isometric & Isotonic Contraction
1. Isometric contraction involves an increase in muscle tension
without a change in muscle length; no external muscle
movement occurs
2. Isotonic contraction is a decrease in muscle length without a
change in muscle tension; muscle tension overcomes external
resistance and moves
3. Isometric and isotonic are both involved in normal muscle
contraction
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VIII. Muscle Metabolism
A. ATP Sources – two main pathways of ATP synthesis are
anaerobic fermentation and aerobic respiration
1. Anaerobic fermentation occurs when a cell metabolizes
glucose the absence of oxygen; lactic acid and 2 ATP are
generated
a. Advantage – allows the cell to produce ATP without oxygen
b. Disadvantage – produces lactic acid, a toxic product that
contributes to muscle fatigue, and depletes glycogen in the liver
2. Aerobic respiration occurs mostly in the mitochondria in the
presence of oxygen; glucose is broken down and its energy is
used to regenerate about 36 ATP molecules. Waste products are
CO2 and H2O.
a. Advantage – produces more ATP and less toxic by-products
b. Disadvantage – requires continuous oxygen supply
3. Immediate Energy for short (15 sec), intense exercise is
provided by two enzyme systems
a. Myokinase - transfers Pi groups from one ADP to another,
forming ATP
b. Creatine kinase obtains Pi from creatine phosphate and
donates it to ADP, forming ATP (and creatine)
4. Short-term Energy – after the immediate energy is exhausted,
for the next minute or so, glucose from the blood and glycogen
stored in muscles is used to produce ATP (anaerobically)
5. Long-term Energy – in the next minute, the respiratory and
cardiovascular systems “catch up” and deliver oxygen to the
muscles for aerobic respiration
6. Fatty acids are broken down to acetyl CoA via beta-oxidation
and used in cell respiration during moderate exercise
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B. Oxygen Debt is the difference between the resting rate of oxygen
consumption and the elevated rate following strenuous exercise
1. At rest or moderate activity, the respiratory and circulatory
systems can supply the muscles with enough oxygen for aerobic
respiration
2. With strenous exercise, there is not enough oxygen for aerobic
respiration, so the muscle uses anerobic respiration for energy
3. Extra oxygen consumed after strenous exercise is used to
a. Replenish the body’s oxygen reserves in myoglobin and
hemoglobin
b. Oxidize lactic acid to pyruvic acid, then to glucose in the
liver, where it is stored as glycogen
C. Muscle Fatigue – progressive weakness and loss of muscle
contractility with use. Causes include:
1. Glycogen and creatine phosphate reserves in muscles and liver
decline
2. ATP synthesis declines
3. Accumulating lactic acid lowers the sarcoplasm pH, inhibiting
enzymes involved in contraction, ATP synthesis, etc.
4. Motor nerves deplete their ACh
D. Muscle Cramps – muscle contraction without relaxation, due to
lack of ATP, which is needed to:
1. Actively transport Ca 2+ back into the SR after contraction
2. Detach myosin heads from actin after contraction
E. Types of Skeletal Muscle Fibers
1. Red slow-twitch (slow oxidative) fibers (dark meat)
a. Look red due to abundant myoglobin, the oxygen storing
protein in muscle
b. Have many capillaries and a large number of mitochondria
for aerobic respiration
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2.
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c. Fibers contract slowly and are resistant to fatigue as long as
oxygen is present
d. Abundant in lower back muscles that contract continuously to
maintain posture
White fast-twitch (fast glycolytic) fibers (white meat)
a. Look pale due to less myoglobin
b. Are about twice the diameter of red fibers
c. Contain more myofilaments and generate more power, but
fatigue quickly
d. Have fewer mitochondria and capillaries but many glycogen
containing glycosomes
e. Anaerobic respiration is their main energy source
f. Common in upper limb muscles that lift heavy objects briefly
Intermediate fast-twitch (fast oxidative) fibers
a. Have diameters, power, and fatigue resistance between red &
white fibers
b. Contract quickly like white fibers
c. Like red fibers, are oxygen dependent, have much myoglobin
and many capillaries
d. Abundant in lower limb muscles for walking
Muscles contain a mixture of the 3 muscle types, so they can do
different things at different times
The proportion of red to white to intermediate fibers one has is
believed to be genetically determined
V. Cardiac Muscle
A. Cardiac muscle tissue is found in the heart wall (myocardium)
B. The cells are involuntary, striated, single uninucleate cells, not
voluntary fused multinucleate cells like skeletal muscle cells
C. A cardiac muscle fiber is a long row of joined cardiac muscle
cells
D. Cardiac muscle contracts via the sliding filament mechanism,
similar to skeletal muscle
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E. Cardiac cells branch and join together at complex junctions, called
intercalated discs, which are composed of
1. Desmosomes and fascia adherans that hold the cells together
2. Gap junctions that allow ions to pass from cell to cell to
synchronize muscle contractions
F. Not all cardiac muscle cells are innervated; they can contract
rhythmically without enervation (autorhythmic)
VI. Smooth Muscle
A. Smooth muscle cells are small, single, spindle shaped, and
unstriated, with a single centrally located nucleus
1. Thick and thin filaments form spiral bundles (myofibrils) within
the cell, but no visible striations and no sarcomeres
2. Tropomyosin is present, but not troponin, instead another Ca2+
binding protein called calmodulin is present
3. No Z-disks are present, thin filaments are attached to the
cytoskeleton via protein dense bodies on the inner sarcolemma
4. Noncontractile intermediate filaments form a cytoskeletal
matrix that supports the contractile filaments
5. Sarcoplasmic reticulum is sparse and there are no T-tubules
6. Ca2+ to activate muscle contraction comes from extracellular
fluid through Ca2+ channels in the sarcolemma
7. Not all smooth muscle is innervated, when nerves are present,
they are autonomic, not somatic motor fibers
B. Two types of smooth muscle are multiunit and single-unit smooth
muscle
1. Multiunit SM is found in large arteries, bronchi, arrector pili
muscles, and the iris
a. Though autonomic, innervation is similar to skeletal muscle
b. Terminal branches of a nerve fiber synapse with individual
muscle cells to form a motor unit
c. Each motor unit contracts independently
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2. Single-unit (visceral) SM is found in most blood vessels, and
the digestive, respiratory, urinary, and reproductive tracts
a. Usually forms an inner circular and outer longitudinal layer in
walls of hollow viscera
b. Muscle cells are joined by gap junctions that allow ions to
flow freely from cell to cell, thus many cells contract as a unit
C. Stimulation of Smooth Muscle
1. SM can contract without nerve stimuli
2. Some SM contracts in response to hormones, CO2, low pH, O2
deficiency, and stretch
3. Single-unit SM in the GI tract has pacemaker cells that set off
wavelike contractions through the muscle layer (peristalsis)
4. SM is innervated by autonomic nerve fibers that can trigger or
modify its contractions
5. SM cells don’t have motor end plates, they have diffuse
junctions
a. Their receptor sites are scattered across their surface
b. One nerve fiber with beadlike varicosities passes along many
muscle cells and stimulates all of them at once
D. Contraction & Relaxation of Smooth Muscle
1. SM contraction is similar to skeletal muscle in that:
a. Actin and myosin interact by a sliding filament mechanism
b. A Rise in Ca2+ level triggers contraction
c. ATP energizes the sliding process
2. SM contraction differs in that:
a. Ca2+ can diffuse through the cell membrane to initiate
contraction
b. Ca2+ interacts with components of the myosin filament,
namely calmodulin and a kinase enzyme to activate myosin
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3. Sequence of SM Contraction Events
a. Ca2+ binds calmodulin
b. Calmodulin is activated
c. Calmodulin activates the kinase enzyme
d. The kinase transfers a phosphate from ATP to myosin
e. Myosin interacts with actin (power & recovery strokes)
f. Relaxation occurs when
1) Intracellular Ca2+ is ‘pumped” out of the muscle cell by
active transport
2) Myosin is dephosphorylated by myosin phosphatase
4. Smooth muscle contraction is slower than skeletal muscle, but
more sustained and resistant to fatigue
a. Small blood vessels and visceral organs maintain a degree of
sustained contraction (tonus) without fatiguing due to:
1) ATP efficient contraction
2) Lower energy requirement than skeletal muscle
3) Slow contraction time (30x longer than skeletal muscle)
b. Because smooth muscle has low energy requirements, it has
1) Few mitochondria
2) Mostly anaerobic ATP pathways
5. Regulation of SM contraction is similar to skeletal mus. in that:
a. Neurotransmitters are released in response to a nerve impulse
b. Neurotransmitters bind to receptors in the SM sarcolemma
c. Ca2+ is released into the sarcoplasm and triggers contraction
6. Smooth muscle regulation is dissimilar to skeletal mus. in that:
a. Not all neural signals to SM result in activation
b. Not all activation is due to neural signals, some SM can
contract in response to chemical & mechanical stimuli
c. SM autonomic motor neurons release two types of
neurotransmitters:
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1) Acetylcholine (ACh)
2) Norepinephrine
d. Both neurotransmitters may be either excitatory or inhibitory,
because there are excitatory and inhibitory receptors in SM
membranes. Examples:
1) ACh contracts smooth muscles in bronchioles
2) Norepinephrine relaxes smooth muscle in bronchioles
3) Norepinephrine contracts smooth muscle in blood vessels
7. Some chemical stimuli affect smooth muscle Ca2+ levels,
causing the muscle cells to contract or relax
a. Hormones, such as gastrin from stomach and small intestine,
cause SM contraction
b. Lack of oxygen or excess CO2 relaxes SM (e.g., in lung
bronchioles)
c. Low blood pH relaxes SM
E. Smooth muscle Response to Stretch
1. Skeletal and cardiac muscle contract rapidly when stretched
2. The smooth muscle “stress-relaxation response” is much
slower, which allows
a. SM fibers can remain stretched to accomodate an enlarged
lumen (e.g., in the stomach and urinary bladder)
b. Digestive materials can pass slowly through the intestines via
peristaltic contractions, allowing time for maximum absorption
of nutrients
F. Muscle length and tension changes
1. Organization of sarcomeres limits the stretch of skeletal muscle
2. Overlapping, irregular arrangement of smooth muscle filaments
allow them to generate force, even when stretched
G. Hyperplasia & Hypertrophy in muscle tissue
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1. All muscle cells experience hypertrophy (increase in size) in
response to stress
2. Most skeletal and cardiac muscle cells do not undergo
hyperplasia (increase in cell number), but some smooth muscle
cells do. Examples:
a. At puberty, estrogen binds to uterine smooth muscle receptors
and stimulates synthesis of more smooth muscle fibers
b. During pregnancy, estrogen stimulates uterine hyperplasia to
accomodate the fetus
VII. Disorders of Muscle Tissue
A. Muscular Dystrophy
1. A group of inherited muscle destroying diseases that usually
appear in childhood
2. Skeletal muscle degenerates and is gradually replaced by
adipose and fibrous tissue
3. Duchenne muscular dystrophy is the most serious and is
inherited as a sex-linked recessive disease, thus affects more
males than females. Patients rarely live beyond age 20
4. Myotonic dystrophy is also inherited and can appear at any
age; symptoms include skeletal muscle spasms, muscle
weakness, and abnormal heart rhythm
B. Myofascial Pain Syndrome
1. Affects up to 50% of all people 30 - 60 yr. old
2. Pain is caused by tightened bands of muscle fibers that twitch
when the skin over them is touched
3. Often associated with strained postural muscles
4. Treated with nonsteroidal anti-inflammatory drugs, stretching,
and massage
C. Fibromyalgia
1. Chronic pain syndrome of unknown cause
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2. Symptoms include severe musculoskeletal pain, fatigue, sleep
abnormalities, and headache
3. To be diagnosed as fibromyalgia, pain must be present in at
least 11 of 18 standardized points on the body
4. Treatments include antidepressants, exercise, and pain relievers