Download Muscular System Overview of Muscle Tissues • Types of Muscle

Muscular System
Overview of Muscle Tissues
• Types of Muscle Tissue
o Skeletal and smooth muscles which are elongated are called muscle fibers
o Myo- and Mys- = “muscle”
o Sarco = “flesh” refers to muscle; i.e., sarcolemma (“muscle” + “husk”)—plasma
membrane of muscle cells
o Skeletal muscle tissue are the longest muscle cells and have obvious stripes called
striations.
Voluntary muscles = only type subject to conscious control.
o Cardiac muscle tissue occurs only in the heart where it constitutes most of the heart
walls.
Cardiac muscles are striated, but not voluntary (i.e., most of us have no
conscious control over how fast our heart beats).
o Smooth muscle tissue is found in the walls of hollow visceral organs (e.g., stomach,
urinary bladder, respiratory passages).
Role is to force fluids and other substances through internal body channels
Smooth muscle cells are elongated fibers (like skeletal muscle fibers), but have
no striations.
Contractions of smooth muscle fibers are slow, sustained, and involuntary.
• Special Characteristics of Muscle Tissue
o Excitability—(i.e., responsiveness or irritability) the ability to receive and respond to a
stimulus (i.e., any change in the environment both inside and outside the body).
o Contractility—ability to shorten forcibly when adequately stimulated.
o Extensibility—ability to be stretched or extended
o Elasticity—ability of a muscle cell to recoil and resume its resting length after being
stretched
• Muscle Functions
o Producing Movement
Just about all movements of the human body and its parts result from muscle
contraction (i.e., skeletal, cardiac and smooth muscle).
o Maintaining Posture and Body Position
Skeletal muscle functions continuously to adjust our posture and positioning in
relation to gravity
o Stabilizing Joints
Skeletal muscles stabilize and strengthen the joints of the skeleton
o Generating Heat
Muscles generate heat as they contract
• This heat is vitally important in maintaining normal body temperature.
• Skeletal muscle accounts for at least 40% of body mass (most
responsible for generating heat).
o Miscellaneous
Skeletal muscles protect internal organs by enclosing them
Smooth muscles regulate passage of internal substances, dilate and constrict
pupils of eyes, and form arrector pili muscles attached to hair follicles (i.e.,
goose bumps).
Skeletal Muscle
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Gross Anatomy of a Skeletal Muscle
o Nerve & Blood Supply
Generally, each muscle is served by one nerve, an artery and one or two veins
Skeletal muscles have a nerve ending that controls its activity
Cardiac and smooth muscles can contract with the absence of nerve stimulation
Skeletal muscles require huge amounts of oxygen via blood supply and give off
large amounts of metabolic waste—removed via veins.
o Connective Tissue Sheaths
Individual muscle fibers are wrapped and held together by several different
connective tissue sheaths
• They give support to each muscle cell and reinforce the whole muscle
preventing the bulging muscles from bursting during exceptionally
strong contractions.
All connective tissue sheaths are continuous with one another as well as with
the tendons that join muscles to bones.
Connective tissue sheaths transmit force from contracting muscles to the bones
to be moved
The sheaths contribute somewhat to the natural elasticity of muscle tissue, and
also provide entry and exit routs for the blood vessles and nerve fibers.
From external to Internal in orientation:
• Epimysium
o Meaning “outside the muscle”; an overcoat of dense irregular
connective tissue that surrounds the whole muscle.
o Sometimes it blends with the deep fascia that lies between
neighboring muscles or the superficial fascia deep to the skin
• Perimysium
o Meaning “around the muscle [fascicles]”;
o Within each skeletal muscle, the muscle fibers are grouped into
fascicles that resemble bundles of sticks that are wrapped
around with perimysium, a layer of fibrous connective tissue.
• Endomysium
o Meaning “within the muscle”; whispy sheath of connective
tissue that surrounds each individual muscle fiber
o It consists of fine areolar connective tissue.
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Microscopic Anatomy of a Skeletal Muscle
o Sarcolemma = plasma membranes surface of a muscle fiber
o Sarcoplasm = cytoplasm of a muscle cell similar to the cytoplasm of other cells, but
contains unusually large amounts of glycosomes (granules of stored glycogen that
provide glucose during periods of muscle cell activity) and myoglobin, a red pigment
that stores oxygen (similar to hemoglobin).
o Myofibrils
Rodlike bundle of contractile filaments found in muscle fibers
Hundreds to thousands of myofibrils are in a single muscle fiber; they account
for about 80% of cellular volume.
Striations—a repeating series of dark and light bands along the length of the
myofibril
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A Bands (dark) and I Bands (light) nearly aligned with one another
Each A band has a lighter region in its midsection called the H Zone (H
for helle = “bright”)
Each H zone is bisected vertically by a dark line called the M Line (M for
middle) formed by molecules of protein myomesin.
The light I bands also have a midline interruption, a dark area called the
Z Disc (or Z line)
Sarcomere
Smallest contractile unit of a muscle fiber (i.e., functional unit of skeletal
muscle).
Region of the myofibril between two successive Z discs (i.e., has an A band
flanked by half an I band at each end).
Within each myofibril, the sarcomers are aligned end-to-end like boxcars in a
train
Myofilaments or filaments
Muscle contraction depends on the myosin- and actin-containing myofilaments.
On a molecular level, each sarcomere is made up of small structures called
myofilaments
• Thick filaments containing myosin that extend the entire length of the A
band
o Each thick filament contains about 300 myosin molecules
bundled together
Each myosin molecule is shaped like a golf club with two
heads (i.e., it has a long rod-like tail and two golf-club
heads (cross-bridges))
Cross-bridge (aka, myosin head) has two important
binding sites: 1) ATP (i.e., adenosine triphosphate; an
organic molecule that stores and releases chemical
energy within the cell composed of adenine, ribose
sugar and three phosphate groups), 2) Actin
The linking of thick and thin filaments together by the
heads of the myosin protein creating a power stroke
(movement creating a muscle contraction)
• Thin filaments containing actin that extend across the I band and
partway into the A band.
o Each thin filament is composed of three molecules:
Actin
• Main component of the thin filament structured
into a double helical chain
• Each actin subunit contains specific sites to
which the myosin heads bind
Tropomyosin
• A regulatory protein entwined around the actin
• It covers the binding sites on the actin subunits
preventing myosin head binding in an
unstimulated muscle
Troponin
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Sarcoplasmic Reticulum
An elaborate smooth endoplasmic reticulum surrounding each myofibril
communicating at the H zone and forming terminal cisternae (“end sacs”) at the
A band-I band junctions
Endoplasmic reticulum (“network within the cytoplasm”) is an extensive system
of interconnected tubes and parallel membranes enclosing fluid-filled cavities
(cisternae).
They play an important role in calcium ion storage and release during muscle
contraction. Calcium provides the final “go” signal for contraction.
o T Tubules
An elongated tube at each A band-I band junction where the sarcolemma of the
muscle cell protrudes deep into the cell interior
T tubules are continuations of the sarcolemma; they conduct nerve impulses to
the deepest regions of the muscle cell and to every sarcomere; these impulses
signal the release of calcium ions from the adjacent terminal cisternae.
Sliding Filament Model of Contraction
o Six Steps of Cross Bridge Cycling
1. The influx of calcium released from the terminal cisternae trigger the exposure
of binding sites on actin
• Presence of an action potential in the muscle cell membrane.
• Release of calcium ions from the terminal cisternae (i.e., end sacs of
the sarcoplasmic reticulum containing calcium ions).
• Calcium ions rush into the cytosol (i.e., a viscous, semitransparent
fluid substance of cytoplasm in which other elements are suspended)
and bind to the troponin.
• There is a change in the conformation of the troponin-tropomyosin
complex.
• This tropomyosin slides over, exposing the binding sites on actin.
2. The binding of myosin to actin
• When a binding site on actin is exposed, an energized myosin head can
bind to it forming a cross bridge.
3. The power stroke of the myosin head causes the sliding of the thin filaments
• The binding of myosin to actin brings about a change in the
conformation of the myosin head, resulting in the release of ADP and
inorganic phosphate.
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A molecule attached to and periodically spaced
along the tropomyosin strand
Functions to move the tropomyosin strand
uncovering the myosin binding sites on the
actin subunits
By themselves, troponin molecules are unable
to move tropomyosin
o After an action potential, calcium ions
are released and bind with the troponin
which cause the tropomyosin strands to
move.
At the same time, the myosin head flexes, pulling the thin filament
inward toward the center of the sarcomere. This movement is called
the power stroke.
• The chemical energy of ATP has been transformed into the mechanical
energy of a contraction.
4. The binding of ATP to the myosin head, which results in the myosin head
disconnecting from actin
• In order to disconnect the myosin head from actin, an ATP molecule
must bind to its site on the myosin head.
• If ATP is depleted in a muscle, the muscle will remain in a state of
“rigor”
o The condition called rigor mortis occurs after death due to
depletion of ATP. Without ATP, the calcium ion pumps no
longer function. The exposed actin binding sites attract myosin
cross bridges which attach but are not able to detach.
o The body becomes stiff due to these bound myosin cross
bridges.
o The Muscles will remain in “rigor” until the cellular proteins
begin to break down, usually within 24-48 hours.
5. The hydrolysis of ATP, which leads to the re-energizing and repositioning of the
myosin head
• The release of the myosin head from actin triggers the hydrolysis (i.e., a
chemical process in which an enzyme uses water to split one molecule
into smaller parts) of the ATP molecule into ADP and Pi
• Energy is transferred from ATP to the myosin head, which returns to its
high-energy conformation
6. The transport of calcium ions back into the sarcoplasmic reticulum
• Calcium is actively transported from the cytosol into the sarcoplasmic
reticulum (i.e., endoplasmic reticulum of the muscle cell; its
interconnecting tubules surround each myofibril like the sleeve of a
loosely knit sweater) by ion pumps (i.e., an integral protein of a cell
membrane that actively transports ions from one side of the membrane
to the other).
o The active transport (i.e., the movement of materials across a
cell membrane against a concentration gradient. This process
uses a carrier protein molecule and requires cellular energy—
usually ATP) of calcium involves specialized ion pumps in the
membrane of the sarcoplasmic reticulum
o These pumps must be energized by ATP
o Note that during a regular muscle contraction, all myosin heads would cycle, but not all
are bound or disconnected at the same time.
Physiology of Skeletal Muscle Fibers
o For a skeletal muscle to contract:
It must be activated (i.e., stimulated by a nerve ending) causing a change in the
membrane potential
Next, it must generate and propagate an electrical current (i.e., action
potential) along its sarcolemma
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A resting sarcolemma (like the plasma membranes of cells) is polarized
(i.e., has a potential difference or voltage across the membrane with the
inside being negative relative to the outside membrane surface).
Finally, a short-lived rise in intracellular calcium ion levels occurs to trigger a
contraction
• Motor neuron = a nerve cell that activates a skeletal muscle fiber
o Motor neurons reside in the brain or spinal cord, but their long
axons (i.e., threadlike extensions bundled with nerves) travel to
a muscle fiber.
• Neuromuscular junction = region where a motor neuron comes into
close contact with a skeletal muscle cell
o As a rule, each muscle fiber has only one neuromuscular
junction, located approximately midway along its length
o Synaptic cleft = the remaining functional space within the
neuromuscular junction between the motor neuron and the
muscle fiber.
o Acetylcholine (ACh) = a neurotransmitter released by the motor
neurons within the synaptic cleft; when released, it stimulates
an electrical impulse creating an action potential
o Acetylcholinesterase = an enzyme located in the synaptic cleft
that binds with and breaks down acetylcholine; terminates
action potential
• Excitation-Contraction Coupling—sequence of events by which
transmission of an action potential along the sarcolemma leads to the
sliding of myofilaments.
• Depolarization—loss or reduction of negative membrane potential
• Repolarization—movement of the membrane potential to the initial,
negative resting (polarized) state.
• Refractory Period—a period during the repolarization phase when the
cell cannot be stimulated again until repolarization is complete.
Events at the Neuromuscular Junction
• Action potential arrives at axon terminal of motor neuron
• Voltage-gated calcium-ion channels open and calcium-ions enter the
axon terminal
• Calcium-ion entry causes the release of acetylcholine out of the axon
terminal
• Acetylcholine diffuses across the synaptic cleft and binds to receptors in
the sarcolemma
• Acetylcholine binding opens ion channels that allow simultaneous
passage of Sodium-ions into the muscle fiber and Potassium-ions out of
the muscle fiber.
o More sodium-ions enter than potassium-ions leave—this
produces a local change in the membrane potential called
depolarization (i.e., the interior of the sarcolemma becomes
slightly less negative. This local electrical event is called the end
plate potential).
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The action potential is propagated (moves along the length of
the sarcolemma) as the local depolarization wave spreads to
adjacent areas of the sarcolemma and opens voltage-gated
sodium channels there. Again, Sodium-ions, normally restricted
from entering, diffuse into the cell following its electrochemical
gradient.
Acetylcholine effects are terminated by its enzymatic breakdown in the
synaptic cleft by acetylcholinesterase.
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Muscle Metabolism
o ATP is the only source of energy used directly for contractile activities including:
Cross bridge movement
Cross bridge detachment
Operation of the calcium pump in the sarcoplasmic reticulum
o Muscles store only 4 to 6 seconds worth of ATP –just enough to get going
o Fortunately, after ATP is hydrolyzed to ADP and inorganic phosphate in muscle fibers, it
is regenerated within a fraction of a second by one or more of three different pathways.
o Muscle fatigue = state of physiological inability to contract even though the muscle still
may be receiving stimuli
o Contractures = a state of continuous contraction because the cross bridges are unable
to detach due to a lack of ATP