 | each muscle fiber is surrounded by a fine layer of areolar (loose)
tissue called the endomysium
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several endomysium-wrapped muscle fibers are joined together in
a bundle called a fascicle which is bound together by a dense fibrous tissue
sheath called the perimysium
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fascicles are bundled together into the muscle itself, held together
by the dense fibrous epimysium.
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superficial to the epimysium is another sheath of connective tissue
called the fascia or muscle belly.
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all of these sheaths are continuous with one another, so that when
muscle fibers contract, the forces are transmitted throughout the muscle.
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each muscle fiber is supplied with a nerve ending to control its
activity
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each muscle is served by an artery and one or more veins
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attachments
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most muscles span joints and are attached to bones in at least
two places
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when a muscle contracts, the muscle's insertion moves toward the
muscle's origin
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muscle attachments are either direct or indirect
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direct attachments (fleshy) : the epimysium
is fused to the periosteum or perichondrium
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| indirect attachments : the connective tissue
wrappings extend beyond the muscle as a tendon or aponeurosis
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tendon : connects the muscle
to a skeletal element
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aponeurosis : connects the muscle
to the fascia of other muscles
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| each skeletal muscle cell is a long cylindrical, multinucleated
cell, a syncytium of hundreds of embryonic cells
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the sarcolemma is the plasma membrane of a muscle fiber
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sarcoplasm is similar to cytoplasm, but it has higher amounts of
stored glycogen and a protein called myoglobin which binds oxygen
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sarcoplasmic reticulum is a specialized smooth endoplasmic reticulum
of a muscle cell; it regulates the intracellular Ca2+ in the cell
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transverse (or "t") tubules are hollow, elongated tubes where the
sarcolemma penetrates into the cell's interior; they help to evenly distribute
the muscle impulse
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myofibrils are rodlike structures that are organized into
sarcomeres
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| a sarcomere, or "unit of muscle," is a contractile unit and is
defined as the region between two "z" lines
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a z-line is a midline interruption of an i-band; it is a coin-shaped
protein that anchors thin (actin) filaments and connects each myofibril to
the next
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the i-band is a light band in a striated muscle; it is light because
only actin (thin) filaments are found in that area
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an a-band is the dark band in striated muscle; it is dark because
of the thick filaments there
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the h-zone is a slightly lighter area in the middle of the a-band;
it is lighter because it is the point where there is no overlap of thin and
thick filaments; they are only visible in relaxed muscle for that reason
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in the middle of the h-zone is the m-line, which is a dark band
where the thick filaments are joined
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the thin filament -- the actin filament -- is actually made up
of three proteins: G-actin, tropomyosin and troponin
| G-actin contains the active sites to which the myosin heads
will bind
| G-actin "beads" join together to form F-actin
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| two strands of F-actin coil around each other
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| two strands of tropomyosin coil around the
two strands of F-actin; they block the active sites during relaxation
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| troponin, a three-polypeptide complex, binds to actin
and to tropomyosin, and helps to control the positioning of tropomyosin
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the thick filament -- the myosin filament -- has two distinctive
features: the myosin tail and the myosin head or cross bridge
| the cross bridge is named for the fact that
it links actin and myosin during contraction
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| the myosin heads contain ATP binding sites
and ATPase enzymes to generate the energy needed during contraction
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| when a muscle contracts, the individual sarcomeres shorten, but
the filaments do not
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the sliding filament theory of contraction was first proposed in
1954 by Hugh Huxley
| he said that thin filaments slide past the
thick ones so that they overlap to a greater degree during contraction
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| during contraction the z-lines move closer together, the h-zone
disappears, and the a-bands gets closer while unchanging in their length
but the i-band gets smaller
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the cause of sliding filaments
| when intracellular levels of calcium are low,
the muscle is relaxed and the active sites on the thin filaments are
blocked
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| when calcium ions become available, they bind
to troponin, moving the tropomyosin molecule and exposing the active sites
and the following occurs in rapid succession:
| cross bridges attach
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| power stroke
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| cross bridge detachment
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| "cocking" of the myosin head
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sliding of the filaments occurs as long as there are calcium ions
available
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as the SR pumps the calcium back, and the active sites are blocked,
relaxation occurs
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the sliding filament mechanism is the best accepted model of muscle
contraction, but there is still some controversy
| actin has ATPase activity and undergoes
conformation changes, so what is their true role?
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| how many ATP molecules are used? one per power
stroke?
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rigor mortis illustrates how cross bridge detachment is ATP
driven
| muscles stiffen 3-4 hours after death (due
to release of calcium ions from the SR)
| peak rigidity is about 12 hours after death
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| 48-60 hours after death, muscles have relaxed
again -- because the actin and myosin are being broken down
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| at death, ATP is no longer being made, and
so when the actin and myosin bind, they can't be detached without ATP
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regulation of muscle contraction
| skeletal muscle cells are stimulated by motor
neurons of the somatic nervous system which form a neuromuscular junction
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| the neuromusclar junction is not a direct
connection, rather it is a very close association separated by a synaptic
cleft
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| the motor end plate is a highly folded part
of the sarcolemma that has a great number of ACh receptors in it.
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| when a nerve impulse reaches the junction,
ACh (acetylcholine) is released into the synaptic cleft, fusing with the
receptors in the motor end plate, causing a muscle impulse, similar to a
nerve impulse
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| the muscle impulse is the result of depolarizing
the sarcolemma
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| the ACh is rapidly destroyed after being taken up
by the receptors in order to allow another muscle impulse; acetylcholinesterase
is responsible for this
| a shortage of acetylcholine
receptors and blood containing antibodies to ACh receptors can result in
myasthenia gravis
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| curare, an arrowhead poison
used by natives in South America, binds to ACh receptors and blocks ACh
attachment, resulting in paralysis
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| the muscle impulse moves along the sarcolemma,
including down into the muscle itself through the t-tubules
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| the impulse triggers the release of calcium
ions from the SR into the sarcoplasm
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| the released calcium ions bind to troponin, allowing
the binding of cross bridges to the now-exposed actin active sites
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| calcium is shuttled back into the SR through
a continuously active ATP-dependent calcium pump
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| when intracellular calcium levels drop too
low to allow contraction, the tropomyosin blockage is reestablished and
relaxation occurs
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| the motor unit is one motor nerve and the hundreds of muscle fibers
that it services
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when a motor neuron fires, all of the fibers it supplies are stimulated
as above
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a motor unit may have as many as several hundred fibers associated
with it, or as few as 4
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the more finely controlled muscles, such as those controlling the
fingers or eyes, have fewer fibers in each motor unit
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the fibers of a motor unit are not clustered together, but spread
throughout the muscle, so stimulation of a single motor unit causes a weak
contraction of the entire muscle
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the response of a muscle to a single, brief, threshold stimulus
is a muscle twitch
| a muscle twitch produces a brief spike on a
myogram
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| it may be strong or weak depending on the number
of motor units involved
| the muscle contracts quickly and then relaxes
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| this is not the way our muscles work, not
normally
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graded responses -- variation in the degree of muscle contraction
-- are how our muscles work
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two ways of grading muscle contraction in vivo : changing the speed
of contraction and changing the number of motor units activated
| if two identical stimuli are delivered to a
muscle in rapid succession, the second twitch will be stronger than the first,
and on a myogram, it will appear to "ride" on the shoulders of the first
contraction -- this is wave summation, and occurs when the muscle is not
completely relaxed before being stimulated again
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| if the stimulus is held constant and the relaxation
time between twitches becomes shorter and shorter, resulting in a "disappearance"
of relaxation on a myogram, the sustained contraction is called tetanus
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| tetanus is a reflection of how muscle contraction
typically works in the body because motor neurons deliver a volley of impulses
rather than just one impulse at a time
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| tetanus can not be sustained indefinitely --
eventually the muscle will become fatigued and unable to contract any
longer
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recruitment is how a muscle can call more than one motor unit into
contraction, resulting in multiple motor unit summation
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threshold stimulus is the stimulus that results in the first obserable
muscle contraction; maximal stimulus is the strongest stimulus that results
in increasing contractile force and represents the point where all of the
motor units are engaged
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muscle tone is the state of semi-contraction that all muscles are
in; it keeps the muscles firm, healthy and ready to act, as well as stabilizing
joints and maintain posture
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isotonic and isometric contractions
| sometimes a muscle contracts but does not
shorten
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| isotonic contraction involves shortening the
muscle fiber and doing work (concentric); or lengthening the muscle fiber
(eccentric)
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| isometric contractions increase tension, but
the muscle no longer changes length
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| posture maintaining muscles work
isometrically
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| isometric contractions occur when a muscle
attempts to move a load greater than the force it is able to develop
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muscle fatigue : the muscle is physiologically unable to contract;
this results from a relative deficit of ATP or an excessive accumulation
of lactic acid or ionic imbalances
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muscle fiber types
| red slow -- thin cells that contain slow-acting
myosin ATPases; have a lot of myoglobin (hence their red color), abundant
mitochondria, rich capillary supply; oxygen dependent, very fatigue resistant
and have high endurance, but don't generate much power
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| white fast -- large pale cells with little
myoglobin, contain fast-acting myosin ATPases and contract rapidly; contain
few mitochondria, large glycogenic reserves, tire quickly, but very
powerful
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| red fast -- also called intermediate fibers
or pink fibers, have fast-acting myosin ATPases, and contact quickly, but
are more like red slow in that they are oxygen dependent, have a high myoglobin
content and fatigue slower than the white but faster than the red slow
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