a&p section 3
Terms in this set (63)
Three types of muscle tissue
skeletal, cardiac, and smooth.
Skeletal muscle function
Locomotion, movement of skeleton, blood, heart, GI tract, urinary and reproductive tract posture. Stabilize joints and generate heat
Skeletal muscle functional characteristics
Excitability or irritability or responsiveness
Gross Anatomy of skeletal muscle in organ
Muscle fibers, blood vessels, nerve fibers, and connective tissue are present.
connective tissue coverings and the 3 types
continuous with one another and tendons
Epimysium, Perimysium, endomysium
function of these connective tissue
reinforce muscle and support cells
entry and exits routes for bv and nerves
transmits force of muscle fiber contraction to bones, which result in movement.
Contribute to elasticity
Dense Irregular C.T.
Fuses with fascia-deep, superficial
fasicles are bundles of muscle fibers visible to the naked eye.
surrounds each muscle fiber within each fascicle
Every muscle is served by
nerves - each muscle fiber is supplied by a branch of nerve fibers
Direct - epimysium of muscle to periosteum or perichondrium
Exp. intercostal mm
Indirect - collagen fibers of epimysium continue as tendon or aponerurosis
Histology of skeletal muscles
myofilaments form striations
multinucleated cells aka fibers
Skeletal muscle fibers looks like
long cylindrical cell with multiple oval nucei just beneath the sarrcolemma or plasma membrane.
the cytoplasm of the muscle cell. They include glycosomes, myoglobin, and myofibrils
rodlike fibers that extend length of cell
contractile elements of skeletal muscle cells.
made up of smaller structures called myofilaments or filaments
myofilaments are composed of
thick filament - myosin
thing filament - actin, tropinin, and tropomyosin
Thick filaments are composed of
myosin which have rodlike tail and 2 gobular heads bundled with tails forming filament and heads outward.
myosin heads which are binding sites for actin, binding sites for ATP, and ATPase enzyme.
Thin filaments are composed of
Actin which ar ebinding sites of myosin heads
Troponin with are regulatory proteins
are rod shaped protein
spirals about actin core to help stiffen and stabilize it.
Relaxed sacromere which block actin active sites so myosin heads cant bind to thin filaments
major protein in thin filament
TnI - inhibitory subunit which binds to actin
TnT - binds to tropomysin and helps it position on actin
TnC - binds calcium ions
Composed of protein called tinin
Connect muscles to z disc.
z disc function
allow muscle to resis excess stretching
keep thick and thin filaments aligned
help cell recoil after stretching.
z disc contains
sheet composed of connectin or alpha-actinin proteins
achors the thin filaments
connects each myofibril to the next throughout the cell
striations of myofilbril
due to alternation of dark a bands and light I bands.
A band consist
stand of protein desmin (aka myomesin) in central portion of thick filament. Serves to hold adjacent thick filaments together. Bisect H zone.
Only visible in relaxced sarcomere.
I band consist of
extends from A band to A band.
Contains z disc or line.
region of a myofibril between two successive z discs.
Smallest contractile unit of muscle fiber.
What are the two sets of intracellular tubes?
The sarcoplasmic reticulum and the T Tubules.
Modified smooth ER
Tubules run along myofibril and surround each myofibril.
Run across myofibril at A-I junction to form terminal cisternae.
Sarcoplasmic reticulum function
store and release calcium ions
Invaginations of sarcolemma form elongated tubes
T Tubles function
system of T Tubles ensures simultaneous contraction of myofibril.
T Tubule between paired terminal cisternae of SR
Somatic Motor neurons
Cell bodies in brain and spinal cord.
Axon (fiber) travels to skeletal muscle nerve
Axon divides into hundreds of axon branches (terminal branches)
Each axon branch ends with a cluster of axon terminals
each branch innervates a different muscle fiber
each branch of a somatic motor neuron's axon forms an ending on single muscle cell.
a large transient depolarization event that is conducted along the membrane of a muscle cell or nerve
Only cells with excitable membranes can generate Action Potential which are muscle cells and neurons
Action Potential 2 phases
Depolarizing phase and re-polarizing phase
The 2 phases of action potential is created by movents of what two ions?
NA+ and K+
depolarization wave (End plate potential EPP) from motor end plate depolarizes adjacent sarcolemma to "threshold voltage"
At thhreshold voltage
voltage-gated channels in sarcolemma triggered to open
Results in generation of the action potential!
Depolarizing phase of AP
Close to peak of +75mV, voltage-gated Na + channels begin closing and voltage-gated K + channels begin opening
So K + diffuses rapidly out of cell down its concentration gradient
Repolarization restores that "patch" of sarcolemma to initial polarized state
+75mV to -90mV
Repolarization restores electrical conditions
Na +-K + ATPase pump works to restore proper Na+-K + (ionic) balance
Expels 3 Na+ ions and reclaims 2 K+ ions
AP moves from "patch" to "patch" down the sarcolemma
spreading in all directions
Each AP serves as depolarizing stimulus to trigger opening of voltage-gated ion channels in next "patch"
Consequently, membrane potential in that region
decreases and depolarization occurs there, effectively spreading the AP
Repolarization wave follows depolarization wave
Period when muscle fiber cannot be stimulated until repolarization is complete
Action potential brief 1-2 ms
Contraction phase 100 ms
Sliding Filament Theory of Contraction- 1954 Huxley
States that during contraction, thin filaments "slide" past the thick ones so that thick and thin filaments overlap to a greater degree
Distance between Z discs decreases
I bands shorten
H zones disappear
A bands move closer together
Cross Bridge Cycle In relaxed state
intracellular Ca +2 levels low. Tropomyosin physically blocks myosin binding sites on actin
Higher concentration of cross bridge cycle
Removal of tropomyosin blockade
Ca +2 binds to troponin
Troponin changes shape conformationally
Moves tropomyosin deeper into groove of actin helix and away from myosin binding sites on actin
Cross Bridge formation
Removal of tropomyosin blockade enables "cocked" myosin heads to bind to actin
Working stroke or power stroke
As myosin head binds to actin, it changes shape and pivots
Pulls thin filament toward center of sarcomere, toward M line
Simultaneously ADP and Pi generated from previous cycle are released from myosin head
Cross bridge detachment
New ATP molecule binds to myosin head, breaking bond between myosin head and actin
Cocking of myosin head
Hydrolysis of ATP by ATPase to ADP and Pi provides energy to return myosin head to high energy or "cocked" position
Myosin head in cocked position ready to take another "step" in pulling thin filaments toward center of sarcomere
Some myosin heads remain in contact with actin because
prevent thin filaments from sliding backwards during next cycle!
Sliding of filaments continues as long as
enought ATP present
EXP. rigor mortis or fatigue
relaxation of muscle fiber
Immediately after AP passes by triad, Ca +2 starts to be reclaimed by calcium pumps in membrane of terminal cisternae and SR
Provided no further AP travel down T tubule
Ca+2 levels decrease in sarcoplasm
Tropinin changes shape
and returns to resting conformation
Tropomyosin re-covers binding sites on actin
Tropomyosin blockade re-established
Sarcomeres and muscle fiber passively return to resting length
Muscle fiber relaxes
Contraction of Skeletal Muscle Fiber
Contracting muscle shortens 30-35%. Contraction of Skeletal Muscle Fiber
total resting length
Cross bridge cycle must occur numerous times
Somatic motor neuron and all muscle fibers it supplies
Motor unit fiber distribution
Muscle fibers of a single motor unit dispersed throughout entire muscle
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