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Skeletal Muscle Physiology
Terms in this set (68)
Place these skeletal muscle structures in order from largest to smallest: sarcomere, myocyte, myofibril
Myocyte, myofibril, sarcomere
What is a sarcomere?
The functional unit of cardiac and skeletal muscle
T/F: A muscle fiber (myocyte) usually runs the whole length of the muscle.
What does the dark band correspond to?
What does the light band correspond to?
Skeletal muscles are divided into regions from Z disc to Z disc. What are the regions and what do they contain (5)?
H band = dark only (thick)
M line = middle of H
I band = Light only (thin); contains Z disk
A band = dark overlapping light; contains M line
Z disk = connects to next sarcomere
What is titin?
One of the largest proteins in the body associated with thick filaments (myosin) helps center them in Z disk. It extends from M line to Z disk; spring-like in nature.
What is nebulin?
Large protein associated with thin filaments sets the length of thin filaments during assembly; also called the Molecular RULER = determines RESTING length
What is alpha actinin?
anochors thin filaments (actin) to the Z disk (similiar to titin's job for thick filaments)
What is the thin filament composed of? What are the jobs of each?
F and G-actin (active site which binds myosin)
troponin (binds actin, tropomyosin and calcium)
tropomyosin (covers the active sites of actin)
What is the thick filament? What is it's job(s)?
myosin = form multiple cross-bridges where the "heads" can bind to G-actin molecules; ATPase enzyme
What regions can troponin's heads be divided into?
C = Calcium binding site
T = tropomyosin binding
I = inhibitory
What is the purpose of the dystrophin-glycoprotein complex?
- dystrophin protein connects thin filaments glycoproteins in the sarcolemma; connects F actin to other proteins in ECM
- provides scaffolding for sarcomeres
- associated with muscular dystrophies i.e. Duschene's Muscular Dystrophy (progressive weakness)
What is excitation coupling?
The mechanism which translates muscles action potential into tension.
T/F: The action potential always precedes the rise in intracellular Ca+ concentration, and this precedes contraction (tension) of the muscle.
How is the action potential initiated?
An alpha motor neuron releases ACh which binds the nicotinic ACh receptor (Nm) on the muscle fiber.
What are the three phases of muscle contraction?
1. Latent phase (AP start => force increase)
2. Contractile phase (increase of force)
3. Relaxation phase (decrease of force)
What must happen to the intracellular Ca in order to have contraction?
Intracellular Ca must increase.
What is the Sarcomplasmic Reticulum? What two parts is it made of?
The SR is a modified ER which sequesters Ca. It is made of T-tubules (sarcolemma invaginations) and terminal cisternae (portions of the SR that contact T-tubules).
Why is Ca important for muscle contraction?
Because Ca binds to the C region of troponin which causes a conformational change that allow the T region of troponin to bind and remove tropomyosin, revealing the G-actin binding sites for the myosin head creating a cross-bridge. At rest tropomyosin covers G-actin.
Where does the Ca come from?
The Ca comes from depolarization of T-tubules' voltage sensitive dihydropyridine (DHP) receptors which open Ca channels called ryanodine receptors on the SR which causes them to release Ca into the sarcoplasm. (In the process, Ca moves down it's conc. gradient.)
What is the significance of the sarcolemma?
It has T-tubules positioned in the center of muscles so action potentials and Ca release spreads quickly. (Myocytes intracellular structure is arranged so that AP and Ca spread rapidly)
T/F: ATP is unneccessary for contraction.
What are four ATP sources? Approximately how long will each last in sustained contraction?
1. Stored ATP = 1-2 sec
2. Phosphocreatine (4-6 sec)
3. Glycolytic metabolism (1 minute)
4. Oxidative Metabolism (longest of all, the other three provide time for this to boot up)
Why do muscles fatigue even with ample ATP? (6)
Glycogen depletion, hyperpolarization of Vm, reduced blood flow, decreased ACh, Lactic acid, central fatigue.
Explain glycogen depletion.
fatigue quickly inferferes with Ca; number 1 reason for muscle fatigue.
Explain hyperpolarization of Vm.
each consecutive depolarization makes the membrane more and more negative, so that it is more difficult to depolarize it.
Explain reduced blood flow.
increased CO2 and K+ activates pain receptors
Explain decreased ACh.
synatpic fatigue, receptors become less active the more they are activated
Explain Lactic acid.
might be an effect from ACh
Explain central fatigue.
attributed to the brain, but whole body effected
T/F: Muscle always have enough ATP, unless you are dead.
True - when we die our body cannot make anymore ATP, thus rigor mortus. Ca+ is still present, however.
What two things must be present for cross-bridge cycling?
ATP and Ca+
What are the steps of cross-bridge cycling?
1. rigor state = myosin and actin tightly bound (brief)
2. ATP binds myosin causing it to release Actin
3. myosin head moves in direction of Z disk/line; ATP hydrolysis
4. mysoin binds next actin = power stroke
5. ADP released from myosin head => back to rigor state
What two things must happen for the skeletal muscle to relax?
1. the alpha motor neuron must stop firing
2. intracellular Ca+ concentrations must decrease
**** ATP must also be present, or rigor state is maintain (death)
What steps are taken to decrease intracellular Ca+?
- Ca+ ATPases on SR (SERCA = smooth endoplasmic reticulum Ca+ ATPase)
- tropomyosin moves to cover actin's myosin binding site
- actin slides back to resting place and sarcomere returns to original length (due to Titin)
What can activate relaxation in skeletal muscles?
Low Ca+ in the SR
What are the three main types of muscle fibers?
Type I (marathon runner), Type IIa and Type IIb (body builder)
Type I: Color, myosin ATPase activity, SR ATPase capacity, diameter, oxidative capacity, fatigue
Color=red (due to amount of O2, red = more O2 = less likely to fatigue)
myosin ATPase activity= slow
SR ATPase capacity=moderate
oxidative capacity= high
Type IIa: Color, myosin ATPase activity, SR ATPase capacity, diameter, oxidative capacity, fatigue
Fast, Oxidative, glycolytic
myosin ATPase activity= fast
SR ATPase capacity=fast
oxidative capacity= moderate
fatigue= fast resistant
Type IIb: Color, myosin ATPase activity, SR ATPase capacity, diameter, oxidative capacity, fatigue
myosin ATPase activity= fastest
SR ATPase capacity=fast
oxidative capacity= low
fatigue= fast fatigable
Skeletal Muscle changes terms:
hypertrophy, atrophy, sarcopenia, hyperplasia
hypertrophy= increase in size/growth = same #
atrophy=decrease in size (usually due to disuse)
sarcopenia=wasting due to disease, also loss of bone mass
hyperplasia= rare athletes split muscles cells increasing #
T/F: It is possible to change between muscle fibers types; example, you can go from having type IIb to type IIa and type IIa to type I.
False, it is possible to go from having type IIb to type IIa but NOT type IIa to type I.
What is a motor unit?
The muscle fibers and the alpha motor neuron that innervates it.
How are motor units recruited?
- in order of size, so smaller motor units are recruited first (smallest motor units control fewer fibers); type I recruited first, if not enough then add to that (think spatial summation)
What does increase in the number of motor units activated increase?
The total tension produced by contraction of a muscle; you don't want to recruit all motor units to begin with if you are only picking up a feather.
What is asynchronous recruitment? Why is it important?
It is the switching of muscle fibers to avoid fatigue = submaximal contraction i.e. some motor units are not used/preserved
T/F: All motor units are the same type of fiber - all in unit 1 are type IIa.
What are the two types of contractions and what determines which is activated?
The two types of contractions are isotonic and isometric. Which type is activated is determined by the load and force generated.
What is an isotonic contraction?
Force produced is great enough to move a load; length of sarcomere shortens; elastic already shortened so so muscle shortens
i.e. pushing a cart, it moves.
What is an isometric contraction?
Force produced is less than load = no movement; length of sacromere is shortens, but elastic elements stretch = maintain length
i.e. pushing against a building, no matter how much you push it's not going anywhere.
What does it mean to say that contraction of a whole skeletal muscle is graded?
This means that increasing the total force increases the frequency of fiber activation and the number of muscle fibers contracting. (Increasing frequency an alpha motor neuron fires increases the tension produced)
What is tentany? What is treppe?
Tetany = The joining/fusing of contractions together in response to consecutive action potentials. This will happen if the muscles had not completely relaxed.
Treppe = opposite of tetany, step-like series of contractions which usually precedes tetany
How do we increase strength of contractions (2)?
1. add more motor units
2. increase the frequency of contractions
T/F: For a single muscle twixh, the tension developed is altered by sarcomere length.
What is the optimal sarcomere length?
Slight overlap between A and I bands - myosin and actin in contact barely. Too much overlap, smaller contraction, no overlap difficult to start the contraction
What is the difference btw the extrafusal fiber and the muscle spindle?
The extrafusal fibers are the muscles themselves and are innervated by alpha motor neurons (responsible for contraction). The muscle spindle is inside of the extrafusal fibers and is innervated by gamma motor neurons (responsible for sensing changes in length and knowing what resting position is).
What are the two parts of the muscle spindle?
1. intrafusal fibers = contractile elements
2. sensory fibers = mechnosensitive elements that monitor changes in muscle LENGTH
What types of muscle have the most muscle spindles?
Those with responsible for fine motor movements (i.e. eyes, hands)
What is the effect of stretch on the muscle spindle? Contraction?
Stretch = increase in firing rate
Contraction = decrease in firing rate
What stimulates muscle spindle reflex? How does this happen?
Stretching of muscle triggers the sensory fibers in the muscle spindle.
- This activates an alpha motor neuron which stimulates CONTRACTION in the muscle's extrafusal fiber = this occurs with a inhibition of the alpha motor neuron's antagonistic muscles.
- gamma motor neuron also activated = intrafusal stimulated to contract
(dropping a book onto someones arms, the drop it's meet with contraction)
What happens if the intrafusal fibers do no contract during muscle spindle reflex?
The sensory fibers would not be able to sense anymore change in muscle length because they would be slack.
What is the effect of muscle spindle activation?
Contraction which has dynamic and static reflex components. Basically, think of the book example, when the arms muscles are stretched they contract extrafusally (dynamic) and then that is maintained intrafusally (static)
T/F: Anytime the alpha motor neuron is activated so is the gamma motor neuron.
True - it allows the muscle spindle to maintain sensititivy to changes in muscle length even when muscle is contracted (muscle may not be contracted all the way, plus needs to eventually go back to it's resting state.)
What are Golgi Tendon Organs (GTO)?
mechanosensitive receptors found at the junction of tendons and muscle; sensitive to changes in force.
What is the Golgi Tendon Organ reflex?
A protective reflex to DECREASE contraction:
- tendons stretch in response to contraction
- too much stretch triggers GTOs
- 1. inhibitory interneuron stimulated which decreases alpha motor neuron activity and contraction is decreased i.e. relaxation
ex. if given a 500 lb weight, do NOT want to hold that so we drop it
Summary of Muscle Tendon Reflex (Stretch Reflex):
- muscle spindles = receptors
- sensitive to changes in length
- contract muscle when activated
Summary of Golgi Tendon Organ Reflex:
- GTOs are receptors
- sensitive to extreme change in tension
- relax muscle when activated
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