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205 > lecture 17- skeletal muscle > Flashcards

lecture 17- skeletal muscle Flashcards

1
Q

what are the two key molecules for cross bridge formation and cycling

A

Ca2+
ATP

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2
Q

how does Ca2+ help with cross bridge cycling?

A

enables myosin heads to attach to actin by revealing myosin binding sites on actin molecules
–> high force cross bridge formation

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3
Q

how does ATP help with cross bridge cycling?

A

ATP binds to myosin and is hydrolyzed to ADP+Pi

ATP hydrolysis provides energy for the myosin “powerstroke”

ATP binding to myosin allows it to detach from actin
–> cross-bridge cycling

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4
Q

what does tropomyosin do when Ca2+ binds to troponin?

A

tropomyosin shifts away from the myosin binding site on actin

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5
Q

explain the 4 steps of the contraction cycle

A
  1. ATP binds to myosin
  2. myosin hydrolyzes ATP
  3. power stroke
  4. myosin releases ADP at the end of the power stroke
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6
Q

do thick and thin filaments change in length?

A

NO

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7
Q

what is true about lengthening and shortening with contraction

A

A band length maintained

I and H band shorten

sarcomere shortens (Z lines closer)

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8
Q

Muscle tension

A

force created by muscle

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9
Q

Load

A

weight or force opposing contraction

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10
Q

contraction

A

creation of tension in muscle

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11
Q

relaxation

A

release of tension

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12
Q

what are the 4 stages of excitation-contraction in skeletal muscle cells

A
  1. events at the NMJ lead to muscle cell depolarization
  2. muscle cell depolarization triggers Ca2+ release from the SR
  3. increased [Ca2+] triggers cross bridge cycling and contraction
  4. Ca2+ is removed from the sarcoplasm
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13
Q

how is an action potential initiated at the neuromuscular junction?

A

somatic motor neuron releases ACh at a neuromuscular junction

net Na+ influx through nAChR (nicotinic acetylcholine receptor) causes end plate potential (EPP) which is a graded potential

the graded potential (EPP) is always suprathreshold

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14
Q

how does muscle cell depolarization trigger Ca2+ release from the SR?

A

action potentials in t-tubule alters conformation of DHP (dihydropyridine) receptor

DHP receptor opens RyR Ca2+ release channels in sarcoplasmic reticulum and Ca2+ enters cytoplasm

Ca2+ binds to troponin allowing actin-myosin binding

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15
Q

The DHP receptor is what type of channel?

A

Ca2+ channel

BUT Ca2+ entry is NOT required for opening of the RyR

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16
Q

how does increased [Ca2+] trigger cross-bridge cycling and contraction?

A

myosin heads execute power stroke

actin filament slides toward the center of the sarcomere

17
Q

how is Ca2+ removed from the sarcoplasm?
(Skeletal muscle relaxation)

A

sarcoplasmic Ca2+-ATPase pumps Ca2+ back into SR

decrease in free cytosolic [Ca2+] causes Ca2+ to unbind from troponin

tropomyosin covers the binding site again.
myosin heads release, making elastic elements pull filaments back into their relaxed position

18
Q

what pumps Ca2+ back into the SR?
(skeletal muscle relaxation)

A

the SR-Ca2+ ATPase (SERCA)

19
Q

a muscle twitch is a single …… cycle

A

contraction-relaxation

20
Q

what does summation allow for in a muscle fiber in terms of contraction?

A

allows for increased force of contraction

21
Q

What would low force of contraction be due to?

A

low glycogen?

excitation-contraction failure such as SR Ca2+ leak?

lactate of acid accumulation?

NMJ failure?

22
Q

is low muscle contraction force due to low ATP?

A

no

23
Q

what is a motor unit?

A

one motor neuron and the muscle fiber it innervates

24
Q

size of motor units is related to…

A

the need for refined movement

more refined movement= fewer fibers per motor unit

25
Q

number and order of motor units recruited is related to…

A

the power needed to generate movement

smaller motor units are always recruited first!!!!

26
Q

larger motor units contain more muscle fibers and thus…

A

generate more force/tension when activated

27
Q

tension can be increased by recruitment of..

A

multiple motor units within a muscle

28
Q

larger muscles have many….

A

motors units, some of which may quite large

29
Q

smaller motor units have…

A

smaller motor neurons

-lower threshold for activation
-activated at lower frequency of stimulation from CNS

30
Q

as frequency of stimulation from from CNS increases

A
  • AP frequency and tension in motor unit increases
  • larger motor units brought to threshold and contribute to total muscle tension
31
Q

total muscle tension reflects the

A

sum of motor unit activation in a motor neuron pool

32
Q

length-tension relationships in skeletal muscle

A

sarcomeres contract with maximal force when they are at their optimal resting length just prior to contraction
- provides optimal number of cross bridges
- normal resting length of muscle usually ensures this happens

205 (19 decks)

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