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Physiology 1 (WPH) > L9: muscle mechanics > Flashcards

L9: muscle mechanics Flashcards

1
Q

isometric contraction =

A

constant length (fixed ends)

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2
Q

isotonic contraction =

A

fixed tension (constant rate of shortening)

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3
Q

roughly how long does a twitch last

A

from 20 to 100 ms depending on fiber type and temperature

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4
Q

what determines the length of a twitch?

A

density of Ca2+ channels on SR
perhaps fiber type
perhaps temperature

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5
Q

what is the latent period of a twitch?

A

time interval required to depolarize cell membrane and activate contractile apparatus in response to a stimulus

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6
Q

mechanical summation

A

muscle contraction tensions can be summed by successive stimuli

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7
Q

how does unfused tetanus contraction tension compare to fused tetanus contraction tension?

A

unfused tension is lower (can only overcome so much elastic force)

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8
Q

why is tetanic tension so much greater than single twitch tension?

A

elastic component must be stretched before full tension is possible, this requires multiple stimuli and more time to reach full contractile tension
-may also involve the rate of A-M cross bridge mobilization)

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9
Q

if there were no elastic elements (completely rigid) in the muscle, how would twitch tension compare to tetanic tension?

A

they would be equal
no elastic components to stretch so full tetanic tension possible in one twitch
(this is called a theoretical “active state” curve)

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10
Q

the maximum force that a contractile unit is capable of generating is called

A

tetanic tension

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11
Q

what is a theoretical “active state” curve

A

elastic components replaced by an inextensible element, so full tetanic tension possible in one twitch

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12
Q

the series elastic component accounts for __ % of entire muscle length

A

2-3% (pretty small, exaggerated in examples)

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13
Q

the series elastic component is due to

A

active elasticity from contraction

e.g. two-way stretch of myosin from middle, stretch of actin filaments and anchors

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14
Q

the parallel elastic component is due to

A

passive elasticity in titin, SR, t-tubules, sarcolemma, etc

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15
Q

between series elastic component and parallel elastic component, which contributes to active tension and which contributes to passive tension

A

SEC - active tension

PEC - passive tension

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16
Q

what is the most significant structure that contributes to the parallel elastic component?

A

titin (anchors myosin to z-line)

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17
Q

during isometric contraction, initial tension response decreases exponentially with time to a lower stable value. why?

A

stress-relaxation

SEC elements are visco-elastic, they lose some elasticity and settle into a lower elasticity when stretched

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18
Q

T/F the series elastic tension is equal and opposite to active tension

A

true

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19
Q

maximum active isometric force is dependent on

A

length of the muscle

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20
Q

the muscle length at which maximum active tension is possible is often called…

A

rest length

because muscles usually rest at this length

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21
Q

lo or lmax is…

A

the length at which the muscle can produce maximum active tension
(also called “rest length” because muscles usually rest at this length)

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22
Q

T/F the strength of a myofibril is the sum of the strength of the individual sarcomeres

A

false
myofibril strength = strength of 1 sarcomere
(chain links in series only as strong as weakest link)

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23
Q

why does lmax or “rest length” occur?

A

this is where actin/myosin overlap is optimal for cross-bridging… at shorter lengths, actin fibrils overlap and can even pass into the wrong half of the sarcomere to interact with the wrong myosin heads, and crumpled titin and thick filaments can crash into z-lines… at longer lengths, myosin heads lose contact with actin

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24
Q

which can operate over a wider window of muscle lengths, smooth or striated muscle?

A

smooth

  • no z-lines
  • side-polarized filaments
  • must aid large luminal volume changes
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25
Q

how do skeletal and cardiac passive length-tension curves compare?

A

cardiac develops passive tension at lower lengths (intercalated disks, mono/binucleated cells)
skeletal muscle can stretch more (syncytium)

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26
Q

is passive tension more significant below rest length for cardiac or skeletal muscle?

A

cardiac develops passive tension at lower lengths (intercalated disks, mono/binucleated cells)
skeletal muscle can stretch more (syncytium)

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27
Q

how do skeletal and smooth active length-tension curves compare?

A

smooth is wider
skeletal muscle loses contractility at ~60% Lrest where smooth muscle still retains about half of its contractility, and can contract down to shorter lengths

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28
Q

when muscle shortens at constant velocity, active tension =

A

force of the load

29
Q

in muscle contraction, “preload” =

A

passive tension due to parallel elastic components (titin, membranes, etc)

30
Q

in muscle contraction, “afterload” =

A

active tension due to contraction / lifting

31
Q

during a contraction, series elastic elements are stretched until

A

tension = weight of load

then shortening and isotonic contraction occurs

32
Q

why does it take a certain time interval (latent period) before contraction begins to lift a load?

A

it takes time to build up isometric tension equal to weight of the load before you can begin to lift and contract isotonically

33
Q

how does increasing the load affect the latent period before isotonic contraction?

A

increases the latent period

because it takes time to build up isometric tension equal to weight of the load

34
Q

how does decreasing the load affect the latent period before isotonic contraction?

A

decreases the latent period

because it takes less time to build up isometric tension equal to weight of the lesser load

35
Q

how is the extent of isotonic shortening affected by increasing the load?

A

less shortening

because the active tension possible decreases as muscle shortens less than Lmax/Lrest

36
Q

how is the extent of isotonic shortening affected by decreasing the load?

A

more shortening

because the active tension possible decreases as muscle shortens less than Lmax/Lrest, and less load can shorten further

37
Q

how does maximum speed of shortening change with increasing load?

A

lower maximal speed of shortening

38
Q

what is the load at maximum velocity of shortening?

A

load = 0 at Vmax for shortening

39
Q

with loads greater than ~160% of the maximum load that a muscle can hold, what happens?

A

muscle lengthening is very rapid due to muscle tearing

40
Q

with loads greater than the maximum load that a muscle can hold, what happens?

A

the muscle lenghtens

41
Q

T/F smooth muscle can generate forces comparable to those of skeletal muscle even though its myosin concentration is only one fifth that of skeletal muscle

A

true
it just shortens more slowly
(molecular basis for this is unknown)

42
Q

which generally have higher velocities of shortening, skeletal or smooth muscle?

A

skeletal

43
Q

which generally generate a greater force, skeletal or smooth muscle?

A

can be comparable

smooth muscle just takes longer

44
Q

what molecule buffers muscular ATP stores?

A

PCr creatine phosphate PCr

45
Q

what enzyme catalyzes the equilibrium between PCr and ATP?

A

CPK creatine phosphokinase

46
Q

what is the equilibrium catalyzed by creatine phosphokinase?

A

CPK

PCr + ADP < - > ATP + Cr

47
Q

what is the equilibrium constant for this reaction and what does that mean?
CPK
PCr + ADP –> ATP + Cr

A

Keq ~ 20
so products favored
so PCr will be used up until its concentration falls to a very low level if ATP is being used

48
Q

how is ATP generated aerobically vs anaerobically?

A 
aerobic = krebs / TCA cycle 
anaerobic = glycolysis
49
Q

2 things that are responsible for the breakdown of ATP during muscle activity

A

actomyosin ATPase

SR Ca++ ATPase pumps

50
Q

give two examples of muscles that adequately match ATP & PCr synthesis to their rate of breakdown

A

cardiac

smooth

51
Q

give an example of a muscle that breaks down ATP & PCr at a much higher rate than it can synthesize it

A

fast, white, skeletal muscles

52
Q

what is oxygen debt

A

the oxygen consumed during recovery to replenish depleted ATP, PCr and glycogen stores

53
Q

the oxygen consumed during recovery to replenish depleted ATP, PCr and glycogen stores

A

what is oxygen debt

54
Q

what is a motor unit

A

motor neuron together with all of the individual muscle fibers it innervates
(motor neuron axon splits into multiple projections that innervate a number of fibers

55
Q

how many motor neurons can a skeletal muscle fiber be innervated by?

A

1

56
Q

how many skeletal muscle fibers can a motor neuron innervate?

A

multiple (axon splits into multiple projections

57
Q

how does a threshold stimulus applied to a motor neuron affect the innervated skeletal muscle fibers?

A

produces a maximal twitch contraction in all skeletal muscle fibers of that motor unit

58
Q

what is a twitch?

A

1 contraction of a motor unit

59
Q

T/F in the case of individual muscle fibers, twitch contraction is always maximal

A

true

threshold stimulus always produces max twitch of that fiber

60
Q

how can the strength of a single contraction of muscle be increased?

A

motor unit summation

61
Q

when is maximum muscle twitch achieved?

A

when all possible motor units are activated

62
Q

what is the “all or none” law of skeletal muscle contraciton

A

a single threshold stimulus produces the same twitch contraction in a motor unit, no matter how much greater than threshold the stimulus may be
(greater contraction must be achieved by summing multiple motor units with a larger stimulus or by temporally summing with wave summation)

63
Q 
white muscle fibers
size
mitochondrial content
blood supply
glycogen content
contraction speed / fatigue
motor unit size
A 
largest fibers
few mitochondria
meager blood supply
lots of glycogen
-fast contractions but easily fatigued
large motor unit
64
Q 
red muscle fibers
size
mitochondrial content
blood supply
glycogen content
contraction speed / fatigue
motor unit size
A 
smaller fibers
lots of mitochondria
good blood supply
little glycogen
slow contractions but good endurance
small motor unit
65
Q 
intermediate muscle fibers
size
mitochondrial content
blood supply
glycogen content
contraction speed / fatigue
A

intermediate in all cases

between red and white

66
Q

T/F the smallest increment that can be added to the force exerted by a skeletal muscle becomes greater as the total force of contraction itself increases

A

true
because small motor units (red fibers) are more excitable and usually activated first, or even constantly
then intermediate fibers are activated
then large motor units (white fibers) are activated, and they are the largest and most powerful

67
Q

what is the “size principle” in motor units

A

the size principle refers to the relationships between skeletal muscle on the spectrum from white to intermediate to red

68
Q

what is an alpha motor neuron

A

the neuron from which many processes project to multiple motor end plates

Physiology 1 (WPH) (18 decks)

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