Skeletal Muscle Physiology Flashcards Preview

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Flashcards in Skeletal Muscle Physiology Deck (96):
1

muscle twitch

mechanical response to action potential

2

latent period

from AP initiation to cross bridge formation
-start of contraction

3

contraction time

beginning of contraction to beginning of relaxation
-until peak tension

active sites exposed to when they begin to be covered up

when Ca2+ is high enough to keep active sites exposed

4

relaxation time

peak tension to complete relaxation

Ca2+ sequestering into the SR

5

total force generated?

tension
sum of forces independently produced by many cycling cross-bridges

can vary with:
initial length of muscle fiber
pattern or frequency of muscle fiber stimulation

6

isometric contraction

muscle length constant
increase in tension, but no shortening

force production is equal to resistance

7

isotonic contraction

contraction occurs at constant load
-not really a constant force

length change occur

8

two phases of isotonic contraction

concentric and eccentric

9

concentric phase

muscle shortens as tension is produced

10

eccentric phase

muscle lengthens as tension is produced

11

length-tension relationship

for isometric contractions
-force production depends on initial fiber length

muscle length influences tension devleopment by determining region of overlap between actin and myosin

12

passive tension

tension prior to muscle contraction

increases as fiber is progressively lengthened because muscle becomes stiffer as it is distended

13

active tension

total tension - passive tension

cuased when cross-bridge cycling occurs in isometric contraction (fixed length)

14

when is active tension maximal?

near 100% of normal muscle length

15

what happens with increased fiber length?

ends of actin are pulled away from each other

greater than 150% - ends of actin are pulled beyond myosin

no interaction/overlaps occur and therefore no development of tension

16

what happens with decreased fiber length?

actin and myosin increase overlap
ends of actin filaments are pushed toward eachother

-tension can develop depending on degree of overlap

shortening to less than 70-85% of resting length
-opposing actin filaments slide over one another and hit Z disks

17

normal resting length?

of sarcomere

maximal overlap between actin and myosin filaments and maximal active tension

18

total tension = ?

passive + active tension

19

force-velocity relationship

in isotonic contractions

shortening velocity decreases as load increases

**lighter loads can be lifted faster

20

maximum velocity?

determined primarily by maximum velocity of myosin ATPase enzyme

Vmax also varies with fiber type

21

smaller the load?

greater the shortening velocity

22

larger the load?

the lower the shortening velocity

23

what is contraction at zero velocity?

ISOMETRIC!

24

at a given fiber length?

there is a hyperbolic relationship between shortening velocity and load

25

what does maximal velocity depend on?

maximal rate of cross-bridge turnover
-not on initial overlap of thin and thick filaments

therefore, it is independent of length

26

longer the initial fiber length?

the larger the maximal load under zero-velocity conditions

aka isometric conditions

27

work = ?

load x displacement

measureable mechanical work
-only when muscle displaces a load

28

power = ?

work/time

maximal at intermediate lads
-where both F and v are moderate

zero load F= 0
maximum load v = 0

29

power also = ?

load x displacement / time

aka load x velocity (Fv)

30

frequency summation?

tension of single fiber can be summed if APs fire rapidly

aka twitch summation

repetitive stimulation leads to increased tension

31

what causes frequency summation?

no fiber relaxation between stimuli due to sustained levels of Ca2+

32

tetanus?

twitches merge to a smooth, sustained, maximal contraction

result of high stimulation frequency
-muscle tension at a plateau

Calcium levels are sustained until tetanic stimuli ceases

tension increases very little at stimulation frequencies greater than the fusion frequency that causes tetanus

33

fusion frequency?

frequency of signals that lead to tetanus

34

motor unit?

single motor neuron and the muscle it innervates

35

whole muscle tension depends on?

size of muscle
number of motor units recruited
size of each motor unit recruited

36

muscles for refined, delicate movements?

few muscle fibers per motor unit

37

muscles performing stronger, coarser movements?

large number of fibers per motor unit

38

MMUS?

multiple motor unit summation

in skeletal muscle - increased force production with summation of multiple fibers

CNS can control how many individual fibers it stimulates

39

motor neuron pool

group of all motor neurons innervating a single muscle

40

asynchronous recruitment

some units develop tension while others relax

delays and prevents muscle fatigue during SUBMAXIMAL contraction

41

contractile strength can vary with?

number of active alpha-motor neurons in pool
frequency of firing of each alpha motor neuron

42

EMG

gross measure of electrical activity

43

hennemans size principle

size of cell body dictates excitability
smaller are more excitable
-threshold reached sooner

small recruited first, followed by larger

44

given excitatory stimulus...

will generate a larger EPSP in motor neurons with smaller cell bodies

45

slow-twitch motor unit

I
small amount of force, prolonged period of time

46

fast-twitch fatigue-resistant motor unit

FR
moderate amount of force, sustained for moderate amount of time

47

fast-twitch fatigable motor unit

FF
larger amount of force, brief period of time

48

order of recruitment of motor units?

I > FR > FF

49

type I motor units?

small cell diameter
fast conduction
high excitability

50

type II motor units?

large cell diameter
very fast conduction
low excitability

51

muscle fatigue

inability to maintain desired power output

decline in force production and shortening velocity

decline in maximal force production - from decreased number of active cross bridges

lower rates of force production and relaxation
-bc of impaired release and uptake of calcium from SR

52

role of fatigue?

protective
-allows contraction to occur at lower rates/forces while preventing extreme changes that can damage

53

muscle fatigue reversible?

yes, with rest

versus damage or weakness which compromise ability to develop force

54

factors contributing to fatigue

motivation, physical fitness, nutrition, type of motor unit recruited

55

central fatigue

changes in CNS
-brain > motor neuron cell bodies

can be opposed by cheering
-seriously though.

56

peripheral fatigue

motor neuron axon > NMJ > fiber

impaired APs, Ca release, depletion of metabolism substrates, accumulation of byproducts

57

peripheral fatigue and time required for recover depend on?

recruitment pattern and fiber type

58

anaerobic sources of ATP?

creatine phosphate (fast)
glycolysis (pretty fast)

59

aerobic source of ATP?

oxidative phosphorylation (slow)

60

slow twitch muscles?

type I

61

fast twitch muscles?

type II

62

different fiber types how?

different myosin heavy chain isoforms

-difference in mATPase activity corresponds to rate of contraction

can be hybrid fibers with intermediate rates

63

type I fibers?

slow oxidative fibers

64

type IIA fibers?

fast-oxidative fibers

65

tyoe IIX fibers?

fast-glycolytic fibers

66

how are skeletal muscle fiber types classified?

pathway for ATP synthesis (ox vs. glycolytic)
rate of ATP hydrolysis (mATPase isoform)
contractile velocity (fast vs. slow)

67

slow-twitch fibers?

smaller cross section
greater oxygen transport ability
more capillaries
appear red (myoglobin)
low glycogen
high mitochondria
resistant to fatigue

68

type IIA fiber characteristics?

fatigue resistant
oxidative metabolism
-red (myoglobin)
-mitochondria high
-abundant glycogen**
more capillaries

ensures adequate ATP generation for rapid depletion with rapid contraction

69

type IIX fiber characteristics?

fatigable
rely on glycolysis
few mitochondria
white (low myoglobin)
high glycolytic enzyme content
high glycogen

70

slow-twitch fibers?

tetanize at lower stimulation frequencies

71

fast-twitch fibers?

develop larger maximal force due to greater twitch tesion and larger motor units

72

proprioception

detailed information sensed about location in space, direction, and speed of movement

73

2 main purposes of proprioception?

identify external objects
accurately guide movement

74

muscle proprioception?

afferent info to regulate skeletal muscle activity

75

muscle spindles

detect changes in muscle length and rate of stretch

76

golgi tendon organs

detect muscle tension in muscle tendon

77

muscle spindle structure?

intrafusal muscle fibers aligned in parallel with force generating extrafusal fibers

78

golgi tendon structure?

aligned in series with extrafusal fibers

79

function of muscle spindle

send proprioceptive info about muscle to CNS
respond to muscle stretch

80

two kinds of intrafusal fibers?

bag and chain

81

two kinds of sensory endings

primary and secondary

82

primary sensory endings?

of group Ia axons
innervate bag fibers (in addition to chain fibers)

sensitive to change in length

83

secondary sensory endings?

of group II axons
innervate mainly chain fibers

transduce static length
-slowly adapting receptors

84

what happens when muscle stretches?

firing rate of sensory fibers increased

85

gamma motor neuron

to contractile region of spindle fiber

86

alpha motor neuron

output to regular skeletal muscle fiber

87

stretch reflex pathway?

afferent from muscle spindle
alpha to skeletal muscle

88

why does muscle spindle also have motor innervation?

alpha motor contract extrafusal fiber and the spindle becomes slackened

gamma not neurons help to maintain the sensitivity of the spindle apparatus

89

sensory response of spindle depends on what?

length of whole muscle AND contractile state of intrafusal fiber

90

reflex

basic neural function, involves simple neural circuits

91

motor reflex

rapid, stereotyped motor response to a particular sensory stimulus

motor neurons receive many synaptic inputs within the brain and spinal cord

92

stretch reflex

type of monosynaptic reflex
-myotatic

group Ia sensory axons terminate monosynaptically on alpha motor neurons innervating same muscle

93

reciprocal innervation

as stretched muscle contracts, parallel circuits inhibit the alpha motor neurons of the antagonist

Ia sensory axons stimulate inhibitory interneurons that synapse with alpha motor neurons of antagonist

94

golgi tendon organs

autogenic inhibition
-protective refelx

group Ib axons in encapsulated collagen matrix

located at musculotendinous junction

increased muscle tension

located at musculotendinous junction


GTOs may respond to passive stretch, but especially respond during active muscle contractions

95

group Ib sensory axons?

in golgi tendon organs

96

autogenic inhibition

GTO circuit inhibits the muscle in which tension increased and excites the antagonist

response usually opposite the stretch reflex

in general, GTO mediated reflexes act to control muscle force and joint stability