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MSK exam 3 AP lower limb > Skeletal muscle physiology > Flashcards

Skeletal muscle physiology Flashcards

1
Q

what is a twitch

A

mechanical response to 1 single AP

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2
Q

what is latent period

A

Time from AP initiation to cross-bridge formation, start of contraction (~1-3 msec)

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3
Q

What is the contraction time/?

A

beginning of contraction to beginning of relaxation (peak tension)

when active sites are exposed until they begin to be covered up

when Ca is high enough to keep active sites exposed

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4
Q

/what is relaxation time

A

Peak tension to complete relaxation (~50msec)
- ↓ tension as active sites are covered

  • time for Ca2+ sequestering into the SR***
    (taken back up into SR or pumped out into ECF)
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5
Q

total force generated = ?

A

sum of forces independently produced by many cycling cross-bridges

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6
Q

number of simultaneously cycling cross-brdiges can vary with?

A

Initial length of muscle fiber (length tension–> isometric action)

Pattern or frequency of muscle fiber stimulation
(twitch summation, multiple-fiber or multiple motor unit summation)

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7
Q

what are the two types of muscle action?

A

Isometric contractions

Isotonic contractions

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8
Q

what is isometric contraction?

A 
SAME LENGTH
(muscle is not shortening but tension is being produced)

stimulation can increase in tension, but no shortening)

force production is equal to resistance

NO MOVEMENT

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9
Q

Isotonic contraction

A

Muscle contraction occurs at a constant load (SAME TONE)

Not really a constant force (dynamic movement)

Length changes during production of tension: muscle length, joint angle & leverage changes with ROM  amount of force production also changes through the ROM

2 phases

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10
Q

what are the two phases of isotonic contraction?

A

concentric = muscle shortens as tension is produced (bicep curl)

eccentric = muscle lengthens as tension is produced (lowering a bucket)

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11
Q

what is the length tension relationship?

A

same length of a give fiber

how much force can be produced at a given starting length of a fiber

***Muscle length influences tension development by determining region of overlap between actin & myosin

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12
Q

what is passive tension

A

tension measured prior to muscle contraction

Passive tension increases as a fiber is progressively lengthened (stretched) because muscle becomes stiffer as it is distended

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13
Q

active tension

A

total tension minus passive tension

how much energy is actually being formed as a result of contraction

When muscle is stimulated to contract at any fixed length (isometric), cross-bridge cycling produces active tension in addition to passive tension

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14
Q

when is active tension maximal?

A

near 100 percent of normal muscle length

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15
Q

what does increase in fiber length cause

A

ends of actin filaments are pulled away from eachother

When length is increased > 150% of resting sarcomere length, ends of actin are pulled beyond myosin filaments

No interaction/overlap occurs between actin and myosin and no development of active tension

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16
Q

what happens when fiber length is decreased?

A

Actin and myosin increase overlap
Ends of actin filaments are pushed toward each other
Tension can develop, depending on degree of overlap

can’t produce a meaningful contraction
tension is decreased b/c opposing actin filaments slide over one another and can hit against opposing Z disks

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17
Q

sarcomere length is near normal resting length .. means what?.

A

Maximal overlap between actin and myosin filaments and maximal active tension

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18
Q

total tension =

A

passive plus active tension

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19
Q

Force velocity relationship?

A

having to do with isotonic contractions
(shortening velocity decreases as load increases)

LIGHTER LOADS CAN BE LIFTED FASTER

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20
Q

what is maximum velocity determined by?

A

by the maximum velocity of myosin ATPase enzyme (Vmax varies with fiber type)

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21
Q

slide 14-15

A

look at

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22
Q

what is work?

A

load x displacement

measurable mechanical work: only when muscle displaces a load

in order to do work you have to move a load a distance so isometric contraction is not doing that

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23
Q

what is power?

A

work/time

rate work is performed

Zero at maximum load
zero at zero load

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24
Q

what is frequency (twitch) summation

A

Tension developed by a single fiber depends on stimulation frequency

Repetitive stimulation: ↑ tension

Contractile responses (twitches) can be summed if APs fire rapidly –> no fiber relaxation between stimuli due to sustained elevation of Ca2+

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25
Q

tetanus is what

A

twitches merge into a smooth, sustained, maximal contraction

DON”T confuse with tetanus (excitability cause by toxin) or tetany (spasms caused by increased excitability associated with hypocalcemia)

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26
Q

low stimulation frequencies?

A

tension developed falls to resting level between individual twitches

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27
Q

what is the duration of each twitch compared with duration of AP

A

twitch is LONG

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28
Q

high stimulation frequency?

A

ual twitches occur so close in time they fuse, causing muscle tension to remain at a steady plateau

slide 19

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29
Q

what is multiple fiber summation

A

?

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30
Q

motor pool

A

all of the motor neurons who supply any muslce fiber that comprises a given muscle (biceps for example)

group of all motor neurons innervating a single muscle

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31
Q

whole muscle tension depends on?

A

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

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32
Q

muscles performed refined, delicate movements have …

A

few muscle fibers per motor unit

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33
Q

muscles performing stronger, coarser movements have …

A

a large number of fibers per motor unit

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34
Q

what is Multiple-fiber/Multiple Motor Unit Summation (MMUS):

A

Excitation of additional motor neuron cell bodies recruits the fibers of the motor unit, adding them to the contractile pool

this is how CNS controls force production for specific situation (By the number of individual fibers that it stimulates)

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35
Q

what is another benefit of multiple fiber summation?

A

DELAY FATIGUE- asynchronous recruitment
Delays or prevents muscle fatigue during submaximal contractions

mechanism allowing whole-muscle force development to be relatively constant

Asynchronous activation of individual motor units by the CNS so that some units develop tension while others relax

36
Q

contractile strength can vary with?

A

number of active alpha motor neurons in pool

frequency of firing of each alpha motor neuron

37
Q

Electromyography?

A

gross measure of electrical activity as it is projected to skin surface!!

records sum of all electrical activity (AP’s of all activated fibers in all activated motor units)

38
Q

Henneman’s size principal?

A

size of cell body dictates excitability

give same input; threshold reached sooner in a smaller motor neuron

39
Q

which type of alpha motor neurons are most easily recruited?

A

SMALLEST - reach threshold quicker

40
Q

how can you differentiate between central and peripheral fatigue on an EMG?

A

experiencing central fatigue–> decreases EMG, output and excitation is decreasing

peripheral fatigue–> ability to produce force is tapering off and off, but still able to maximally recruit all the motor units possible , so still have electrical activity

41
Q

what determines degree of use and what does degree of use influence?

A

motor unit excitability determines degree of use

degree of use influences fiber type

42
Q

small motor units are recruited with MINIMAL neuronal stimulation meaning what?

A

A given excitatory stimulus will generate a larger EPSP in motor neurons with smaller cell bodies

as neuronal stimulation intensifies, larger motor neurons are recrutied

43
Q

slow twitch motor units? (I) type I

A

Small amount of force, prolonged period of time

smaller motor neurons, supplying a fewer amount of muscle fibers for a long period of time

smal cell diameter

fast conduction velocity

high excitability

44
Q

fast twitch fatigue resistant FR type II

A

produce moderate amount of force but can’t do this as long as slow twitch

45
Q

fast twitch fatiguable FF type II

A

large amount of force produced

only for brief period of time

46
Q

what is the order of recruitment of motor units?

A

I–> FR–> FF

47
Q

fast twitch fibers in general characteristics

A

large cell diameter

very fast conduction velocity

low excitability

48
Q

propteries of muscle cells of type I motor units

A

few number of fibers

moderate fiber diameter

low force of unit

oxidative metabolic profile

moderate contraction velocity

low fatigability

49
Q

properties of muscle cells of type II motor units

A

many number of fibers

large fiber diameter

high force of unit

glycolytic metabolic profile

fast contraction velocity

high fatigability

50
Q

watch videos

A

only can watch on campus

51
Q

muscle fatigue is what?

A

Inability to maintain desired power output, decline in force production & shortening velocity

52
Q

why does a decline is maximal force happen?

A

results from reduction in number of active cross-bridges

Lower rates of force production and relaxation: due to impaired release and reuptake of Ca2+ from the sarcoplasmic reticulum (SR)

53
Q

why have muscle fatigue?

A

protective role

Allows contractions to occur at lower rates/forces while preventing extreme changes that could cause damage

54
Q

is muscle fatigue reversible?

A

yes with rest

55
Q

various factors contribute to fatigue

A

Motivation, physical fitness, nutritional status, and the type of motor units (i.e., fibers) recruited with respect to the intensity and duration of activity

56
Q

what is central fatigue

A

anything that is going on within the CNS

from brain–> motor neuron cell bodies in ventral horn of spinal cord

possible involvement of:

  • -Altered input from muscle sensory nerve fibers
  • -Reduced excitatory input to motor control centers of the brain & SC
  • –Resulting in altered excitability of α - and γ - motor neurons

CONTRIBUTIONS of these factors vary by individual and type of activity
slide 35

57
Q

what is peripheral fatigue

A

Motor neuron axon –> NMJ –> Fiber
Possible involvement of:
Impaired initiation/propagation of muscle APs
Impaired release/handling of [Ca2+ ]i
Depletion of energy metabolism substrates
Accumulation of metabolic byproducts

Peripheral fatigue and the time required for recovery varies with recruitment pattern of motor units & metabolic properties of muscle fiber type

58
Q

3 sources of ATP in skeletal muscle

A

Anarobic sources:

Creatinine phosphate
Glycolysis

Aerobic source:
oxidative phosphorylation

slide 37

59
Q

what are the different characteristics/functions of skeletal muscle fibers?

A

strength, speed and fatigability

60
Q

slide 40

A

look at

61
Q

3 major human skeletla muscle fiber types

A

slow oxidative type I
fast oxidative type IIA
fast glycolytic Type IIX fibers

62
Q

classificationof muscle fibers by….

A

Pathway for ATP synthesis (oxidative versus glycolytic)

Rate of ATP hydrolysis (mATPase isoform)

Contractile Velocity (fast vs. slow)

63
Q

slow twitch fibers

A

greater oxygen transport capability as oxidative metabolism as primary source of ATP

small cross sectional area

can act for longer periods of time

“red fibers” due to myoglobin content

slide 42

64
Q

type IIA

A

fatigue resistant

oxidative metabolism
similar to Type I in myoglobin content (red) and metabolic machinery
similar number of mitochondria vs. slow-twitch fibers

abundant glycogen

Ensures adequate ATP generation to compensate for the increased rate of ATP hydrolysis for rapid contraction

65
Q

type IIX

A

fatigable (couch potato fibers)
high proportion in untrained individuals

rely on glycolysis

few mitochondria (white fibers)

low concentrations of myoglobin and oxidative enzymes

high glycolytic enzyme content

66
Q

can you train to change from type I to type II fibers

A

NO

but what you can do with training is switch between fast fiber population

so cannot go from slow to fast
but fast type Iix to fast type IIa

67
Q

see slide 44 for review of types of fibers

A

do it

68
Q

why do slow twitch fibers tetanize at lower stimulation frequencies ?

A

due to longer duration of contraction

69
Q

why do fast twitch fibers develop larger maximal force ?

A

due to greater twitch tension and large motor units

don’t maintain that level of tension that long

70
Q

how can you identify fiber types based on ATPase enzyme

A

staining intensity (having to do with pH) gives you an idea of what type of ATPase enzyme

don’t need to know

71
Q

what does peripheral fatigue involve?

A

ATP depletion

lactic acid accumulation

glycogen depletion

(not going to be assessed on it)
slide 48-50

72
Q

Factors influencing tension development in skeletal muscle ***

A

Number of fibers contracting

  • motor units recruited
  • muscle fibers per motor unit
  • size of muscle

Tension developed by each contracting fiber

  • frequency of stimulation (twitch summation and tetanus)
  • fiber length at onset of contraction
  • extent of fatigue
  • thickness of fiber

slide 51 ***

73
Q

changes that can occur with training (fiber adaptations)

A

skeletal muscle plasticity

response to changes in functional demands include:

  • fiber size (RIPPED)
  • fiber type composition (change between type II fibers X and A)
  • mitochondrial content
  • capillary density
  • oxidative enzymes
74
Q

what is proprioception

A

detailed information sensed about location in space

75
Q

2 main purposes of proprioceptors

A
  1. Identification of external objects

2. Accurately guide movement

76
Q

what are the muscle proprioceptors?

A

1 Muscle spindles

2 Golgi tendon organs

77
Q

what do muscle spindles do and what are they?

A

detect changes in muscle length and rate of stretch

1> send proprioceptive information about muscle –> CNS
2> respond to muscle stretching

these are INTRAFUSAL muscle fibers aligned in PARALLEL with force generating extrafusal fibers
(this is why they can give info about length)

78
Q

what do golgi tendon organs do and what is their position?

A

detect muscle tension in muscle tendon (near MTJ)

these are aligned in SERIES with extrafusal fibers (so as the tension is developed and pulled, they can detect differences in muscle force)

79
Q

when does firing rate of sensory fibers of muscle spindle fibers increase?

A

when the muscle is stretched

80
Q

what is the difference between primary and secondary endings

A

primary sensitive to change in length

secondary sensitive to static length

81
Q

why does the spindle need to have motor innervation

A

b/c the intrafusal fiber must also be contracted to match the extrafusal fibers

For continued maintenance of spindle sensitivity, γ-motor neurons cause intrafusal fibers to contract in parallel with extrafusal fibers

Sensory response of the spindle depends on BOTH the length of the whole muscle AND on the contractile state of the intrafusal fiber itself

82
Q

what is the monosynaptic reflex

A

involves 1 synapse

When skeletal m. is abruptly stretched, a rapid, reflexive contraction of the same muscle can occur

Contraction increases muscle tension and opposes the initial stretch

Stretch reflex is especially strong in extensor muscles

1 alpha motor neuron that supplies the muscle is stimulated

83
Q

slide 63 on relfexes

A

look at

84
Q

reciprocal innervation

A

slide 64

85
Q

golgi tendon organ receptor

A

protective of muscle where stimulus is detected to be too much tension

slide 66

Autogenic inhibition

MSK exam 3 AP lower limb (15 decks)

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