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MSK exam 3 > Skeletal Muscle Physiology > Flashcards

Skeletal Muscle Physiology Flashcards

1
Q

Describe Type IIX fibers

A
  • Fatigable
  • Rely on glycolysis
  • Few mitochondria; low concentrations of myoglobin and oxidative enzymes (white)
  • High glycolytic enzyme content
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2
Q

At what fruequencys do slow twitch fibers tetanize?

Why?

A

Lower stimulation frequencies, partly due to longer duration of contraction.

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3
Q

Why do fast twitch fibers develop larger maximal force?

A

Greater twitch tension and larger motor units

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4
Q

Identify fiber type of each graph

A
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5
Q

Which fiber types are represented on the fatigability graphs shown?

A

(Left to right) - Type I, Type IIa, Type IIX(b)

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6
Q

Speed of shortening is correlated with…

A

myosin ATPase activity and firing rate

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7
Q

What changes can be seen as a result of adaptation to training?

A

•Skeletal muscle plasticity

•Response to changes in functional demands
–Fiber-type composition, MHC isoform content
–Fiber size
–Satellite cells and myonuclei
–Myonuclear domain
–Mitochondrial content
–Capillary density
–Oxidative enzymes

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8
Q

The two main purposes of proprioceptors are?

A
  1. Identification of external objects
  2. Accurately guide movement
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9
Q

What are the two muscle proprioceptors, what do they do?

A
  1. Muscle spindles:Detect changes in muscle length and rate of stretch
  2. Golgi tendon organs:Detect muscle tension/strain in m. tendon (near MTJ)

•
•

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10
Q

Describe the fiber orientation of muscle spindles

A

Intrafusal muscle fibers aligned in parallel with force-generating extrafusal fibers

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11
Q

How are golgi tendon organs aligned?

A

aligned in series with extrafusal fibers

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12
Q

Describe the sensory components of muscle spindle fibers

A

•Afferent and efferent innervation
(primarily sensory in function)

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13
Q

Describe the shown phases of the pathways involved in the monosynaptic stretch reflex and coactivation of alpha and gamma motor neurons

A
  1. Afferent input from sensory endings of muscle spindle fiber
  2. Alpha motor neuron output to regular skeletal-muscle fiber
  3. Stretch reflex pathway
  4. Gamma motor-neuron output to contractile end region of spindle fiber
  5. Descending pathways coactivating alpha and gamma motor neurons
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14
Q

What is alpha-gamma coactivation important for?

A

Important for maintenance of Proprioceptor sensitivity

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15
Q

What are the 3 types of basic spinal reflexes?

A
  1. monosyaptic reflex/stretch reflex
  2. reciprocal inhibition
  3. autogenic inhibition
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16
Q

Define motor reflex

A

Rapid, stereotyped motor response to a particular sensory stimulus

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

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17
Q

What are the 5 Components of a Reflex Arc?

A
  1. Receptor
  2. Afferent Pathway
  3. Integration Center (CNS)
  4. Efferent Pathway
  5. Effector
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18
Q

Describe the stretch reflex

What is being activated here?

A
  • Most simple reflex, involves 1 synapse
  • When skeletal m. is abruptly stretched,a rapid, reflexive contraction of the same muscle can occur
  • Contraction increases muscle tension & opposes the initial stretch
  • Stretch reflex is especially strong in extensor muscles

Activation of muscle spindle fibers

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19
Q

What are the 5 components of this reflex arc?

A
  1. Receptor - muscle spindle
  2. Afferent - AP to spinal cord
  3. Integration center - Spinal cord, (interneurons if not monosynaptic, which this is)
  4. Efferent - AP of alpha motor neuron
  5. Effector - Biceps Brachii (Plus inhibition of triceps brachii)
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20
Q

What are the two types of inhibition we covered?

A
  1. Reciprocal inhibition
  2. Autogenic Inhibition
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21
Q

Describe reciprocal inhibition

A

As a stretched muscle is being stimulated to contract, parallel circuits inhibit the α- motor neurons of the antagonist

Group Ia afferent axons excite inhibitory interneurons which synapse with α-motor neurons of the antagonist muscle.

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22
Q

Describe Autogenic inhibition

A

The GTO circuit inhibits the muscle with increased tension & excites the antagonist (multiple synapses)

GTO group Ib afferent axons excite both excitatory AND inhibitory interneurons

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23
Q

Describe the golgi tendon organ reflexes

A

GTO response is generally opposite the stretch reflex

GTO-mediated reflexes may act to control muscle force & joint stability

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24
Q

Describe the nerve endings and location of the golgi tendon organ

A
  • Bare nerve endings of group Ib axons
  • Endings invested in encapsulated collagen matrix
  • Located at musculotendinous junction

mechanosensitive nerve endings of the GTO → action potential

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25
Q

Define Twitch…

A

Mechanical response to 1 AP (~100msec)

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26
Q

Describe the latent phase of muscle twitch

A

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

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27
Q

Describe the contraction time of muscle twitch

A

Beginning of contraction to beginning of relaxation (peak tension) (~50msec)

  • when active sites are exposed until they begin to be covered up
  • when [Ca2+] is high enough to keepactive sites exposed
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28
Q

Describe the relaxation time of muscle twitch

A

Peak tension to complete relaxation (~50msec)

  • ↓ tension as active sites are covered
  • time for Ca2+ sequestering into the SR
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29
Q

What is the total force developed by a muscle?

A

•Total force (tension) generated = Sum of forces independently produced by many cycling cross-bridges

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30
Q

The Number of simultaneously cycling cross-bridges can vary with:

A

– Initial length of muscle fiber: (Length-Tension Relationship)

–Pattern or frequency of muscle fiber stimulation
•Frequency or Twitch Summation (frequency)
•Multiple-Fiber or Multiple Motor Unit Summation

31
Q

Describe the factors influencing tension development in skeletal muscle with regard to the number of fibers contracting.

A
32
Q

Describe the factors influencing tension development in skeletal muscle with regard to the tension developed by each fiber.

A
33
Q

Describe isometric contractions

A
  • Muscle length is constant (“same length”)
  • Stimulation causes increase in tension, but no shortening
  • Force production is equal to resistance (no movement)
34
Q

Describe isotonic contractions

A
  • Muscle contraction occurs at “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 à amount of force production also changes through the ROM

35
Q

What are the 2 phases of isotonic contractions?

A

Concentric - muscle shortens as tension is produced

Eccentric - muscle lengthens as tension is produced

36
Q

How does muscle length influence tension development? What is force production dependent on in isometric contractions?

A

By determining region of overlap between actin and myosin.

Force production in isometric contraction depends on initial fiber length.

37
Q

What is total tension?

A

Passive + active tension

38
Q

Describe passive tension

A
  • Tension measured prior to muscle contraction
  • Passive tension increases as a fiber is progressively lengthened (stretched). Muscle becomes stiffer as it is distended.
39
Q

Describe active tension

A

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

  • Small when fiber length is < ~70-85 % of resting length
  • Small when fiber length is > ~ 130-150 % of resting length
40
Q

When is active tension maximal?

A

Near 100% of normal muscle length, where there is maximal overlap of actin and myosin filaments

41
Q

What is maximum velocity (Vmax) determined by?

A

Determined primarily by the maximum velocity of the myosin ATPase enzyme

 - Vmax varies with fiber type
42
Q

Define Work

A

work = load x displacement

43
Q

define power

A

power = work/time

44
Q

Where is power maximal? Where is it zero?

A

At intermediate loads, where both forve and velocity are moderate.

Zero at maximum load, where V = 0, and zero load where F = 0

45
Q

Describe the frequency (twitch) summation known as tetanus (not to be confused with the pathological conditions due to toxins or hypocalcemia)

A

twitches merge into a smooth sustained maximal contraction

46
Q

Describe low stimulation frequencies

A

Tension developed falls to resting level between individual twitches

  • Duration of each twitch is long vs. duration of AP
  • Possible to initiate a second action potential before the tension of the first contraction has fully decreased
  • The next AP stimulates a twitch which is added to the residual tension of the first twitch = greater total tension
47
Q

Describe high stimulation frequencies

A

Individual twitches fuse resulting in a steady plateau of muscle tension

• Time between successive APs is too short to return enough Ca2+ to the SR to lower [Ca2+]i below relaxation level

  • ↑[Ca2+]i is sustained until tetanic stimulus ceases
  • Tension increases very little at stimulation frequencies greater than the fusion frequency that causes tetanus
48
Q

What three things does whole muscle tension depend on?

A
  • Size of the muscle
  • Number motor units recruited
  • Size of each motor unit being recruited
49
Q

What muscles have few muscle fibers per motor unit?

A

muscles performing refined, delicate movements

50
Q

What muscles have larger numbers of fibers per motor unit?

A

muscles performing stronger, coarser movements

51
Q

Define multiple-fiber/multiple motor unit summation

What is the motor neuron pool?

A

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

Group of all motor neurons innervating a single muscle

52
Q

How can CNS control the degree of force?

A

through the number of individual fibers it stimulates

53
Q

What purposes does asynchronous recruitment serve?

A
  • Asynchronous activation of individual motor units by the CNS so that some units develop tension while others relax
  • Delays or prevents muscle fatigue during submaximal contractions
54
Q

What does Henneman’s size principle say?

A

Progressive recruitment small —> larger motor units

55
Q

Characterize the stimulation required to recruit small motor units

A

• Small motor units are recruited with minimal neuronal stimulation:
A given excitatory stimulus will generate a larger EPSP in motor neurons with smaller cell bodies

56
Q

What are the motor unit types? Describe them, and their order of recruitment.

A

•Slow-Twitch Motor Units (I):
–Small amount of force, prolonged period of time

•Fast-Twitch Fatigue-Resistant Motor Units (FR):
–Moderate amount of force, sustained for moderate amount of time

•Fast-Twitch Fatigable Motor Units (FF):
–Large amount of force, brief period of time•Order of Recruitment:

I ---\> FR ---\> FF
57
Q

Describe the following properties of type I and II nerves

Cell Diameter

Conduction velocity

Excitability

A
58
Q

Type I

Small cell diameter

Fast conduction velocity

High excitability

Type II

Large cell diameter

Very fast conduction velocity

low excitability

A
59
Q

Define muscle fatigue

A

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

60
Q

During fatigue, what causes the decline in maximal force production?

What causes the diminished rates of force production and relaxation?

A
  • Decline in maximal force production: 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
61
Q

What 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

62
Q

Describe central fatigue

A

Reflects changes in the CNS (Brain à motor neuron cell bodies)

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

63
Q

What can help individuals overcome central fatigue?

A

Training

Ex: Minor role in limiting performance of highly trained athletes who have learned to pace and are mentally conditioned to stress and discomfort

Will

Ex: External sensory input (cheering) can increase force production and performance, indicating that pathways superior to corticospinal outputs can oppose central fatigue

64
Q

Describe 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

65
Q

What factors lead to variability in peripheral fatigue and recovery time?

A

Recruitment pattern of motor units

Metabolic properties of muscle fiber type

66
Q

What are the three sources of ATP in skeletal muscle.

A

Anaerobic - fast

  1. creatine phosphate
  2. Glycolysis

Aerobic

Oxidative phosphorylation

67
Q

Describe creatine posphate’s role in ATP production

A

–Creatine phosphate + ADP → Creatine + ATP

(<< 1 minute of exercise)

68
Q

What are the 3 major human skeletal muscle fiber types?

A

–Slow oxidative (Type I) fibers
–Fast-oxidative (Type IIA) fibers
–Fast-glycolytic (Type IIX) fibers

69
Q

Differences in what correspond to the rate of contraction a muscle fiber is capable of?

A

mATPase

70
Q

Describe slow-twitch fibers

A

•Generally smaller cross-sectional area (CSA)
•Greater oxygen transport capability as oxidative metabolism is 1° source of ATP
–Greater capillary density
–Appear red because due to high concentration of myoglobin ( oxygen-binding protein) in the sarcoplasm
–Low glycogen content and glycolytic enzyme activity
–High mitochondrial and oxidative enzyme content
–Slow but efficient contraction
–Resistant to fatigue

71
Q

Describe type IIA fast twitch fibers

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

72
Q

What are the two types of fast-twitch fibers?

A

Type IIA and Type IIX (Previously IIB)

73
Q
A

MSK exam 3 (12 decks)

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