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KNES 451 > modulation of spinal reflexes > Flashcards

modulation of spinal reflexes Flashcards

1
Q

what are the numbers of bag and chain fibres in muscle spindles

A

2-3 bag
3-5 chain (variable)

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2
Q

what is the continuum of intrafusal fibres

A

not exclusively dynamic or static
- can have a mix of both to determine the sensitivty
- more dynamic = more sensitive to velocity of muscle stretch
- more static = more sensitive to amount of muscle stretch

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3
Q

why is proprioception faster than vision

A

no chemical cascades need to happen
all mechanical

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4
Q

how do spindle receptors sense muscle length

A
  • muscle stretch deforms and opens stretch sensitive ion channels
  • cytoskeletal strands connected to the channels on the inside of the membrane lengthen forcing the channels to open
  • influx of ions excites the spindle receptor causing it to fire APs
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5
Q

what are the muscle spindle firing patterns (assuming no yMN activity) when the muscle is stretched, relaxed, and passively shortened

A

stretched
- causes spindle recptors to increase their firing rates

relaxed
- maintain a high tonic firing rate with regular APs
- slower firing rate than stretched

passive shortening
- reduces the firing rate of spindle receptors

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6
Q

what is reciprocal inhibition

A

stretch of agonist muscle produces reflex contraction of agonist/synergist muscles and inhibition of antagonists

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7
Q

what are the properties of short latency reflexes (SLRs)

A
  • stimulus driven
  • recruit only spinal circuits
  • simple processing capabilities
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8
Q

what are the two ways SLRs scale

A

amount and rate of muscle stretch
- amplitude increases with larger stretch

background muscle activity (automatic gain scaling)
- amplitude increases with higher background muscle activity (even with same perturbation)

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9
Q

why do SLRs scale with higher background activity

A

need to recruit larger MUs because the small MUs are already recruited in the background
(hennemans and ohms law)

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10
Q

why do SLRs scale with amount and rate of stretch (larger responses to larger / longer stretches)

A

above threshold for a longer time = more APs = more NT released = more excitation of aMNs
(prolonged increases in receptor and membrane potential)

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11
Q

what is hennemans size principle

A

smaller, less fatiguable (slow twitch MUs are recruited first because they are higher resistance)

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12
Q

what is ohm’s law

A

membrane resistance is inversely proportional to the cell’s SA (number of parallel ion channels)

small MNs have higher membrane resistance (fewer parallel ion channels)

large MNs have low membrane resistance (leaky)

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13
Q

what is the difference in recruitment between small and large MUs based on ohm’s law

A

large
- hard to reach threshold for recruitment because the ions leak faster
- recruited by larger background muscle activity

small
- see larger changes in membrane potential with the same synaptic input because of higher membrane resistance (resistance to leak)

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14
Q

what is the difference in effect of the same EPSP between rest and increase in background muscle activity

A

at rest - series of EPSPs doesn’t elicit reflexive firing
increase BMA - tonic subthreshold increase in muscle activation causes the same EPSP to cause the aMN to fire and result in reflexive muscle contraction
(BMA pushes aMNs closer to the threshold membrane potential and increases likelihood of firing after receiving EPSP)

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15
Q

what is the effect of coactivation before contraction

A

increase the net force of contraction
- increase agonist
- increase inhibition of antagonist

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16
Q

what are the 3 main ways to modulate spinal reflexes

A
  1. alter excitability of spinal MNs
  2. alter excitability of pools of spinal interneurons
  3. increase sensitivity to sensory feedback in the spinal cord
17
Q

what are the 3 ways to alter excitability of spinal MNs

A
  1. descending pathways from M1 and premotor (CSTs)
  2. supraspinal control of spinal MNs
  3. gain scaling of spinal stretch reflexes
18
Q

what is supraspinal control of spinal MNs

A

brain has direct control over the excitability of the spinal cord (descending control)

19
Q

how does supraspinal control of spinal MNs alter their excitability

A

by altering the tonic excitability or background activity of spinal MN, the brain has direct control over the sensitivity of spinal processing of sensory feedback

20
Q

what is the relationship between background / reflex EMG and force applied

A

linear relationship between force production and amplitude of EMG
- higher forces = slower velocity

21
Q

why does automatic gain scaling occur

A

caused by recruitment of larger MUs with larger amounts of background EMG

22
Q

why does the timing of peaks (SLR and LLR) stay relatively constant regardless of MUs recruited

A

same circuit is used (same # of synapses, etc)

23
Q

what are the 2 main factors in altering the excitability of pools of spinal interneurons

A
  1. supraspinal control of spinal MNs (renshaw cells)
  2. pre synaptic inhibition of spinal MNs
24
Q

what are renshaw cells

A
  • specific class of inhibitory interneurons
  • located in the intermediate zones and ventral horn of the spinal cord
  • can inhibit several populations of MNs
25
Q

what makes renshaw cells different from different inhibitory interneurons

A

always inhibit the aMNs that excite them (loop back onto the saem aMN)

26
Q

why are renshaw cells recurrent connections (loop backs) important

A

form a negative feedback system that helps stabilise the firing rate sof the MNs during mvmt and voluntary contractions
- important for reflexes because there are two levels of control over aMNs (also under direct control of descending MNs from cerebral cortex)

27
Q

how do renshaw cells work

A

pre synaptic inhibition
- inhibit aMN and limits NT release at the junction between aMN and muscle (motor end plate)

balance of excitatory and inhibitory inputs determines whether it inhibits or excites the targeted aMN

28
Q

what is the effect of renshaw cells in inhibition

A

reduction in Ca2+ uptake
decreased release of NT
reduction of EPSP of targeted skeletal muscle
inhibition of target

29
Q

what occurs in the absence of renshaw cells

A

influx of Ca2+ causes APs
leads to release of NT
EPSP in targeted skeletal muscle

30
Q

what is the importance of pre synaptic inhibition during voluntary motor actions

A

keeps activity of aMNs in check during mvmt
reduces the gain of spinal reflexes to enable smooth and efficient control of mvmt

31
Q

what is the reflex sensitivity of spinal interneurons during posture control, mvmt, holding, and passive mvmts

A

higher during posture control than mvmt
returns to posture control level while holding
unaltered during passive mvmts

voluntary mvmts suppress reflex sensitivity of neurons in the spinal cord to permit mvmt

32
Q

what are yMNs and what do then do

A

smaller diameter than aMN
control spindle sensitivity to rate and amount of stretch
innervate only intrafusal fibres

33
Q

what are bMN and what do they do

A

can help maintain sensitivity
innervate both intra and extrafusal fibres

34
Q

what happens to spindle firing rate if there is only aMN activity

A

spindle stops firing when intrafusal fibres shorten

35
Q

what occurs during alpha gamma coactivation

A

yMN maintains length of intrafusal fibres
spindle maintains sensitivity
can increase firing rates and sensitivity to changes in muscle length

36
Q

what do static yMNs respond to

A

alter sensitivity to the amount of muscle stretch
- higher steady state response

37
Q

what do dynamic yMNs respond to

A

alter sensitivity to the rate of muscle stretch
- higher response during phasic (change) in stretch
- firing rate may remain elevated during steady state because static have been stimulated

38
Q

what is the dynamic index

A

= peak firing rate – steady state response

KNES 451 (11 decks)

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