Perception and Sensorimotor Control 3

Question 1

what do muscle spindles compromise of?

Answer 1

spindles comprise 8 - 10 intrafusal fires arranged parallel with the force generating extrafusal fibres

Question 2

most spindles contain what proportions of nuclear bag and nuclear chain fibres?

Answer 2

2/3 nuclear bag fibres; 6 nuclear chain fibres

Question 3

large diameter sensory axons (T1 afferents) do what?

Answer 3

large diameter sensory axons (T1 afferents) coil around the middle of each class of intrafusal fibre forming an ‘annulospiral primary ending’

Question 4

what do the T2 afferents do?

Answer 4

almost as large in diameter as T1 afferents, T2 afferents are mainly on nuclear chain fibres –> ‘flower-spray endings –> short, outside the middle region of the fibre

Question 5

is the conduction velocity of T1 and T2 afferents fast or slow, and what is the implication of this ?

Answer 5

fast –> mediate rapid reflex adjustments when the muscle is stretched

Question 6

how to intrafusal fibres get innervated?

Answer 6

stretch causes deformation –> initiates AP by activating mechanotransductor channels in T1 and T2 axon endings, innervating the spindle

Question 7

what do T1 afferents respond to and how?

Answer 7

• respond phasically to small stretches
• as afferent activity is dominated by signals transduced by the dynamic sub-type of nuclear bag fibres, they are sensitive to the velocity of stretch

Question 8

what do T2 afferents respond to and how?

Answer 8

• innervate nuclear bag fibres & nuclear chain fibres
• signal the level of sustained fibre stretch by firing tonically at a frequency proportional to the degree of stretch, with little dynamic frequency

Question 9

what does reciprocal innervation of nuclear chain & nuclear bag fibres result in?

Answer 9

reciprocal innervation results in rapid contraction of the stretched muscle & simultaneous relaxation of the antagonist –> rapid changes in muscle length

Question 10

what do gamma motor neurones do?

Answer 10

control the functional characteristics of the muscle spindles by modulating their level of excitability

Question 11

what do gamma motor neurones do during when the muscle stretches, and when they shorted

Answer 11

• stretch: spindle stretches –> rate of discharge in afferent fibres inc^
• shortening: spindle relieved of tension –> sensory axons remain active

Question 12

where to gamma motor neurones terminate

Answer 12

gamma motor neurones terminate on the contractile poles of intrafusal fibres, and the activation of these neurones causes intrafusal fibre contraction, maintaining tension on the middle of the intrafusal muscle fibres

Question 13

there are dynamic and static motor neurones, what is the difference between the two?

Answer 13

dynamic: when they fire, dynamic response of group 1a neurones are activated
static: when they fire, the dynamic component of the type 1a fibres is reduced, and the static response is increased, and static response of type 2 is also increased

Question 14

what does co-activation of alpha and gamma motor neurones allow for?

Answer 14

co-activation of alpha and gamma motor neurones results in spindles function at all muscle lengths during movement and postural alignment

Question 15

what is the ‘Gain of Myotatic Reflex’?

Answer 15

the gain of myotetic reflex refers to the amount of force generated in response to certain stretch of the intrafusal fibres –> if gain is high, then the small stretch applied will produce large increases in the number of alpha MN’s recruited and inc^ in firing rate –> greater stretch required to generate the same force

Question 16

what does voluntary stretching cause?

Answer 16

voluntary stretching causes reduction in the gain of the myotatic reflex to facilitate the lengthening of elastic elements

thus, alpha & gamma MN’s co-activate by higher centres to prevent spindles from being unloaded or over-activated

Question 17

apart from gamma MN’s, what other factor sets the gain of the stretch reflex?

Answer 17

also depends on the level of excitability of alpha MN’s that serve as the efferent side of the reflex loop

local circuits in the spinal cord can change the stretch reflex again by excitation & inhibition of alpha and/or gamma MN’s