Reflexes 2

Question 1

what are 3 main features of muscle reflexes?

Answer 1

1. they are automatic and fast
2. they do not require conscious intervention for movement control
3. they produce stereotyped response to a given stimulus (which means they can be used for diagnosis)

Question 2

what are features of a monosynaptic reflex arc?

Answer 2

+ sensory receptor
+ afferent component (sensory input)
+ central component (spinal processing)
+ efferent component (motor output)

Question 3

what are features of polysynaptic reflex arc?

Answer 3

one or more interneurons connect afferent (sensory) and efferent (motor) signals

+ sensory receptor
+ afferent component (sensory input)
+ central component (spinal processing)
+ efferent component (motor output)

Question 4

what are the two classes of reflexes?

Answer 4

1. intrasegmental

2. intersegmental

Question 5

what does it mean if a reflex is classified as intrasegmental?

Answer 5

the reflex arc is restricted to one or two spinal cord segments

Question 6

what does it mean if a reflex is classified as intersegmental?

Answer 6

the reflex arc involves a number of spinal cord segments

Question 7

what does the central portion of the spindle provide?

Answer 7

provides the sensory output for muscle length, however it is sensitive only over a relatively small change in length

Question 8

when is the spindle less sensitive?

Answer 8

when the muscle is completely contracted

Question 9

what are the steps of the muscle reflex mechanism?

Answer 9

1. activation of sensory receptors
2. conduction to CNS along nerve fibres from receptors
3. transmission across neuron/neuron synapses in CNS
4. conduction from CNS along nerve fibres to muscle
5. activation of muscle contractile machinery

Question 10

what is the structure of muscle spindles?

Answer 10

• extrafusal fibres
• intrafusal fibres (2 types, bag and chain, depending on nuclei location)
• nucleated portion of intrafusal fibres
• spindle capsule
• gamma motor neuron (efferent)
• Grp II sensory fibre (afferent)
• Grp 1A sensory fibre (afferent)

Question 11

what are the 3 portions of the spindle fibre?

Answer 11

1x less elastic/contractile sensory portion (central)

2x contractile (gamma MNs) and elastic portion (ends)

Question 12

what are features of the central portion of the spindle?

Answer 12

• provides sensory output for muscle length

- sensitive only over a relatively small change in length

Question 13

describe spindle sensitivity when almost completely contracted:

Answer 13

• spindle sensitivity is at the most compressed part of its range and is LESS SENSITIVE

to prevent this:
- length of spindle adjusted to keep it within its most sensitive range by extending or shortening contractile spindle ends

Question 14

how is the CNS involved in adjusting the length of the spindle?

Answer 14

CNS activates the gamma motor efferents to the spindle in concert with the alpha motor neuron activation of the extrafusal muscle fibres

Question 15

what are the 2 types of spindle fibres?

Answer 15

1. chain fibres

2. bag fibres

Question 16

what are features of chain fibres?

Answer 16

• innervated by both 1 A and II sensory nerve fibres
• most common fibre type in spindle
• so called because fibres’ nuclei are arranged in central portion

Question 17

what are features of bag fibres?

Answer 17

• only innervated by type 1A fibres
• only 1 or 2 per spindle
• so called because of swelling in the equatorial region which houses the nuclei

Question 18

what info do 1A fibres provide?

Answer 18

info about rate of change in length

Question 19

what info do type II fibres provide?

Answer 19

info about total length

Question 20

what are features of mechanism of reciprocal innervation?

Answer 20

1. influence of spindle sensory nerve fibres from one (homonymous) muscle on motor neurones projecting to antagonist muscles
2. the effect is opposite to that on their own muscle
3. relaxes antagonist muscles and facilitates shortening oh homonymous muscle
4. requires involvement of a local inhibitory neuron in the reflex onto the antagonist muscles

Question 21

what is the definition of reciprocal innervation?

Answer 21

innervation so that the contraction of a muscle or set of muscles (as of a joint) is accompanied by the simultaneous inhibition of an antagonistic muscle or set of muscles

Question 22

what happens when a voluntary signal for muscular contraction leaves the spinal cord?

Answer 22

• it travels to both intrafusal (via gamma MNs) and extrafusal (via alpha MNs) causing both sets of fibres to contract
• spindle fibre ends shorten in concert with extrafusal fibres, which keeps central position constant in length, so anulospiral sensory endings do not detect a change in muscle length

Question 23

what happens in voluntary muscle lengthening?

Answer 23

• actually passive but involves inhibition
• intrafusal ends inhibited (excited less)
• can stretch but no change in centre so no change in signal

Question 24

what are functional features of anulospiral endings?

Answer 24

• if anulospiral endings do not change their length, there is no change in sensory nerve activity
• only unpredicted changes in muscle length alter extension of anulospiral endings
• therefore unpredicted changes in muscle length cause a change in sensory nerve activity

Question 25

what do muscle spindles monitor?

Answer 25

- muscle length - rate of change in length - duration of change

Question 26

what do muscle spindles do?

Answer 26

mediate monosynaptic and polysynaptic stretch reflexes

Question 27

how do muscle spindles lie in relation to extrafusal fibres?

Answer 27

parallel

Question 28

how is length monitoring achieved?

Answer 28

by activation of both principal (group 1A) sensory fibres and secondary (group II) sensory fibres on both bad and chain intrafusal fibre types

Question 29

how is rate of change in length and duration achieved?

Answer 29

by activation of only principal (group 1A) sensory fibres on both bad and chain intrafusal fibre types

Question 30

what is the purpose of stretch reflexes?

Answer 30

to minimise unexpected changes in muscle length

Question 31

what is the role of gamma MNs in relation to spindle sensitivity

Answer 31

- gamma MNs automatically produced: cause contraction of ends of intrafusal muscle fibres - adapts working range of intrafusal fibres