Sensory receptors 3

Question 1

what is the most common example of a stretch reflex?

Answer 1

knee jerk/ patellar tendon

Question 2

what does a stretch activate?

Answer 2

• sensory nerves in the muscle spindles increasing number of APs (in afferent nerves projecting in to spinal cord through dorsal horn)

Question 3

what are the 3 types of connections that spindle sensory afferents make?

Answer 3

1. Monosynaptic reflex
2. reciprocal inhibition
3. spindle afferent information ascends in DORSAL columns, connects with somatosensory complex and tells brain about muscle length

Question 4

what is a monosynaptic reflex?

Answer 4

• directly activates alpha motor neuroens
• causes contraction of the agonist muscle
• NO interneurones involved
• simplex reflex arc with a single synapse (classic negative feedback)

Question 5

what are the main steps for a monosynaptic reflex? (5)

Answer 5

1. muscle stretch
2. stimulates muscle spindles
3. ions flow through the membranes depolarising nerves
4. reflex muscle contraction
5. muscle shortens back to previous length

Question 6

what is reciprocal inhibition?

Answer 6

• when agonist muscle contracts, the antagonist muscle relaxes (stretches)
• muscles need antagonists and agonists to move
• spindle afferents connect with and activate inhibitory interneurones which decrease activation of alpha motor neurones to antagonists which then relaxes

Question 7

what has a smaller diameter, 1a or 1b afferent neurones?

Answer 7

1b

Question 8

what happens when a muscle contracts, in relation to GTOs?

Answer 8

• muscle contracts and shortens
• this pulls on the GTO stretching it
• 1b sensory nerves from GTO increase firing of APs

Question 9

what does inverse stretch reflex/clasp-knife reflex cause?

Answer 9

1. activation of inhibitory interneurones to the agonist muscle
2. activation of excitatory interneurones to the antagonist muscle

Question 10

What does a clasp- knife reflex mean?

Answer 10

by greatly increasing the tension in the GTO, it leads to collapse in resistance (protective mechanism) e.g. when arm wrestling, arm will fall down eventually

Question 11

what reflexes are polysynaptic and protective?

Answer 11

flexor and withdrawal reflexes by using info from nociceptors

Question 12

What does flexor/ withdrawal reflex do?

Answer 12

withdraws part of the body from the painful stimulus and in towards the body (flex the affected part away from pain)

Question 13

what 2 things to increases sensory APs from pain receptors cause?

Answer 13

1. increased activity in flexor muscles of the affected part (via excitatory internuerones)
   BUT ALSO
2. via excitatory and inhibitory interneurones, antagonistic extensors are inhibited to make the quick reflex even quicker

Question 14

what sort of flexion occurs in response to pain?

Answer 14

ipsilateral (towards same side of body)

Question 15

what effect does the pain stimulus have on the reflex?

Answer 15

the bigger the pain stimulus, the bigger the branching and the quicker the response

Question 16

what else do excitatory interneurones excite as a protective mechanism other than causing flexion of the affected limb?

Answer 16

excite the contralateral extensors ( contralateral limb)

Question 17

what is inhibited during contralateral extension?

Answer 17

contralateral flexors (to maintain posture when extending a limb to bear body weight)

Question 18

why is flexor-crossed extensor reflex slower than stretch reflex?

Answer 18

nocicpetive sensory fibres have a smaller diameter than muscle spindle afferents in stretch reflex

Question 19

can reflexes be over-ridden consciously?

Answer 19

Yes

Question 20

what occurs if excessive load is put on the muscle?

Answer 20

1. neuron from GTO fires sensing too much tension
2. motor neuron is inhibited
3. muscle relaxes
4. load is released

Question 21

why is it that we can override a reflex?

Answer 21

-descending voluntary excitation of alpha motor neurones can override the inhibition from GTOs and maintain contraction for a long time

Question 22

activity of gamma motor neurones depends on what pathways?

Answer 22

entirely on DESCENDING pathways

Question 23

what does very high gamma motor neurone activation of muscle spindles cause?

Answer 23

muscle to become extremely resistant to stretch and is “spastic”

Question 24

why is stretch reflex is important clinically?

Answer 24

allows doctors to evaluate integrity of whole spinal cord circuit and spinal level localisation of a problem

Question 25

what happens the more you stimulate sensory receptors? (e.g. pushing on palms in apart motion)

Answer 25

the more exaggerated the sensory neurones can become in other areas of the body
This is called FACILITATION (more excitation)

Question 26

what reflex is seen shortly after brain death diagnosis and can be helpful to inform the timing of transplants?

Answer 26

Lazarus reflex