Reflexes

Question 1

What is the simplest reflex of all?

Answer 1

The stretch reflex which is found in all muscles

Question 2

What is the best example of the stretch reflex?

Answer 2

Patella tendon or knee-jerk reflex

Question 3

What does the knee jerk reflex use information from?

Answer 3

The muscle spindles which monitor muscle length

Question 4

How does a stretch reflex work?

Answer 4

• Follows a sharp tap to the inelastic tendon
• Force is transmitted to the muscle fibres- they are more elastic than tendons and so are more able to stretch
• Stretch activates the sensory nerves in the muscle spindle
• This increases the number of APs in afferent nerves projecting in to spinal cord through the dorsal horn

Question 5

How many types of connections do spindle sensory afferents divide and make?

Answer 5

3

Question 6

What is a monosynaptic reflex?

Answer 6

• Many spindle sensory afferents activate the a-motorneurone pool to the muscle which was stretched which causes rapid contraction of the agonist muscle.
• There are no interneurons onvolved

Question 7

How is the stretch reflex a classic negative feedback loop?

Answer 7

• Muscle stretch
• Stimulates muscle spindles
• Reflex muscle contraction
• Muscle shortens back to previous length

Question 8

What is reciprocal inhibition?

Answer 8

• Sensory fibres from the stretched spindle also connect indirectly with and influence the antagonist muscles
• So when the agonist muscle contracts, the antagonist muscle relaxes
• Because spindle afferents connect with and activate inhibitory interneurons which decrease activation of a-motorneurones to the antagonist which then relaxes

Question 9

How does spindle afferent information tell the brain about the length of muscles?

Answer 9

It ascends in dorsal columns and connects with somatosensory cortex

Question 10

What is the inverse stretch reflex?

Answer 10

The clasp knife or Golgi tendon organ reflex

Question 11

What happens in the clasp knife reflex?

Answer 11

• Caused by afferent nerves from the Golgi tendon organs which monitor muscle tension
• Muscle contracts and shortens, this pulls on the tendon and the 1b sensory nerves from GTOs which increase firing of APs

Question 12

What does the firing of APs in the clasp knife reflex cause?

Answer 12

• Activation of inhibitory interneurons to the agonist muscle and a decrease in contraction strength
• Activation of excitatory interneurons to antagonist muscles

Question 13

Describe the clasp knife reflex?

Answer 13

• Agonist muscle is inhibited- relaxes
• Antagonist muscle is activated- contracts
• This reflex is polysynaptic and protective
• It prevents muscles contracting so hard that the tendon insertion is torn away from the bone

Question 14

Why does the clasp-knife reflex exist?

Answer 14

-It greatly increases tension in tendon which leads to a collapse of resistance like a spring loaded knife which acts as a protective mechanism

Question 15

Describe the flexor(withdrawal) reflex with crossed extension.

Answer 15

• Flexor or withdrawal reflex use info from pain receptors in skin, muscles and joints
• They are polysynaptic and protective
• They withdraw part of the body away from a painful stimulus and towards the body, flexing the affected part

Question 16

How does the withdrawal reflex work?

Answer 16

Increased sensory APs from pain receptors cause:

• Increase activity in the flexor muscles of the affected part via a number of excitatory interneurons
• At the same time, via a number of excitatory and inhibitory interneurons, the antagonistic extensors are inhibited

Question 17

How can the flexor reflex be described?

Answer 17

Ipsilateral flexion in response to pain

Question 18

Describe the circuitry of flexor withdrawal reflex?

Answer 18

• Small diameter Ao nociceptive fibres triggering pain enter cord.
• They branch a lot and so activate interneurons in several spinal segments
• Which activate a motorneurones controlling all the flexor muscles of the affected limb

Question 19

Why does the contralateral limb extend in the withdrawal reflex?

Answer 19

To avoid falling over

Question 20

How does the contralateral limb extend?

Answer 20

• Several excitatory interneurons which cross the spinal cord excite the contralateral extensors
• At the same time, via several interneurons, there is inhibition of the contralateral flexors
• Sensory information ascends to the brain in the contralateral spinothalamic tract

Question 21

Why is the flexor-crossed extensor reflex far slower than the stretch reflex?

Answer 21

• There are several interneurons in the pathway each with a small synaptic delay
• Nociceptive sensory fibres have smaller diameter than muscle spindle afferents and so conduct more slowly

Question 22

What can you do to reflexes consciously?

Answer 22

Over ride them

Question 23

How can you override a reflex?

Answer 23

-One a motorneurone receives >10,000 synapses
-Many are from descending cortical excitatory and inhibitory inputs, with continual integration of EPSPs and IPSPs
So if you are holding something heavy but important, descending voluntary excitation of a motorneurones can override the inhibition from the GTOs and maintain contraction

Question 24

What does high y-motorneurone activation of muscle spindles cause?

Answer 24

Muscles become extremely resistant to stretch and is spastic

Question 25

How can the stretch reflex be overridden?

Answer 25

Strong descending inhibition hyperpolarises a-motorneurones and the stretch reflex can not be evoked

Question 26

What is the clinical relevance of stretch reflex?

Answer 26

- Assessing integrity of whole spinal cord circuit | - Allows spinal level localisation of a problem

Question 27

How can the stretch reflex be used to localise a problem?

Answer 27

- In stretch reflex, spindle input is highly localised and affects only a-motorneurones of 1 or 2 spinal segments - In withdrawal reflex, pain fibre input is diffused and spreads through several spinal segments - The more powerful the pain stimulus, the greater the spinal spread and the larger the reponse

Question 28

What does facilitation do?

Answer 28

Increases the effects of sensory inputs

Question 29

How does the pain fibre input facilitate the action of the muscles spindles?

Answer 29

Maintains the a-motoneurones in a more depolarised state

Question 30

What is an example of facilitation?

Answer 30

Withdrawing whole arm if full hand in contact with hot surface instead of just a hand if a finger was in contact

Question 31

What is facilitation?

Answer 31

Activation of groups of neurones that provide sufficient excitation to bring other neurones to threshold