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Science for Medicine > Reflexes > Flashcards

Reflexes Flashcards

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1
Q

What is the simplest reflex of all?

A

The stretch reflex which is found in all muscles

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2
Q

What is the best example of the stretch reflex?

A

Patella tendon or knee-jerk reflex

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3
Q

What does the knee jerk reflex use information from?

A

The muscle spindles which monitor muscle length

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4
Q

How does a stretch reflex work?

A
  • 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
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5
Q

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

A

3

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6
Q

What is a monosynaptic reflex?

A
  • 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
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7
Q

How is the stretch reflex a classic negative feedback loop?

A
  • Muscle stretch
  • Stimulates muscle spindles
  • Reflex muscle contraction
  • Muscle shortens back to previous length
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8
Q

What is reciprocal inhibition?

A
  • 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
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9
Q

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

A

It ascends in dorsal columns and connects with somatosensory cortex

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10
Q

What is the inverse stretch reflex?

A

The clasp knife or Golgi tendon organ reflex

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11
Q

What happens in the clasp knife reflex?

A
  • 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
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12
Q

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

A
  • Activation of inhibitory interneurons to the agonist muscle and a decrease in contraction strength
  • Activation of excitatory interneurons to antagonist muscles
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13
Q

Describe the clasp knife reflex?

A
  • 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
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14
Q

Why does the clasp-knife reflex exist?

A

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

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15
Q

Describe the flexor(withdrawal) reflex with crossed extension.

A
  • 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
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16
Q

How does the withdrawal reflex work?

A

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
17
Q

How can the flexor reflex be described?

A

Ipsilateral flexion in response to pain

18
Q

Describe the circuitry of flexor withdrawal reflex?

A
  • 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
19
Q

Why does the contralateral limb extend in the withdrawal reflex?

A

To avoid falling over

20
Q

How does the contralateral limb extend?

A
  • 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
21
Q

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

A
  • 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
22
Q

What can you do to reflexes consciously?

A

Over ride them

23
Q

How can you override a reflex?

A

-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

24
Q

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

A

Muscles become extremely resistant to stretch and is spastic

25
Q

How can the stretch reflex be overridden?

A

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

26
Q

What is the clinical relevance of stretch reflex?

A
  • Assessing integrity of whole spinal cord circuit

- Allows spinal level localisation of a problem

27
Q

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

A
  • 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
28
Q

What does facilitation do?

A

Increases the effects of sensory inputs

29
Q

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

A

Maintains the a-motoneurones in a more depolarised state

30
Q

What is an example of facilitation?

A

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

31
Q

What is facilitation?

A

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

Science for Medicine (50 decks)

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