Reflexes Flashcards
What is the simplest reflex of all?
The stretch reflex which is found in all muscles
What is the best example of the stretch reflex?
Patella tendon or knee-jerk reflex
What does the knee jerk reflex use information from?
The muscle spindles which monitor muscle length
How does a stretch reflex work?
- 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
How many types of connections do spindle sensory afferents divide and make?
3
What is a monosynaptic reflex?
- 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
How is the stretch reflex a classic negative feedback loop?
- Muscle stretch
- Stimulates muscle spindles
- Reflex muscle contraction
- Muscle shortens back to previous length
What is reciprocal inhibition?
- 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
How does spindle afferent information tell the brain about the length of muscles?
It ascends in dorsal columns and connects with somatosensory cortex
What is the inverse stretch reflex?
The clasp knife or Golgi tendon organ reflex
What happens in the clasp knife reflex?
- 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
What does the firing of APs in the clasp knife reflex cause?
- Activation of inhibitory interneurons to the agonist muscle and a decrease in contraction strength
- Activation of excitatory interneurons to antagonist muscles
Describe the clasp knife reflex?
- 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
Why does the clasp-knife reflex exist?
-It greatly increases tension in tendon which leads to a collapse of resistance like a spring loaded knife which acts as a protective mechanism
Describe the flexor(withdrawal) reflex with crossed extension.
- 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
How does the withdrawal reflex work?
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
How can the flexor reflex be described?
Ipsilateral flexion in response to pain
Describe the circuitry of flexor withdrawal reflex?
- 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
Why does the contralateral limb extend in the withdrawal reflex?
To avoid falling over
How does the contralateral limb extend?
- 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
Why is the flexor-crossed extensor reflex far slower than the stretch reflex?
- 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
What can you do to reflexes consciously?
Over ride them
How can you override a reflex?
-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
What does high y-motorneurone activation of muscle spindles cause?
Muscles become extremely resistant to stretch and is spastic
How can the stretch reflex be overridden?
Strong descending inhibition hyperpolarises a-motorneurones and the stretch reflex can not be evoked
What is the clinical relevance of stretch reflex?
- Assessing integrity of whole spinal cord circuit
- Allows spinal level localisation of a problem
How can the stretch reflex be used to localise a problem?
- 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
What does facilitation do?
Increases the effects of sensory inputs
How does the pain fibre input facilitate the action of the muscles spindles?
Maintains the a-motoneurones in a more depolarised state
What is an example of facilitation?
Withdrawing whole arm if full hand in contact with hot surface instead of just a hand if a finger was in contact
What is facilitation?
Activation of groups of neurones that provide sufficient excitation to bring other neurones to threshold
