reflexes mw%% (X Flashcards
Stretch reflex
- 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 the spinal cord through the dorsal horn.
Spindle sensory afferents divide and make 3 types of connections
1.Many directly activate the a-motoneurone pool to the muscle which was stretched : causes rapid contraction of the AGONIST muscle.
This type is MONOSYNAPTIC REFLEX with no interneurones involved.
- But muscles need to use agonists and antagonists to move joints
So the 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 (stretches).
- Because spindle afferents connect with and activate inhibitory interneurones which decrease the activation of a-motoneurones to the antagonist which then relaxes
- this is called RECIPROCAL INHIBITION.
- Spindle afferent information also ascends in dorsal columns, connects with somatosensory cortex to tell the brain about length of muscles
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Inverse stretch reflex - (clasp-knife or Golgi-tendon organ reflex)
Caused by afferent nerves from the Golgi tendon organs (GTO) – which monitor muscle tension.
Muscle contracts and shortens – this pulls on the tendon and the 1b sensory nerves from GTOs increase firing of APs : this causes
- Activation of inhibitory interneurones to the agonist muscle and a decrease in contraction strength.
- Activation of excitatory interneurones to antagonist muscles.
- Again, information about muscle tension ascends in the dorsal columns to the somatosensory cortex.
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Flexor (Withdrawal) reflex - with crossed extension
- Flexor or withdrawal reflexes use information from pain receptors (nociceptors) in skin, muscles and joints.
- They are polysynaptic and protective
- They withdraw part of the body away from the painful stimulus and in towards the body - so flex the affected part.
Mechanism of flexor reflex
Increased sensory APs from pain receptors cause:
- Increase activity in the flexor muscles of the affected part via a number of excitatory interneurones.
- At the same time, via a number of excitatory and inhibitory interneurones, the antagonistic extensors are inhibited.
Contralateral limb
- BUT, if you withdraw limb from a tack and do nothing else, you would fall over – which is NOT protective!
To prevent this the contralateral limb extends via:-
- Several excitatory interneurones which cross the spinal cord excite the contralateral extensors
- At the same time, via several interneurones, there is inhibition of the contralateral flexors
- This helps to maintain an upright posture by extending the limb to bear the body weight.
- Sensory information ascends to the brain in the contralateral spinothalamic tract.
Facts
- Reflexes are “hard-wired” building blocks integrated into voluntary movement control
- They interact with other reflexes but can be over-ridden consciously,ask someone to hold an increasing load
- Load stretches the muscle and its spindles, causing reflex contraction to restore arm position (stretch reflex).
- Excess load – GTO reflex activated
- If the load was not blocks, but a child would you inevitably drop it because of the Golgi tendon reflex?
- NO! reflexes can be over-ridden
Inhibition of GTO
- One α motoneurone receives >10,000 synapses (in red above)
- 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 alpha motoneurones can override the inhibition from the GTOs and maintain contraction.
The stretch reflex also can be overridden
- Strong descending inhibition hyperpolarizes α-motoneurones and the stretch reflex can not be evoked.
- So when testing reflexes, subjects are distracted to prevent voluntary effects on the reflex responses.
- SO, the absence of reflex responses in an uncooperative patient may not be evidence of peripheral nerve damage.
Picture
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Facilitation
- When A and B are active together there is sufficient excitation to bring neurones in C to threshold
- A alone = 4 motoneurones activated
- B alone = 4 motoneurones activated
- A + B =12 motoneurones activated
- This is Facilitation
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