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1
Q

Stretch reflex

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 the spinal cord through the dorsal horn.
2
Q

Spindle sensory afferents divide and make 3 types of connections

A

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.

  1. 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.
  1. Spindle afferent information also ascends in dorsal columns, connects with somatosensory cortex to tell the brain about length of muscles
3
Q

Inverse stretch reflex - (clasp-knife or Golgi-tendon organ reflex)

A

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

  1. Activation of inhibitory interneurones to the agonist muscle and a decrease in contraction strength.
  2. Activation of excitatory interneurones to antagonist muscles.
  3. Again, information about muscle tension ascends in the dorsal columns to the somatosensory cortex.
4
Q

Flexor (Withdrawal) reflex - with crossed extension

A
  • 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.
5
Q

Mechanism of flexor reflex

A

Increased sensory APs from pain receptors cause:

  1. Increase­ activity in the flexor muscles of the affected part via a number of excitatory interneurones.
  2. At the same time, via a number of excitatory and inhibitory interneurones, the antagonistic extensors are inhibited.
6
Q

Contralateral limb

A
  • 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:-

  1. Several excitatory interneurones which cross the spinal cord excite the contralateral extensors
  2. At the same time, via several interneurones, there is inhibition of the contralateral flexors
  3. This helps to maintain an upright posture by extending the limb to bear the body weight.
  4. Sensory information ascends to the brain in the contralateral spinothalamic tract.
7
Q

Facts

A
  • 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
8
Q

Inhibition of GTO

A
  • 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.
9
Q

The stretch reflex also can be overridden

A
  • 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.
10
Q

Picture

A
11
Q

Facilitation

A
  • 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