Reflexes Flashcards
the stretch reflex is found in
ALL muscles.
Best example is the patellar tendon or knee-jerk reflex.
A Stretch reflex =
follows a sharp tap to an 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 which
increases thee number of action potentials in afferent nerves projecting through the dorsal horn into the spinal cord.
spindle sensory afferents divide to make 3 types of connections
MONOSYNAPTIC REFLEX
RECIPROCAL INHIBITION.
Inverse stretch reflex
MONOSYNAPTIC REFLEX
no interneurones involved – it is the only such connection known.
- many directly activate the alpha-motoneurone pool to the muscle which was stretched: causes rapid contraction of the AGONIST muscle.
The stretch reflex is a
classic negative feedback loop.
Muscle stretch - stimulates muscle spindles
Causes reflex muscle contraction – muscle shortens to previous length
RECIPROCAL INHIBITION.
spindle afferents connect with and activate inhibitory interneurones which decrease activation of alpha-motoneurones to the antagonist muscle - which then relaxes.
- muscles use agonists and antagonists to move joints - 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).
Spindle afferent firing also travels
up the dorsal columns to the somatosensory cortex - to tell the brain about length of muscles
Inverse stretch reflex
(clasp-knife or Golgi-tendon organ reflex)
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.
clasp-knife reflex
- because greatly increasing tension in tendon leads to a collapse of resistance
- like a spring-loaded knife opening up
- it is a protective mechanism to prevent muscle damage
Golgi-tendon organ reflex : inverse stretch reflex : clasp-knife reflex
agonist muscle is inhibited – and relaxes rapidly
antagonist muscle is activated – and contracts
this reflex is polysynaptic (disynaptic) and protective
it prevents muscles contracting so hard that the tendon insertion is torn away from the bone
Flexor (Withdrawal) reflex - with crossed extension.
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.
Increased sensory APs from pain receptors cause
- increased 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
FLEXOR REFLEX
ipsilateral flexion in response to pain
Circuitry of flexor withdrawal reflex
- Small diameter Aδ nociceptive fibres triggering pain enter cord
- they branch a lot and activate interneurons in several spinal segments above entry point
- which activate α motoneurones controlling all the flexor muscles of the affected limb
contralateral limb extends
and prevents you from falling over say when you step on a pin so you can stay balanced.
Sensory information ascends to the brain in the
contralateral spinothalamic tract.
- This basic circuitry is similar to that for walking
- hardwired at spinal cord level.
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
the flexor and crossed extensor reflex is
far slower than the stretch reflex.
this is because:
- there are several interneurons in the pathway - each with a small synaptic delay
- nociceptive sensory fibres have a smaller diameter than muscle spindle afferents and so conduct more slowly
reflexes are “hard-wired” building blocks integrated into the voluntary control of movement and they interact with other reflexes: but
can be over-ridden consciously.
example of this is load- if something is precious you are going to hold on to it until you can safetly put it down somewhere - eg a baby
can the GTO reflex be overridden?
The GTO reflex can be over-ridden by voluntary input from the CNS
if excess load is placed n muscle
golgi tendon reflex is activated (motor neuron inhibited)causing relaxation and thus protecting muscle.
when holding heavy loads descending voluntary excitation of
alpha motoneurones can override the inhibition from the GTOs and maintain contraction.
stretch reflex can be overriidden when
strong descending inhibition hyperpolarises alpha motorneurons and the stretch reflex cannot be evoked.
activation of gamma motor neurons depends on
descending pathways.
high gamma-motoneurone activation of muscle spindles, causes
muscles to become extremely resistant to stretch and muscle is “spastic”.
- Many neurological disorders are associated with altered gamma-motoneurone discharge to spindles
reflexes allow
spinal level localisation of a problem
in stretch reflex, spindle input is
highly localised and affects only alpha motoneurones at one or two spinal segments
in withdrawal reflex, pain fibre input is
diffuse and spreads through several spinal segments - the more powerful the pain stimulus, the greater the spinal spread and the larger the response.
Facilitation
increases the effects of sensory inputs
- finger on a hot plate - just withdraw hand
- whole hand in contact with hot surface, whole arm will be withdrawn and extension of the contralateral arm too.
Facilitation occurs between
- similar inputs eg. pain fibre inputs
- diverse inputs eg. burnt hand on a hotplate and biceps stretch reflex
Facilitation explained
A alone = 4 motoneurones activated
C alone = 4 motoneurones activated
A + C =12 motoneurones activated
(B is in the middle)
When A and C are active together there is sufficient excitation to bring neurones in B to threshold
