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Flashcards in Spinal Reflexes Deck (26):

What is the simplest reflex?

The stretch reflex


What provides information for the stretch reflex?

Muscle spindles (monitor muscle length)


How does a muscle stretch reflex affect muscle spindles?

The muscle stretch causes the spindle to stretch
This activates 1a sensory nerves in the muscle spindle
This increases the number of APs in afferent nerves


How many connections do spindle sensory afferent fibres make?



How does a stretch reflex affect the stretched muscle?

The first connection of the afferent fibres activates the alpha-motorneuron fibres to the stretched muscle
This causes rapid contraction of the AGONIST muscle.
This is a monosynaptic reflex


How do afferent sensory fibres connect ot antagonist muscles?

spindle afferent fibres activate inhibitory interneurons
They decrease activation of alpha-motorneurons to antagonist muscles.
Tbis causes the antagonist to relax (reciprocal inhibition)


How do spindle afferent fibres connect to the brain?

They ascend in dorsal columns and connect with somatosensory cortex to tell the brain about the muscle length.


How does the inverse stretch reflex affect the golgi tendon organ?

- Muscles contracts & shortens
- Tendon is pulled and 1b sensory nerves from GTO fire more APs


How does the inverse stretch reflex affect muscles?

The afferent nerve fibres from the GTO affect:
- Activates inhibitory interneurones to the agonist muscle (weakens contraction)
- Activates excitatoy interneurones to antagonist muscles


How does information from the inverse stretch reflex reach the brain?

Info about the muscle tension ascends the 1b sensory afferent fibres in dorsal columns to somatosensory cortex.


What is the purpose of the inverse stretch reflex?

Prevents muscle contracting so hard the tendon tears from the bone (think sudden release of muscle tension when losing at arm wrestling)


Is the inverse stretch reflex poly- or mono- synaptic?



What receptors are involed in the Flexor (withrdrawal) reflex?

Nociceptors or pain receptors in the skin, muscles and joints


Whats the purpose of the flexor (withdrawal) reflex?

To withdraw the body part from painful stimulus (and towards the body).


What does increased APs from pain receptors cause in flexor muscles?

Increased activity via excitatory neurons


What does the flexor reflex do to antagonistic extensor muscles?

The antagonistic extensors are inhibited.


How do flexor reflexes cause large scale withdrawl movements?

The nociceptor fibres branch alot
Activate interneurons in multiple spinal segments
Activate alpha motorneurones controlling all flexor muscles of the affected limb.


What does the flexor (withdrawal) reflex do to prevent you hurting yourself when you withdraw a limb?

Triggers contralateral limb extension to balance you out. (crosed extension)


How does the flexor reflex control contralateral limb extension?

- Excitatory interneurons cross the - spinal cord and excite the contralateral extensor muscles
- Inhibitory interneuorns cross the cord and inhibit the contralateral flexor muscles.


How does sensory info reach the brain regarding the contralateral limb extension?

Sensory info ascends to the brain in the contralateral spinothalamic tract


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

IT involves several interneurons with a small synaptic delay
Also the nociceptor fibres are smaller in diameter than the afferent fibres from spindles/GTOs so transmit slower.


How can we override reflexes consciously?

Each alpha-motorneuron receives > 10000 synapses continually integrating.
descending excitation/inhibition from the brain can overpower inhibition/excitation from the GTOs/Spindles/nociceptors.


What do gamma-motorneurons depend on?

Descending pathways, no reflex APs reach tehm


What does abnormally high gamma-motorneuron activation of muscle spindles cause?

The muscle spindles become resistant to passive stretch and the gamma-motorneurons triger random stretch reflexes (spastic)
This is problem with the CNS


Whats the clinical relevance of the stretch reflex?

- Useful for assessing integrity of whole spinal cord circuit
- allows spinal level localisation of a problem (reflexes evoked above a cetain level but not below show segmental trauma to spine)


What is facilitation?

Multiple small pain receptors depolarise enough to trigger depolarisation in nearby ones too which causes an exaggerated response.