Spinal Reflexes

Question 1

What is the simplest reflex?

Answer 1

The stretch reflex

Question 2

What provides information for the stretch reflex?

Answer 2

Muscle spindles (monitor muscle length)

Question 3

How does a muscle stretch reflex affect muscle spindles?

Answer 3

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

Question 4

How many connections do spindle sensory afferent fibres make?

Answer 4

3

Question 5

How does a stretch reflex affect the stretched muscle?

Answer 5

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

Question 6

How do afferent sensory fibres connect ot antagonist muscles?

Answer 6

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

Question 7

How do spindle afferent fibres connect to the brain?

Answer 7

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

Question 8

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

Answer 8

• Muscles contracts & shortens

- Tendon is pulled and 1b sensory nerves from GTO fire more APs

Question 9

How does the inverse stretch reflex affect muscles?

Answer 9

The afferent nerve fibres from the GTO affect:

• Activates inhibitory interneurones to the agonist muscle (weakens contraction)
• Activates excitatoy interneurones to antagonist muscles

Question 10

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

Answer 10

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

Question 11

What is the purpose of the inverse stretch reflex?

Answer 11

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

Question 12

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

Answer 12

Polysynaptic

Question 13

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

Answer 13

Nociceptors or pain receptors in the skin, muscles and joints

Question 14

Whats the purpose of the flexor (withdrawal) reflex?

Answer 14

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

Question 15

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

Answer 15

Increased activity via excitatory neurons

Question 16

What does the flexor reflex do to antagonistic extensor muscles?

Answer 16

The antagonistic extensors are inhibited.

Question 17

How do flexor reflexes cause large scale withdrawl movements?

Answer 17

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

Question 18

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

Answer 18

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

Question 19

How does the flexor reflex control contralateral limb extension?

Answer 19

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

Question 20

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

Answer 20

Sensory info ascends to the brain in the contralateral spinothalamic tract

Question 21

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

Answer 21

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.

Question 22

How can we override reflexes consciously?

Answer 22

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

Question 23

What do gamma-motorneurons depend on?

Answer 23

Descending pathways, no reflex APs reach tehm

Question 24

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

Answer 24

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

Question 25

Whats the clinical relevance of the stretch reflex?

Answer 25

• 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)

Question 26

What is facilitation?

Answer 26

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