Myotatic Reflexes

Question 1

Myotatic (stretch) reflex

Answer 1

Contraction of a stretched muscle to protect from tearing
Initiated by muscle spindle
Monosynaptic (this is the only monosynaptic reflex)

Question 2

Primary afferent neuron of myotatic reflex

Answer 2

Ia fiber
Innervates both nuclear bag and chain
Sensitive to both change in length and rate of change
Synapses on alpha motor neuron of stretched muscle as well as inhibitory interneuron for alpha motor neuron of antagonist muscle

Question 3

Secondary afferent neuron of myotatic reflex

Answer 3

Group II fiber
Innervates only nuclear chain fiber
Sensitive only to length of muscle

Question 4

Intrafusal muscle fibers

Answer 4

Innervated by gamma motor neuron
Control the length of the sensory portion
This increases sensitivity of the Ia and II fibers to stretch

Question 5

Gamma motor neuron

Answer 5

Smaller, a little slower than alpha MN
Innervates contractile component (intrafusal fibers) via NMJ
Causes contraction
Controls sensitivity of muscle spindle
Activity does not lead to motion- only alpha motor neuron leads to motion

Question 6

Motor end plate

Answer 6

Located on post synaptic cells in NMJ
No voltage gated channels on the plate
Ach opens nicotinic cholinergic channels

Question 7

Neurotransmitter used by afferent neuron synapsing in spinal cord in myotatic reflex

Answer 7

EAA- binds to non NMDA receptors

Question 8

Golgi tendon reflex

Answer 8

Abrupt relaxation of a strongly contracted muscle
Polysynaptic
Prevents damage to a contracted muscle due to excessive force development

Question 9

Golgi tendon organ

Answer 9

Afferent leg of the Golgi tendon reflex
Innervated by Ib fiber
Activated by tension in tendon

Question 10

Golgi tendon reflex process

Answer 10

Ib fiber synapses on interneuron in spinal cord
Release EAA binding to non-NMDA receptor
Interneuron synapses on alpha motor neuron of the contraction muscle and releases GABA
GABA binds GABAa receptors on alpha motor neuron and hyperpolarizes it
Reflex is high threshold- doesn’t kick in until muscle is about to tear

Question 11

Spinal shock

Answer 11

A period of areflexia immediately after transection of spinal cord
Partly due to loss of excitatory bias from CNS

Question 12

Spasticity

Answer 12

Muscle contraction which resists motion in response to passive motion of the limbs
i.e. if I raise your left arm, a stretch reflex will be activated that opposes my attempt to raise your arm quite effectively
Caused by increase in gamma motor neuron firing due to loss of inhibition from reticular system in brainstem- causes increased sensitivity to stretch and any attempt to move muscle will cause reflex
Cutting afferents to muscle spindle will abolish spasticity

Question 13

Cause of rigidity

Answer 13

Increase in alpha motor neuron excitability

It is independent of afferent input, so cutting afferents will not help

Question 14

Decorticate rigidity

Answer 14

If stroke damages internal capsule, input from the cortex to motor pathways in disrupted
Posture is dependent on head location
Head in neutral position- both legs are extended (if bilateral damage), both arms somewhat flexed
Head turned to side- legs remain extended, arm on side the head is turned to flexes further while contralateral arm extends
Unilateral damage w/head turned- arm flexes further when head is turned towards affected side, and extends when turned away
These head position findings may be due to loss of inhibition of the red nucleus and increased rubrospinal pathway activity