Motor Control: Reflexes

Question 1

What are the cortical reflexes?

Answer 1

Placing reaction, hopping reaction

Question 2

What are the brainstem/midbrain reflexes?

Answer 2

Vestibular, righting reflex, suckle, yawn, eye/head movements

Question 3

What are the spinal reflexes?

Answer 3

Stretch (myotatic), golgi tendon reflex, crossed extensor

Question 4

What are the components of the intrafusal muscle spindle?

Answer 4

Sensory component - nuclear bag and nuclear chain

Motor component - intrafusal muscle fibers (contractile elements)

Question 5

What are group Ia fibers?

Answer 5

Respond to passive stretch and relay length of muscle and rate of change. Are highly myelinated, thick, and fast. Associated with nuclear bag and nuclear chain

Question 6

What are group Ib fibers?

Answer 6

Involved in relaxing contracted muscles for the golgi tendon organ reflex, are less myelinated, thinner, and slower (but still fast)

Question 7

What are group II fibers?

Answer 7

Relay length of muscle only, associated with nuclear chain only

Question 8

What is spinal shock?

Answer 8

A period of areflexia after a spinal cord transection

Question 9

How do reflexes return after spinal shock?

Answer 9

Axonal sprouting of interneurons below the level of the transection. Interneurons also start to express a self-exciting serotonin receptor (5HTC ionotropic receptor, allows Na+ in, stays open even w/o serotonin present)

Question 10

Describe the steps of the basic stretch reflex

Answer 10

Stretch muscle - increase AP in Ia fiber - leads to increased EAA release at synapse w/ alpha motor neuron - increase AP of alpha motor neuron - increase ACh release at NMJ - muscle contraction decreases stretch of muscle

Question 11

How does damage to the brain stem adjust sensitivity of stretch reflex?

Answer 11

Change gamma motor neuron firing rate

Question 12

Describe how damage to the cortex in inhibitory areas leads to hyperreflexia

Answer 12

Damage to cortex in inhibitory area - decreased activation of inhibitory area in brainstem = unopposed excitation - alpha MN send input to gamma MN - increase gamma MN firing rate - contraction of intrafusal fibers - increased sensitivity of Ia fiber to stretch

Question 13

What is rigidity vs spasticity?

Answer 13

Rigidity - resists motion in all directions (maintained muscle contraction - alpha MN continually activated)
Spasticity - resists motion in a given direction (hyperactive myotatic reflex - gamma MN continually activated)