1.8 Motor Neurons

Question 1

Explain the route of an upper motor neuron

Answer 1

Travels from cortex to the spinal cord

Question 2

Which tract do the upper motor neurons follow?

Answer 2

Corticospinal Tract

Question 3

Where do the upper motor neurons decussate?

Answer 3

Upper medulla - for the lateral corticospinal tract

spinal level - for the anterior corticospinal tract

Question 4

What is a lower motor neuron, and what else is it known as (which innervate skeletal muscles)?

Answer 4

“alpha” motor neuron - synapse and exit from the ventral horn

Question 5

What are the properties of an alpha motor neuron?

Answer 5

Cell body in the CNS, thickly myelinated, large diameter and multipolar

Question 6

What is the difference between extrafusal and intrafusal fibers?

Answer 6

Extrafusal fubres - normal muscle fibres

Intrafusal fibres - sit on top of extrafusal fibres and monital muscle length

Question 7

Define a motor unit

Answer 7

one alpha motor unit, 50-100 extrafusal fibres and 8-10 intrafusal fibres

Question 8

Explain the three types of motor unit

Answer 8

Slow - type 1 - continuous extended use
Fast fatigue resistant - type 2a
Fast fatiguable - type 2b

Question 9

Explain the withdrawal reflex…

Answer 9

Withdraw from a painful and dangerous stimulus 
Polysynaptic - 4-5 interneurones 
Ipsilateral 
A delta - III 
Higher cortical control 
Interneurones inhibit antagonists

Question 10

Explain the crossed extension reflex…

Answer 10

Maintain balance during withdrawal reflex 
Polysynaptic 
Contralateral 
A delta - III 
Inhibits flexor, activates extensor

Question 11

Explain the stretch reflex…

Answer 11

Muscle spindle involved in proprioception
Prevents over stretching of muscle - muscle fibre length
Ia - A alpha
Monosynaptic
Negative feedback

Question 12

What is the function of gamma motor neurons?

Answer 12

They are special motor neurones - innervate only muscles - 30% fibres going to the muscle - cell bodies in the anterior part of the spinal cord - can adjust muscle spindle sensitivity

Question 13

What triggers the golgi tendon reflex?

Answer 13

The stretching of a muscle tendon during muscle flexion
Ib - A alpa
Changes protein shape - AP
Excites extensory and inhibits flexor - prevents from over stretching.

Question 14

Explain what the Renshaw cell is and its function

Answer 14

Self inhibiting interneuron

Question 15

What other type of inhibition is there?

Answer 15

Reciprocal - inhibits antagonis

Presynaptic