Spinal Motor Systems

Question 1

What is the major motor area of the spinal cord?

Answer 1

Ventral horn

This is where the α-motoneurons that innervate muscles are located

Question 2

Where do most synapses onto motorneurons come from?

Answer 2

Spinal interneurons located in the intermediate zone grey matter

Question 3

What is the motorneuron pool?

Answer 3

The group of 200-500 motorneurons that innervate a given muscle

They are located close together in the ventral horn over several spinal segments extending rostrocaudally

Question 4

What is the somatotopic organisation?

Answer 4

Distal muscles represented laterally

Question 5

What is a motor unit?

Answer 5

The motor neuron and the muscle fibres it innervates

Question 6

What are the three categories of motor units?

Answer 6

1. Slow
2. Fast fatiguable
3. Fast fatigue-resistant

Question 7

What is a slow motor unit?

Answer 7

It innervates small muscle fibres

Specialised for oxidative metabolism

Highly vascular with lots of myoglobin and mitochondria

Generates low forces with slow twitches

Fatigue-resistant

Question 8

What is a fast fatiguable motor unit?

Answer 8

It innervates large muscle fibres (so high force)

Highly glycolytic

Fatigue easily

Question 9

What is a fast fatigue resistant motor unit?

Answer 9

Intermediate

Question 10

How is force generation controlled?

Answer 10

1. Motor neuron firing rate - rate coding

2. Motor neuron recruitment

Question 11

What is the ‘size principle’ of motor neuron recruitment?

Answer 11

Motor units are recruited in an orderly sequence

Lowest force motor units first

Highest force motor units last

Ensures force always increased by increments of finest available force

Question 12

What is the basis of the ‘size principle’?

Answer 12

Developmental plasticity

Motor neurons with low firing thresholds are successful in innervating small numbers of muscle fibres

Their firing rate induces them to become slow, fatigue resistant and low force

Motor neurons with high firing thresholds innervate many muscle fibres, inducing them to be high force but fatiguable

This is because they are not recruited as often due to their higher firing threshold

Question 13

What are the three sources of synaptic input to motor neurons?

Answer 13

1. Afferent fibres from muscle spindles
2. Descending fibres
3. Spinal interneurons