Lecture 4 - Motor neuron pools and their functional organisation

Question 1

Where are motor units located?

Answer 1

Ventral horn of the spinal cord

Question 2

List some features of motor neurons.

Answer 2

• Large neurons

- Extensive dendritic trees

Question 3

What can affect motor neurons?

Answer 3

Feedback from sensory inputs

Question 4

What three inputs converge onto motor neurons? [3]

Answer 4

• Sensory fibres
• Interneurons
• Descending pathways

Question 5

What is a motor neuron pool?

Answer 5

Groups of motor neurons supplying an individual muscle

Question 6

How are motor neuron pools arranged?

Answer 6

• Longitudinal columns

- Span several segments

Question 7

What is the proximal-distal rule?

Answer 7

• Medial motor neuron pools innervate proximal muscles

- Lateral motor neuron polls innervate distal muscles

Question 8

What is the final common pathway?

Answer 8

Motor neurons

Question 9

What are the two main types of motor neuron? [2]

Answer 9

• Alpha

- Gamma

Question 10

What are the features of alpha motor neurons?

Answer 10

• Supply extrafusal muscle fibres

- Control muscle force generation

Question 11

What are the features of gamma motor neurons?

Answer 11

• Supply intrafusal muscle fibres

- Control muscle spindle reqsponsiveness

Question 12

What is special about beta motor neurons?

Answer 12

They supple both extrafusal and intrafusal muscle fibres

Question 13

How are motor neurons recruited?

Answer 13

Small > Large

Question 14

How are motor units recruited?

Answer 14

S > FR > FF

Question 15

Why are small motor neurons recruited first?

Answer 15

• High density of synaptic inputs

- High electrical input resistance

Question 16

Low [A] motor neurons will give a high [B] which leads to an [C].

Answer 16

A - Resistance
B - ESPS
C - Action potential

Question 17

What are the advantages of the size principle?

Answer 17

• Non-fatigable muscle fibres are used for most tasks (conserve energy)
• Increments of contractile force can be achieved

Question 18

How do muscle work together?

Answer 18

• Form antagonistic pairs

- Help joints to open and close

Question 19

What is Torque?

Answer 19

A force that rotates and joint

Question 20

What does the overall rotation of a joint depend on?

Answer 20

The combination of the Torque that rotates it clockwise and the Torque that rotates it anticlockwise

Question 21

What happens during the continuous low level activation of a flexor?

Answer 21

• A slow rise in contractile force

- Takes time to generate enough force to give contraction

Question 22

What happens during asymetrical activation of a flexor and extensor?

Answer 22

• Large initial burst of activity in flexor (large posative torque)
• Delayed activation of extensor (negative torque)

Question 23

What happens during coactivation of flexor and extensor (with a slight delay)?

Answer 23

It allows for very rapid movement