Neuromuscular control

Question 1

What is an alpha motor neuron?

Answer 1

LMN of brainstem and spinal cord

Question 2

What muscles do alpha motor neurons innervate?

Answer 2

Extrafusal muscles - skeletal muscles that cause contraction
activation causes contraction

Question 3

Motor neuron pool

Answer 3

contains all the alpha motor neurons innervating a single muscle

Question 4

What is a motor unit?

Answer 4

A single motor neuron and all the muscle fibres it innervates ~ 600
stimulation of one unit causes contraction of all muscle fibres

Question 5

What are the three types of motor units?

Answer 5

Slow (Type 1),
Fast & Fatigue Resistant (2a),
Fast & Fatiguable (2b)

Question 6

What are the structural properties of slow type compared to others?

Answer 6

Small cell body diameter, small dendrites, thin axons, slowest velocity

Question 7

How are motor units types classified?

Answer 7

• Tension generated
• Contraction speed
• Fatiguability

Question 8

Which muscle type generates the greatest force

Answer 8

fast fatiguable 2b, low duration

Question 9

What are the two mechanisms to regulate muscle force?

Answer 9

Recruitment
Rate coding

Question 10

What is recruitment?

Answer 10

units are recruited in order
First small units (slow), and then more units as demand increases. This allows fine control

Question 11

What is rate coding?

Answer 11

units fire at a range of frq
as demand increases, motor units fire more.
slow units fire until too fast to recover (summation) and then faster units take over

Question 12

What are neurotrophic factors?

Answer 12

Growth factors that travel down motor neurons to supply muscle fibres that are innervated

Question 13

What is the function of neurotrophic factors?

Answer 13

Promote growth of neurons after injury and prevent death

Question 14

What is it called when muscle fibres are swapped with innervation type and what effect does it cause?

Answer 14

Cross innervation - slow muscle types can become fast and vice versa
This is because muscle fibre characteristics are dependent on the nerve that innervates them

Question 15

Explain plasticity of muscle fibres and 3 conditions they may occur in

Answer 15

Muscle fibres changing properties under certain conditions
Training - fatiguable to fatigue resistant
Spinal cord injury/ severe deconditioning - slow to fast (less use of postural muscles)
Ageing - preferential loss of fast type

Question 16

What is a reflex?

Answer 16

Automatic response to stimulus -
involuntary coordinated pattern of muscle contraction due to peripheral stimuli

Question 17

Difference between reflex and voluntary movement

Answer 17

Once released, cannot be stopped

Question 18

What is the descending (supraspinal) control of reflexes with an example?

Answer 18

Reflex strength can be controlled by higher motor areas - Jendrassik manoeuvre - clenched teeth, making fist, pulling apart fingers exacerbate knee jerk reflex

Question 19

Explain how descending (supraspinal) control of reflexes occurs

Answer 19

Both inhibitory and excitatory regulation on stretch reflex in higher CNS centres - inhibitory control usually, but decerebration in the Jendrassik manoeuvre allows excitatory control

Brain normally dampens down reflexes, can be diminished if the brain is disconnected e.g. via stroke. Damage to motor cortex can cause positive signs

Question 20

What is hyperreflexia and how does it occur?

Answer 20

Overactive reflexes, loss of descending inhibition from upper motor neuron lesion

Question 21

What is clonus and how does it occur?

Answer 21

Involuntary oscillation of muscle contraction, loss of descending inhibition from upper motor lesion

Question 22

What is Babinski’s sign and how does it occur?

Answer 22

Blunt instrument on sole stimulates big toe - adults - downward curl, infant - upward curl. Abnormal (positive sign) - adult - upwards curl // upper motor neuron lesion

Question 23

What is hyporeflexia and how does it occur?

Answer 23

Below normal reflexes, lower motor neuron disease