Week 3: Electromyography and Reflexes

Question 1

What is all overt movement controlled by?

Answer 1

Skeletal muscle.

Question 2

What activates muscles?

Answer 2

Action potentials travelling down the axons of the alpha motor neurons.

Question 3

Define ‘motor unit’.

Answer 3

One motor neuron and the group of muscle fibres it innervates.

Question 4

The alpha motor neurons that innervate a muscle are referred to as what?

Answer 4

The muscle’s “motor neuron pool”.

Question 5

The cell bodies of the motor neurons can be found where?

Answer 5

The ventral horn of the spinal cord.

Question 6

Define electromyography and motor unit potential.

Answer 6

MOTOR UNIT POTENTIAL.
- All muscle fibres in a motor unit generate an individual action potential, creating a single summed action potential.

ELECTROMYOGRAPHY
- The technique of extracellular recording of motor unit potentials.

Question 7

The size of the motor unit potential varies proportional to what.

Answer 7

Size of the motor unit (number of muscle fibres innervated).

Question 8

The size of the incremental force generated varies proportional to what.

Answer 8

Size of the motor unit (number of muscle fibres innervated).

Question 9

The total force generated by a muscle depends on what two things:

Answer 9

1. Number of motor units active.

2. Size of motor units active.

Question 10

When the strength of excitation of the motor neuron pool is increased, what motor units are fired off first, smaller or larger?

Answer 10

Smaller.

• This ensures that small motor units are used for small forces.

Question 11

Describe what is happening in the tendon reflex?

Answer 11

• Tap imparts a stretch on the muscle.
• Length change sensed by the muscle spindle sensory neurone.
• A reflex excitation of the motor neurons is caused.
• The stretch of the muscle is opposed by active contraction of the muscle.
• -> Muscle length is regulated.

Question 12

Muscle tone is…

Answer 12

The steady state of contraction of muscle.

Question 13

How does the inverse myotactic reflex differ from the tendon reflex?

Answer 13

• This reflex occurs in response to rise in muscle tension, not change in length.
• Reflex inhibition, not excitation is caused, leading to sudden relaxation of the muscle when tension rises.

Question 14

Give an example of a polysynaptic cutaneous reflex and describe how it arises.

Answer 14

• Crossed extensor reflex.
• Painful stimulus causes limb to withdraw.
• Flexors in this limb contract and extensors relax.
• Opposite occurs in other limb.