Neuroscience Motor Unit Blair Calancie

Question 1

What are some characteristics of LMNs?

Answer 1

1. Large cell body (50um)
2. Extensive dendritic tree
3. Large axon diameter
4. Long axon (1+ m)
5. Myelinated with Schwann cells
6. Fast impulses (up to 60 m/s)

Question 2

What kinds of motor neurons are found in the medial ventral horn?

Answer 2

proximal muscles, ie shoulder

Question 3

What kinds of motor neurons are found in the lateral ventral horn?

Answer 3

Distal muscles, is hand

Question 4

What is a lower motor neuron pool?

Answer 4

Total of all LMNs innervating a given muscle

Question 5

What is a motor unit?

Answer 5

Made up of:

1. Cell body
2. Axon **innervates ONLY ONE muscle
3. All the muscles that the axon innervates

Question 6

What is a muscle unit?

Answer 6

Collection of nerve fibers innervated by the one axon, 6-10 in eye, 1500+ in quadriceps

Question 7

What is the general theme of muscle contraction?

Answer 7

Internal shortening

Question 8

What is this? One single stimulus to the motor unit’s axon.

Answer 8

Twitch

Question 9

What is this? Repeated stimuli to the axon, before the mechanical force calls to zero.

Answer 9

Tetanus

Question 10

What is the influence of rate on motor twitch force?

Answer 10

Temporal summation. The shorter the interval, the higher the peak force of contraction that can be achieved (to a limit)

Question 11

Explain the “recruitment” method of modulating force generation.

Answer 11

Recruitment of a previously silent motor unit:

• -cause a motor unit to begin discharging
• -stronger inputs leads to recruitment of more powerful motor units
• -size principle - smallest MNs are the first to be recruited (cascade)

Question 12

Explain the “rate coding” method of modulating force generation.

Answer 12

Rate-coding of an already firing motor unit:

• -increase the firing/discharge rate of an already-recruited motor unit
• -partially fused force output
• -fully fused (ie tetanic) force output at highest firing rates

Question 13

Motor unit properties vary with ___ of the alpha-motorneuron (LMN).

Answer 13

SIZE

Question 14

How do the sizes of fast fatiguable, fast fatigue-resistant and slow muscles compare?

Answer 14

FF&raquo_space; FR > S

Question 15

With increased motor unit size, alpha motor neurons exhibit INCREASED:

Answer 15

1. Cell body size
2. Dendritic complexity
3. Number of axonal branches
4. Axonal diameter / faster conduction
5. Short-term EPSP potentiation with repeated activation

Question 16

With increased motor unit size, alpha motor neurons exhibit DECREASED:

Answer 16

1. Input resistance
2. Excitability
3. Ia EPSP amplitude
4. PSP decay constant
5. Duration of after-hyperpolarization

Question 17

Why is EMG (electromyogram) easy to record?

Answer 17

The MUSCLE UNIT of a single MOTOR UNIT serves as a natural BIOLOGIC AMPLIFIER of a nerve action potential.