L18: The Motor Unit

Question 1

What are some names of lower motor neurons?

Answer 1

• alpha-motoneuron
• gamma-motoneuron
• spinal motoneuron

Question 2

Is there somatotopy associated with motoneurons in spinal cord?

Answer 2

medial ventral horn –> proximal/axial muscles

lateral ventral horn –> distal muscles

Question 3

Define motor neuron pool

Answer 3

total of all lower motor neurons innervating a given muscle

EX: abductor pollicus brevis has 80 motoneurons that innervate it soo these 80 motoneurons comprise the motor neuron pool of the abductor pollicus brevis.

Question 4

A motor unit is made of 3 components. List them

Answer 4

1. cell body
2. its axon
3. all the muscle fibers that axon innervates

Question 5

List 5 characteristics of a motor unit

Answer 5

1. axon ONLY innervates ONE muscle
2. collection of muscles innervated by that one axon
3. properties of muscle fibers within a muscle unit are similar/identical
4. all muscle fibers of muscle unit contract simulatenously when axon fires (exceptions: disease process)
5. size of muscle unit varies trememndously

Question 6

In force generation, what is needed to relax the muscles?

Answer 6

ATP; without ATP –> rigor mortis

Question 7

What are 2 ways to modulate force generation?

Answer 7

1) Recruitment of a previously silent motor unit to cause it to fire (***size principle: small ones are recruited first)
2) Rate coding of an already-firing motor unit to increase firing rate which may lead to tetanus

Question 8

There are 3 different types of motor units (slow-twitch, fast/fatigue resistant, fast/fatiguable). Explain the characteristics of the slow-twitch muscle fibers.

Answer 8

• Type 1
• fire more slowly
• uses O2 to make ATP for long period of muscle contractions
• “marathoners”
• no/little force loss with repeated stimulations

Question 9

There are 3 different types of motor units (slow-twitch, fast/fatigue resistant, fast/fatiguable). Explain the characteristics of the fast, fatigue resistant fibers muscle fibers.

Answer 9

• Type IIa
• uses both anerobic & aerobic respiration
• fast rate of force increase during twich
• moderate force loss with repeated twitches.

Question 10

There are 3 different types of motor units (slow-twitch, fast/fatigue resistant, fast/fatiguable). Explain the characteristics of the fast, fatiguable muscle fibers.

Answer 10

• Type IIb
• anaerobic
• fastest rate of force increase during twitch
• rapid force loss with repeated stimulus

Question 11

Explain how the total force for a twitch varies among the 3 different fiber types.

Answer 11

• For a twitch, total force varies a lot!
• Slow fiber –> low force
• Fast, Fatigue resistant –> higher force
• Fast, Fatiguable –> highest force

Question 12

Explain how the time to reach peak force differs among the 3 different muscle fiber types.

Answer 12

Type IIb (fast, fatiguable) fibers reach peak force the fastest at 20-40 ms.

Type IIa (fast fatigue-resistant) fibers reach peak force at 30-80 ms.

Type I (slow) fibers reach peak force at 60-120 ms.

Question 13

Explain how using different rates of stimulation can cause different motor twitch forces. (5 Hz vs 20 Hz vs 80 Hz vs 100 Hz)

Answer 13

5 Hz –> single muscle twitches
20 Hz –> temporal summation
80 Hz –> unfused tetanus
100 Hz –> fused tetanus

Question 14

In a fatigue test, what kind of results will you expect from the slow, fast/fatigue resistant, fast fatiguable fibers?

Answer 14

Type IIb (fast, fatiguable) fibers will experience a very rapid force drop

Type IIa (fast, fatigue resistant) fibers will experience eventual force drop.

Type I (slow) fibers will experience little to no force drop.

Question 15

What happens if you don’t turn off the stimulation on the muscle fibers?

Answer 15

Within 10 minutes of continuous stimulation, there will be 100% of fiber death because NEED RELAXATION for blood delivery.

Question 16

Are smaller or larger motor units more excitable?

Answer 16

Small motor units are more excitable. Hence, smaller motor units are recruited first in modulating force generation than large ones.

Question 17

By increasing motor unit size, these motor units will exhibit increased _______ (list 5)

Answer 17

increased:

1) cell body size
2) dendritic complexity
3) short-term EPSP potentiation w/ repeated activation
4) axonal diameter (faster conduction)
5) # of axon branches (more muscle fibers innervated)

Question 18

What will increasing motor unit size decrease in the motor neuron?

Answer 18

DECREASED EXCITABILITY

Question 19

Motor unit recruitment is not random (size principle: smaller motor units are recruited before larger motor units). What would happen if motor unit during movements was random?

Answer 19

loss of movement precision

loss of function

Question 20

What are the 4 things that determine motor unit type?

Answer 20

1) activity pattern can influence motor unit type but takes a lot of input (no way for humans to have the time, motivation, and energy to achieve this change)
2) muscle fibers are poly-innervated at birth, and very quickly after birth, start losing some connections, depending on activity.
3) muscle fibers can split & regenerate that can change muscle properties/type post-injury
4) Genetics

Question 21

What is electromyogram (EMG) used for?

Answer 21

to diagnose neuromuscular diseases

Question 22

Why is it easy fr electromyogram to work?

Answer 22

muscles act as biological amplifier taking tiny source of axon & branch off many times (much easier to record than just an axon)

Question 23

What can EMG electrodes influence?

Answer 23

• -volume of muscle recorded
• -signal characteristics (freq response, sensitivity to noise: more sensitive electrode is more sensitive to noise)
• -EMG electrode designs are either SURFACE or NEEDLE (non-selective vs selective)

Question 24

Electrode designs are either surface or needle. Of the needle electrode designs, there are non-selective vs selective. Which is considered the standard for EMG exams?

Answer 24

Selective needle electrode design (concentric bipolar) is standard for EMG exams.

Question 25

Can EMG exams see recruitment & rate-coding?

Answer 25

Yes. ***if pt cannot recruit as many or any motomeurons, then it’s suggestive of disease process.

Question 26

Explain which muscle types are associated with smaller or larger loads.

Answer 26

Small loads require less force, slow twitch fatigue resistant muscle fibers are recruited first. Larger motor units are typically composed of faster muscle fibers that generate higher force