L18: The Motor Unit Flashcards
What are some names of lower motor neurons?
- alpha-motoneuron
- gamma-motoneuron
- spinal motoneuron
Is there somatotopy associated with motoneurons in spinal cord?
medial ventral horn –> proximal/axial muscles
lateral ventral horn –> distal muscles
Define motor neuron pool
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.
A motor unit is made of 3 components. List them
- cell body
- its axon
- all the muscle fibers that axon innervates
List 5 characteristics of a motor unit
- axon ONLY innervates ONE muscle
- collection of muscles innervated by that one axon
- properties of muscle fibers within a muscle unit are similar/identical
- all muscle fibers of muscle unit contract simulatenously when axon fires (exceptions: disease process)
- size of muscle unit varies trememndously
In force generation, what is needed to relax the muscles?
ATP; without ATP –> rigor mortis
What are 2 ways to modulate force generation?
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
There are 3 different types of motor units (slow-twitch, fast/fatigue resistant, fast/fatiguable). Explain the characteristics of the slow-twitch muscle fibers.
- Type 1
- fire more slowly
- uses O2 to make ATP for long period of muscle contractions
- “marathoners”
- no/little force loss with repeated stimulations
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.
- Type IIa
- uses both anerobic & aerobic respiration
- fast rate of force increase during twich
- moderate force loss with repeated twitches.
There are 3 different types of motor units (slow-twitch, fast/fatigue resistant, fast/fatiguable). Explain the characteristics of the fast, fatiguable muscle fibers.
- Type IIb
- anaerobic
- fastest rate of force increase during twitch
- rapid force loss with repeated stimulus
Explain how the total force for a twitch varies among the 3 different fiber types.
- For a twitch, total force varies a lot!
- Slow fiber –> low force
- Fast, Fatigue resistant –> higher force
- Fast, Fatiguable –> highest force
Explain how the time to reach peak force differs among the 3 different muscle fiber types.
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.
Explain how using different rates of stimulation can cause different motor twitch forces. (5 Hz vs 20 Hz vs 80 Hz vs 100 Hz)
5 Hz –> single muscle twitches
20 Hz –> temporal summation
80 Hz –> unfused tetanus
100 Hz –> fused tetanus
In a fatigue test, what kind of results will you expect from the slow, fast/fatigue resistant, fast fatiguable fibers?
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.
What happens if you don’t turn off the stimulation on the muscle fibers?
Within 10 minutes of continuous stimulation, there will be 100% of fiber death because NEED RELAXATION for blood delivery.
Are smaller or larger motor units more excitable?
Small motor units are more excitable. Hence, smaller motor units are recruited first in modulating force generation than large ones.
By increasing motor unit size, these motor units will exhibit increased _______ (list 5)
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)
What will increasing motor unit size decrease in the motor neuron?
DECREASED EXCITABILITY
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?
loss of movement precision
loss of function
What are the 4 things that determine motor unit type?
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
What is electromyogram (EMG) used for?
to diagnose neuromuscular diseases
Why is it easy fr electromyogram to work?
muscles act as biological amplifier taking tiny source of axon & branch off many times (much easier to record than just an axon)
What can EMG electrodes influence?
- -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)
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?
Selective needle electrode design (concentric bipolar) is standard for EMG exams.
Can EMG exams see recruitment & rate-coding?
Yes. ***if pt cannot recruit as many or any motomeurons, then it’s suggestive of disease process.
Explain which muscle types are associated with smaller or larger loads.
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
