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MOTOR SYSTEMS:

MOTOR UNIT PROPERTIES

MODULE 2: NEUROSCIENCE

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Cortical motor system and action planning

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Motor Unit Properties

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Motor Unit Types

SECTION OVERVIEW

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WHAT WE KNOW...

The motor unit consists of a neuron and multiple muscle fibers.

Multiple motor units make up the muscles.

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What are the properties of the motor unit?

How do the properties influence motor unit recruitment?

Can motor unit properties be altered?

KEY QUESTIONS:

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QUICK REMINDER

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CONTRACTILE SPEED AND FATIGABILITY

Slow Twitch: smaller force, less fatigable

Fast Twitch: larger force, quickly fatigues

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Contractile speed is a measure of the time it takes for a motor unit to reach full contraction from signal onset. Slow twitch fibers exert smaller forces than fast twitch fibers. On a graph of force vs time, this is what you would see when comparing a force exerted over time for a slow twitch fiber vs a fast twitch fiber. Slow twitch fibers have a greater time to peak but can maintain force longer (iow: they are less fatigable). This makes sense when you consider the energy source of slow twitch fibers: aerobic respiration. As long as there is a supply of oxygen, aerobic respiration will provide the energy needed for slow twitch muscles. Anaerobic respiration can only occur as long as there is an immediate source of glucose, so it makes sense that fast twitch fibers would tire more quickly.

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MOTOR UNIT FORCE

strength of a signal = spatial and temporal summation of all muscle fiber action potentials

in the graphs, as frequency increases so does the force detected

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The strength of a signal coming from a motor unit represents a spatial and temporal summation of all muscle fiber action potentials. Thus, exerting a higher force can be achieved through increasing the frequency of action potentials in a single motor unit, or increasing the number of motor units that fire. These graphs depict the readout for time vs force for slow twitch and fast twitch muscle fibers. The first thing to note is that the scales for both axes are different for each graph. The maximum force for slow twitch only reaches 100, and the maximum for fast twitch reaches 500 (this reflects what we reviewed before about the properties of slow twitch and fast twitch fibers). Also note that the time scale is different, which makes sense given what we just learned about contraction time and muscle force. The different lines on each of the graphs show the force for different frequencies of stimulation (which in turn produces different frequencies of action potentials). As the frequency increases, so does the force of the motor units.

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MOTOR UNIT RECRUITMENT PRINCIPLES

Motor units are recruited according to size and therefore contractile force. Small, less forceful motor units are recruited first in an action. When the same action calls for a muscle to exert more force, larger motor units are then recruited.

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“RULE OF FIVES”

Start of activation:

Motor Unit A firing at 5 Hz

Middle of activation:

Motor Unit A firing at 10 Hz

Motor Unit B firing at 5 Hz

Maximum of activation:

Motor Unit A firing at 15 Hz

Motor Unit B firing at 10 Hz

Motor Unit C firing at 5 Hz

A

B

A

B

C

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Research has established a ‘rule of fives’ that can help us understand motor unit recruitment. Let’s say a muscle action takes 3 motor units in order to occur. Motor unit A is the smallest and motor unit C is the largest. In the beginning of the movement, a look at the action potentials would look like the first image, with just action potentials from motor unit A firing at 5 Hz. To increase the force of a movement, we need to increase the frequency of the action potentials. Thus, the frequency of motor unit A will increase as needed. Once the frequency of firing for motor unit A reaches 10 Hz, motor unit B will be activated at 5 Hz. Once the frequency of firing for motor unit a reaches 15 and b reaches 10, motor unit c will be activated. This is a simplified way to think about motor unit recruitment occurring at different levels depending on the force needed.

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ALTERATION OF MOTOR UNIT PROPERTIES

Can be achieved through...

Exercise!

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Summary/What’s Next

Summary:

Contractile speed, maximal force and fatigability are determined by the muscle fiber type. Recruitment order is determined by the size of the motor unit, with smaller motor units being recruited first. The rule of fives helps us understand when motor units are recruited, and finally motor unit properties can be altered by exercise.

What’s next:

Module 3

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THANKS FOR LISTENING!

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