Advanced Anatomy & Physiology
Unit 3 – Neuromuscular and endocrine systems
Lesson 3.2 – Motor Units
Advanced Anatomy and Physiology
Lesson 3.2 Motor units
Step 1 – Introduction
In this lesson we’ll learn about motor units:
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What they are composed of
The different types
How they are recruited
Step 2 – The motor unit
The stimulation of motor units initiates and controls the activity of muscles.
A motor unit is made up of a motor neuron and the muscle fibres it innervates or
communicates with.
There are two types of motor unit: slow twitch motor units and fast twitch motor
units.
All of the muscles fibres within a motor unit are of the same type.
Step 3 – The motor unit
The motor neuron in a slow twitch motor unit has a small cell body and typically
innervates 10–180 muscle fibres.
May 2015
© Future Fit Training, 2015
Page 1 of 3
Advanced Anatomy and Physiology
Lesson 3.2 Motor units
The motor neuron in a fast twitch motor unit has a large cell body, many axons
and innervates 300–800 muscle fibres. Fast twitch motor units reach peak tension
quicker than slow twitch motor units and generate more force.
Step 4 – The motor unit
Each motor neuron innervates from a few, to several thousand muscle fibres
depending on the function of the muscle. For example, muscles used for fine
control, such as movement of the eye, may have one motor neuron innervating
only fifteen fibres.
Muscles involved in more general movement, such as the gastrocnemius calf
muscle, can have one motor neuron innervating almost two thousand fibres.
Step 5 – Motor unit recruitment
In order for our muscles to produce force, motor units have to be activated, or
recruited. Let’s now have a look at the process involved in recruiting motor units.
Motor units work in an all-or-none fashion. That is, when a motor unit is recruited
into activity, the threshold for stimulation has to be met or exceeded. When this
happens all of the muscle fibres in that motor unit work to their maximum.
If the threshold stimulation does not occur, no fibres in the unit act.
Step 6 – Motor unit recruitment
It seems logical that activating more motor units will produce more force. However,
it is not quite as simple as that. When performing a task such as lifting weights,
motor units are recruited in a specific order. The order of this recruitment directly
relates to the size of the motor neuron.
The slow twitch motor units which have small neurons are recruited first. As the
force needed to lift the weight increases, fast twitch ‘a’ units are recruited, followed
by the fast twitch ‘b’ units at higher intensities.
This pattern of recruitment is the same in endurance events, such as running a
marathon. As the slow twitch units fatigue, fast twitch units are recruited.
Step 7 – Activity
Use the words from the list below to complete the statement.
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10–180
Fast twitch a and b units
Innervates
Muscle fibres
Slow twitch motor units
May 2015
© Future Fit Training, 2015
Page 2 of 3
Advanced Anatomy and Physiology
Lesson 3.2 Motor units
A motor unit is made up of a motor neuron and ………... The motor neuron ……….
the muscle fibres making your muscles move.
The motor neuron in a slow twitch motor unit innervates …………. muscle fibres.
The motor neuron in a fast twitch motor unit innervates 300–800 muscle fibres.
Motor units are recruited in a specific order. …………………….. are recruited first
and as more force is needed, ……………………. are recruited.
Feedback: Remember the numbers of muscle fibres are approximate.
Step 8 – Summary
Now that you have completed this lesson you will have a better understanding of
the following:
A motor unit consists of a neuron and the muscle fibres it innervates.
When the threshold of stimulation is reached, all the muscles fibres in a motor unit
are activated.
Motor units are recruited in the order 1) Slow twitch 2) Fast twitch type a 3) Fast
twitch type b.
May 2015
© Future Fit Training, 2015
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