How to remember the nervous system: What is a motor unit

This blog will teach you how to remember the nervous system structure and explain what is a Motor unit, ready for your level 2 and level 3 anatomy exam

You’ll discover:

  • The ACE acronym to remember the nervous system structure
  • What is a motor unit?
  • 5-minute Video Tutorial explaining motor units
  • What are motor neurones and muscle fibres?
  • Three Nervous System Mock Questions to test your knowledge

ACE Acronym: How to remember the nervous system structure

The nervous system’s primary function is to send and receive messages all around our body so we can react to the world around us.

In order to do this, there are specific components of the nervous system, each with a different role.

To remember the order we use the ACE acronym

A = Afferent Neuron
C = Central Nervous System
E = Efferent Neuron

Lets have a look at each of these:

Afferent or Sensory Neurons

The sensory neuron carries information from our 5 senses up to our central nervous system.

These are continually collecting sensory information and carrying it to our CNS for interpretation

The Central Nervous System

When we need to move a muscle, our Central Nervous System (brain and spinal cord) is responsible for deciding what muscles need to be activated and when.

Whether you want to do a squat or just lift a pencil, your central nervous system is “deciding” what muscles need to contract in order for this to happen.

Once the Central Nervous System has “decided” what muscle needs to contract, the information needs to get to the muscle. This is the job of the motor neuron.

Efferent or Motor Neurons

The motor neuron carries messages from our Central nervous system to our muscles/ organs.

A motor neuron carries information from the central nervous system to the muscles. 

The information that is carried, determines whether the muscle will contract or not. 

This is a simple “ON or OFF” option sent down the motor neuron.

Each motor neuron is attached to a group of muscle fibres and it sends an electrical impulse that stimulates all of these fibres to contract at once.

This is none as the All or None law – either ALL the muscle fibres (within one motor unit) contracts or none of them …

like a light switch, i’s either on or off.

Explanation: What is a motor unit?

A motor unit is made up of a single neuron [nerve cell]  that innervates a group of skeletal muscles.

The neurons receive signals from the brain and stimulate all the muscle fibres in that particular motor unit.

There are two components that make up a motor unit and you need to know both of these ready for your anatomy exam.

  1. motor neuron 
  2. The muscle fibres that are innervated by the motor neuron.

This 5-minute video shows you what a motor unit looks like, and explains what this really means for you and your clients:

5 minute video tutorial: What Is A Motor Unit?

What is a Motor Unit?

What is a Muscle Fibre?

The second component of the motor unit is the muscle fibre. As you have just learned these fibres are innervated by the motor neurone.

The primary function of a muscle fibre is muscle contraction. The fibres contract, making our muscles shorter in length, which in turn moves our joints. 

It is important to know that when the motor neurone signals for the muscle fibres to contract, it is not the entire muscle that contracts, just the muscle fibres that are innervated by that specific motor neurone.

If you don’t already know the difference between a muscle fibre, myofilament and a muscle belly, then watch this video to learn

Test your knowledge with 3 Nervous System Unit Mock Questions

Look at the 3 Nervous System Unit Mock questions below and jot down your answer on a scrap paper or as a note in your phone.

Then scroll down to reveal the answers.

[NOTE: The answers are below the 3rd questions]

1. What two components create a motor unit?
A. Central Nervous System and Motor neuron
B. Actin and Myosin
C. Motor Neuron and Muscle Fibres
D. Sensory Neuron and Muscle Fibres

2. Which part of the nervous system carries information to the CNS for interpretation?
A. Efferent Neuron
B. Afferent Neuron
C. Motor Neuron
D. Motor unit

3. Why would more motor units be recruited?
A. Less force required/ lighter weight
B. More force required/ heavier weight
C. To relax the muscle
D. Fatigue in the muscle

What’s the CORRECT answer?

Answers to the mock questions are :

Question 1= C, Question 2 = B, Question 3 = B

If you want more mock questions like this, then you can download more Free Mock Questions: DOWNLOAD NOW

Mock Questions

In summary

  • “A motor unit is a Motor Neuron and all of the Muscle Fibres it innervates”
  • To lift a heavier force, you recruit more Motor Units, so that more muscle fibres can be innervated
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