A motor unit, motor neuron, and muscle fibres are tricky topics, and I can see why
It’s tough to understand from a few pages in a course manual, I’m sure you’ll agree?
Let’s make this module super easy and jump into today’s web class:
What we’re really talking about here is, module 8 of the Level 2 Anatomy and Physiology exam: The Nervous System. You can expect 4 to 6 exam questions around this module in your final exam so it’s key you understand the “Ins & Outs” to ensure you pass.
First Up, PRESS PLAY
and take notes as you go through the 3-minute video making this complex topic, super simple!
You need to know:
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 level 2 anatomy exam.
- A motor neuron
- The muscle fibres that are innervated by the motor neuron.
What is a Motor Neuron?
[NOTE: There are two different types of neurons in the body. This blog specifically looks at one of them; the motor neuron. The second neuron which is briefly mentioned in the video is a sensory neuron. This neuron carries information from our 5 senses up to our 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. So, 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 neurone.
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.
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 everything about the muscle structure WATCH NOW.
Let’s give this a real-life example!
To recap, the motor unit is the motor neuron and all the muscle fibres that it innervates.
As we said a moment ago, muscle fibres can only switch on or off, there are no half measures. For that reason, muscle contraction follows the “all or none law“. For example, if you wanted to lift a heavier weight you must recruit MORE motor units and likewise, if you wanted to lift a lighter weight you would recruit FEWER motor units.
If you imagine or remember back to a time when you had to lift up a heavy box but you thought it was quite light. You think it’s a light box, you try to pick it up and you can’t lift it, it doesn’t move!
The reason for this is that you’ve only recruited enough motor units to be able to lift what you deemed as a light box. When you try again, you can actually lift it because on the second attempt you’ve recruited more motor units.
The motor unit causes a muscle contraction. If you need more contraction, then you need more Motor UNITS
Now Test Yourself:
3x Nervous System Level 2 Anatomy and Physiology Mock Questions below:
[NOTE: The answers are below the 3rd questions]
QUESTION 1: What is a Motor unit?
A. The nerve cell that sends information to the muscles
B. The nerve cell that sends information to the central nervous system
C. A motor neurone and all of the muscle fibres it innervates
D. A sensory neurone and all of the muscle fibres that it innervates
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QUESTION 2: What must occur in order to lift more weight during a resistance exercise?
A. Recruit more motor units
B. Recruit less motor units
C. Send a more powerful signal down via the motor neurones
D. A stronger contraction of the muscle fibres
QUESTIONS 3: What is the role of a motor neurone?
A. Analyse information from the senses
B. Decide which muscles should contract
C. Deliver signals from the Sensory Receptors to the CNS
D. Deliver signals from the CNS to the Muscle Fibres
Answers to the mock questions are :
Question 1= C, Question 2 = A, Question 3 = D
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