1. Skeletal Muscle Contraction

2. Learning Objectives
To understand the structure of a motor unit and its role in muscle contractions.
To understand how nervous stimulation controls muscle contractions and the role of neurotransmitters.
To know the ‘all or none law’ and understand its meaning.

3. Structure of a motor unit
A group of fibres and its neurone is called a motor unit. A motor unit can contain anything between 10 & 1,000 muscle fibres.
A motor unit is made up of myofibrils which are individually wrapped.
A muscle is made up of many motor units. In large muscle groups such as the leg or the arm one motor unit services around 1,000 muscle fibres.
Groups of motor units work together to produce the contraction of a single muscle from an electrical impulse reaching a single motor unit.

4. A Neurone

5. Nervous stimulation of a motor unit
The transmission of neural messages along a neurone is an electrochemical process.
When a neurone is not conducting an impulse it has a resting potential.
An action potential occurs when sufficient number of sodium ions are allowed to diffuse into the neurone. This depolarizes the axon.
This allows the electrical impulse to travel along the neurone.

6. Neurotransmitters
Chemical messengers that transmit signals across a chemical synapse from the neurone to the muscle fibres. This is known as the neuromuscular junction.
Neurotransmitters are released into the synaptic cleft where they are received by receptors on the target cells.

7. ‘All or None’ Law
States that there must be a minimum level of depolarization for an action potential to occur. Without reaching this level no impulse will be propagated.
It also applies to the contraction of fibres within a motor unit. When a motor unit activates, all of the fibres within the unit contract and at full force, there is no strong or weak contraction. The strength of the resultant whole muscular contraction depends upon the number of motor units recruited.

9. Home Study
Represent pictorially the process of skeletal muscle contraction from the brain to the muscle fibres. Must be done on paper to hand in – not typed! You can make something if you wish, draw a poster or present it however you want.