Motor nerves and units

Motor nerves (also known as motor neurones) are made up of three parts:

  • The cell body
  • The dendrite
  • The axon

motor nerves and unitsClick to enlarge

The motor neurone, and the fibres it stimulates, is called a motor unit, which is the functional unit of skeletal muscle.

The number of fibres innervated by a single motor nerve varies depending on the precision of movement required. For example, in the eye, which requires a great deal of control and precision in order to focus, possesses between one and five fibres per motor nerve, while the rectus femoris muscle of the quadriceps group in the thigh requires greater power, and therefore possesses up to 2,000 fibres per motor neurone. All muscle fibres within a particular motor unit are usually of the same type, either fast twitch or slow twitch. Motor units are recruited depending on the activity being undertaken, and the recruitment is based on twitch response time or speed of contraction needed.

Muscle relaxation is a passive process. The cross bridges uncouple, causing the sarcomere to lengthen, and return to its pre-contracted length.

motor unitClick to enlarge

Each fibre within a motor unit contracts according to the all or none law. This principle states that when a motor unit receives a stimulus of sufficient intensity to bring forth a response, all the muscle fibres within the unit will contract at the same time, and to the maximum possible extent. If the stimulus is not of sufficient intensity, the muscle fibres will not respond, and contraction will not take place. The degree to which a muscle contracts is dependent on several factors, including the number of motor units recruited by the brain. This will determine the force that can be generated within the muscle.