2. Why do you think that your body uses both different motor axon firing frequen

Question

2. Why do you think that your body uses both different motor axon firing frequency as well as motor unit size to control the force production of a muscle? Consid would happen in each of the following scenarios: er what a. Action potential frequency did not change muscle fiber contraction and the only way to change force production was through motor units of different sizes. There was one motor unit for the whole muscle and frequency of motor axon firing was the only way to change force production. b.

Explanation / Answer

A motor neuron and the skeletal muscle fibers supplied by that motor neuron's axon terminals is called a motor unit.

Group of motor units work together to co-ordinate the contractions of a single muscle. All of the motor units within a muscle constitutes a motor pool.

All the muscle fibers in a motor unit are composed of same fiber type. When a motor unit is activated, all of its fibers contract. The force of a muscle contraction is controlled by the number of activated motor units.

Features of a motor unit:

1. In a particular muscle, say for example biceps brachii, the number of muscle fibers in a motor unit vary.

2. The number of motor units will also vary from muscle to muscle. For example, thigh muscles (quadriceps femoris) have thousands of muscle fibers in each unit. However, in extraocular muscles like superior oblique has ten only.

3. The force of contraction of muscles is regulated by the number of motor units activated. Thus, where the need of strong muscle contraction is required, there will be more motor units as in gluteus maximus. ( same as scenario (a)

4. Recruitment of motor neurons in an orderly manner is done by the Central Nervous System. The motor units are recruited from smallest to largest based on the size of load. For smaller loads, slow-twitch, low force and fatigue- resistent muscle fibers are activated earlier to fast twitch, high force and less fatigue resistent fibres.

This explains scenario (b).

5. Spatial recruitment : Activation of more motor units to produce a greater force. Larger motor units contract along with small motor units. Thus all muscle fibers in a single muscle are activated producing maximum contraction. In this case, action potential frequency will remain same.

6. Temporal recruitment : Deals with the frequency of activation of muscle fiber contractions. Consecutive stimulation of motor units cause muscle to twitch more frequently until the twitches fuse temporally.

Thus, it is the central nervous system which will recruit he motor units to control the force of muscle contraction.

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