part 7b4

Whole-Muscle Contraction

whole muscles are made up of many muscle fibers organized into motor units.

All the muscle fibers in a single motor unit are of the same fiber type.

fiber type designation to the motor unit and refer to

slow-oxidative motor units fast-oxidative motor unit fast glycolytic motor units.

Depending on the proportions of the fiber types present, muscles can differ considerably in their maximal contraction speed, strength, and fatigability

Eg. the muscles of the back and legs, muscles in the arms

Control of Muscle Tension The total tension a muscle can

develop depends upon two factors:

1. the amount of tension developed by each fiber.

2. the number of fibers contracting at any time.

By controlling these two factors, the nervous system controls whole-muscle tension, as well as the shortening velocity.

The number of fibers contracting at any time depends

1. the number of fibers in each motor unit (motor unit size),

1. the number of active motor units.

Motor unit size varies considerably from one muscle to another.

The force produced by a single fiber, as we have seen earlier, depends in part on the fiber diameter— the greater the diameter, the greater the force.

The process of increasing the number of motor units that are active in a muscle at any given time is called recruitment.

achieved by increasing the excitatory synaptic input to the motor neurons

The greater the number of active motor neurons, the more motor units recruited, and the greater the muscle tension.

Motor neuron size plays an important role in the recruitment of motor units.

Control of Shortening Velocity the velocity at which a single muscle

fiber shortens is determined by:

1. the load on the fiber2. whether the fiber is a fast fiber or a

slow fiber.

Translated to a whole muscle, these characteristics become:

1. the load on the whole muscle2. the types of motor units in the

muscle