Question

In your own words, briefly describe how and why the size of motor units vary in muscles which require strength vs. muscles which require precision.

Answer 1

Motor unit:

A motor unit consists of a single motor neuron its axonal branches and all the muscle fibers supplied by them.

Size of a motor unit:

The size of the motor unit can be large or small depending upon the number of muscle fibers innervated by it.

In the intrinsic muscles of the hand, one motor neuron innervates less than 10 muscle fibers so, stimulation of a motor unit produces a small rise in tension.

The back muscles have hundreds or even thousands of muscle fibers per motor unit, where activation of a single motor neuron causes a large increase in tension.

Size principle:

The large diameter, fast conducting motor neurons innervates the muscle fibers of fast motor units, and the small diameter, slow conducting motor neurons innovates muscle fibers of slow motor units. This is called the size principle.

Recruitment of motor units:

The process of activating more number of motor number of units is called the recruitment of motor units.

In hand muscles, gradual activation of the unit produces a stepwise increase in muscle tension because with activation of each new unit a small amount of tension is added. As the tension goes up in a graded manner, it can precisely be regulated. Therefore, the muscles carrying out skilled humans have smaller units for which the finer regulation of muscle tension is of primary importance.

In back muscles, with the recruitment of additional motor units, each time of the amount of tension is added. Therefore, the back muscles are useful in the maintenance of posture, as there is a need to develop greater tension to rise to the downward pull of gravity.

Innervation ratio of muscle unit:

The number of muscle fibers supplied by a single neuron is known as the innervation ratio of a motor unit. The ratio is low in muscle concerned with precise movement.

The extraocular and hand muscle has a ratio of 3-6 (total 3000 motor units) whereas gastrocnemius and back muscle (concern with posture) has an innervation ratio of 2000 (total 600 units).