Nerve supply of skeletal muscles

The nerve supplying a muscle is called motor nerve. In fact it is a mixed nerve and consists of the following types of fibers.

Motor fibers

These fibers make up to 60% of a nerve to skeletal muscle. They comprise of:

• Large myelinated alpha efferents which supply extrafusal muscle fibers.
• Smaller myelinated gamma efferents which supply intrafusal fibers of the muscle spindles which refine and control muscle contraction.
• The fine non-myelinated autonomic efferents which supply smooth muscle fibers of the blood vessels.

Sensory fibers

They form up to 40% of a nerve to the muscle and comprise of:

• Myelinated fibers distributed to muscle spindles for proprioception, also to tendons.

The axon of the motor nerve branches many times after entering a target muscle. Each of these branches makes its way to different set of muscle fibers (motor unit, see below). Near the midpoint of the muscle fiber, the each branch splits again, forming a small cluster of terminal branches, which form the neuromuscular junction.

Neuromuscular Junction

Neuromuscular Junction is a synapse between the motor neuron and the muscle fiber. It is an important structure through which the impulse is transferred from the motor nerve to the muscle fiber. This phenomenon eventually results in muscle contraction. Visit the page: Neuromuscular Junction for detailed study.

Muscle Spindles

Muscle spindles are spindle-shaped sensory end organs of the skeletal muscle. Each spindle contains 6 to 13 intrafusal muscle fibers which are of two types, the larger nuclear bag fibers, and the smaller nuclear chain fibers. The spindle is innervated by both the sensory and motor nerves. The sensory endings are of two types, the primary sensory endings (annulospiral endings) around the central nuclear region of the intrafusal fibers, and the secondary sensory endings {flower spray endings) beyond the nuclear region on either side of these fibers.

The motor nerve supply of the spindle is derived from gamma motor neurons of the spinal cord. Muscles spindles act as stretch receptors. They record and help regulate the degree and rate of contraction of the extrafusal fibers by influencing the alpha neurons.

Motor Point

It is the site where the motor nerve enters the muscle. It may be one or more than one. Electrical stimulation at the motor point is more effective.

Motor unit (myone)

It is defined as a single alpha motor neuron together with the muscle fibers supplied by it. The size of motor unit depends upon the precision of muscle control. Small motor units (5-10 muscle fibers) are found in muscles of fine movements (extra¬ocular muscles). Large motor units (100-2000 muscle fibers) are found in muscles of gross movements (proximal limb muscles).

Composite/hybrid muscle

Muscle supplied by two different motor nerves with different root values is called a composite or hybrid muscle. Examples of composite muscles are adductor magnus, flexor digitorum profundus and pectoralis major.