Skeletal Muscle-Nerve Supply

 Describe the nerve supply of skeletal muscle.

Skeletal muscle is supplied by somatic nerves. The nerve to a skeletal muscle is a mixed nerve containing 60% motor and 40% sensory fibers.

Motor supply

Extrafusal fibers of the muscle which produce movements are innervated by α motor neurons of the anterior horn of spinal cord or motor nuclei of cranial nerves( in case of head & neck muscles). Intrafusal fibers of the muscle spindle are innervated by the gamma motor  neurons.

Sensory supply

Sensory nerve fibers innervate muscle spindles and tendons, ligaments, joint capsules to carry proprioceptive impulses to central nervous system.

 Describe Muscle Spindle.

Muscle spindles are sensory end organs present within the skeletal muscles which  provide proprioceptive information to sensory nerves that carry this information to central nervous system to regulate the contaction of extrafusal fibers.

• Muscle spindle consist of a bundle of  6-13 small muscle fibers called intrafusal fibers, surrounded by connective tissue capsule (main skeletal muscle fibers are called extrafusal fibers).
• Infrafusal muscle fibers are of two types:
  • Nuclear chain fibers: In these fibers, nuclei are arranged in a single row in the middle unexpanded part.
  • Nuclear bag fibers: In these fibers, nuclei are aggregated in the middle expanded part.
• The sensory innervation of the muscle spindle is by two types of nerve endings:
  • Annulospiral nerve endings: they wind spirally around the central nuclear regions of intrafusal fibers.
  • Flower spray endings: they terminate at the ends of infrafusal fibers.
• The motor innervation of infrafusal muscle fibers is by γ (gamma) motor neurons of spinal cord.

Describe strech reflex.

• When the extrafusal muscle fibers are streched, the intrafusal fibers of muscle spindle are elongated, thereby stimulating nerve ending around them.
• The sensory nerves carry the information from the muscle spindle to the spinal cord.
• They synapse with alpha motor neurons which innervate extrafusal fibers.
• This results in contraction of extrafusal fibers.

Define motor unit.

Motor Unit:  α motor neuron along with all the muscle fibers innervated by it constitutes a ‘motor unit’.

• Trunk muscles have few large motor units and can perform only gross movements, whereas extraocular muscles of eye and muscles of thumb have smaller motor units (a single motor neuron innervates 5-10 muscle fibers) and are capable of delicate and precise movements.

 Define Motor point.

Motor point: It is the point of entrance of the nerve into the muscle. Usually a nerve enters the deep surface of the muscle. Electrical stimulation of the muscle is most effective at the motor point.

 Define Muscle tone.

Muscle tone: Skeletal muscle fibers even in resting condition remain in a state of partial contraction. This is referred to as muscle tone.

 Define Myotome.

 Myotome: Muscle fibers supplied by a spinal nerve constitute a myotome.

Applied Aspects