Peripheral Nervous System: Overview

The peripheral somatic sensory neurons are bipolar neurons. The sensory neuron cell bodies reside in the dorsal root ganglion, that is, situated posterolateral to the spinal cord, usually at or near the intervertebral foramen. Extending out from the cell body are two axon extensions; one extending distally to the skin or other organs and one extending proximally through the dorsal root into the posterior horn of the spinal cord. The relationship between the sensory neuron cell bodies within the dorsal root ganglion and the sensory axons is important; when the axon branch (either the distal or proximal branch) is separated from the cell body, the separated axonal component undergoes axonal (wallerian) degeneration. As a result, an injury at the proximal nerve root (“pre-” or proximal to the ganglion) will lead to sensory deficits even though the distal sensory axon remains intact. The clinical implications of this occur during electrophysiologic testing with nerve conduction studies in which a distal sensory nerve is studied. In the presence of sensory loss in an extremity, a preserved or normal sensory nerve conduction study response suggests that the sensory loss is caused by a disorder involving the sensory pathway, either in the proximal root (proximal to the ganglion) or the central somatosensory conduction pathways in the spinal cord or brain and not the peripheral nerves per se.

Several structurally different types of sensory neurons exist within a somatic nerve. The largest sensory neurons are type IAα and Ib fibers, which are large, myelinated fibers whose function is to transmit proprioceptive signals from muscle spindle endings, Golgi tendon organs, and proprioceptive sensory receptors. Type II (Aβ) fibers are smaller myelinated fibers that transmit signals from the muscle spindles and skin receptors controlling touch and pressure. Type III (Aδ) fibers are small myelinated fibers that play a role in sharp pain, temperature, touch and pressure, and visceral sensations. In addition, small unmyelinated fibers, type IV (C), contribute to sensation of somatic and visceral pain.

Somatic motor neurons are unipolar neurons. The motor neuron cell bodies reside in the anterior gray matter (anterior horns) of the spinal cord. A single axon extends distally from each anterior horn cell, through the ventral root, and joins the peripheral nerve that innervates individual muscles. The motor conduction pathway continues along the nerve to the nerve terminal. At this site, an action potential traveling through the motor axon to the nerve terminal leads to a cascade of reactions at the neuromuscular junction, where the nerve terminal is adjacent to the muscle fiber. Through these reactions, an action potential is generated along the muscle fibers, resulting in muscle fiber contraction. The integrity of the motor unit (an anterior horn cell and its axon, nerve terminal, neuromuscular junction, and innervated muscle fibers) can be tested during electrophysiologic testing with motor nerve conduction studies.

Motor neurons are typically large in diameter and myelinated. The major type of motor neuron, the alpha motor neuron, innervates the extrafusal striated muscle fibers and, primarily, is responsible for muscle fiber contraction. Smaller, myelinated gamma motor neurons innervate the intrafusal fibers of muscle spindles.

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