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Knowledge of histology is vital to understanding cell function and the composition of the tissue layers and planes, as well as being relevant to clinical practice of regional anesthesia. The primary objective of this chapter is to provide a basic understanding of the structure, classification, and organization of peripheral nerves.

The peripheral nervous system consists of sensory afferent (centripetal) nerve fibers connecting receptors to the central nervous system (CNS) and motor efferent (centrifugal) nerve fibers connecting the CNS to muscle or glands. The system includes somatic and autonomic nerves, as well as their associated Schwann cells and connective tissue sheaths. All lie peripheral to the pial covering of the CNS, through which the central and peripheral nerve fibers are continuous.1

The connection between the CNS and the peripheral structures derived from the somites and neural crest is formed by axon growth from the dorsal root ganglia into the alar plate of the neural tube and by axon outgrowth from neurons in the basal plate. These distally growing motor axons join the peripherally growing sensory axons of the dorsal root ganglia to form nerves innervating the somite at the same level, Figure 4–1. Nerves formed in this fashion at spinal level are the spinal nerves; those formed at the posterior fossa and supratentorial levels are the cranial nerves. Both types are composed of sensory and motor axons. Motor axons from the CNS innervate the muscles and autonomic ganglia, whereas sensory axons innervate receptors in the skin, muscle, bone, and viscerae. As the embryo develops and cells forming bone, muscles, skin, and internal organs migrate to their adult locations, these neural processes will follow suit in order to establish the peripheral nerves' innervation pattern. Fibers innervating tissue derived from somites (muscles and skin) are described as somatic; fibers innervating endodermal or other mesodermal derivatives (internal organs) are called visceral.

Fig. 4-1

Anatomy of the peripheral nerve. Shown are the cell body, axon with its myelin sheath, nodes of Ranvier, synaptic terminals, and the direction of the transmission.

Axons are guided to their targets by apical growth cones. The growth cone, which is thought to move by means of filopodia, is believed to guide the axon to its destination by sensing molecular markers that designate the correct route. This activity of the growth cone is called path finding. Once the growth cone reaches its target, it halts and forms a synapse. Numerous mechanisms have been proposed to explain the ability of neurons to establish correct connections with each other and with end organs.2

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Clinical Pearls
Motor Axons
  • The motor axons can arise from either the somatic or autonomic system. Somatic motor neurons innervate skeletal muscle.
  • Alpha motor neurons innervate the extrafusal fibers. Smaller γ motor neurons innervate the intrafusal fibers. The perikarya of these neurons are located in specific brainstem ...

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