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.
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
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 ...