• Steven Deschner, MD

        INTRODUCTION

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.

        MORPHOLOGIC ORGANIZATION OF PERIPHERAL NERVOUS SYSTEM

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 píate of the neural tube and by axon outgrowth from neurons in the basal píate. 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.

Figure 4-1. Anatomy ofthe peripheral nerve. Shown arethecell body, axon with its myelin sheath, nodes of Ranvier, synaptic termináis, and the direction ofthe 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

Clinical Pearls

        Similarly to the organization of the CNS tracts, both myelinated and unmyelinated fibers occur peripherally too.^3–4 A myelinated nerve fiber is surrounded by the plasma membrane of Schwann cell (lemmocyte). The plasma membrane spirals around the axon arranging the membranes in concentric layers. The structure is called a myelin sheath. In the myelinated nerve fiber a single axon is enclosed by a series of Schwann cells arranged along its length. The region where two adjacent Schwann cells abut and the myelin is interrupted is referred to as a node of Ranvier (Figure 4-2). Individual axons, generally less than 1.0 pm in diameter, that indent the surface of the Schwann cell and become embedded in separate troughs are known as unmyelinated nerve fibers. Each Schwann cell can sheathe many axons in this way. However, a single Schwann cell does not sheathe the entire length of a group of axons. Instead the sheath is formed by a chain of Schwann cells, the axons being passed on from cell to cell. Thus all axons in the peripheral nervous system are invaginated into Schwann cell surfaces, but myelin sheaths only form around larger axons, which represent only a small portion of peripheral nerve fibers.^1–3

        Nerve trunks and their principal branches (Figures 4-3 and 4-4) consist of parallel bundles of nerve fibers (nerve fascicles, fasciculi). The size, number, and pattern of fasciculi vary in different nerves and at different levels along their paths. The axon-associated surface glycoprotein neurofascin is implicated in axonal growth and fasciculation as revealed by antibody perturbation experiments. We will examine the basic organization of the peripheral nerves using the sural nerve as an example. The sural nerve includes 9-21 fascicles and comprises 4600-9600 myelinated nerve fibers and 19,000-45,000 unmyelinated axons^5,6 depending, on age of the nerve.

        Peripheral nerves have three separate connective tissue sheathes.³ On the outside of each peripheral nerve there is a dense irregular connective tissue sheath, the epineurium. Perineurium surrounds each fascicle of nerve fibers. Individual nerve fibers are embedded in a loose, delicate connective tissue (endoneurium), filling the space bounded by the perineurium. The connective tissue sheaths support nerve fibers and their associated blood and lymphatic vessels (see Figure 4-3).

        Both central and peripheral fibers present challenges for light microscopists because of the small size of unmyelinated fibers (Figure 4-5) and a disruption of sheaths of myelinated fibers that commonly occurs due to treatment with lipid solvents prior to sectioning. Various methods have been devised to overcome these problems. It is electron microscopy, however, that has added to our knowledge perhaps more than any method.

        NERVE FIBERS

As axons course through body tissues, they are associated with Schwann cells. The axon with its associated Schwann cells forms a nerve fiber. A nerve fiber is the basic structural and functional unit of peripheral nerves (Figure 4-6).

Figure 4-2. Nodes of Ranvier.

Figure 4-3. Connective tissue sheaths and fascicles of the human ulnar nerve as seen on light microscopy (Hematoxylin-eosin × 400).

Figure 4-4. Myelinated and unmyelinated nerve fibers bundled in a fascicle by perineurial membrane. Light microscopic image of rat sciatic nerve in cross section (Hematoxylin-eosin × 400).

Figure 4-5. Organization of the nerve shown in longitudinal section (Bodian × 400).

Figure 4-6. Schematic diagram of an axon with its accompanying connective tissue sheaths.

        Fresh myelinated fibers appear as homogeneous, glisteningtubes. In stained preparations (Figure 4-7) appearance of various constituents of the nerve fiber differs according to the technique applied (Table 4-1). Motor nerve fibers of the skeletal muscles are thick and heavily myelinated; those of visceral smooth muscle are thin, lightly myelinated, or without myelin. Tactile fibers are medium-sized and moderately myelinated, whereas pain and taste fibers are thinner, with less myelin or none at all. In comparison, olfactory nerve filaments are always unmyelinated.^1,3

Figure 4-7. Exaggerated Schwann cells shown in cross section (x400).

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