Chapter 4: Histology of the Peripheral Nerves and Light Microscopy

INTRODUCTION

Microscopic anatomy that emphasizes structure-function relations is important to the clinical practice of regional anesthesia. This chapter provides basis for understanding of the structure, classification, and organization of the peripheral nerves and insight into how the characteristics of the peripheral nerves (Figure 4–1) relate to the clinical practice of regional anesthesia.

ORGANIZATION OF THE PERIPHERAL NERVOUS SYSTEM

The nervous system enables the body to respond to continuous changes in its external and internal environments. It controls and integrates the functional activities of the organs and organ systems.

Nervous system cells consist of neurons and neuroglia. Neurons transmit nerve impulses to and from the central nervous system (CNS), thereby integrating motor and sensory functions. Neuroglial cells support and protect the neurons. In the CNS, myelin is produced by oligodendrocytes and in the peripheral nervous system (PNS) by the Schwann cells. Although both Schwann cells and oligodendrocytes are in charge of axon myelination, they have distinct morphological and molecular properties and different embryonic origins, the neural crest and the neural tube, respectively.¹

The PNS consists of peripheral nerves (craniospinal, somatic, autonomic) with their associated ganglia and connective tissue investments. All lie peripheral to the pial covering of the CNS.^2,3,4,5,6

Peripheral nerves contain fascicles of nerve fibers consisting of axons. In peripheral nerve fibers, axons are ensheathed by Schwann cells, which may or may not form myelin around the axons, depending on their diameter. Nerve fibers are grouped into fascicles of variable numbers. The size, number, and pattern of fascicles vary in different nerves and at different levels along their paths. Generally, their number increases and their size decreases at some distance proximal to the branching point.

NEURONS

A neuron is the structural and functional unit of the nervous system. It includes the cell body, dendrites, and axon.

The cell body (perykarion) is the dilated region of the neuron that contains a large, euchromatic nucleus with a prominent nucleolus and surrounding perinuclear cytoplasm (Figure 4–2). The perinuclear cytoplasm contains abundant rough-surfaced endoplasmic reticulum and free ribosomes. On light microscopy, the rough endoplasmic reticulum with rosettes of free ribosomes appears as small bodies, called Nissl bodies. The perinuclear cytoplasm contains numerous mitochondria, a large perinuclear Golgi apparatus, liposomes, microtubules, neurofilaments, transport vesicles, and inclusions. The presence of the euchromatic nucleus, large nucleolus, prominent Golgi apparatus, and Nissl bodies indicates the high level of anabolic activity needed to maintain these large cells.