Chapter Summary

I. SPINAL CORD

A. Gray Matter

The gray matter, centrally located and more or less in the shape of an H, has two dorsal horns and two ventral horns. Ventral horns display numerous multipolar (motor) cell bodies. The perikaryon possesses a large, clear nucleus and a dense nucleolus. Its cytoplasm is filled with clumps of basophilic Nissl substance (rough endoplasmic reticulum) that extends into dendrites but not into the axon. The origin of the axon is indicated by the axon hillock of the soma. Numerous small nuclei abound in the gray matter; they belong to the various neuroglia. The nerve fibers and neuroglial processes in the gray matter are referred to as the neuropil. The right and left halves of the gray matter are connected to each other by the gray commissure, which houses the central canal lined by simple cuboidal ependymal cells.

B. White Matter

The white matter of the spinal cord is peripherally located and consists of ascending and descending fibers. These fibers are mostly myelinated (by oligodendroglia), accounting for the coloration in live tissue. Nuclei noted in white matter belong to the various neuroglia.

C. Meninges

The meninges of the spinal cord form three layers. The most intimate layer is the pia mater, surrounded by the arachnoid, which, in turn, is invested by the thick, collagenous dura mater.

(or cerebellar islands). These mainly represent areas of synapses on granule cell dendrites.

B. Medullary Substance

The medullary substance (internal white mass) is the region of white matter deep to the granular layer of the cerebellum, composed mostly of myelinated fibers and associated neuroglial cells.

III. CEREBRUM

A. Cortex

The cerebral cortex is composed of gray matter, mostly subdivided into six layers, with each housing neurons whose morphology is characteristic of that particular layer. The major neuronal types are pyramidal cells, stellate (granule) cells, horizontal cells, and inverted

(Martinotti) cells. The following description refers to the neocortex and is presented from superficial to deep order. The first layer is just deep to the pia mater, whereas the sixth level is the deepest cortical layer, bordering the central white matter of the cerebrum.

1. Molecular Layer

Composed of horizontal cells and cell processes.

2. External Granular Layer

Consists mostly of granule (stellate) cells, tightly packed.

3. External Pyramidal Layer

Large pyramidal cells and granule (stellate) cells.

4. Internal Granular Layer

Closely packed granule (stellate) cells, most of which are small, although some are larger.

II. CEREBELLUM

A. Cortex

The cortex of the cerebellum consists of an outer molecular layer and an inner granular layer with a single layer of Purkinje cells interposed between them. The perikaryons of the molecular layer are small and relatively few in number. Most of the fibers are unmyelinated. Purkinje cells are easily distinguished by their location, large size, and extensive dendritic arborization. The granular layer displays crowded arrays of nuclei belonging to granule cells and intervening clear regions known as glomeruli

5. Internal Pyramidal Layer

Medium and large pyramidal cells constitute this layer.

6. Multiform Layer

Consisting of various cell shapes, many of which are fusiform. This layer also houses Martinotti cells.

B. White Matter

Deep to the cerebral cortex is the subcortical white matter, composed mostly of myelinated fibers and associated neuroglial cells.

172 N E R V O U S T I S S U E

IV. CHOROID PLEXUS

The choroid plexus consists of tufts of small vascular elements (derived from the pia-arachnoid) that are covered by modified ependymal cells (simple cuboidal in shape). These structures, located in the ventricles of the brain, are responsible for the formation of the cerebrospinal fluid.

V. DORSAL ROOT GANGLION (DRG)

A. Neurons

The somata of these cells are pseudounipolar, with large nuclei and nucleoli. Surrounding each soma are capsule cells, recognized by their small, round nuclei. Fibroblasts (satellite cells) are also evident. Synapses do not occur in the DRG.

B. Fibers

Fibers are mostly myelinated and travel in bundles through the DRG.

C. Connective Tissue

The DRG is surrounded by collagenous connective tissue, whose septa penetrate the substance of the ganglion.

VI. PERIPHERAL NERVE

A. Longitudinal Section

The parallel fibers stain a pale pink with hematoxylin and eosin, although Schwann cells and occasional fibroblast nuclei are clearly evident. The most characteristic feature is the apparent wavy, zigzag course of the nerve fibers. At low magnification, the perineurium is clearly distinguishable, whereas at high magnification the nodes of Ranvier may be recognizable.

B. Transverse Section

The most characteristic feature of transverse sections of nerve fibers is the numerous, small, irregular circles with a centrally located dot. Thin spokes appear to traverse the empty-look- ing space between the dot and the circumference of the circle. These represent the neurolemma, the extracted myelin (myelin proteins), and the central axon. Occasionally, cres- cent-shaped nuclei hug the myelin; these belong to Schwann cells. The endoneurium may show evidence of nuclei of fibroblasts also. At lower magnification, the perineuria of several fascicles of nerve fibers are clearly distinguishable. When stained with OsO4, the myelin sheath stands out as dark, round structures with lightly staining centers.

CHAPTER OUTLINE

Graphics

Graphic 8-1 Artery and Vein p. 182

Graphic 8-2 Capillary Types p. 183

Tables

Plates