Nervous tissue is the fourth basic tissue type of the body and is organized into two basic systems: (1) the Central Nervous System (CNS) and (2) the Peripheral Nervous System (PNS). The peripheral system responds to stimuli and sends impulses to the central system (brain and spinal cord). These impulses are interpreted by the CNS and then other impulses initiated in the CNS travel back through the PNS to effector organs to produce the proper response.
The basic cell of the nervous system is called a neuron. Structurally, a neuron consists of: (1) the cell body, containing the nucleus and synthetic organelles; (2) the axon, a long cytoplasmic process associated with the cell body used to communicate with target organs; and (3) the dendrites, shorter cytoplasmic processes off the cell body used to communicate between neurons.
Central Nervous System
The spinal cord is composed of grey and white matter (Spinal Cord 1). The grey matter is composed of nerve cell bodies and in a cross-sectional cut appears as a darker stained "H"-like central area. The white matter, composed entirely of axonal projections, surrounds the grey matter and is lighter staining (Spinal Cord 2).
Like the spinal cord, the brain is composed of areas of grey and white matter. In some areas an addition outer gray layer is present. We will study two regions of the brain, in which this additional grey matter has a distinct morphology.
Starting at the surface of the cortex and moving inward, you can identify the first three layers of cell bodies (Cerebral Cortex 1 ): (1) superficial molecular layer, containing only a few small cell bodies; (2) outer granular layer, containing small round cell bodies; (3) pyramidal cell layer, containing cell bodies triangular in shape (Cerebral Cortex 2). In total, the cerebral cortex consists of six layers with the inner most three represented by an inner granular layer, internal pyramidal layer, and polymorphic cell layer, the innermost layer containing cell bodies of many shapes.
This portion of the brain's gray matter is arranged into three layers (Cerebellar Cortex 1): (1) the superficial molecular layer containing mostly unmyelinated axons and few cell bodies; (2) a deeper layer of large flask-shaped cells called Purkinje cells (Cerebellar Cortex 2); and (3) an inner granular layer containing many small cell bodies. The purkinje cells send long dendritic projections into the molecular layer (Cerebellar Cortex 3).
The Meninges - Connective Coverings of the Brain
The brain is enclosed in three layers of connective tissue. The outer most (dura mater) consists of dense connective tissue. Underlying the dura is the arachnoid layer, often described as a "roof with pillars" made of dense connective tissue. Spaces within the arachnoid are filled with cerebrospinal fluid. The inner most layer, the pia mater, consists of loose connective tissue on the surface of the brain and lining channels which penetrate the brain carrying the vascular system (Pia Mater).
Regular connective tissue types surround the components of the central nervous system, but they are not found within the tissue. The glial cells, derived from neuroectoderm, serve roles of connective tissue within the CNS tissue. Three types are found: (1) microglia, which represent macrophages of the CNS; (2) oligodendrocytes, which myelinate the axons within the CNS; and (3) astrocytes, which are a fibroblast-like supportive cell (Astrocytes). These cells are characterized as fibrous astrocytes with unbranched processes and protoplasmic astrocytes with branched processes.
The choroid plexus produces the cerebrospinal fluid. It consists of a small tuft of capillaries surrounded by epithelium which "hangs" in the brain ventricles (Choroid Plexus 1). The capillaries are covered by a layer of simple cuboidal epithelium, the ependymal cells, which also surround the ventricular space (Choroid Plexus 2).
Peripheral Nervous System
Axonal projections travel in bundles through the body. These bundles are encapsulated in fibroconnective tissue in a manner similar to that seen in muscle tissue. Entire nerve bundles are surrounded by the epineurium (Nerve Bundle 1). Branching from the epineurium and dividing the nerve bundle into fascicles is the perineurium (Nerve Bundle 2). Finally each individual axon is surropunded by the endoneurium (Nerve Bundle 3).
Observed in cross section, in a routine H & E preparation, a clear area will be seen around each axon. This is the location of the myelin sheath, which due to its high lipid content is dissolved in this type of preparation (Nerve Bundle 4). Only in specimens fixed in osmium tetroxide (OsO4) will the myelin be preserved and, in this case, stained black (Nerve Bundle 5). The myelin sheath is also visible in teased preparations. Here the gaps in the sheath, the Nodes of Ranvier, are easily observed (Teased Nerve). Many of the nuclei seen within a nerve bundle are fibroblast nuclei. Their presence is needed to synthesize and maintain the connective tissue wrappings. Other nuclei represent the Schwann Cells, a PNS glial cell responsible for myelination of nerve processes (Nerve Bundle 4 ). These cell types can also be distinguished in nerves cut in longitudental section (Nerve Bundle 5). Fibroblasts contain darker stained nuclei when compared to Schwaan cells (Nerve Bundle 6).
Usually more than one nerve is needed to reach from the CNS to or from the peripheral effector organs. These chains of nerve fibers interconnect in structures called ganglions (Ganglion 1). Within the ganglion very large cells are visible. These are the cell bodies of the neurons (Ganglion 2). Within the cell body you should be able to see the Nissl substance, an accumulation of the basophilic stain of ribosomes (Ganglion 3). The cell bodies are supported by small surrounding cells called capsule cells or amphicytes. Between this arrangement, you will find a number of bundles of axonal projections. Ganglions of the sympathetic system (Ganglion 4) are easily distinguished from others by the presence of lipofuscin pigment within the cytoplasm of the cell bodies (Ganglion 5).