Section 6: Nervous Tissue

Ronald A. Bergman, Ph.D., Adel K. Afifi, M.D., Paul M. Heidger, Jr., Ph.D.
Peer Review Status: Externally Peer Reviewed

Plates

The nervous system is developed entirely from ectoderm. It manifests optimally the two properties of protoplasm, irritability and conductivity, and is one of the most highly differentiated tissues in the body.

Neural tissue is made up of cells and their processes. Cells of the nervous system fall into two general categories: (1) nerve cells or neurons, and (2) supporting and satellite cells. In addition, neural tissue contains blood vessels and protective coverings (meninges).

The study of neural tissue is facilitated by several stains, none of which alone is capable of revealing all the desired details of structure. Because of the affinity of nerve cells and their processes for silver solutions (argyrophilia), silver impregnation methods are frequently used to demonstrate them.

The Golgi silver methods selectively impregnate relatively few cells, but they accomplish this most completely. These methods are good for outlining the external shape of nerve cells and their processes (especially dendrites) but do not reveal details of internal cell structure such as neurofibrils and Nissl bodies.

The Cajal and Bielschowsky silver methods are later developments of the silver impregnation methods. They are used to demonstrate axons, neurofibrils, and nerve endings, including synapses. Originally used on blocks of tissues, they have been modified for use on mounted sections. The most useful modification is that of Bodian in which activated protargol (silver proteinate) is used (Plates 98 and 113).

The Nissl substance (cytoplasmic ribonucleoprotein) of nerve cells is revealed using basic aniline (cationic) dyes, also called Nissl stains, such as cresyl violet, gallocyanin, and toluidine blue (Plates 1 and 90), which bind to nucleic acid and demonstrate nuclei, nucleoli, and cytoplasmic Nissl substance of neurons.

Demonstration of the myelin sheath is accomplished by a variety of methods, including osmium tetroxide, Pal-Weigert, Weil, and Marchi techniques. The Marchi method is used to demonstrate degenerating myelin (Plates 325 and 327), whereas the Pal-Weigert and Weil methods stain normal myelin (Plates 317 and 319).

A neuron (nerve cell) consists of the cell body (perikaryon) and all its processes. Neurons ranging in diameter from 4 to 135 µm are generally larger than other cells in the body. The shape of neurons varies with the number and arrangement of their processes. In general, three types of neurons are recognized: (1) Unipolar or pseudounipolar neurons have spherical cell bodies with single processes that later bifurcate. Such cells are found in the dorsal root ganglia (Plates 103 and 106). (2) Bipolar neurons are spindle-shaped, with one process at each end. Such neurons are found in certain peripheral ganglia, such as in the acoustic and olfactory systems (Plates 105 and 298). (3) Multipolar neurons have polygonal cell bodies and many processes. Such neurons are encountered in the autonomic ganglia (Plate 107) and central nervous system (Plates 10 and 89).

Nuclei of neurons are usually large, rounded, and centrally located and are characterized by well- defined, strongly RNA-positive nucleoli (Plate 90). Bi- and trinucleated neurons are found rarely in some autonomic ganglia (Plate 109).

The cytoplasm of neurons is rich in Nissl bodies, which are particularly coarse in the somatic motor neurons (Plates 1 and 90). Electron microscopy has shown the Nissl substance to be composed of ribonucleoprotein bound to membrane (granular encloplasmic reticulum). Nissl material extends into the proximal portions of dendrites but is absent from axons and axon hillocks. Nissl substance undergoes definite changes in response to axonal injury. In addition to the Nissl substance, neuronal cytoplasm is rich in mitochondria and contains a prominent perinuclear Golgi apparatus (Plate 7). The Golgi area of the neuron is the site where carbohydrates are linked to proteins in the synthesis of glycoproteins. Neurofibrils are seen in the cytoplasm of neurons and their processes (Plate 10). They are made up of subunits (neurofilaments), which are 7.5 to 10 nm in thickness and thus beyond the limit of resolution of the light microscope. Neurofilaments are made of structural proteins similar to those of the intermediate filaments of other types of cells. The argyrophilic neurofibrils are unique to nerve cells. In addition to neurofilaments, neuronal cytoplasm contains microtubules similar in external diameter (about 25 nm) to those observed in other types of cells. They are involved in the rapid transport of protein molecules through axons and dendrites. In addition to the aforementioned cell organelles, neuronal cytoplasm may contain lipid droplets, glycogen, pigment granules (Plate 13), and secretory products (hormones; Plate 116). Pigment granules increase in number with age. Some types of neurons, such as Purkinje cells of the cerebellum, do not contain pigment granules.

Neurons in the central nervous system have a variety of shapes. They may be stellate in the anterior horn of the spinal cord (Plate 89) or flask-shaped, as in the Purkinje cells of the cerebellum (Plate 93). Neurons in the peripheral ganglia are surrounded by satellite cells forming a capsule around the neuron. Those located in sensory ganglia are unipolar (pseudounipolar), whereas those in autonomic ganglia are multipolar.

The cell body of a neuron is its trophic center. Separation of a process from the cell body results in the death of that process. Neuronal processes are extensions of the cell body and serve to initiate or conduct nerve impulses. Dendrites generally receive and then conduct impulses toward the cell body, whereas axons conduct them away from the cell body. In unipolar neurons, in which the single process bifurcates into a peripheral and a central branch, both branches are structurally axon-like. In bipolar neurons, the effector and receptor portions of the neuron are found at the extreme ends of the two processes, and the entire intermediate portion is conductive. Multipolar neurons have several dendrites arising from the cell body (Plate 91) and one axon that arises from the cell body or from the base of a dendrite (Plate 89). Dendrites branch repeatedly, and their surfaces are studded with spines or gemmules (Plate 88), thus expanding the receptive cell surface. It is estimated that some neurons receive as many as 100,000 axon terminals on their dendritic expansion. A striking example of the vast dendritic expansion is seen in Golgi preparations of the Purkinje cell of the cerebellum (Plate 99).

Axons are more slender than dendrites (Plate 87) and are more uniform in diameter. The region of origin of the axon from a nerve cell is termed the axon hillock and is devoid of Nissl substance. It is the most excitable part of the neuron and the site at which the nerve impulse is initiated. Distally, each axon breaks up into simple or complex arborizations, the telodendria, which end on other neurons, glands, or muscle (Plates 91 and 117, 118 and 119). Axons invariably acquire sheaths along their course. The axon and its sheath are referred to as a nerve fiber. Nerve fibers that run together in a bundle and share a common origin and destination in the central nervous system constitute a tract (Plates 319 and 327). A nerve fiber bundle in the peripheral nervous system constitutes a nerve (Plate 115). Nerve fibers may be myelinated or unmyelinated. Myelin sheaths are elaborated and maintained by oligodendroglia in the central nervous system, and by Schwann cells in the peripheral nervous system. Unmyelinated and myelinated peripheral nerve fibers are in intimate contact with Schwann cell cytoplasm and nucleus (the neurolemmal sheath or sheath of Schwann), and the plasma membrane is covered by a prominent polysaccharide surface coat. The relationship of such nerve fibers to oligodendroglia in the central nervous system is not quite so intimate. The myelin sheath around an axon is interrupted at regular intervals known as the nodes of Ranvier (Plate 103). The nodes are the site of voltage-gated sodium channels and ionic movement of impulse conduction. The flow of an electrical impulse along the nerve fiber thus skips from one node of Ranvier to the next. Myelin sheaths serve to insulate axons between nodes and thus speed up conduction of the nerve impulse between nodes of Ranvier (saltatory conduction). Myelin is made up of a lipid-protein complex. Some of the lipid is usually lost during tissue preparation, leaving behind a resistant proteo-lipid, neurokeratin (Plate 114) unless special methods are used to preserve it (Plate 115).

In addition to the myelin sheath and the sheath of Schwann, peripheral nerve fibers are surrounded by connective tissue, the endoneurium. The endoneurium is continuous with the more abundant connective tissue perineurium, which envelops bundles of nerve fibers. The nerve trunk is ensheathed in turn by the epineurium.

Nerve fibers, both axons and myelin sheaths, vary in size. The size of the nerve fiber (axon and its myelin coat) bears a direct relationship to its rate of impulse conduction. Large and heavily myelinated fibers conduct faster than small, unmyelinated ones.

Axons that branch at their termination to establish synapses on other neurons (dendrites, perikarya, or other axons) or muscle (Plates 91, 117, and 118) come in close proximity to, but not in contact with, the post-synaptic components of the synapse. Synaptic junctions vary in configuration, from the bouton-type (end bulb) of synapse (Plate 91), to the side-to-side contact seen in the climbing fiber system of the cerebellum (Plate 95), to the basket-type seen in the cerebellum (Plates 95 and 96).

Supporting cells of the nervous system include the capsule or satellite cells of peripheral ganglia, ependyma, neuroglia, and Schwann cells.

Satellite cells surround neurons of peripheral ganglia, forming a capsule one cell layer thick (Plate 102). They are derived from neural crest elements and are continuous with a neurolemmal (Schwann) sheath.

Ependymal cells line the cavities of the brain and spinal cord (Plate 131). A specialized form of ependymal cell is seen in some areas of the nervous system (subcommissural organ).

Neuroglia are the "supporting elements" of the central nervous system. Three cell types are found: (1) astrocytes with their two varieties, protoplasmic and fibrous; (2) oligodendroglia; and (3) microglia.

The astrocytes, as their name implies, are star-shaped cells with relatively lightly staining nuclei and processes closely applied to capillary blood vessels (perivascular end-feet or footplates). Other end- feet are applied to the pia mater. Two varieties are distinguished on the basis of the morphology of their processes. The protoplasmic variety, found mostly in gray matter, have plump and abundant cell processes that branch repeatedly (Plate 128). The fibrous variety, found mostly in white matter, have more slender but well-defined and fewer cell processes. They are longer and straighter than are those of the protoplasmic variety (Plate 129). Both varieties of astrocyte play a role in metabolite transfer within the central nervous system. The fibrous astrocytes, in addition, play a role in healing and scar formation in the nervous system. The cytoplasm of astrocytes contains glial filaments made up of glial fibrillary acidic protein (GFAP). Special histochemical stains for GFAP help identify astrocytes in tissue sections.

Oligodendroglia are smaller than astrocytes and have a denser nucleus and cytoplasm. As their name indicates, they have few delicate processes (Plate 130). These glial cells are seen adjacent to myelinated nerve fibers in the white matter or forming satellite cells to the neurons in the gray matter. Oligodendroglia elaborate central nervous system myelin.

Microglia are the smallest of the neuroglia, and, unlike the ectodermally derived macroglia (astrocytes and oligodendroglia), they are formed from the mesoderm. They are dense cells with deeply staining elongated nuclei (Plate 128) and are frequently seen in gray matter in close proximity to neurons. The perikaryon of a microglial cell is irregular in shape, and, if elongated, the few processes emanate from both of its poles. Microglia are believed to be the scavenger cells of the central nervous system.

The central nervous system is covered by three protective coats (meninges): (1) The outermost layer is the dura mater, made up of a vascular dense fibrous connective tissue. (2) The middle layer is the arachnoid, a non-vascular delicate connective tissue coat. (3) The innermost layer is the pia mater, a delicate vascular layer adherent to the surface of the brain and spinal cord. Between the pia and arachnoid membranes is the subarachnoid space, in which the cerebrospinal fluid circulates. The small arteries and capillaries of the pia mater in certain regions of the ventricular system form tufts, which invaginate into the ventricular cavity (choroid plexus). The invaginated tufts are lined by cuboidal epithelium. The chorold plexus elaborates cerebrospinal fluid.

Peripherally located receptors constantly feed information into the central nervous system. These receptors may convey general sensation such as touch, pain, thermal sense, pressure, position, and movement or specialized sensations such as vision, audition, taste, and smell. The latter variety are discussed in Section 16. Illustrations of most of the former are seen in this section. Such receptors are found as (1) free nerve endings in epithelia or connective tissue, or as (2) encapsulated endings, in which the neural component of the receptor is surrounded by a connective tissue sheath of varying thickness. The sheath is continuous with the endoneurium and perineurium of the nerve fiber. Examples of such encapsulated endings are Meissner's (Plates 123 and 138), Krause's (Plate 126), genital (Plate 127), and Pacinian corpuscles (Plate 124), the neuromuscular spindle (Plates 120 and 121), and the Golgi tendon organ (Plate 122).

Stimulation of any of these receptors results in the initiation of a nerve impulse that travels to the central nervous system. The translation of this impulse into a conscious sensation is a function of the brain.

Although doubt has been cast upon the functional specificity of the different varieties of receptors, it is still generally believed that free nerve endings respond optimally to sensations of pain, and possibly touch and thermal sense; Meissner's corpuscles respond optimally to touch sensations, whereas Pacinian corpuscles respond optimally to pressure sensibility. Krause's end-bulb and Ruffini receptors are believed to be cold and warmth receptors, respectively. The receptors in muscle and tendon are concerned with movement and posture. They respond to stretch and tension resulting from muscular contraction or passive stretch of muscles.

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