Biological Chemistry - Berezov, T. T., Korovkin, B. F. 1998

Nervous Tissue
Neuron Structure

Nervous Tissue shares common features inherent to Cells of any tissue, as well as specific characteristics determined by The Nature of the Functions performed by The Nervous system in the intact Organism. These features manifest themselves both in the Chemical Composition and in the metabolic profile of nervous tissue.

Nervous tissue consists of three cellular elements: Neurons (Nerve Cells); neuroglia, a system of cells directly surrounding nerve cells in the Brain AND SPINAL cord; and mesenchymal elements, which include microglia—glial macrophages (Ortega cells).

The bulk of the brain is represented by the first Two Types of cellular elements. Neurons are concentrated in the Gray matter (accounting for 60–65% of the brain mass), whereas the White matter OF the CNS and peripheral nerves consist primarily of neuroglial elements and their derivative, myelin.

A neuron has a Cell body, numerous branching short processes known as dendrites, and a single long process called an axon, the length of which can reach several tens of centimeters (Fig. 19.1).

The volume of Cytoplasm contained within the processes of a nerve cell can several times exceed its amount in The Cell body. The neuron body is surrounded by a Plasma Membrane, or Plasmalemma (Fig. 19.2). In close association with the plasmalemma* in the neuron body and the proximal segments of dendrites lies the so-called subsurface membranous Structure. These are cisternae located parallel to The surface of the plasmalemma and separated from it by a very narrow light zone. It is hypothesized that these cisternae play an important role in neuronal METABOLISM. The main ULTRASTRUCTURE OF THE neuronal cytoplasm is The Endoplasmic reticulum—a system of membrane-bound vesicles, tubules, and flattened sacs, or cisternae. The membranes of the endoplasmic reticulum are linked in a specific way with the plasmalemma and the nuclear envelope of the neuron.

Granules localized on the membranes of the endoplasmic reticulum, as well as those located freely in the cytoplasm, are Ribosomes.

A characteristic structural feature of the nerve cell is the basophilic substance (Nissl substance), which consists of Ribonucleic Acids and Proteins. A network of fine filaments—neurofibrils—is also revealed in the cytoplasm, forming a dense interconnected meshwork. Neurofibrils represent the structural manifestation of the orderly linear orientation of protein molecules.

* Upon neuronal excitation, the permeability of The Plasma Membrane changes.

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Fig. 19.1. Structure of a neuron (diagram based on Schmitt).

1 - dendrites; 2 - neuron body; 3 - axon; 4 - myelin sheath; 5 - nodes of Ranvier; 6 - terminals.

An important component of the neuronal cytoplasm is the lamellar complex (Golgi apparatus), which is the primary site of cellular lipid components. One of the distinguishing features of Mitochondria isolated from nerve cells is that they contain fewer Enzymes involved in the Oxidation of Fatty acids and Amino Acids compared to mitochondria from other tissues.

Lysosomes are consistently found in the CNS and perform the same functions as lysosomes in other Organs and tissues.

The size of the neuronal Nucleus ranges from 3 to 18 µm, reaching up to ¹/₄ of the cell body size in large neurons.