Neurons and Associated Cells

Abstract

Some 100–200 billion ([1–2] × 1011) neurons (nerve cells), as well as many more glial cells, are integrated into the structural and functional fabric that is the brain. They exhibit a wide diversity of form and sizes. The neuron is the basic unit of the nervous system and is composed of four structurally defined regions: a cell body (soma) that emits a single nerve process called an axon, which ends at presynaptic terminals, and a variable number of branching processes called dendrites (Figs. 2.1 and 2.2). Each axon, including its collateral branches, usually terminates as an arbor of fine fibers; each fiber ends as an enlargement called a bouton, which is part of a synaptic junction. At the other end of the neuron, there is a three-dimensional dendritic field, formed by the branching of the dendrites (Fig. 2.2).