Cells in the CNS can be divided into two general types—neurons and neuroglia. Neurons constitute the basic unit for neural communication and are the primary target of psychotropic agents. Neuroglia are considered primarily supportive (e.g., provide regulatory and metabolic functions and structural support) but also have an increasingly recognized role in modulating neural communication. Some estimates suggest that brain volume is somewhat evenly divided between neurons and neuroglia, although neuroglia out number neurons in some brain areas by 10:1.


Neurons are responsible for most communication and information processing in the nervous system. Neurons can be small, less than 1 mm, or very large, up to 1 m in length. There are numerous ways to classify neurons, but one approach useful for psychopharmacologists classifies neurons into three general groups according to their relative size and functional characteristics.


The principal parts of an idealized neuron are shown in Figure 2.1.

Spinal Cord Motorneuron
Figure 2.1: A spinal cord motorneuron. From Feldman and Quenzer (1984).

Not shown in the figure are a number of organelles that make necessary contributions to cell function. Some of these structures are found in all cells, while others are unique to neurons.

Neuroglial Cells

Unlike neurons, neuroglia continue to divide and multiple throughout the life of the organism. Traditionally glial cells were thought to provide mainly a supportive function (e.g., structural support), but there is now evidence that they also participate in metabolic processes and provide important regulatory functions for neuronal activity (e.g., absorb excess K+). Because of their ability to multiply, the role of glial cells in response to injury has long been recognized. This same ability to multiply may also give glial cells a role in long-term neuroadaptive effects (e.g., pharmacological tolerance).