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

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.

• Long hierarchical circuits (also called projection neurons, principal neurons, and relay cells). These neurons usually have a single, long axon that interconnects distant cells. Examples of these circuits include the major sensory and motor pathways. Fast-acting neurotransmitters, such as acetylcholine and the amino acid neurotransmitters, are often used by these neurons.
• Local circuit neurons (also called intrinsic neurons and interneurons). These small neurons usually perform a type of information processing where the actions of afferent neurons are modified (e.g., propagated, enhanced, or inhibited). Examples of these circuits include interneurons which modulate spinal reflects. Fast-acting neurotransmitters, such as the amino acid neurotransmitters glycine and GABA, are frequently used by these neurons.
• Single-source, divergent neurons. These neurons have axons which diverge and send projections to many brain areas, thus affecting a large number of other cells. Examples of these neurons include functional brain pathways involved in the regulation of motivation and emotion. Slower-acting neurotransmitters and neuromodulators, such as the monoamines and most of the neuropeptides, are usually used by these neurons.

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

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.

• Mitochondria

involved in cell metabolism providing "energy" for cell various functions
• Golgi Apparatus

involved in protein refinement
• Lysosomes

consist of an aggregate of enzymes that "digest" organic compounds and can convert proteins to amino acids and glycogen to glucose
• Endoplasmic Reticulum

rough: contains ribosomes that are involved in protein synthesis
smooth: involved in axoplasmic transport

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).

• Astrocytes

metabolic functions
regulate the levels of certain substances
phagocytosis
structural support
• Microglia

phagocytosis (especially in response to cell injury)
• Oligodendrocytes (CNS only)

form myelin sheath
structural support
possible metabolic functions
• Schawnn Cells (PNS only)

form myelin sheath
structural support