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.
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
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
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.
involved in cell metabolism providing "energy" for cell various functions
involved in protein refinement
consist of an aggregate of enzymes that "digest" organic compounds
and can convert proteins to amino acids and glycogen to glucose
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
regulate the levels of certain substances
phagocytosis (especially in response to cell injury)
Oligodendrocytes (CNS only)
form myelin sheath
possible metabolic functions
Schawnn Cells (PNS only)
form myelin sheath