Cells are the basic structural and functional units of life. Some life forms are more complex than others and require a vast array of specialized cell types in order to carry out their required physical functions.

In humans and many other animals, some cells contribute to what is called the nervous system, which is responsible for communication of the organism both internally and with the outside environment. The cells that make up the majority of this system are called neurons, or simply nerve cells.

The nervous system can be subdivided both anatomically and functionally. In both the central nervous system (CNS), which includes the nerves of the brain and spinal cord, and the peripheral nervous system (PNS), which includes all other neurons, clusters of cell bodies are observed.

These clusters of cell bodies (also known as somata; this is the Latin plural of soma, and the s_oma_ definition in English is "body") go by different names in their respective locations.

Cells are the smallest units of living things that, by themselves, display all of the properties of life. In some cases this is literally necessary, because some organisms, such as bacteria, consist of only a single cell.

Almost all of these organisms belong to the classification known as prokaryotes, which have cells that include a bare minimum of essential components: genetic material (i.e., DNA), a cell membrane to keep the whole thing together, cytoplasm (the gel-like matrix forming the majority of the cell's mass) and ribosomes, which manufacture proteins.

In contrast, the cells of the more complex organisms in the domain of eukaryotes (plants, animals, protists and fungi) are laden with specialized, membrane-bound components called organelles. These include mitochondria, which are the "powerhouses" of oxygen-based respiration and the chloroplasts of plants, which enable photosynthesis.

Although all eukaryotic cells have a number of elements in common, they vary widely in appearance and function depending on the tissue to which they contribute. This is perhaps more true of nerve cells than of any other cell in the human body, as these cells have unique shapes, interactions with their neighbors, protein properties and more.

A neuron, or nerve cell, is a perfect example of the "form meets function" maxim that is so wondrously evident in the biology world. Not only are neurons different from other types of cells in appearance and shape, but they vary considerably from each other, depending on where they exist in the nervous system.

A neuron consists of three main parts: the cell body, or soma; dendrites, which are branch-like extensions of the cytoplasm that receive input from other neurons; and an axon (usually just one), which transmits input to the end of the neuron, where substances called neurotransmitters are released and activate other neurons, usually at their dendrites.

Because of the way neurons are shaped and the way they are often grouped together in the body, the cell bodies of neurons are often found in distinct anatomical clusters, with the axons and dendrites relegated to the structural periphery. This aggregation of cell bodies allows for the high-level processing of nervous-system impulses both within the CNS and outside it in the PNS.

As noted, the human nervous system can be divided into the CNS and the PNS. This is an anatomical division, meaning that it accounts for where the neurons in each "system"are but says nothing about what they do. Nerve cells can, however, also be divided into motor neurons (or "motoneurons"), sensory neurons and interneurons.

Also called efferent ("outward-carrying") and afferent ("inward-carrying" neurons, these neurons are bundled in the PNS into nerves, which are parallel-running axons of neurons. A cross-section of a nerve would reveal a great many individual axons. The CNS has analogous structures called tracts.

Motor, or efferent, neurons can be divided into somatic (i.e., voluntary) neurons, which are under your conscious control, and autonomic neurons, which control involuntary functions such as heartbeat.

The autonomic nervous system is the branch of the PNS concerned with unconscious functions, and itself includes the sympathetic ("fight-or-flight") and parasympathetic ("relax-and-digest") divisions. The cell bodies of both types of autonomic neurons are found in clusters called ganglia.

Clusters of cell bodies found in the CNS are called nuclei. This is somewhat confusing, because the term nucleus as applied to individual cells refers to the part of the eukaryotic cell that contains DNA. Clusters of cell bodies found in the PNS, on the other hand, are called ganglia (singular: ganglion).

Aggregations of cell bodies can be noteworthy for their dense packing of somata, or they can be called a "cluster" even if they are somewhat more physically dispersed as long as they maintain a characteristic appearance. This grouping appearance sets nuclei apart from regions where cell organization assumes a different form.

For example, in the cerebral cortex of the brain, the cell bodies of neurons are arranged in layers instead of clusters.

You have probably heard of "gray matter" and "white matter" used in reference to the brain, perhaps in a slang sense. They are actually scientific terms, however!

Gray matter refers to the nerve cell bodies of CNS neurons and their dendrites and axons. White matter refers to material made almost entirely of axons, which look whitish on examination because they are heavy in a fatty substance called myelin.

Your brain contains hundreds of individually labeled clusters of cell bodies. These include the paired basal nuclei, which include the caudate nucleus, the putamen, and the globus pallidus. The thalamus is surrounded by a reticular nucleus, which is a nucleus consisting of the bodies of inhibitory neurons. The caudate and putamen together are called the striatum, which lies just next to the globus pallidus (actually a pair of structures and also called the lenticular nuclei) on each side of the brain.

Note: the basal nuclei are commonly called the basal ganglia, which is best avoided because of the general "CNS-nuclei, PNS-ganglia" scheme.

Clusters of cell bodies in the PNS are called ganglia, and include both sympathetic ganglia and parasympathetic ganglia. Other ganglia called dorsal root ganglia are found close to the spinal cord and carry sensory impulses from organs (for example, the skin or the inside of the gut) to integrating centers.

A typical sympathetic ganglion can have 20,000 to 30,000 individual cell bodies. These run in close proximity to the spinal cord, making their easy reach from the CNS a major factor in the rapid sympathetic response to environmental threats and the like.

When your heart starts to race and you unconsciously start to breathe harder in response to experiencing fear, this is the work of sympathetic nerves and ganglia.

Parasympathetic ganglia tend to be far smaller and also lie on or near the organs that they actually innervate (i.e., provide nervous impulses to).

An example is the ciliary ganglion, which constricts the pupil of the eye. The neurons that constrict the pupil, in the oculomotor nerve, run near sympathetic fibers from a different ganglion that dilate the pupil, thus demonstrating the complementary nature of the autonomic nervous system.