Table 5.    Some neuroanatomical terminology

Cellular components

A neuron is a cell specialized for communication with other cells. It typically has a cell-body containing the nucleus and other common organelles, and one or more long extensions of the cytoplasm: the dendrites and the axon.

A neurite is any cytoplasmic process of a neuron: a dendrite or an axon.

An axon is a cytoplasmic extension of uniform diameter that conducts action potentials or (also called impulses), which are the most rapidly neuronal signals. A neuron has only one axon; it branches terminally and may also have collateral branches nearer to the cell-body.

A myelin sheath is a multi-layered membranous formation that intimately invests many types of axon, allowing action potentials to propagate much more rapidly than they can in unmyelinated axons. Myelin accounts for the pale color of white matter in the brain.

A nerve fiber is an axon together with its myelin sheath (if present) and associated neuroglial cells.

A dendrite is a cytoplasmic process that is widest at its union with the neuronal cell body and becomes narrower with increased length and repeated branching. Dendrites increase the surface area of the cell to accommodate numerous afferent synaptic terminals. The signals that move along a dendrite are graded variations in the electrical potential difference across the cell membrane; they propagate more slowly than the impulses that travel along an axon.

A synapse is a site of functional contact between cytoplasmic processes of neurons (e.g. a branch of an axon with a branch of a dendrite) or between a terminal branch of an axon and a cell that is not a neuron (e.g. a muscle fiber). A chemical neurotransmitter is released by the presynaptic neurite and acts on a receptor on the postsynaptic neurite. Changes in the receptor and tend to excite or inhibit the postsynaptic cell. At any time the activity or inactivity of the postsynaptic cell is determined by summation of its excitatory and inhibitory synaptic inputs.

Receptor. This word has two meanings. At the cellular level it is a molecule that responds to a chemical signal, such as a synaptic transmitter. On a larger scale, a receptor is a sense organ, which may be large (as is the eye) or small (as are terminal branches of axons in the skin).

A neuroglial or glial cell is a cell in the nervous system that is not a neuron. They outnumber the neurons 10:1 and have several well understood functions. Central glia include astrocytes, oligodendrocytes and microglia. The glial cells in a nerve, which intimately invest the axons and recognize their need for myelination, are called Schwann cells; these occur also in ganglia, along with satellite cells associated with the neuronal cell bodies. Neuroglial cells have a variety of important functions, but are outside the scope of these notes.

Anatomical terms

Gray matter is central nervous tissue that contains neuronal cell bodies. Dendrites, axons and synapses are also present in gray matter.

A nucleus is a circumscribed region of gray matter, often named from its location, appearance or function.

A ganglion is, strictly speaking, a collection of neuronal cell bodies in the peripheral nervous system: the sensory ganglia of the dorsal spinal roots and of some cranial nerves, and the sympathetic, parasympathetic and enteric ganglia that innervate smooth and cardiac muscle and secretory cells. In the central nervous system, certain large nuclei of the forebrain and upper brain stem are called the basal ganglia, and the cells in the retina whose axons enter the optic nerve are known as ganglion cells. (The retina and optic nerve are parts of the central nervous system.)

White matter is central nervous tissue that is largely composed of myelinated axons. These fibers are organized into fasciculi, capsules, peduncles and tracts.

A sulcus is a groove; a gyrus is a convexity, delimited by sulci.

A fasciculus is any bundle of nerve fibers, central or peripheral.

A capsule is a large, flattened region of white matter.

A peduncle (the word means "stalk") is a stout bundle of white matter that physically as well as functionally joins two major parts of the brain together.

A tract is a region of the central nervous system largely occupied by a population of axons that all have the same origin and destination, which often form the name. For example, the spinothalamic tract consists of the axons of neurons whose cell bodies are in the spinal cord, and the fibers of this tract end in the thalamus.

A root is a bundle of axons, with associated supporting cells and connective tissue, that traverses the subarachnoid space. A dorsal (sensory) and a ventral (motor) spinal root join, at each intervertebral foramen, to form a mixed spinal nerve. Cranial nerves also have roots, but these are usually named as nerves.

A nerve is a bundle of axons, with associated supporting cells and connective tissue, that is not a root and is outside the central-peripheral nervous system boundary. The optic nerve is badly named because it is part of the brain, as is the retina.

A ramus is a branch (Latin) of a nerve. The communicating rami (rami communicantes) carry axons between spinal nerves and sympathetic ganglia.

A pathway is a set of interconnected groups of neurons that serves a particular function. For example, the visual pathway, which conducts signals from the retina to the cerebral cortex, is composed of various nuclei and tracts.

Afferent means carrying towards, whereas efferent means carrying away from. These terms are relative. For example, the motor fibers in a nerve are efferents of the spinal cord and afferent to muscles.

Rostral means "higher" (literally, towards the beak), and caudal means "lower" (towards the tail). When describing levels of the central nervous system, these adjectives are preferred to the "superior" and "inferior" of ordinary anatomical terminology.