Collections of Neurons

gray matter A generic term for a collection of neuronal cell bodies in the CNS. When a
freshly dissected brain is cut open, neurons appear gray.

cortex A collection of neurons that forms a thin sheet, usually at the brain's surface.
"Cortex" is Latin for "bark." Example: cerebral cortex, the sheet of neurons found just
under the surface of the cerebrum.

nucleus A clearly distinguishable mass of neurons, usually deep in the brain {not to be
confused with the nucleus of a cell). "Nucleus" is from the Latin word for "nut."
Example: the lateral geniculate nucleus, a cell group in the brain stem that relays
information from the eye to the cerebral cortex.

substantia A group of related neurons deep within the brain, but usually with less
distinct borders than those of nuclei. Example: the substantia nigra (from the Latin for
"black substance"), a brain stem cell group involved in the control of voluntary
movement.

locus (plural: loci) A small, well-defined group of cells. Example: the locus coeruleus
(Latin for "blue spot"), a brain stem cell group involved in the control of wakefulness
and behavioral arousal.

ganglion (plural: ganglia) A collection of neurons in the PNS. "Ganglion" is from the
Greek for "knot." Example: the dorsal root ganglia, which contain the cell bodies of
sensory axons entering the spinal cord via the dorsal roots. Only one cell group in the
CNS goes by this name, the basal ganglia, which are structures lying deep within the
cerebrum that control movement.
 

Collections of Axons

nerve A bundle of axons in the PNS. Only one collection of CNS axons is called a nerve:
the optic nerve.

white matter A generic term for a collection of CNS axons. When a freshly dissected
brain is cut open, axons appear white.

tract A collection of CNS axons having a common site of origin and a common destination.
Example: the corticospinal tract, which originates in the cerebral cortex and ends in
the spinal cord.

bundle A collection of axons that run together but that do not necessarily have the same
origin and destination. Example: the medial forebrain bundle, which connects cells
scattered within the cerebrum and brain stem.

capsule A collection of axons that connect the cerebrum with the brain stem. Example:
the internal capsule, which connects the brain stem with the cerebral cortex.

commissure Any collection of axons that connect one side of the brain with the other
side.

lemniscus A tract that meanders through the brain like a ribbon. Example: the medial
lemniscus, which brings touch information from the spinal cord through the brain stem.