In vertebrate anatomy, a ganglion is a tissue mass that contains the dendrites and cell bodies (or "somas") of nerve cells, in most case ones belonging to the peripheral nervous system. Within the central nervous system such a mass is often called a nucleus. An interconnected group of ganglia is called a plexus.
There are two major groups of ganglia: spinal ganglia and autonomic ganglia. The former contains the cell bodies of sensory nerves and the latter contains the cell bodies of autonomic nerves.
In the autonomic nervous system, fibers extending from the CNS to the ganglion are known as preganglionic fibers, while those from the ganglion to the effector organ are called postganglionic fibers.
See also: nervous system, neuron, ganglion cyst