A nerve cell: the functional unit of the nervous system. Structurally, the neuron is made up of a cell body or soma and one or more long processes: a single axon and dendrites (see illus.). The cell body contains the nucleus and usual cytoplasmic organelles with an exceptionally large amount of rough endoplasmic reticulum, called Nissl substance in the neuron. The longest cell process is the axon, which is capable of transmitting propagated nerve impulses. There may be none, one, or many dendrites composing part of a neuron. If there is no dendrite, it is a unipolar neuron; with one dendrite, it is a bipolar neuron; if there is more than one dendrite, it is a multipolar neuron. The dendrites are shorter and more branched than the axon. Dorsal-root spinal ganglia and most cranial nerve ganglia have unusual pseudounipolar neurons. Here a single process leaves the soma and then bifurcates, sending a long peripheral process to skin, muscle, or viscera and sending a central process into the spinal cord or brain. Both processes can conduct nerve impulses. These pseudounipolar neurons are always sensory. In most neurons only the axon propagates nerve impulses; the dendrites and somas are also irritable but do not propagate nerve impulses.