<P> The peripheral nervous system (PNS) is a collective term for the nervous system structures that do not lie within the CNS . The large majority of the axon bundles called nerves are considered to belong to the PNS, even when the cell bodies of the neurons to which they belong reside within the brain or spinal cord . The PNS is divided into somatic and visceral parts . The somatic part consists of the nerves that innervate the skin, joints, and muscles . The cell bodies of somatic sensory neurons lie in dorsal root ganglia of the spinal cord . The visceral part, also known as the autonomic nervous system, contains neurons that innervate the internal organs, blood vessels, and glands . The autonomic nervous system itself consists of two parts: the sympathetic nervous system and the parasympathetic nervous system . Some authors also include sensory neurons whose cell bodies lie in the periphery (for senses such as hearing) as part of the PNS; others, however, omit them . </P> <P> The vertebrate nervous system can also be divided into areas called grey matter ("gray matter" in American spelling) and white matter . Grey matter (which is only grey in preserved tissue, and is better described as pink or light brown in living tissue) contains a high proportion of cell bodies of neurons . White matter is composed mainly of myelinated axons, and takes its color from the myelin . White matter includes all of the nerves, and much of the interior of the brain and spinal cord . Grey matter is found in clusters of neurons in the brain and spinal cord, and in cortical layers that line their surfaces . There is an anatomical convention that a cluster of neurons in the brain or spinal cord is called a nucleus, whereas a cluster of neurons in the periphery is called a ganglion . There are, however, a few exceptions to this rule, notably including the part of the forebrain called the basal ganglia . </P> <P> Sponges have no cells connected to each other by synaptic junctions, that is, no neurons, and therefore no nervous system . They do, however, have homologs of many genes that play key roles in synaptic function . Recent studies have shown that sponge cells express a group of proteins that cluster together to form a structure resembling a postsynaptic density (the signal - receiving part of a synapse). However, the function of this structure is currently unclear . Although sponge cells do not show synaptic transmission, they do communicate with each other via calcium waves and other impulses, which mediate some simple actions such as whole - body contraction . </P> <P> Jellyfish, comb jellies, and related animals have diffuse nerve nets rather than a central nervous system . In most jellyfish the nerve net is spread more or less evenly across the body; in comb jellies it is concentrated near the mouth . The nerve nets consist of sensory neurons, which pick up chemical, tactile, and visual signals; motor neurons, which can activate contractions of the body wall; and intermediate neurons, which detect patterns of activity in the sensory neurons and, in response, send signals to groups of motor neurons . In some cases groups of intermediate neurons are clustered into discrete ganglia . </P>

The central nervous system (cns) is divided into two main divisions. what are they