GANGLION

plural Ganglia, dense aggregate of nerve-cell bodies present in most animals more advanced than cnidarians. In flatworms (e.g., Planaria) there are two lateral neuronal cords that carry impulses to and from a pair of ganglia at the head of the animal. In more advanced groups, such as earthworms and arthropods, there are pairs of ganglia at intervals along the body that largely control the actions of each body segment, as well as a larger, dorsal pair in the head. This latter pair of ganglia, considered the brain in these more advanced invertebrates, acts mostly to inhibit responses rather than to direct them. In vertebrates the term ganglion denotes a cluster of neural bodies outside the central nervous system; a spinal ganglion is such a cluster of nerve bodies positioned along the spinal cord at the root of a spinal nerve. The dorsal root ganglia contain the cell bodies of afferent nerve fibres (those carrying impulses toward the central nervous system); efferent motor neurons are present in the ventral root ganglia.