Neurons, excitable nervous system cells, generally consist of three main parts: the cell body, the dendrites, and the axon. The cell body, also known as the perikaryon, is the metabolic center of a neuron. It comprises the nucleus surrounded by cytoplasm containing other organelles, including mitochondria. Its cytoskeleton includes neurofibrils, bundles of intermediate filaments that support neurons, and neurotubules, along which materials are transported to the axon. The cell body also has Nissl bodies, clusters of ribosomes and rough endoplasmic reticulum that give the characteristic gray color to gray matter. Groups of cell bodies in the central nervous system are called nuclei, but in the peripheral nervous system, they are called ganglia. Dendrites are slender cellular processes that emerge from the cell body. They are highly branched and are often covered with short dendritic spines that form synapses with neighboring neurons. Dendrites contain receptors that bind to chemical messengers released by the surrounding cells and relay this information to the cell body through short-distance impulses called graded potentials.