Neurons are specialized cells capable of tranmsitting an electrical signal, and form the building blocks of the nervous system. Neurons are further specialized into sensory neurons, motor neurons and interneurons (interconnections between neurons forming a pathway). Multiple neurons are grouped into pathways, which are known as nerves in the peripheral nervous system and by various names in the central nervous system (e.g. tract, fasciculus, lemniscus, peduncle). As with all cells, neurons have a cell body (soma) with cytoplasm and a nucleus. The neuron cell body has tree-like extensions, dendrites, that along with the cell body, receive signals from sensory receptors or other neurons. An axon transmits the signal from the cell body to its axon terminals. Most axons are insulated with a thin layer of cells (myelin sheath) to conserve and speed electrical transmission. Myelinated axons appear as white matter, while neuron cell bodies appear as gray matter. From the axon terminals, the signal is passed to other nerves via neurotransmitters across an inter-neuron space known as the synapse.