The nervous system is composed of two main divisions: the central nervous system (CNS) containing the brain and spinal cord, and the peripheral nervous system (PNS) containing nerves. Sensory neurons carry messages from receptors to the CNS for integration, then motor neurons carry output commands to effectors. Motor actions are either somatic, occurring in skeletal muscle and being voluntary, or autonomic, occurring involuntarily in smooth and cardiac muscle. The cells of the nervous system are neurons, which transmit nerve impulses, and neuroglia which are supporting cells like astrocytes and oligodendrocytes. Neurons have a cell body containing the nucleus, dendrites receiving signals, and an axon transmitting signals