The central nervous system is composed of the brain and spinal cord. The spinal cord contains sensory neurons, motor neurons, and interneurons that enable spinal reflexes, allowing the body to quickly remove itself from danger. The peripheral nervous system includes the autonomic nervous system, which regulates internal organs, and the somatic nervous system, which connects the brain to skeletal muscles. Neurons transmit electrochemical signals via neurotransmitters released at synapses between neurons.