The peripheral nervous system (PNS) has two components: the somatic nervous system and the autonomic nervous system. The PNS consists of all of the nerves that lie outside the brain and spinal cord. Nerves are bundles of neuron fibers (axons) that are grouped together to carry information to and from the same structure. The somatic nervous system is made up of nerves that connect to voluntary skeletal muscles and to sensory receptors. It is composed of afferent nerves that carry information to the central nervous system (spinal cord) and efferent fibers that carry neural impulses away from the central nervous system. The autonomic nervous system also consists of two components: the sympathetic division and the parasympathetic division. This system mediates much of the physiological arousal (such as rapid heart beat, tremor, or sweat) experienced by a fearful person in an emergency situation. The sympathetic nervous system mobilizes the body to respond to emergencies. The parasympathetic nervous system generally helps to conserve the body's energy. It controls normal operations of the body such as digestion, blood pressure, and heart rate. It helps the body return to normal activity after an emergency.