AUTONOMIC NERVOUS SYSTEM

also called vegetative, or visceral, nervous system, in vertebrate animals, the part of the nervous system that controls and regulates the internal organs without any conscious recognition or effort by the organism. It is composed of two antagonistic sets of nerves, the sympathetic and parasympathetic nervous systems. The former connects the internal organs to the brain by spinal nerves. When stimulated, these nerves prepare the organism for stress: the heart rate increases, and blood flow to the muscles is increased and that to the skin is decreased. The nerve fibres of the parasympathetic nervous system are the cranial nerves, especially the vagus, and the lumbar spinal nerves. When stimulated, they increase digestive secretions and reduce the heartbeat.