The Autonomic Nervous System

The autonomic nervous system (ANS) is the involuntary, largely self-regulating division of the peripheral nervous system that controls the activity of the internal organs — smooth muscle in the walls of blood vessels and hollow organs, cardiac muscle of the heart, and the secretory glands — generally without requiring any conscious effort or awareness.

The ANS is organised into two anatomically and functionally distinct divisions that typically act in opposition (antagonistically) on the same target organs, allowing fine, balanced control. The sympathetic division has its motor neurons arising from the thoracic and lumbar segments of the spinal cord, and it is broadly associated with preparing the body to deal with stressful, exciting or emergency situations — often summarised as the 'fight or flight' response. When the sympathetic division is activated, it typically increases heart rate and the force of the heartbeat, dilates the pupils to let in more light, dilates the airways (bronchioles) to improve airflow, redirects blood towards skeletal muscle, and slows down non-urgent activities such as digestion; it also stimulates the adrenal medulla to release the hormone adrenaline into the bloodstream, which reinforces and prolongs these effects throughout the body. The parasympathetic division, in contrast, has its motor neurons arising mainly from certain cranial nerves (notably the vagus) and from the sacral segments of the spinal cord, and it is broadly associated with conserving and restoring the body's resources during calm conditions — often summarised as 'rest and digest'. Parasympathetic activity typically slows the heart rate, constricts the pupils, and stimulates digestive secretions and gut movement. …