Autonomic nervous system: anatomy, physiology, and relevance in anaesthesia and critical care medicine

Abstract

### Key points The autonomic nervous system (ANS) is the part of the nervous system that regulates involuntary functions.1 Examples are the heartbeat, the digestive functions of the intestines, control of respiration, and secretion by glands. The organization of the ANS is on the basis of the reflex arc and it has an afferent limb, efferent limb, and a central integrating system.1 The afferent limb transmits information from the periphery to the central nervous system (CNS). The receptors are present in the abdominal and thoracic viscera.1 The transmissions from these receptors are conducted along neural pathways into the spinal cord via the dorsal root ganglion or to the brain stem via cranial nerves. Baroreceptors and chemoreceptors are examples of the afferent pathway. These are present in the aortic arch and carotid sinus. The sensory impulses from these receptors are transmitted via glossopharyngeal and vagus nerves to the brain stem. The efferent limb is made up of preganglionic and post-ganglionic fibres and an autonomic ganglion. The efferent limb is further subdivided based on its anatomic and physiological differences into sympathetic and parasympathetic components. A useful generalization is that the sympathetic system responds for ‘flight-or-fight’ and prepares the body for …