Physiology

Autonomic nervous system (ANS)

The ANS is a complex system of neurones that controls the body's internal milieu. It is not under voluntary control and is anatomically distinct from the somatic nervous system. Its efferent limb controls individual organs and smooth muscle, while its afferent limb relays information (occasionally in somatic nerves) concerning visceral sensation and may result in reflex arcs.

The hypothalamus is the central point of integration of the ANS, but is itself under the control of the neocortex. However, not all autonomic activity involves the hypothalamus: locally, the gut coordinates its secretions; some reflex activity is processed within the spinal cord; and the control of vital functions by baroreceptors is processed within the medulla. The ANS is divided into the parasympathetic and sympathetic nervous systems.

Parasympathetic nervous system (PNS)

The PNS is made up of pre- and post-ganglionic fibres. The pre-ganglionic fibres arise from two locations (Figure 12.1):

• Cranial nerves (III, VII, IX, X) – which supply the eye, salivary glands, heart, bronchi, upper gastrointestinal tract (to the splenic flexure) and ureters

• Sacral fibres (S2,3,4) – which supply distal bowel, bladder and genitals

All these fibres synapse within ganglia that are close to, or within, the effector organ. The post-ganglionic neurone releases acetylcholine, which acts via nicotinic receptors.

The PNS may be modulated by anticholinergics (see Chapter 18) and anticholinesterases (see Chapter 11).

Sympathetic nervous system (SNS)

The SNS is also made up of pre- and post-ganglionic fibres.