The autonomic nervous system (ANS) comprises a system of efferent nerves that regulate the involuntary functioning of most organs, including the heart and vasculature. The cardiovascular effects of the ANS are deployed for two purposes.

First, the ANS provides the effector arm of the cardiovascular reflexes, which respond mainly to activation of receptors in the cardiovascular system (see Chapter 27). They are designed to maintain an appropriate blood pressure, and have a crucial role in homeostatic adjustments to postural changes (see Chapter 22), haemorrhage (see Chapter 31) and changes in blood gases. The autonomic circulation is able to override local vascular control mechanisms in order to serve the needs of the body as a whole.

Second, ANS function is also regulated by signals initiated within the brain as it reacts to environmental stimuli or emotional stress. The brain can selectively modify or override the cardiovascular reflexes, producing specific patterns of cardiovascular adjustments, which are sometimes coupled with behavioural responses. Complex responses of this type are involved in exercise (see Chapter 30), thermoregulation (see Chapter 25), the ‘fight or flight’ (defence) response and ‘playing dead’.

The ANS is divided into sympathetic and parasympathetic branches. The nervous pathways of both branches of the ANS consist of two sets of neurones arranged in series. Preganglionic neurones originate in the central nervous system and terminate in peripheral ganglia, where they synapse with postganglionic neurones innervating the target organs.

The Sympathetic System

Sympathetic preganglionic neurones originate in the intermediolateral (IML) columns of the spinal cord. These neurones exit the spinal cord through ventral roots of segments T₁–L₂, and synapse with the postganglionic fibres in either paravertebral or prevertebral

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