Pharmacology for Anaesthesia and Intensive Care

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SECTION III Cardiovascular drugs

12

Sympathomimetics

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 (Figure12.1):


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 recep-

tors.

The PNS may be modulated by anticholinergics (see Chapter 18 ) and anti-

cholinesterases (see Chapter 11 ).

Sympathetic nervous system (SNS)

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

fibres arise within the lateral horns of the spinal cord at the thoracic and upper

lumbar levels (T1–L2) and pass into the anterior primary rami, and via the white

rami communicans into the sympathetic chain or ganglia where they may either

synapse at that or an adjacent level, or pass anteriorly through a splanchnic nerve to

synapse in a prevertebral ganglion (Figure12.2). The unmyelinated post-ganglionic