The role of the autonomic nervous system

There are two branches of the autonomic nervous system (ANS): the sympathetic (SNS) and the parasympathetic (PNS). The heart is supplied by both: by the vagus nerve from the PNS and the cardiac accelerator nerves from the SNS. Stimulation of the PNS slows the heart and reduces the force of contraction, while stimulation of the SNS has the opposite effect, speeding the heart and increasing the force of contraction. Depression of one half of the system produces similar results to stimulation of the other half (Hopkins 1999). In health the PNS is dominant, slowing the heart to its normal rate of about 70 beats per minute. In hearts that have no nervous supply (such as transplanted hearts), this 'parasympathetic brake' is absent and the resting heart rate is about 90-100 beats per minute (Lamb et al. 1991).

The side-effects of the drugs acting on the ANS (seen in Table 10.2) are related to the physiology of the organs supplied by its branches (Hinchliff et al. 1996, Hopkins 1999, Prosser et al. 2000).

Table 10.2 Effects of the autonomic nervous system.

Organ

Sympathetic stimulation (parasympathetic blockade)

Parasympathetic stimulation (sympathetic blockade)

Heart

Increased rate and

Decreased rate and

contractility

contractility

Blood vessels

Constriction

Dilation

Lungs

Bronchial dilation

Bronchial constriction

Eye

Pupils dilate

Pupils constrict

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