Actions of the peripheral nervous system

There are a large variety of neurotransmitters in the CNS, but as far as the peripheral nervous system is concerned we need only consider two—acetylcholine and noradrenaline (Fig. 11.4). 

Noradrenaline is released at target organs and leads to the contraction of cardiac muscle and an increase in heart rate. It relaxes smooth muscle and reduces the contractions of the GIT and urinary tracts. It also reduces salivation and reduces dilation of the peripheral blood vessels. 

In general, the sympathetic nervous system promotes the fight or flight response by shutting down the body s housekeeping roles (digestion, defecation, urination, etc.), and stimulating the heart. The stimulation of the adrenal medulla releases the hormone adrenaline which reinforces the action of noradrenaline. 

The stimulation of the parasympathetic system leads to the opposite effects from those of the sympathetic system. Acetyl choline is released at the target organs and reacts with receptors specific to it and not to noradrenaline. 

Note that the sympathetic and parasympathetic nervous systems oppose each other in their actions and could be looked upon as a brake and an accelerator. The analogy is not quite apt since both systems are always operating and the overall result depends on which effect is the stronger. 

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It has been reported that GABA was absorbed in the gastrointestinal tract as a specialized transport mechanism which obeys Michaelis-Menten and first-order kinetics [52]. Since the blood-brain barrier is impermeable to GABA [53], the hypotensive effect of peripherally administered GABA is due to the actions of the peripheral nervous system and those of several... 

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