The autonomic nervous system controls many involuntary bodily functions such as arterial blood pressure regulation, heart rate and pupillary dilatation. A large proportion of its work is on physiological systems which require ongoing assessment and correction - homeostatic maintenance of an internally stable environment.
The autonomic nervous system is divided into two limbs with generally opposing functional effects:
Both have efferent - motor - and afferent - sensory - limbs. They share parts of their anatomical pathways both within the central and peripheral nervous systems. However, parasympathetic fibres emerge from the brain stem and sacral region of the spinal cord - the craniosacral outflow - whereas the sympathetic nervous system emerges from the thoracic and lumbar segments of the spinal cord - thoracolumbar outflow.
One key difference between the autonomic and somatic nervous systems is that the efferent pathways of the former consist of two neurones in series between a central nervous system neurone and a target cell. In the somatic nervous system, there is only one. In the ANS, the synapse of the first and second neurones in the circuit is within a ganglion. Therefore, the proximal neurone that commences in the CNS is termed pre-ganglionic whereas the distal one that terminates on the effector cell is termed post-ganglionic. The anatomical position of the ganglion varies between the sympathetic and parasympathetic nervous systems. The pre-ganglionic neurotransmitter is acetylcholine for both limbs; however, the postganglionic neurotransmitter varies:
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