The autonomic nervous system (ANS or visceral nervous system or involuntary nervous system) is the part of the peripheral nervous system that acts as a control system functioning largely below the level of consciousness and controls visceral functions Figure 2. The ANS affects heart rate, digestion, respiratory rate, salivation, perspiration, pupillary dilation, micturition (urination), and sexual arousal. Whereas most of its actions are involuntary, some, such as breathing, work in tandem with the conscious mind.
It is classically divided into two subsystems: the parasympathetic nervous system (PSNS) and sympathetic nervous system (SNS). Recently, a third subsystem of neurons has been identified referred to as "non-adrenergic and non-cholinergic" neurons (because they use nitric oxide as a neurotransmitter). These have been found to be integral in autonomic function, particularly in the gut and the lungs.
With regard to function, the ANS is usually divided into sensory (afferent) and motor (efferent) subsystems. Within these systems, however, there are inhibitory and excitatory synapses between neurons. The enteric nervous system is sometimes considered part of the autonomic nervous system, and sometimes considered an independent system. It receives extensive innervation from both the SNS and the PSNS.
Regulation of Enteric Function
The effect of parasympathetic stimulation is to increase activity in the entire enteric nervous system The proximal half of the nervous system is innervated from the cranial parasympathetic nerve fibers via the vagal nerve. The distal half is innervated via the Sacral Parasympathetic nerves. The later gives a rich supply to the Sigmoid colon, rectum, and anus, and are important in controlling defecation.
The fibers originate in the sympathetic ganglia of T-5 to L-2 and terminate mainly on the enteric plexuses, but also a few nerves terminate in the mucosa it self. Stimulation of the enteric nerves by the sympathetic system inhibits GI activity. It does this in a minor way with the direct effect of its secreted norepinephrine, and in a major way by inhibiting action in the enteric plexuses.
Numerous afferent sensory fibers innervate the gut. Some have their cell bodies in the enteric plexus, and some in the spinal cord. As well as sending information concerning irritation and over distension, they can also pick up the presence of chemical signals in the gut. Eighty percent of the fibers in the vagus nerve are afferent and these send signals to the medulla for processing.