Dual innervation


A variety of organs throughout the human body are designed with both sympathetic and parasympathetic innervation. The result of this dual innervation process is likely to result in either cohesive unification, an incohesive antagonistic response, or an unilateral cooperative response.


The most glaringly obvious antagonistic response to dual innervation is the heart. More specifically, the sinoartial node of the pacemaker. The sinoartial node of the pacemaker is dually innervated by both divisions. Sympathetic neurons are responsible for an increase in heart rate through the release of adrenergic stimulation. The parasympathetic neurons release cholinergic stimulation and this results in a decrease in the heart’s rhythm.

The GI tract is another prime example of antagonistic effects of dual innervation. The parasympathetic nerves release the adrenergic stimulation in order to increase peristaltic action. Alternatively, the GI tract can be slowed and nearly stopped by the release of cholinergic stimulation from the sympathetic division.

Sympathetic and parasympathetic innervation is necessary in order for the muscular effects of the eye, such as pupil dilation, to occur. It can be readily compared to the flexor and extensor muscles that control musculoskeletal motions. The iris is in part made up of antagonistic muscle layers, which accounts for this comparison. Stimulation provided by the sympathetic nerve endings causes the papillary dilator muscle to respond. This in turn creates dilation. Additionally, the parasympathetic nerve endings also cause a response by the papillary constrictor muscles which in turn create the pupil’s constriction.


Dual innervation
Image: Dual Innervation


In some areas of the body, the sympathetic and parasympathetic effects are complimentary. A perfect example of this is the salivation process. Parasympathetic nerves send impulses to begin or increase exocrine glandular secretions through the GI tract, including salivary glands. The sympathetic nerves are responsible for sending impulses throughout the GI tract which result in the constriction of blood vessels. The decrease in the blood flow throughout the GI tract results in a thicker saliva produced with more viscous-ness and more speed.


The urinary system and the reproductive system are excellent example of the cooperative effects of the parasympathetic and sympathetic nerve stimulations. In the reproductive system, a penile erection is the result of both divisions communicating simultaneously. The sympathetic nerves send out impulses which prepare and send out ejaculatory responses. The parasympathetic nerve impulses are responsible for the vasodilation which occurs to cause an erection.

In the urinary system, the bladder’s action is known to be independent of any nerve stimulation once a body has learned to control the process of urination. However, Action potentials from the parasympathetic nerves are responsible for the contractions which occur that signal a need to urinate, as well as the final contractions that lead directly to urination. During the process of urination, action potentials via the sympathetic nerves assist in the release of urine from the body and exhibits an increase in muscle tone of the urinary tract to help assist the urine from the body.

Interestingly, the emotional status of the human mind has a direct impact on the state of sympathetic nerve actions. Highly emotional states of the human mind result in higher and more sensitive sympathetic nerve responses. This means that emotional individuals may experience bladder contractions despite the fact that the body has not yet reached the potential stage for reflex action triggered by the sympathetic nerves.
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