The guts are controlled by multiple nerves, including the vagus which is a cranial nerve that does not involve the spinal cord. The gut has its own nervous system composed of mesenteric neurons situated in the linings of the gut. Scientists who study the mesenteric nervous system say with justification that there are more neurons in the mesenteric system than in the spinal cord.

In general, spinal cord injury may be associated with constipation but these can be overcome with local stimulation that can increase mesenteric activit Even after complete spinal cord injury or damage to the sacral spinal cord (conus injury), the gut usually continues to move materials. Sacral injuries, however, may compromise the function of the anal sphincters. It is perhaps worthwhile explaining the anal sphincter system.

People have two anal sphincter: the internal and the external sphincter.

The internal sphincter is a thin white band of muscle that is involuntary. It is innervated by two sets of nerves: sympathetic and parasympathetic. The sympathetic innervation causes the sphincter to contract and comes from the hypogastric nerve and the pelvic plexus (a local set of neurons). The role of the parasympathetic innervation is not well understood but it comes from the S1, S2, and S3 roots through the pelvic plexus. You cannot control the internal sphincter and it is normally in a contracted state, preventing fecal matter from escaping. However, when the rectum is full (of fecal material or gas, etc.) the internal sphincter will relax, allowing the contents of the rectum to come out.

The external sphincter is a thick red circular muscle that is voluntary. You can contract the external sphincter voluntarily and it is controlled by the S2, S3, and S4 roots of the spinal cord, through the inferior rectal branch of the pudendal nerve. Part but not all of the activity of the sphincter and rectal muscles comes from the spinal cord. So, for example, elimination of the spinal contribution by anesthesia of the spinal cord reduces rectal pressure by only 50%.

Rectal distension causes both the internal and external sphincters to relax but the external sphincter contracts reflexively if fecal contents contact the anal canal. However, if gas rather than solid fecal material contacts the anal sphincter, the external sphincter does not contract as much, allowing flatus (fart) to exit. The reflexive contraction of the external sphincter in response to contact with solid fecal matter is called the "sampling reflex" and it requires sensory input from the anal canal to the sacral spinal cord. Loss of this reflex can contribute to fecal incontinence.

The external sphincter also contracts when you cough, sneeze, or stand up. It contracts when the skin around the anus is gently stroked, called the cutaneous anal reflex or the "anal wink". This reflex is suppressed in several circustances:
• It is suppressed during sleep and at very high rectal volumes.
• It is absent in a number of diseases that affect nervous control of the gut, including Hirschsprung's disease and Chagas disease
• It can be suppressed after surgical excision of rectal mucosa.
• It is lost after damage to the S5, the lowest spinal cord segment. It is partly suppressed after damage to the S2, S3, or S4 segments or roots.

Continence does not depend on the internal or external anal sphincters alone. One can have flaccid anal sphincters and still be continent most of the time. In children, a functional and intact puborectal muscle can provide continence. However, the puborectal muscle alone is usually not sufficient to provide consistent continence in situations, particularly where the rectal content is partly flatus.

Defecation is the act of eliminating fecal material in the rectum. Voluntary defecation involves a fairly complex series of activities, including relaxation of the pelvic floor muscles, abdomenal muscle contraction to increase abdomenal pressure, and suppression of the sampling reflex (the reflexive contraction of the external sphincter upon contact of fecal matter in the anal canal).

After spinal cord injury, voluntary defecation is usually impaired. To initiate defecation, digital manipulation of the external anal sphincter may be necessary. Spinal cord injury to suprasacral segments (above the sacral spinal cord and roots) can lead to overactive anal reflexes that the defecation reflex. Squatting may help because this straightens the anorectal angle which increases the diameter of the anal canal. A detailed explanation can be found in http://www.fascrs.org/displaycommon....articlenbr=133

So, given the above discussion, you can perhaps understand why it is not easy to select a particular nerve or part of the rectal system to connect to. The reflexes that are responsible for proper functioning of the system are complex and involve multiple muscles, the spinal cord, and local neurons.

Wise.