Bowel Movement

Bowel Movement and Stool Characteristics
The occurrence of strong and coordinated contractions (mass movements) allows the migration of stool from distal colonic segments into the rectum. These contractions occur more frequently in the morning after awakening and after a meal (gastrocolic reflex). In contrast, colonic motility is reduced at night to avoid incontinence (Andrews & Storr, 2011) The volume and consistency of the feces are other important factors affecting anal continence. Leakage of gas or liquid stools is more difficult to control, while hard stools are difficult to expel even in the presence of damaged or poorly functioning anal sphincters(Giannini et al., 2014).

Defecation Sensory perception and physiological coordination
of pelvic floor muscles are important components of defecation, which depends on both involuntary and voluntary mechanisms. The site for the integration and control of defecation is located in the lumbosacral spinal cord and is modulated by higher centers (brainstem and cerebral cortex). Alterations of this brain–gut axis can lead to important dysfunctions: in fact the voluntary control of defecation is lost in patients who have a spinal cord injury with interrupted corticospinal connections (Furness, 2012). At rest, the pressure in the rectum is lower than in the anal canal, but once the rectum has received the fecal mass from the distal colon, its intraluminal pressure increases and the rectal walls are stretched. The pressure becomes higher than pressure of the anal canal, and rectal distension activates the rectoanal inhibitory reflex causing the relaxation of the IAS so that the feces, according to the new pressure gradient, come down to the upper anal canal where the sensory receptors in the anal mucosa can discriminate between flatus and liquid or solid stools. This mechanism is called “sampling” and determines both the urgency of defecation and the reflected contraction of the EAS, which prevents the loss of stools. When the conscious perception of the stimulus to defecate is realized, if the passage of stools must be prevented, a voluntary contraction of the EAS and puborectalis muscle forces the stool back into the high rectum. Here the feces are temporarily stored so that the urgency to defecate temporarily disappears and anal sphincters recover their basal tone. In contrast, if the time and the place are appropriate, the subject sits on the toilet and by the Valsalva maneuver increases the abdominal pressure contracting the abdominal muscles. Simultaneously the EAS and puborectalis muscles voluntarily relax and the anorectal angle opens and the stool can be expelled through the anus. The “closing reflex” (transient contractions of the EAS and puborectalis muscle) after defecation closes the anal canal, restoring its basal tone (Ramalingam & Mortensen, 2005)
Role of The Nervous System Hormones, paracrine substances, enteric nervous system, autonomic (sympathetic and parasympathetic) nervous system, and cerebral cortex together regulate colorectal motility and sensitivity.
The enteric nervous system is composed of the myenteric plexus (Auerbach’s plexus) and the submucosal plexus (Meissner’s plexus), and it consists of a network of nervous fibers, ganglion cells (sensory and effectors neurons), and interneurons richly interconnected by reflex arcs located in the wall of the gastrointestinal tract and directed to innervate smooth muscle cells. The effector neurons of the myenteric plexus may be excitatory or inhibitory according to the substances released in contact with smooth muscle cells. Excitatory neurons release acetylcholine, substance P, and other tachykinins, while inhibitory neurons release vasoactive intestinal peptide and nitric oxide, which cause relaxation of smooth muscle cells As a result of this organization, the enteric nervous system acts like a semi-autonomous system: it is able to coordinate most of the activities, even in the absence of an extrinsic control. Extrinsic innervation is provided by the sympathetic and parasympathetic nerves, which have only a modulatory function on the contractile activity. Sympathetic innervation originates from postganglionic fibers of the hypogastric plexus and it has an inhibitory effect on the motor function, making connections with neurons of the enteric nervous system, which in turn sends fibers to smooth muscle cells, inhibiting the contraction. On the other hand, parasympathetic fibers, which originate from the sacral plexus (S2–S4) and run into the pudendal nerve, send preganglionic fibers to neurons of the intramural plexus, which in turn sends fibers to smooth muscle cells, stimulating the contractile function. Also important are the intrinsic reflexes located in the colon and rectal wall, as well as throughout the gastrointestinal tract; the colocolonic reflex is finely organized so
that the stimulation of an intestinal segment causes contraction of the segments upstream and distension of segments downstream. The gastrocolic reflex acts to produce an increase in colonic motility and mass movements in response to the presence of food in the stomach. Therefore, it is evident that continence and defecation are the effects of the integration of many functions involving the colon, anorectum, pelvic floor muscles, and nervous system (Giannini et al., 2014).
Conclusions The ability to retain stools, distinguish them from flatus, and allow defecation, is a complex process controlled by several anatomic factors including the pelvic floor musculature and the anorectum, with its complex innervation including the somatic, autonomic and enteric nervous systems (Giannini et al., 2014).