Gastrointestinal Physiology

Defecation Reflex

Maj Umbreen Ahmed

Functional Anatomy of Rectum•Rectum ( means straight)

–Curved –No sacculations, appendices epiploicae, taeniae coli–Upper part’s diameter is same as sigmoid colon–Lower part dilated to form rectal ampulla–3 lateral curves–3 valves produced by circular muscle fibers

•May be concerned with separation of flatus from fecal material

–Nerve supply •Sympathetic----hypogastric plexus•Parasympathetic----S2 & S3 or S3 & S4 by pelvic

splanchnic nerves•Both convey pain sensations

Functional Anatomy of Anal Canal

•Tube like

•Internal anal sphincter–circular smooth muscle fibers–continuation of circular muscle of rectum–lie immediately inside the anus

•External anal sphincter–skeletal muscle fibers–striated voluntary muscle–Surrounds the internal sphincter and extends distal to it

Internal Anal Sphincter

•Supplied by autonomic nervous system

•Parasympathetic–Inhibitory to internal sphincter

•Sympathetic–Motor to internal sphincter

External Anal Sphincter•Nerve supply

–Inferior rectal branch of pudendal nerve–Somatic nervous system---voluntary control

•the external sphincter is usually kept continuously constricted unless conscious signals inhibit the

constriction.

•Slow twitch muscle fibers are abundant in external sphincter which show constant electromyographic activity

Anal Continence

•Contraction of puborectalis–maintenance of angle between rectum and anal canal

•Abdominal pressure falttening ant rectal wall over the upper end of canal

•Presence of mucosal cushions in anal canal•Tonic contraction of external sphincter

–Voluntary control•Tonic contraction of internal sphincter

–can only maintain continence of there is no distension–Involuntary control

Defecation

•Defecation is a reflex by which feces are expelled from the large intestine through anus

•Defecation is the temporary reflex interruption of the anal continence by which the feces are expelled from

the large intestine through anus

Defecation

•Rectum is mostly empty–Weak functional sphincter---juncture b/w sigmoid

colon and rectum–Sharp angulation -----additional resistance

Defecation

•Urge starts--- rectal pressure increases to 18 mm Hg•Intrinsic reflex

–In response to entry of feces in the rectum–Mediated by myenteric plexus–Weak reflex

•Extrinsic reflex–Parasympathetic defecation reflex–Strong reflex–Reinforce intrinsic reflex

Defecation reflexesIntrinsic reflex

•Intrinsic reflex mediated by the local enteric nervous system–Weak–Integrated in myenteric plexus

•Stimulus-------stretch of the rectal wallpresence of feces in the rectum

•Receptors---nerve endings

•Afferent signals---- spread through the myenteric plexus

•Response--- initiate peristaltic waves in the descending colon, sigmoid, and rectum, forcing feces toward the anus.

–As the peristaltic wave approaches the anus•the internal anal sphincter is relaxed by inhibitory signals from the

myenteric plexus•if the external anal sphincter is also voluntarily relaxed at the same time,

defecation occurs.

Defecation reflexesExtrinsic reflex

•a parasympathetic defecation reflex•Receptors--- nerve endings in the rectum•Afferents---parasympathetics•Center--- sacral segments of the spinal cord Efferents-

--parasympathetics (pelvic nerves) •Effectors---smooth muscle of the descending colon,

sigmoid, rectum, and anus•Response---- greatly intensify the peristaltic waves as

well as relax the internal anal sphincter

Defecation reflexesExtrinsic reflex

•other effects•taking a deep breath•closure of the glottis•contraction of the abdominal wall muscles to force

the fecal contents of the colon downward•pelvic floor to relax downward and pull outward on

the anal ring to evaginate the feces

Constipation

•slow movement of feces through the large intestine•dry, hard feces in the descending colon --- over-

absorption of fluid

•Causes–intestinal obstruction

•tumors •adhesions that constrict the intestines •ulcers

Constipation

•Causes–functional cause

•irregular bowel habits (inhibition of the normal defecation reflexes)

–spasm of a small segment of the sigmoid colon•motility even normally is weak in the large

intestine•cause serious constipation

Diarrhea

•rapid movement of fecal matter through the large intestine

•Causes of diarrhea–Enteritis.

•Cholera–cholera toxin directly stimulates excessive secretion

of electrolytes and fluid from the crypts of Lieberkühn in the distal ileum and colon.

–10 to 12 liters per day–loss of fluid and electrolytes can be debilitating

Diarrhea

–Psychogenic Diarrhea. •periods of nervous tension •emotional diarrhea •caused by excessive stimulation of the

parasympathetic nervous system–motility –excess secretion of mucus in the distal colon

Diarrhea

–Ulcerative Colitis. •extensive areas of the walls of the large intestine

become inflamed and ulcerated. •mass movements occur much of the day rather

than for the usual 10 to 30 minutes.•colon’s secretions are greatly increased •repeated diarrheal bowel movements.

MegacolonHirschsprung’s disease

•Severe constipation •bowel movements occur only once every several

days/once a week.•Tremendous quantities of fecal matter accumulate in

the colon•colon sometimes distend to a diameter of 3 to 4

inches.

MegacolonHirschsprung’s disease

•Cause of megacolon •lack of or deficiency of ganglion cells in the

myenteric plexus in a segment of the sigmoid colon. •Defecation reflexes or strong peristaltic motility

cannot occur in this area of the large intestine•sigmoid colon becomes small and almost spastic•feces accumulate proximal to this area•causing megacolon in the ascending, transverse, and

descending colons