97 Defecation
Learning Objectives
- Describe the defecation reflex and how it is affected by somatic and autonomic innervation.
When undigested material reaches the distal colon and rectum it triggers a coordinated series of events known collectively as the defecation reflex. As the rectum fills, its walls stretch and activate mechanoreceptors in the rectal mucosa. These stretch signals travel via afferent fibers in the pelvic nerves to the sacral segments of the spinal cord (S2–S4). Within the spinal cord they recruit both autonomic and somatic pathways that together determine whether defecation occurs now or can be temporarily postponed.
Parasympathetic efferent fibers arising in the sacral spinal cord return along the same pelvic nerves. These fibers release acetylcholine at muscarinic receptors on the smooth muscle of the rectum and sigmoid colon. The result is enhanced peristaltic contractions that push fecal matter further toward the anal canal. At the same time, the internal anal sphincter, which is composed of smooth muscle, receives a parasympathetic signal that causes it to relax. Relaxation of this sphincter reduces resistance at the outlet of the rectum.
Sympathetic fibers, originating from thoracolumbar spinal segments, also innervate the distal colon and internal anal sphincter. Under resting conditions the sympathetic input maintains a low level of tone in the rectal smooth muscle and keeps the internal anal sphincter contracted. During periods of social or physical stress, increased sympathetic activity further tightens the internal sphincter and suppresses peristalsis. This sympathetic inhibition allows the body to delay defecation when conditions are not appropriate.
While the internal anal sphincter responds to smooth muscle input from autonomic nerves, voluntary control rests with the external anal sphincter and pelvic floor muscles. The external anal sphincter is made of skeletal muscle and is innervated by somatic motor fibers of the pudendal nerve, which also arises from the sacral plexus (S2–S4). When the rectum is distended, afferent signals seated in the spinal cord ascend to the brainstem and cortex. Conscious awareness of rectal filling enables an individual to decide whether to contract or relax the external sphincter.
If the decision is made to postpone defecation, the cerebral cortex sends inhibitory signals down through the reticulospinal tracts to the motor neurons of the pudendal nerve. This causes contraction of the external anal sphincter and the puborectalis muscle, a sling of muscle at the anorectal junction. Contraction of these skeletal muscles straightens the angle between the rectum and anus and increases outlet resistance, thereby maintaining continence even as the rectum continues to fill.
When the situation is appropriate, cortical control is reversed. Voluntary relaxation of the external anal sphincter and puborectalis muscle decreases outlet resistance. The anorectal angle straightens, allowing stool to move more easily toward and through the anus. At this point the parasympathetic-driven peristalsis and the relaxed internal sphincter produce a coordinated wave of propulsion that expels fecal matter.
An important feature of the defecation reflex is its positive feedback loop. Once the internal sphincter relaxes and the external sphincter begins to open, the increase in rectal distension further intensifies parasympathetic signals to the colon and rectum. This in turn amplifies peristaltic contractions and sphincter relaxation until defecation concludes. After evacuation, sensory input from the now-empty rectum diminishes, autonomic outflow returns to its baseline balance, and both sphincters regain tone under sympathetic and somatic influence.
Disorders of either autonomic or somatic innervation can disrupt this reflex. Damage to the pelvic nerves impairs parasympathetic stimulation, reducing rectal peristalsis and preventing internal sphincter relaxation. The result is constipation and overflow incontinence. Injury to the pudendal nerve or damage to the cortical pathways leads to weakness of the external sphincter and pelvic floor muscles, causing fecal incontinence.
In summary, defecation depends on the interplay of sensory detection of rectal stretch, autonomic control of smooth muscle tone in both the rectum and internal anal sphincter, and somatic control of the external anal sphincter and pelvic floor. Parasympathetic fibers promote propulsive contractions and sphincter relaxation. Sympathetic fibers generally inhibit these actions, preserving continence when necessary. Voluntary signals from the brain enable conscious decision making, permitting defecation only when conditions are socially and physically appropriate. Together these mechanisms ensure that waste elimination is both effective and under the control of the individual.