Receptor apparatus and urge formation (Afferent pathway)
The main triggering mechanism of the bowel evacuation reflex is mechanical stretch of the walls. When fecal masses fill the descending, sigmoid, and rectum, they cause progressive stimulation of local stretch receptors. An additional trigger is the direct irritation of mucosal chemoreceptors.
Upon receiving the signal about the stretch, sensory impulses travel along the afferent fibers of the posterior root nerves. They enter the spinal cord at the level of the lumbar and sacral segments. From there, ascending pathways transmit information to the brain:
- In the hypothalamic center, subcortical (unconscious) information processing and basic autonomic regulation take place.
- In the cerebral cortex, a conscious sensation is formed — «the urge to defecate».
Sphincter apparatus of the colon
The retention of contents within the bowel and the process of its elimination depend on the coordinated work of two anal sphincters. When the rectum is not subjected to significant mechanical irritation, both sphincters remain in a state of continuous tonic tension, preventing the contents from leaving the bowel spontaneously.
- Internal sphincter. Consists exclusively of smooth muscle, functioning involuntarily and independently of human will. Its tone is maintained through tonic influences from sympathetic neurons of the first and second lumbar segments of the spinal cord ($L_1, L_2$). These signals are mediated through $\alpha$-adrenergic receptors.
- External sphincter. Formed by striated muscle tissue, enabling strict conscious control over it. Somatic innervation of this muscle is provided via the pudendal nerve (n. pudendus).
Efferent motor regulation
Descending efferent pathways ensure the execution of motor commands from the brain and spinal cord. Cortical pyramidal neurons are responsible for the voluntary control of the act of defecation, while the hypothalamic center handles autonomic regulation. Motor signals descend and synapse onto motor neurons located in the anterior horns of the spinal cord.
Peripheral innervation of the effector organs (bowel musculature and sphincters) is carried out in three directions:
- Sympathetic innervation. Passes through the hypogastric nerves, with the inferior mesenteric ganglion acting as the relay station. Sympathetic fibers innervate sections of the colon.
- Parasympathetic innervation. Mediated via the pelvic nerves. They provide innervation to the descending, sigmoid, and rectum, and also transmit signals to the smooth-muscle internal sphincter.
- Somatic innervation. Carried out via somatic fibers of the pudendal nerve (n. pudendus). They specifically innervate the external striated sphincter, allowing a person to consciously control the retention of fecal masses.
Impact of emotions, hormones, and pathological conditions
Bowel function can be actively influenced by humoral factors and psychoemotional state. Hormones such as pituitrin and thyroidin can enhance defecation.
Negative emotions, such as intense fear, can provoke involuntary relaxation of the sphincters (a condition known as «fear diarrhea»). Conversely, prolonged negative experiences can completely suppress the reflex urge to defecate.
Common pathological conditions include toxic effects causing diarrhea and increased frequency of urges. A distinct category is tenesmus — painful false urges characterized by a distortion of the normal physiological ratio: the urge is present, but no full act of evacuation occurs.
Disorders in spinal cord injuries
The clinical picture of neurological defecation disorders depends directly on the level of the nervous system lesion:
- Lesion above the spinal centers (transection above the lumbar region). Connection with the cerebral cortex is severed. Urges are no longer perceived consciously, and cortical control is lost. As a result, defecation occurs automatically (involuntarily) due to local reflexes.
- Lesion or destruction of the spinal centers themselves (transection below the spinal center). Leads to complete paralysis of the rectal sphincter apparatus. A pronounced clinical picture develops — «gaping anus». Fecal retention becomes completely impossible (true incontinence occurs).
Briefly: answers to topical questions
In which specific segments of the spinal cord is the parasympathetic defecation center located?
The parasympathetic defecation center is located in the sacral segments of the spinal cord at the level of S2–S4. Neurons in this region provide autonomic regulation and initiate the process of bowel evacuation. Efferent fibers travel to effector organs as part of the pelvic nerves, stimulating rectal peristalsis and relaxing the internal sphincter. Together, these mechanisms ensure normal evacuation of fecal masses.
Where is the cortical center responsible for the conscious control of defecation localized?
Conscious control of defecation is provided by descending cortical influences; the central command is associated with the excitation of pyramidal cells in the cortex, predominantly the precentral gyrus. This excitation spreads downward to the spinal centers. Pyramidal neurons provide voluntary control, while the external anal sphincter is controlled via the pudendal nerves.
What exact effect (excitatory or inhibitory) does the parasympathetic nervous system exert on the internal sphincter?
The parasympathetic nervous system exerts an inhibitory effect on the internal smooth-muscle sphincter of the rectum. Upon excitation of the sacral parasympathetic neurons, the internal sphincter relaxes. This process occurs simultaneously with the stimulation of peristalsis in the distal colon and rectum, creating the necessary conditions for the unhindered expulsion of fecal masses.
To remember the innervation of the external sphincter, pay attention to the nerve's name: the pudendal nerve (n. pudendus) provides conscious control over the "shameful" (voluntary) aspects of fecal retention.
Class and exam questions
What is the difference between the regulation of the internal and external anal sphincters?
The internal sphincter consists of smooth muscle and is controlled autonomically (primarily by sympathetic fibers from L1-L2). The external sphincter is formed by striated tissue and is controlled consciously via the somatic pudendal nerve.
What happens if the spinal cord is transected above the lumbar region?
Cortical (conscious) control over the process will be completely lost. Emptying of the rectum will become an automatic (involuntary) reflex act.
What is tenesmus?
These are pathological false urges to defecate. In tenesmus, the normal ratio is disrupted: the patient experiences an intense urge, but no full act of evacuation takes place.
What else to study on this topic
- Mechanisms of receptor excitation: interaction of mechanoreceptors and chemoreceptors of the colon.
- The role of α-adrenergic receptors in the tonic contraction of the smooth-muscle sphincter.
- Integration of defecation reflexes at the hypothalamic and cerebral cortex levels.
- The effect of hormones (thyroidin, pituitrin) on gastrointestinal motility.
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