Stimulation of which receptors in the wall of the rectum causes the defecation reflex?

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Stimulation of stretch receptors in the rectal wall triggers the defecation reflex. When the rectum fills with stool, its walls stretch and these stretch receptors detect that mechanical deformation. Afferent signals then travel to the spinal cord and evoke a parasympathetic reflex that increases rectal and sigmoid colon motility and relaxes the internal anal sphincter, creating the urge to defecate and facilitating its passage. The trigger here is the mechanical stretch of the rectal wall, not chemical changes or temperature, so chemoreceptors and thermoreceptors aren’t the initiating receptors.