In a patient with a history of depression, anxiety, and fatigue, does the sensation of fullness during bowel movements originate from the internal or external anal sphincter muscle?

Sensation of Fullness During Bowel Movements

The sensation of rectal fullness originates from the rectum itself, not from either the internal or external anal sphincter muscles. 1

Anatomical Basis of Rectal Sensation

The rectum is sensitive to distension, though the exact mechanism—whether from stretching of the gut wall, reflex contraction, or distortion of adjacent structures—remains incompletely understood. 1 Importantly:

• No specific sensory receptors exist in the rectal mucosa on histological examination 1
• Myelinated and non-myelinated nerve fibers are present adjacent to the rectal mucosa, but no intraepithelial sensory fibers arise from these 1
• Rectal sensation travels via the parasympathetic system to sacral nerve roots S2, S3, and S4 1

Rectal vs. Anal Canal Sensation

The sensory innervation differs markedly between these two regions:

Rectal Sensation

• The rectum perceives distension and fullness but is insensitive to pain, touch, or temperature stimuli that would cause sensation on skin 1
• Three distinct sensory thresholds are recognized: constant sensation of fullness, urge to defecate, and maximum tolerated volume 1

Anal Canal Sensation

• The anal canal has profuse innervation with specialized sensory nerve endings including Meissner's corpuscles (touch), Krause end-bulbs (temperature), Golgi-Mazzoni bodies and pacinian corpuscles (tension/pressure), and genital corpuscles (friction) 1
• Anal sensation travels via the inferior hemorrhoidal branches of the pudendal nerve to S2, S3, and S4 1
• The anal canal perceives touch, pain, and temperature—not fullness 1

Clinical Relevance in Patients with Depression and Anxiety

In your patient with depression, anxiety, and fatigue, altered rectal sensation may be particularly relevant:

• Between 20-60% of patients with IBS demonstrate enhanced visceral perception to mechanical distension 2
• Visceral hypersensitivity correlates positively with symptom severity, even after adjusting for psychological comorbidity 2
• Depression is associated with elevated first rectal sensory threshold volumes, which predicts poor response to biofeedback therapy 2
• Patients with lower baseline thresholds for first rectal sensation and urge are more likely to respond to therapeutic interventions 2

Sphincter Function vs. Sensation

While the sphincters don't generate the sensation of fullness, they play critical roles in continence:

• The internal anal sphincter (IAS) generates >70% of resting anal pressure and maintains tone through autonomic innervation 3, 4
• The IAS receives dual autonomic innervation primarily from inferior rectal branches of the pelvic plexus (sympathetic-dominant) and the myenteric plexus (parasympathetic) 5, 6
• The external anal sphincter is under voluntary control and contributes to squeeze pressure, not sensation of fullness 1

Common Clinical Pitfall

Do not attribute sensations of rectal fullness to sphincter dysfunction. The fullness sensation is a rectal phenomenon mediated by parasympathetic afferents, while sphincters primarily control continence through motor function. 1 Altered rectal sensation should be evaluated through rectal sensory testing (balloon distension or mucosal electrosensitivity), not sphincter pressure measurements alone. 2, 1