J Neurogastroenterol Motil.  2019 Apr;25(2):189-204. 10.5056/jnm18172.

Control of Motility in the Internal Anal Sphincter

• Keef KD
• Cobine CA

• KMID: 2449091
• DOI: http://doi.org/10.5056/jnm18172

Abstract

The internal anal sphincter (IAS) plays an important role in the maintenance of fecal continence since it generates tone and is responsible for > 70% of resting anal pressure. During normal defecation the IAS relaxes. Historically, tone generation in gastrointestinal muscles was attributed to mechanisms arising directly from smooth muscle cells, ie, myogenic activity. However, slow waves are now known to play a fundamental role in regulating gastrointestinal motility and these electrical events are generated by the interstitial cells of Cajal. Recently, interstitial cells of Cajal, as well as slow waves, have also been identified in the IAS making them viable candidates for tone generation. In this review we discuss four different mechanisms that likely contribute to tone generation in the IAS. Three of these involve membrane potential, L-type Ca²âº channels and electromechanical coupling (ie, summation of asynchronous phasic activity, partial tetanus, and window current), whereas the fourth involves the regulation of myofilament Ca²âº sensitivity. Contractile activity in the IAS is also modulated by sympathetic motor neurons that significantly increase tone and anal pressure, as well as inhibitory motor neurons (particularly nitrergic and vasoactive intestinal peptidergic) that abolish contraction and assist with normal defecation. Alterations in IAS motility are associated with disorders such as fecal incontinence and anal fissures that significantly decrease the quality of life. Understanding in greater detail how tone is regulated in the IAS is important for developing more effective treatment strategies for these debilitating defecation disorders.

Keyword

Interstitial cells of Cajal; Receptor, platelet-derived growth factor alpha; Slow waves; Smooth muscle; Tone

MeSH Terms

Anal Canal*
Defecation
Fecal Incontinence
Gastrointestinal Motility
Interstitial Cells of Cajal
Membrane Potentials
Motor Neurons
Muscle, Smooth
Muscles
Myocytes, Smooth Muscle
Myofibrils
Quality of Life
Receptor, Platelet-Derived Growth Factor alpha
Tetanus
Receptor, Platelet-Derived Growth Factor alpha