Circular muscular layer of colon

The circular muscular layer of colon forms the inner component of the muscularis externa and consists of a continuous sheet of smooth muscle fibres arranged circumferentially around the bowel wall. Unlike the outer longitudinal muscle layer, which is concentrated into the three teniae coli, the circular muscle layer is uniformly distributed throughout the entire circumference of the colon. Its coordinated contractions are responsible for segmenting, mixing, and propelling colonic contents, thereby facilitating the absorption of water and electrolytes.

Contraction of the circular muscle layer produces localized constrictions that contribute to the formation of the characteristic semilunar folds (plicae semilunares coli) seen within the lumen of the colon. Working together with the teniae coli, it helps create the sacculated appearance of the large intestine known as haustra. Distally, the circular muscle layer becomes progressively thicker and continues into the anal canal, where it forms the internal anal sphincter, an involuntary smooth muscle sphincter that plays an important role in maintaining faecal continence.