Muscular layer of rectum

The muscular layer of rectum consists of an outer longitudinal layer and an inner circular layer of smooth muscles. It is continuous with the muscular coat of the sigmoid colon but undergoes important modifications within the rectum and anal canal that are related to continence and defecation.

1. Outer longitudinal muscular layer of rectum

The outer longitudinal layer is formed by the expansion and fusion of the three taeniae coli at the rectosigmoid junction. Unlike the colon, where the longitudinal muscle is arranged in three distinct bands, the longitudinal fibres in the rectum spread out to form a continuous muscular layer that completely surrounds the rectum. This layer is thicker on the anterior and posterior surfaces than on the lateral surfaces.

Rectococcygeal muscles

• Posteriorly, additional bundles of smooth muscle arise from the anterior surfaces of the second and third coccygeal vertebrae and pass downward and forward to blend with the longitudinal muscle fibres of the posterior wall of the anal canal. These fibres are known as the rectococcygeal muscles and help maintain the position of the anorectal junction.

2. Inner circular muscular layer of rectum

The inner circular layer forms a continuous sheath around the rectum and is considerably thicker than in the colon.

Transverse rectal folds

• Localized thickenings of this layer, together with the overlying mucosa and submucosa, contribute to the formation of the transverse rectal folds (valves of Houston).

Internal anal sphincter

• Inferiorly, within the anal canal, the circular muscle layer becomes markedly thickened and forms the internal anal sphincter, an involuntary smooth muscle sphincter responsible for maintaining most of the resting anal tone and contributing significantly to faecal continence.

Functions

Functionally, the muscular layer of rectum is responsible for the propulsion and storage of faecal contents. Coordinated contractions of the circular and longitudinal muscle layers generate rectal peristalsis during defecation, while the tonic contraction of the internal anal sphincter helps maintain continence between episodes of defecation.