Final answer:
The muscularis mucosae is a thin layer of smooth muscle deep to the lamina propria that facilitates the release of secretions from the mucosa and increases contact of materials in the lumen with the mucosa.
Step-by-step explanation:
The subject of this question is Biology and the grade level is High School.
The thin layer of smooth muscle deep to the lamina propria is called the muscularis mucosae. It is responsible for causing slight movement in the mucosa, which helps in the release of secretions from the mucosa into the lumen. Additionally, the contractions of the muscularis mucosae increase contact of materials in the lumen with the mucosa. This layer is found in the organs of the gastrointestinal (GI) tract and contributes to digestion and absorption of nutrients.