- The walls of the gastrointestinal (GI) tract are primarily composed of smooth muscle.
- Unlike skeletal muscle, which is voluntary and striated, smooth muscle is involuntary and non-striated, allowing for the autonomous and rhythmic contractions necessary for the movement of food through the digestive system, a process known as peristalsis.

- Smooth muscle fibers are arranged in layers, typically an inner circular layer and an outer longitudinal layer, which work together to facilitate the coordinated contractions that propel content along the GI tract.
- This muscle type is controlled by the autonomic nervous system and local enteric nervous system, ensuring that digestion occurs efficiently without conscious effort.

In contrast, skeletal muscle is found in the upper esophagus and external anal sphincter, where voluntary control is necessary, but the predominant muscle type in the GI tract is smooth muscle. Cardiac muscle is confined to the heart, and striated muscle generally refers to skeletal or cardiac muscle, both distinct from smooth muscle.

Key Points:
- The GI tract's muscular walls predominantly contain smooth muscle.
- Smooth muscle is involuntary and non-striated.
- It facilitates peristalsis and coordinated movement of food.
- Controlled by the autonomic and enteric nervous systems.
- Skeletal muscle is limited to specific areas like the upper esophagus and anal sphincter.

Reference: Gray's Anatomy, 42nd Edition, Volume 1, Chapter on Alimentary Canal (Page 1234)