What type of muscle makes up the bulk of the gastrointestinal tract and regulates automatic activity?

Smooth muscle makes up the bulk of the gastrointestinal tract and is responsible for regulating automatic activity within this system. Unlike skeletal muscle, which is under voluntary control and is associated with movements of the skeleton, smooth muscle operates involuntarily, which means it functions without conscious effort.

Smooth muscle fibers are organized in layers that can contract rhythmically and involuntarily to facilitate the movement of food and liquid through the digestive system, a process known as peristalsis. This capability is crucial for digestion, allowing for the mixing of food with digestive juices, which aids in nutrient absorption.

In contrast to smooth muscle, cardiac muscle is specialized for the heart and is also involuntary, but it is not involved in the gastrointestinal tract. Voluntary muscle refers to skeletal muscle, which is not involved in the automatic movements of the digestive system. Thus, the characteristics of smooth muscle make it the correct choice in this context.