Final answer:
Smooth muscle performs the slow, rhythmic contractions that facilitate involuntary movements in the internal organs. It includes two types – single-unit and multi-unit – with the single-unit type being more common in visceral organs except the heart, which has cardiac muscle.
Step-by-step explanation:
The muscle that performs slow, sometimes rhythmic contractions is the smooth muscle. This type of muscle tissue also known as involuntary muscle, is responsible for involuntary movements within the internal organs. Smooth muscle is found within the walls of various structures such as the esophagus, stomach, intestines, bronchi, and blood vessels, among others. It operates under the control of the autonomic nervous system, hormones, or even spontaneously, allowing it to maintain muscle tone and support peristalsis which moves food through the digestive system. There are two types of smooth muscle: single-unit and multi-unit. The single-unit type, also known as visceral muscle, contracts as a whole and features the stress-relaxation response that's critical for organ function as they expand and fill. Visceral smooth muscle produces slow, steady contractions, enabling substances to move through the body. On the other hand, cardiac muscle, although not a smooth muscle, also exhibits rhythmic contractions through its unique property called autorhythmicity.