Final answer:
Cardiac muscle is the type of muscle cell with visible striations but operates involuntarily, meaning it is not under conscious control and is found only in the heart.
Step-by-step explanation:
The muscle cell type that has visible striations but is not under voluntary control is cardiac muscle. Unlike skeletal muscle, which is also striated and under voluntary control, cardiac muscle operates involuntarily, meaning it cannot be consciously controlled. Found solely in the heart, cardiac muscle fibers are unique as they are connected by intercalated discs, which facilitate the rapid transmission of electrical impulses necessary for heart contractions.
As for other muscle types, smooth muscle lacks the striations seen in cardiac and skeletal muscles and is also not under voluntary control. It is found in various parts of the body, including the walls of blood vessels and the digestive system, where it controls things like blood flow, blood pressure, and movement of food through the intestines.