It is usually found as a sheath of muscle fibers surrounding a tube-like structure. When viewed from a microscope, there is no striation presented in the muscle, so it is called involuntary nonstriated muscle.

它通常作为围绕管样结构的肌纤维鞘而存在。用显微镜观察时，肌肉上并不存在横纹，所以它被称为非随意的无横纹肌。