smooth muscle

Meaning: Smooth muscle is nonstriated muscle made of small, spindle-shaped cells, usually with one central nucleus. It lines the walls of hollow organs and vessels such as the gut, bronchioles, bladder, and arteries. It is mostly involuntary: the autonomic nervous system, hormones, local signals, and stretch control it, and some smooth muscle contracts on its own (myogenic activity). Actin and myosin are not arranged in sarcomeres, so no striations appear. There is no troponin. Instead, Ca²⁺ binds calmodulin, which activates myosin light chain kinase; the kinase phosphorylates myosin, allowing cross-bridge cycling. Contraction is slow and can be sustained at low energy cost.

On the MCAT: Expect a comparison of skeletal, cardiac, and smooth muscle by striations, number of nuclei, nervous control, and how Ca²⁺ regulates contraction. A passage may ask what happens when Ca²⁺ entry into vascular smooth muscle is blocked: less contraction, so vessels dilate and resistance falls.

Common confusion: Students assume smooth muscle uses troponin like skeletal muscle, or that involuntary means smooth. Smooth muscle regulates contraction through calmodulin and myosin light chain kinase, and cardiac muscle is involuntary yet striated.