Practice Questions

Acetylcholinesterase breaks down acetylcholine (ACh) in the synaptic cleft, allowing the muscle to relax. If blocked, ACh remains in the cleft, continuously binding to receptors and causing sustained depolarization and continuous muscle contraction (spasms/tetanus).

In striated muscles, actin filaments are anchored to Z-lines. In smooth muscle, which lacks Z-lines and sarcomeres, the actin filaments are anchored to protein structures called dense bodies, which are distributed throughout the cytoplasm and attached to the sarcolemma.

During embryonic development, skeletal muscle fibers are formed by the end-to-end fusion of many individual precursor cells called myoblasts. Because the cells fuse but their nuclei remain, the resulting mature muscle fiber is a massive multinucleated syncytium.

Cardiac muscle evolved to combine the involuntary, continuous activity characteristic of smooth muscle (autonomic control) with the high contractile strength and organized sarcomere structure characteristic of striated skeletal muscle.

Single-unit (visceral) smooth muscle cells are linked by gap junctions, contract as a syncytium, are found in hollow organs, and often exhibit pacemaker activity. Multi-unit smooth muscle (e.g., in the iris) consists of independent fibers that require independent neural stimulation.

When a skeletal muscle is stimulated rapidly, the twitches summate. If the stimuli are rapid enough that the muscle only partially relaxes between them, it enters a state of sustained, quivering contraction known as incomplete (or unfused) tetanus.

Skeletal muscle contraction relies almost entirely on intracellular calcium released from the well-developed sarcoplasmic reticulum. Cardiac and smooth muscles heavily depend on the influx of extracellular calcium to trigger contraction (calcium-induced calcium release).

Arteriolar diameter is controlled by vascular smooth muscle. Smooth muscle cells are spindle-shaped, uninucleated, involuntary, and lack the organized sarcomeres that create striations in skeletal and cardiac muscle.

Cardiac muscle has an unusually long action potential and absolute refractory period (lasting almost as long as the contraction itself). This biological adaptation prevents multiple action potentials from summing and causing tetanus (sustained contraction), which would be fatal as the heart must relax to fill with blood.

Smooth muscle has the slowest speed of contraction and relaxation, allowing for long, sustained contractions. Cardiac muscle has an intermediate speed, and skeletal muscle has the fastest contraction speed, allowing for rapid, voluntary movements.