Practice Questions

Cardiac muscle is highly dependent on aerobic respiration to prevent fatigue, thus it contains a much higher density of mitochondria (up to 35% of cell volume) and a rich blood supply compared to skeletal muscle. It can also readily utilize lactic acid produced by skeletal muscles during exercise.

The muscle maintaining tonus in hollow organs is smooth muscle. Unlike striated muscles, smooth muscle lacks distinct A and I bands and Z-lines. Instead, actin and myosin filaments are arranged in a diagonal, lattice-like network anchored to dense bodies.

Skeletal muscle can undergo limited repair via satellite cells. Smooth muscle has the greatest regenerative capacity and can undergo both hypertrophy and hyperplasia. Cardiac muscle has virtually no regenerative capacity, typically replacing damaged tissue with non-contractile fibrous scar tissue.

Somatic motor neurons exclusively innervate skeletal muscles, which are responsible for voluntary movements and breathing (diaphragm and intercostal muscles). Smooth muscles (stomach, arteries) and cardiac muscle (myocardium) are innervated by the autonomic nervous system.

Tissue X is skeletal muscle (fatigues easily), Tissue Z is cardiac muscle (contracts rhythmically without fatigue), and Tissue Y is smooth muscle, which is highly specialized for slow, sustained contractions (tonus) with minimal energy expenditure and no fatigue.

The parasympathetic nervous system (via the vagus nerve) slows down the heart rate (cardiac muscle) but stimulates the smooth muscles of the gastrointestinal tract to promote digestion (peristalsis). It has no direct effect on skeletal muscle, which is controlled by the somatic nervous system.

Shivering thermogenesis is mediated by skeletal muscles. Skeletal muscle fibers are characterized by being long, cylindrical, unbranched, and multinucleated, with the nuclei located peripherally just beneath the sarcolemma.

Visceral (smooth) muscle lacks the troponin complex found in skeletal (voluntary) and cardiac muscles. Instead, calcium binds to the regulatory protein calmodulin, which then activates myosin light-chain kinase (MLCK) to phosphorylate myosin and initiate contraction.

The tissue described is cardiac muscle. The specialized dark-staining regions are intercalated discs, which contain gap junctions. These gap junctions allow ions to pass freely between cells, facilitating the rapid spread of depolarization so the heart can contract as a functional syncytium.

Smooth muscle relies heavily on the influx of extracellular calcium for contraction, whereas skeletal muscle relies almost exclusively on intracellular calcium stored in the sarcoplasmic reticulum. Cardiac muscle requires both, but smooth muscle is most immediately affected by the lack of extracellular calcium.