Practice Questions

Skeletal muscle is almost entirely dependent on the intracellular calcium stored in its highly developed sarcoplasmic reticulum (SR) for contraction. Cardiac and smooth muscles rely heavily on extracellular calcium and have less developed SRs, making them less immediately vulnerable to SR destruction.

Skeletal muscle is exclusively innervated and controlled by the somatic nervous system (voluntary control). The autonomic nervous system (sympathetic and parasympathetic) controls cardiac muscle, smooth muscle, and glands.

Visceral (single-unit) smooth muscle cells in the digestive tract are electrically connected by gap junctions. This allows action potentials generated by pacemaker cells or stretching to spread rapidly from cell to cell, resulting in the coordinated wave-like contraction of peristalsis.

The release of the hormone epinephrine from the adrenal medulla (endocrine response) acting synergistically with the sympathetic nervous system to increase heart rate (cardiac muscle) and dilate airways (smooth muscle) is a classic involuntary neuroendocrine integration.

Smooth muscle contractions are characterized by being slow, prolonged, and highly energy-efficient. The smooth muscle "latch state" allows it to maintain high tension with very low ATP consumption, preventing fatigue over long periods (e.g., maintaining vascular tone).

Intercalated discs contain two key structures: desmosomes, which mechanically bind the cardiac cells together so they don't tear apart during contraction, and gap junctions, which electrically couple the cells, allowing the action potential to spread rapidly across the entire heart.

The iris of the eye is composed of multi-unit smooth muscle. Unlike single-unit smooth muscle, the fibers in multi-unit smooth muscle are structurally independent, richly innervated by autonomic nerves, and contract in a highly localized, precise manner without gap junction transmission.

Smooth muscle lacks the extensive sarcoplasmic reticulum and true T-tubules of skeletal muscle. Instead, the sarcolemma has pouch-like infoldings called caveolae that contain a high density of calcium channels, facilitating the rapid influx of extracellular calcium needed for contraction.

For short, intense bursts of energy, skeletal muscle relies on the phosphagen system. Creatine phosphate stores high-energy phosphate bonds that can quickly be transferred to ADP to regenerate ATP, providing immediate energy before glycolysis fully kicks in.

Both skeletal and cardiac muscles are striated (have light and dark bands). However, cardiac muscle cells are branched, usually have a single central nucleus, and are uniquely joined end-to-end by intercalated discs, which are absent in skeletal muscle.