Muscles

In smooth muscle, calcium binds to calmodulin. The resulting calcium-calmodulin complex binds to and activates myosin light-chain kinase (MLCK), which then phosphorylates the myosin heads, allowing them to bind to actin and initiate contraction.

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.

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.

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.

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.

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.

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.

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.

Smooth muscle lacks true T-tubules and has a poorly developed sarcoplasmic reticulum. Instead of T-tubules, it has small invaginations of the sarcolemma called caveolae, which concentrate extracellular calcium necessary for contraction.

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.

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.

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.

Correct: Cardiac muscle possesses intrinsic rhythmicity and resists fatigue, allowing continuous contraction. A: Skeletal muscle fatigues comparatively quickly. B: Smooth muscle contracts slowly and is not intrinsically rhythmic in the same manner. D: Tendons do not contract. Concept: Functional adaptation of cardiac muscle.

Correct: Cardiac muscle is striated and usually has one centrally placed nucleus. A: Smooth muscle lacks striations. B: Skeletal muscle has multiple peripheral nuclei. D: Ligaments are connective tissues. Concept: Microscopic comparison of muscles.

Correct: Smooth muscle contracts slowly and involuntarily in the walls of internal organs. A & D: Skeletal muscle is voluntary. B: Cardiac muscle is limited to the heart. Concept: Physiological role of smooth muscle.

Correct: Cardiac muscle fibers branch and connect through intercalated discs. A & D: Describe skeletal muscle. C: Describes smooth muscle. Concept: Structural comparison of muscle tissues.

Correct: Skeletal muscles attach to bones and allow voluntary movement. A: Cardiac muscle functions only in the heart. B & D: Smooth (visceral) muscles control internal organs. Concept: Functional classification of muscles.

Correct: Smooth muscle consists of spindle-shaped, non-striated, uninucleate cells found in hollow organs. A: Skeletal muscle is striated and multinucleated. B: Cardiac muscle is branched and found only in the heart. D: Tendons are connective tissue. Concept: Identification of smooth muscle.

Correct: Skeletal muscles are attached to bones and produce voluntary body movements. A: Cardiac muscle pumps blood. B & D: Smooth (visceral) muscles control internal organs. Concept: Voluntary muscle action.