MCQs

The plateau phase of the cardiac action potential is caused by the opening of slow voltage-gated L-type calcium channels. The influx of extracellular calcium balances the efflux of potassium, prolonging the depolarization and ensuring a long refractory period that prevents tetanus.

Parasympathetic stimulation via the vagus nerve releases acetylcholine, which binds to muscarinic receptors on the heart's pacemaker cells, slowing the heart rate (decreasing cardiac muscle activity). In contrast, it generally stimulates digestive smooth muscle.

Smooth muscle does contain actin and myosin, and they slide against each other to cause contraction. However, these filaments are not arranged in the highly ordered, repeating, registered units (sarcomeres) that create the alternating light and dark bands (striations) seen in skeletal and cardiac muscle.

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.

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.