Muscles

Smooth muscle contractions are characteristically very slow to develop and slow to relax. A single smooth muscle twitch can last anywhere from 1 to 3 seconds, which is up to 30 times longer than a typical skeletal muscle twitch, allowing for prolonged, sustained tension.

Cardiac muscle has an absolute requirement for aerobic metabolism to produce ATP. It contains very little glycogen and relies almost entirely on continuous oxygen supply. Skeletal and smooth muscles can rely more heavily on anaerobic glycolysis and can tolerate hypoxia much longer without cell death.

Calcium channel blockers prevent the influx of extracellular calcium. Since both cardiac muscle and vascular smooth muscle rely heavily on extracellular calcium for contraction, these drugs decrease heart rate/contractility and cause vasodilation, effectively lowering blood pressure and reducing heart strain.

Mature cardiac muscle cells (cardiomyocytes) are terminally differentiated and generally lose their ability to divide. Skeletal muscle cannot divide but has satellite cells for limited repair. Smooth muscle retains a significant capacity to divide and regenerate (hyperplasia).

Spindle-shaped cells (tapered at both ends) are characteristic of smooth muscle. This shape allows them to pack tightly together into continuous sheets (often in circular and longitudinal layers) that form the walls of hollow organs, facilitating functions like peristalsis and regulating vessel diameter.

Muscle hypertrophy (growth due to exercise) occurs not by cell division (hyperplasia), but by the enlargement of existing individual muscle fibers. The cells synthesize more actin and myosin, creating more myofibrils, which increases the thickness and strength of the fiber.

In skeletal muscle, calcium binds to troponin on the actin filament to expose binding sites. In smooth muscle, calcium binds to calmodulin in the cytosol, which then directly activates an enzyme (myosin light-chain kinase) that phosphorylates the myosin head to initiate contraction.

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.

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.

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.

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.

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.

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).

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.

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.

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.

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.

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.

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.