Muscles

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.

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.

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.

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).

Both skeletal and cardiac muscles are striated. This striation is due to the highly organized, identical structural arrangement of actin and myosin filaments into repeating contractile units called sarcomeres, bounded by Z-lines.

Acetylcholine at the neuromuscular junction of skeletal muscle binds to nicotinic receptors to cause depolarization. In the heart, it binds to muscarinic receptors on pacemaker cells, opening potassium channels, which hyperpolarizes the cells and slows the heart rate.

Cardiac muscle represents a biological exception: it is striated like skeletal muscle but is completely involuntary in its control, like smooth muscle. It operates automatically without conscious thought.

The stress-relaxation response is a unique characteristic of smooth muscle, allowing hollow organs like the urinary bladder and stomach to expand and store contents without immediately triggering a strong reflex contraction that would expel the contents.

A motor unit is the fundamental functional unit of skeletal muscle control. It is defined as a single alpha motor neuron and all the individual skeletal muscle fibers it stimulates. When the neuron fires, all fibers in the unit contract synchronously.

Smooth muscle has the slowest speed of contraction and relaxation, allowing for long, sustained contractions. Cardiac muscle has an intermediate speed, and skeletal muscle has the fastest contraction speed, allowing for rapid, voluntary movements.

Cardiac muscle has an unusually long action potential and absolute refractory period (lasting almost as long as the contraction itself). This biological adaptation prevents multiple action potentials from summing and causing tetanus (sustained contraction), which would be fatal as the heart must relax to fill with blood.

Arteriolar diameter is controlled by vascular smooth muscle. Smooth muscle cells are spindle-shaped, uninucleated, involuntary, and lack the organized sarcomeres that create striations in skeletal and cardiac muscle.

Skeletal muscle contraction relies almost entirely on intracellular calcium released from the well-developed sarcoplasmic reticulum. Cardiac and smooth muscles heavily depend on the influx of extracellular calcium to trigger contraction (calcium-induced calcium release).

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.

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.

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.

Gap junctions are crucial for the rapid cell-to-cell transmission of electrical impulses in single-unit smooth muscle (responsible for peristalsis) and cardiac muscle (responsible for coordinated heartbeats). Skeletal muscles lack gap junctions and are individually innervated.

Cardiac muscle is myogenic, possessing specialized pacemaker cells (like the SA node) that spontaneously generate action potentials. Certain types of single-unit smooth muscle (e.g., in the gut) also exhibit spontaneous pacemaker activity (slow waves). Skeletal muscle is entirely neurogenic.

The described cells are skeletal muscle fibers, characterized by being syncytial (multinucleated) with nuclei at the periphery. Their primary function is voluntary movement. Loss of this tissue would prevent somatic motor functions, leaving autonomic functions intact.