Practice Questions

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.