Skeletal Muscle Structure and Contraction

Flashcards covering skeletal muscle anatomy and the excitation-contraction coupling sequence (excitation, coupling, contraction, relaxation).

What are skeletal muscles (pelinal muscles) and how are they organized?

They are voluntary muscles controlled by the nervous system; organized into fascicles, which are bundles of muscle fibers surrounded by connective tissue, blood vessels, and nerves.

What is a motor unit?

A motor nerve axon and the one or more skeletal muscle fibers it innervates.

What are myofibrils and sarcomeres?

Myofibrils are thread-like structures inside a muscle fiber; they are organized into contractile units called sarcomeres.

What are the thick and thin filaments, and which proteins form them?

Thick filaments are made of myosin; thin filaments are made of actin.

What are cross bridges and what is their role in contraction?

The protruding heads of myosin form cross bridges that bind to actin and ATP, enabling contraction.

What do tropomyosin and troponin do at rest?

They inhibit contraction by blocking myosin binding sites on actin.

Where does skeletal muscle contraction begin and what surrounds each myofibril?

It begins at the sarcolemma; transversely oriented t-tubules surround each myofibril and connect to the sarcoplasmic reticulum.

What is the sarcoplasmic reticulum and what does it store?

The sarcoplasmic reticulum stores calcium and releases it into the sarcoplasm via voltage-gated channels.

What happens during excitation in muscle contraction?

A motor nerve fires an action potential at the neuromuscular junction, depolarizing the sarcolemma and propagating the impulse into the t tubules to excite the muscle fiber.

What happens during coupling in muscle contraction?

The action potential depolarizes the T tubules, causing the sarcoplasmic reticulum to release calcium into the sarcoplasm; troponin binds calcium and moves away from tropomyosin.

What happens during contraction in muscle contraction?

Tropomyosin shifts away from actin to expose myosin binding sites; myosin binds actin using ATP, forming cross-bridges and generating the contraction.

What is the sliding filament mechanism?

Released energy causes myosin heads to rotate and pull the thin (actin) filaments inward, sliding past the thick (myosin) filaments and shortening the sarcomere.

What happens during relaxation?

Calcium ions are pumped back into the sarcoplasmic reticulum; myosin–actin bonds break; cross-bridges disengage; the sarcomere lengthens and the muscle relaxes.