describe the sliding filament theory and define myofibrils, myosin, actin, and sarcomeres?

The cells of skeletal muscles are long fiber-like structures. They contain many nuclei and are subdivided into smaller structures called myofibrils. Myofibrils are composed of 2 kinds of myofilaments. The thin filaments are made of 2 strands of the protein actin and one strand of a regulatory protein coiled together. The thick filaments are staggered arrays of myosin molecules.

Units of organization of skeletal muscle. The filaments are organized into structures called sarcomeres. Sarcomeres are constructed in the following manner:

Z lines are at the borders of the sarcomere. They align in adjacent myofibrils.

I bands are areas near the edge of the sarcomere containing only thin filaments.

A bands are regions where thick and thin filaments overlap and correspond to the length of the thick filaments.

H zones are areas in the center of the A bands containing only thick filaments.

The theory:  The thin filaments ratchet across thick filaments to pull the Z lines together and shorten the sarcomere. The myofilaments themselves do not contract.

Myosin molecules on thick filaments attach to the actin on the thin filaments to form a crossbridge. The cross bridges then bend inward pulling the thin filament toward the center of the sarcomere. These cross bridges are broken and reformed further down .

Energy for cross bridge formation comes from the hydrolysis of ATP by the head region of the myosin.

Skeletal muscle contracts when stimulated by motor neurons.