THE MUSCLE CELL

General Function Muscles are used for movement Muscles work antagonistically with one contracting (biceps) while an opposing muscle relaxes (triceps) Work in conjunction with the tendons, bones, and ligaments

Muscle Bundles & Cells Many individual contractile filaments called Myofibrils are contained within one muscle cell. Groups of muscle cells are bundled by connective tissue

The Contractile Unit Myofibrils contain the many contractile units called Sarcomeres An individual sarcomere contains a visible Z-line, A and I bands, and H-zone

The Sarcomere The Z-lines create the border at the ends of one sarcomere The I-bands exist in the light area just inside the Z-line The A-bands are the dark areas making up most of the sarcomere The H-zone is the light area in the middle.

Molecular Structure The sarcomere is composed of proteins, actin and myosin The actin (thin) filaments extend from the Z-lines The myosin (thick) filaments exist through the middle

Sarcomere Contraction: The Sliding Filament Theory During contraction, myosin molecules, powered by ATP, form cross bridges with surrounding actin molecules, forcing the actin filaments toward the center The sarcomere and muscle shortens in length

Nervous Control Motor neurons form neuro-muscular junctions with small muscle cell bundles Thus, many muscle cells act in concert during contractions

The Sarcoplasmic Reticulum The sarcoplasmic reticulum stores large amounts of Ca ions When a nerve impulse stimulates the sarcoplasmic reticulum it releases its ions The ions cause molecules on the actin to move exposing the bonding site for myosin