1. THE MUSCLE CELL

2. 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

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

4. 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

5. 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.

6. 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

7. 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

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

9. 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