Locomotion and Movement – Solutions

1. Define the sliding filament theory of muscle contraction.

Solution

The sliding filament theory explains the process of muscle contraction, during which the thin filaments slide over the thick filaments, which shortens the myofibril. The contraction cycle involves five interlocking steps:

STEP 1: Exposure of active sites. The calcium ions entering the sarcoplasm blind to troponin. This binding weakens the bond between the troponin-tropomyosin complex and actin. This allows cross-bridges to form.

STEP 2: Attachment of cross-bridges. When the active sites are exposed, the myosin cross-bridge bind to them.

STEP 3: Pivoting. In the resting sarcomere, each cross-bridge points away from the M line. In this position, the myosin head is “cocked” like the spring in a mousetrap, and energy is obtained by breaking down ATP into ADP and a phosphate group. In the cocked position, both the ADP and the phosphate are still bound to the myosin head.