GLOSSARY

The I band is the light region of the sarcomere under the microscope, containing only thin actin filaments, with the Z line passing through its centre; each I band actually belongs to two adjacent sarcomeres.
The letter I comes from isotropic, because this region transmits light uniformly in a polarising microscope and appears light.

Characteristics

• Contents: actin, tropomyosin, troponin, and the elastic part of titin.
• No overlap with myosin: which is why it appears lighter.

Changes During Contraction and Stretch