Which region of the sarcomere contains only actin thin filaments or microfilaments?

I bands

Explanation:

The contractile cells of skeletal muscles are very long and multinucleated. The cell membrane is called sarcolemma. Much of the cell is occupied by contractile elements, the myofibrils, which are arranged bundles parallel to the axis of contraction. Each myofibril is about 1 mm in diameter. Myofibrils are immersed in a cytosol called sarcoplasma that contains glycogen, glycolytic enzymes ATP, ADP, AMP, phosphocreatine, creatine, various amino acids and inorganic electrolytes.

In the red muscles are many mitochondria arranged regularly.

An electron mycograph of a myofibril presents a functional unit called a sarcomere that is repeated every 2, 3 uM along the axis of the myofibril. There is a regular alternation of a dark band A and a light band I. The central part of band A, called zone H, is less dense than the rest of the band. In the middle of zone H there is a dark line M. Band I is divided in two parts by a very dense Z line.

The cross section of the myofibrils reveals two kinds of filaments: thick filaments of around 150 Aº in diameter and other thin ones of about 70 Aº in diameter. The thick filaments are myosin and the thin ones contain actin, tropomyosin and troponin. Band I consists only of thin filaments (actin) while in zone H of band A only thick filaments are present.