STRUCTURE OF A MYOFIBRIL -

1. The dark bands of the myofibril are termed the A-bands (Anisotropic bands).
2. Each A-band has at its middle a light zone called H-zone (Henson's line).
3. The light bands are also called I-bands (Isotropic bands).
4. Each I-band has at its centre a dark membrane termed the Krause's membrane or Dobie's line or Z disc or Z-line (Zwischenscheibe line).
5. The sarcolemma is invaginated to form T -tubules (Transverse tubules).
6. The T-tubules are present at the level of Z-line.
7. The T-tubules form a simple mechanism of transportation of nuirients and carry the signals for contraction of the myofibrils from the sarcolemma to the interior.
8. The part of the myofibril between two successive Z-lines is called sarcomere.
9. Therefore, the sarcomere comprises A-band and half of each adjacent I-band.
10. A sarcomere is about 2 to 3 mm long.
11. The sarcomere is the functional unit of contractile system in muscle. In fact each sarcomere is a bundle of primary and secondary filaments.

(i)       Primary Filaments (= Myosin Filaments). The primary filaments are thicker and confined to the A-bands only.

They are free at both the ends. They are composed of myosin (protein) and bear minute projections called cross- bridges of the protein meromyosin.

(ii)      Secondary Filaments (= Actin Filaments). The secondary filaments are thinner and occur in I-bands, but extend for some distance into the A-bands between the primary filaments. This partial over lapping of the primary fila- ments by the secondary filaments gives dark appearance to the Abands. The secondary filaments are composed of the proteins actin, tropomysin and troponin. They are attached to the Z-lines by one end and are free at the other end. The secondary filaments are more numerous than the primary filaments. Six secondary filaments surround each primary filament. "

1. The major component of muscle is water. Potassium is the most abundant mineral element in muscle.
2. Other minerals such as sodium, calcium, phosphorus and magnesium are present only in traces.
3. Muscles store glycogen. They have oxygen carrying pigment myoglobin or "muscle haemoglobin".
4. Muscles also contain ATP, phosphocreatine, creatine, urea, etc.