Sarcoplasm and Myofibrils
Sarcoplasm
Sarcoplasm is the fluid present inside each fiber in which the organelles like sarcosomes(mitochondria), sarcoplasmic reticulum and myofibrilare suspended.
Large number of sarcosomes is placed in between the myofibrils, indicates the large quantity of energy (ATP) requirement for the contraction of fibrils.
Cytoplasm of the muscle cell is called sarcoplasm which contains myoglobin. It is an O2 storage protein and has a higher affinity for O2 than haemoglobin. Myoglobin binds with O2 from oxygenated hemoglobin in the blood stream. This O2 is used by mitochondria for aerobic metabolism. Oxygenated myoglobin is red in colour and myoglobin concentration varies from 1-4 mg/g in mammals.
Myofibrils
Myofibrils which are elongated protein threads are the contractile organelles. They lie parallel with the long axis of the muscle cell. Myofibrils occasionally branch and this branching is a growth mechanism.
Not covered by any membrane and are insoluble at the ionic strength of the sarcoplasm. 12 to 14 proteins have been identified in the myofibrils: Actin (15% by wt), Myosin(45%), Tropomyosin (4%), Troponin (4%), C- Protien (2.5%), Actinin (3%), M-Protein (3%), Creatine kinase(1%), Desmin (1%), Filamim (1%), Titin (10%), Nebulin (3%), Gelsolin (1%) and Para myosin (6.5%).
Myofibrils are cross striated, i.e they consist of alternating light and dark bands – transversely striated.
The dark band is anisotropic or birefringent called ‘A’ band. The light band is isotropic called ‘I’ band. The I band is bisected by a dark disc called ‘Z’ disc or ‘Z’ line. (The distance from ‘Z’ disc to next is called sarcomere and is 2.5 to 2.8 μm in resting mammalian muscle.
During contraction the sarcomere length reduced to 2 or even to a maximum of 1.8m). A contractile unit of the myofibril is the sarcomere. In the middle of the dark A band there seen H zone or Hensons band.
Myofibrils consist of two kinds of filaments – thick and thin in an interdigitating fashion. Thick filaments got double the thickness over thin filaments (14-16nm) and have a length of 1.5 μm. Thin filament got 1 μm length anchored to the Z disc.
Thick filaments got cross bridge or protrusions projecting outside. Thick filament – myosin and thin filament – actin, tropomyosin and troponin. Z disc composed of actinin, actin and tropomyosin. ‘M’ line is present which is a transverse structure connecting thick filaments at their centre is made up of M protein and creatine kinase.