VPB 121: Veterinary Physiology - II (2+1)

Sarcoplasm and Myofibrils

SARCOPLASM AND MYOFIBRIL

Sarcoplasm

• It is the fluid present inside each fibre in which the organelles like sarcosomes (mitochondria), sarcoplasmic reticulum and myofibril are 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.