THE STRUCTURE OF THE STRIATED MUSCLE FIBRE

Summary 1. The muscle fibre is regarded as a bundle of myofibrils, embedded in a viscous matrix, the sarcoplasm, the whole being surrounded by a delicate sheath, the sarcolemma. 2. The sarcolemma is a thin, apparently structureless membrane. Preliminary study with the electron microscope suggests that it may be less than o‐i /u. thick. No fibrils can be seen with the electron microscope, but a large number of small nodules lie scattered over the membrane. The sarcolemma is probably not collagenous in nature. 3. The sarcoplasm is a viscous, protein‐containing gel which surrounds the myofibrils. Mitochondria, glycogen and fat droplets are scattered throughout the sarcoplasm. Several enzymes have been isolated from what is believed to be the sarcoplasm. This suggests that the latter may be the site of important metabolic activity. The question of sarcoplasmic contractility is not yet settled. There is good evidence that it may occur in certain cells (myoblasts, striated fibres of tadpoles and insect larvae). 4. The evidence in favour of the existence of myofibrils as contractile units is discussed. The fact that individual myofibrils or groups of myofibrils can be made to contract independently, and sometimes asynchronously, is strong support for this view. 5. The electron microscope shows that the myofibrils consist of relatively straight protein chains. Several different striations (A, I, Z, M, N, H) can be recognized. There is no gross difference in orientation between the protein chains in the various bands, nor is there any evidence of a spiral structure. 6. Several aspects of Szent‐Gyorgyi's work are discussed. While this work has led to a better understanding of the muscle proteins, the theories of muscle structure arising from it have found no general acceptance. 7. Other theories attempting to account for the existence of the cross‐striations are reviewed. It is suggested that there is a periodic distribution of different chemical substances in the different bands. This may affect the physical, mechanical, and chemical properties of the myosin in those bands, and may account for the differences in birefringence between the A and I bands. 8. Evidence is presented against the existence of a Z ‘ membrane’ or other transverse membranes uniting the myofibrils. No structural connexion is believed to exist between the sarcolemma and the underlying sarcoplasm and myofibrils. The relationship may be a purely viscous or frictional one. 9. Reasons are suggested for the perfect alinement of the striations in neighbouring myofibrils. A full explanation may only be possible as a result of a better understanding of intermolecular and colloidal forces.