[Fiber-type morphology and function of the triads in frog (Rana esculenta) skeletal muscle)]

Abstract

1. Owing to differing structural characteristics of the contractile substance, the muscle fibres have been divided into the three types A, B and C in former papers. This distinction seems to be corroborated by our investigations into the different structure regarding the traids. As for the A-fibres, they are structured in terms of the T-system being connected in its entire length with the SR-cisternae and circling the myofibrils at the level of the Z-layer. In the B-fibres, this permanent couping of the two membrane systems is partially interrupted so that the T-tubules are arranged round the myofibrils in such a way that they are isolated or only coupled on one side with the SR-cisternae. Apart from the triads we also find diads. There is a totally different arrangement of the membrane systems in the C-fibres. In this instance the T-tubules are not only arranged transversally but also vertically along the contractile elements. They are surrounded by an "SR-labyrinth" which forms individual minor cisternae which are lateraly coupled with the T-tubules. So the axis of these triads is turned by 90 degrees as compared to the A and B fibres. As a result of this different arrangement, these triads always appear in cross-sections, not however, in longitudinal sections as is the case with the A and B fibres. The tirads have a varying shape in the cross-sections depending on the level of the section and due to the fact that the cisternae are not always coupled congruently with the T-tubules. 2. In our discussion we have tried to related these differing shapes and arrangements of the triads in the fibre types A, B and C to known physiological findings. Therefore we deduced that the excitation transmittance and calcium release can be correlated with the anomal rectification of the triads, which has been localized in the region where the T-tubules and SR-cisternae are coupled. However, we can only reckon with a solution once the morphology and function of the "feet" and the eletronmicroscopically "blank" spaces which fill the gap-junction between the T-tubules and the SR-cisternae have been explained. Whatever function the free surface of the T-tubules has remains open. It is directly adjoining the sarcoplasm and we are tryping to relate it to the delayed rectification which appears on the fibre membrane. 3. Moreover from the arrangement of the SR-cisternae i- the individual fibre types we can deduce th intrafibrillar directions of expansion of the calcium after its release and thus the process of the excitation in the A, B and C fibres. It appears that calcium is being directed homogeneously from the SR-cisternae of the A-fibres to the actinfilaments. here the morphological appearance of the twitch fibre presents itself to us. In principle this pattern of expansion of calcium in the B-fibres remains consistent. Owing to the interruption between the T-system and the SR-cisternae we may assume that the process of contraction is delayed in contrast to the A-fibres...