The transverse tubular system of cat intrafusal muscle fibres

Summary A modified staining technique for transverse tubular and sarcoplasmic reticular systems was used to investigate their occurrence in different types of intrafusal muscle fibres in cat tail spindles. Intrafusal muscle fibres can be divided into three basic regions, namely, the periaxial space (A-region), intracapsular area (B-region) and the extracapsular area (C-region); the components of these systems were seen to vary in structure and distribution. The occurrence of these systems also varied among the different types of intrafusal muscle fibres, namely, the bag₁, bag₂ and chain fibres. In bag₁ fibres components were sparse in the A-region, increased slightly in the B-region, but were most developed in the C-region; triads were consistently located at the border between A- and I-bands. In bag₂ fibres membrane components were noted in the A-region but were more abundant in the B-region where some tubular components showed transverse and longitudinal branches linked together in the form of a network; membrane systems diminished towards the C-region. The majority of triads were located within the A-bands. In chain fibres the membrane systems occurred more commonly in the A-region, while in the B- and C-regions, the transverse tubular system possessed numerous transverse and longitudinal branches forming irregularly distributed tubular networks. Some tubular branches were dilated, while other branches terminated as sacs among arrays of the sarcoplasmic reticular system in I-bands. Some transverse tubules bifurcated into two branches with numbers of dilated sarcoplasmic reticular cisternae lying on either side, or between, the branches. Triads sometimes occurred between A- and I-bands, but, generally, were situated well within A-bands.