A comparative study on the fine structure of tongue and cricothyroid muscle of the bat, Myotis grisescens, as revealed by thin section and freeze-fracture techniques

Abstract

Tongue and cricothyroid muscle fibers of the bat, Myotis grisescens, revealed differences which included sarcomere lengths, myofibrillar diameters, amounts of sarcoplasmic reticulum (SR) per unit volume, and triadic disposition. In the cricothyroid muscle fiber sarcomere lengths were longer, myofibrillar cross-sectional diameters were less, amounts of SR per unit volume were greater, and triads were located within the A-band level, rather than at the A/I junction as occurs in tongue muscle fibers. Triads of both muscle fiber types appeared morphologically similar. Tannic acid processing highlighted plasmalemmal and T-tubule outer laminae of the unit membranes as well as material within the T tubules not otherwise seen without tannic acid processing. Size (7–8 nm) and numbers of plasmalemmal (1500–2000 μm2) and SR (2500–3000 μm2) PF particles were the same in both muscles. T-Tubule particles at triadic junctions were 10–15 nm in size and were found on the EF face in both muscle fiber types. Orthogonal arrays of 5- to 6-nm size particles were found on the PF face of the cricothyroid muscle fibers only. It is concluded from this study and on the basis of previous evidence that the orthogonal arrays of 5- to 6-nm particles may be a consistent feature of muscle fibers whose excitation—contraction—relaxation cycles fall above a certain level. It is further concluded that it is the characteristics of the myofibrillae, such as sarcomere length, cross-sectional diameter, triadic positions, and amounts of SR per unit volume of muscle, which best reflect contraction—relaxation speeds in the muscle studies here.