Slow muscle fibers in the tensor tympani muscle of the guinea pig

Smooth muscle in the annulus fibrosus of the tympanic membrane: physiological effects on sound transmission in the gerbil

In a wide variety of mammals, the rim of the tympanic membrane (annulus fibrosus) has an array of contractile elements, either smooth muscle [Henson and Henson, J. Assoc. Res. Otolaryngol. 1 (2000) 25-32] or myofibroblasts [Kuijpers et al., Hear. Res. 128 (1999) 80-88]. These elements are anchored peripherally to the bony tympanic ring and centrally to incoming fibers of the pars tensa. Their arrangement suggests that they are involved in the control of tympanic membrane tension. In this study, cochlear microphonic (CM) threshold changes were recorded in gerbils to study the physiological effects of these contractile elements. It was demonstrated that the application of substances known to make smooth muscle contract (vanadate and norepinephrine) caused concentration-dependent elevations in CM thresholds. Maximum changes of 8-9 dB occurred with the lowest frequency tested (2.16 kHz). The application of muscle-relaxing drugs reversed these effects. Controls showed that the threshold changes were not induced by effects on middle or inner ear structures. These results add to emerging evidence that the tympanic membrane has intrinsic control of tension and is potentially able to have some control over energy levels reaching the cochlea.

Rattlesnake shaker muscle: I. A light microscopic and histochemical study

Rattlesnake body and shaker muscles were studied using light microscopy and histochemistry. Five myofiber types are distinguishable in the body musculature. The majority are large diameter fast twitch fibers with high alkaline-stable ATPase activity and few mitochondria. In the shaker muscle the major fiber differs from all body fibers in that myofibrils do not entirely fill the fibers. The myofibrils branch repeatedly with one another, which leaves large areas of sarcoplasm devoid of filaments and gives the fibers a characteristic mottled appearance. Mitochondria and glycogen deposits are very numerous. Shaker fibers have high alkaline stable ATPase activity and, in addition, stain intensely for NADH-TR and alpha GPD. Myofibers of the shaker muscle are unusual in that they are extremely fast contracting yet are highly fatigue resistant.