The effect of smooth muscle antagonists on the sound-induced motion of the tympanic membrane

Abstract

The pars tensa of the tympanic membrane is composed of three layers: an epidermal layer, a fibrous layer, and a mucosal layer. Recent studies (Kuijpers et al, 1999; Henson and Henson, 2000; Henson et al, 2005) suggest that the fibrous layer in several mammalian species contains contractile fibers, which are located primarily within the thickened border of the pars tensa known as the annulus fibrosis. These contractile fibers resemble smooth muscle fibers. Yang and Henson (2002) studied the physiological effects of pharmacological modulators on the pars tensa of the annulus fibrosis by measuring the sound-induced cochlear response. Their results suggest a dose-dependent change in cochlear response after application of sodium orthovanadate and norepinephrine. Application of saline induced no change in cochlear response. Based on their data, Yang and Henson proposed that the pharmacological agents altered the function of the smooth muscle fibers of the annulus fibrosis to produce a mechanical change in the tympanic membrane. In this study two measurements, cochlear response and Laser Doppler Vibrometry, were used to assess the sound-induced velocity of the tympanic membrane of the gerbil before and after application of saline and varying concentrations of three smooth muscle antagonists (sodium orthovanadate, norepinephrine, and carbachol) to the pars tensa. It was demonstrated that applications of saline and varying concentrations of sodium orthovanadate were associated with both increases and decreases in the magnitude of the cochlear response in two out of three ears tested. There was no evidence of a dose-dependent change in the cochlear response.

(cont.) Applications of saline and varying concentrations of sodium orthovanadate, norepinephrine, and carbachol were associated with increases and decreases in the magnitude of the Laser Doppler Vibrometry response in eight of fourteen ears tested. Evidence of a dose-dependent change in Laser Doppler Vibrometry results was obtained in one ear. The results of this study suggest that application of any substance to the tympanic membrane may or may not be associated with an increase or decrease in the cochlear response or Laser Doppler Vibrometry response, and thus, the source of mechanical changes observed at the tympanic membrane is not necessarily the smooth muscle fibers of the annulus fibrosis.