Eardrum Displacement Following Stapedius Muscle Contraction

Simultaneous monitoring in human subjects on the same ear of eardrum displacement by tympanomanometry, and acoustical impedance by the Madsen bridge, provided information concerning contraction of the stapedius muscle and its effect on eardrum displacement. Extensive control procedures were employed to elicit only the stapedius lower-intensity auditory stimulation, electrocutaneous stimulation of the homolateral external ear canal, and anesthetization of nerves leading to the tensor tympani. Extremely small biphasic and monophasic eardrum movements were seen in the stapedius-only ear to auditory and electrocutaneous stimulation. At higher sound intensities relatively large inward and biphasic movements of the eardrum occurred in the normal ear, unquestionably due to contraction of the tensor tympani. Biphasic responses did not occur in the tensor tympani-only ear only monophasic inward responses. Upon air-jet stimulation to the orbit of the eye, these subjects had an accentuated tensor response in that large inward movements of the eardrum occurred as compared to those in normal ears Estimates of the actual eardrum displacements were calculated based on a model of the external ear canal and eardrum.