Seismic sensitivity has been little studied in vertebrates. Recent neurophysiological work has found frogs to be exceptionally sensitive to substrate vibrations and has established that the sacculus and lagena of the inner ear are involved in these responses. A specialized musculo-skeletal system, termed the opercularis system, is responsible for driving inner ear responses to vibration. The opercularis system consists of a movable cartilaginous element, the operculum, lying next to the inner ear, and an opercularis muscle arising from the operculum and inserting on the shoulder girdle skeleton. Work on the bullfrog suggests that substrate motion imparted to the forelimbs and shoulder region produces movement of the opercularis muscle and, consequently, movement of the operculum. Opercular motion apparently produces compression waves within the inner ear fluids that stimulate the appropriate sensory cells. Given the documented sensitivity of frogs to seismic stimuli, the opercularis system of amphibians represents a good model system for analyzing how vibration energy may reach and act on the inner ear. Many biomechanical aspects of opercularis system function need to be investigated. Proposed research includes analysis of: 1) the significance of differences in motion of the head (including the inner ear) and shoulder region in both phase and amplitude in determining sensitivity, including analysis of possible structural resonances in the shoulder girdle that might enhance responsiveness; 2) the role of opercularis muscle activity patterns and tension in determining sensitivity of the system; 3) responsiveness of the opercularis system to vibrational motion in all three dimensions (representative of the types of motion that may be encountered in nature); and 4) the functional significance of structural diversity of the system observed in different amphibian species. The proposed research represents the first comprehensive study of the biomechanics of a vertebrate seismic sensory system, and will provide important information concerning the mechanical basis of poorly understood but potentially very significant sensory modality in vertebrates. It will also provide basic information on the functional capabilities of the vertebrate inner ear, and will have bearing on understanding the mechanics of vibration transmission in body tissues that may be relevant to the general problem of vibratory stimulation of inner ear endorgans, such as the phenomenon of bone conduction hearing in humans.
