In general, the disclosure describes techniques for modifying therapy provided to a patient by a medical device. where, R8, R9, R10, and R11 are each independently selected from H, F, Cl, Br, CN, NO2, alkyl, haloalkyl, and alkoxy groups; X and Y are each independently O, S, N, or P; and Z\u2032 is a linkage between X and Y, and at least one of R8, R9, R10, and R11 is other than H. The information may be recovered from the salient features of this overall response. A control bearing is arranged between the control signal emitter and the control spring, which guides the control signal emitter and which supports the control spring. The membrane extends into the tract without friction thereby reducing the discomfort and the risk of trauma or injury to the animal.