Mechano-electrical transduction in the turtle utricle

Two classes of mechano-sensory hair cell are present in the vestibular system of mammals, birds and reptiles. Type I hair cells are bottle-shaped and make synaptic contact with afferent calyx terminals. Type II hair cells are cylindrical and contact small bouton afferent terminals. Voltage-dependent basolateral currents have been found to differ between the two cell types and these properties are believed to contribute to the shaping of primary afferent responses. Type I hair cells have a low input resistance, which may be related to the low gain of calyx afferents. In the turtle utricle, type I vestibular hair cells are found only in a narrow band of the sensory epithelium called the striola, whereas type II hair cells are found in both striolar and extrastriolar regions. We have made whole cell patch clamp recordings from type I hair cells, type II hair cells and calyx fibers in order to better understand the processing of vestibular signals. Here we describe responses of hair cells to hair bundle displacement with a stiff glass probe. Mechano-electrical transduction (MET) currents were largest in type I hair cells, where the mean peak amplitude was approximately 500 pA. MET currents in all hair cells showed rapid and slow adaptation.