Figure 8.

Cutaneous mechanoreceptors respond to tactile stimuli in touchdown mutants. A, A representation of the preparation used to record electrical and tactile-evoked responses in RBs. The dashed line designates the dorsal midline. The light gray area indicates the region in which the skin and muscle were removed to allow access to a contralateral RB cell body with an intact peripheral neurite (white). The stimulating probe was placed on the skin directly overlying the peripheral neurite. B, RB responses to depolarizing current injections (2 ms) to the cell body, and extracellular electrical stimulation (1 ms) of the peripheral neurite showing that the RBs are excitable in wild-type and touchdown mutant larvae. C, Touchdown mutants possess both type I and type II RBs (see text). In type I RBs, a subthreshold tactile stimulus (black) applied to the peripheral neurite resulted in an observable generator potential (arrows) at the onset and offset of the stimulus. In these RBs, increasing the amplitude of stimuli eventually triggered action potentials (cyan) at the onset and the offset of the stimulus. In type II RBs, activation by tactile stimuli produced action potentials (cyan) without visible generator potentials (arrowheads) in response to subthreshold stimuli (black).