Figure 9.

Elevation of extracellular divalent cations rescues touch responsiveness in touchdown mutants. A, Video images of 55 hpf touchdown mutant larvae in 5 mm Ca²⁺ showing no response to touch when the skin was not removed (top), and exhibiting swimming in response to touch when the skin over several rostral somites (∼5–10) had been peeled, allowing rapid diffusion of elevated divalent cations into the CNS (bottom). 0 ms is the video frame preceding contact of the embryo by the stimulating probe. Scale bar, 1 mm. B, Average responsiveness of unpeeled touchdown mutant larvae in the presence of 5 mm Ca²⁺ to tactile stimuli compared with peeled mutant larvae in the presence of 5 mm Ca²⁺, Ba²⁺, Sr²⁺, or Mg²⁺ (n = 15 from three clutches). Ca²⁺, Sr²⁺, and, to a lesser extent, Ba²⁺, but not Mg²⁺, increased responsiveness to touch of peeled touchdown mutants. *p < 0.05, t test.