Figure 1.

Tadpole behavior, swimming network, and responses to skin stimulation. A, Hatchling tadpole. B, Frames at 13.3 ms intervals from a 300 fps video show a tadpole viewed from above bending first to the left, then swimming forward when touched with a hair on the right side (at arrow). C, Simplified swimming network. Circles represent columns of 30–150 neurons. Synapses are made onto all neuron types inside each box, including neurons of the same type as the presynaptic neuron (Roberts et al., 2010). The skin is innervated by touch-sensitive RB neurons which excite sensory pathway dla and dlc neurons. The symmetrical half-centers of the rhythm generator network have: motoneurons (mn), recurrent inhibitory aINs (Li et al., 2004b), reciprocal inhibitory cINs, and electrically coupled excitatory dINs. The activation of dIN NMDARs transforms the dINs into pacemakers. D, Diagram showing the CNS and electrodes (hb, hindbrain; mb, midbrain). E, Right skin stimulus (↑) leads to spike in a sensory pathway dlc, then excitation and firing in a dIN on the left, then firing in motor nerve on the right (mn R). Swimming started on the left. F, Similar stimulus leads to excitation of dINs on both sides, but the right side dIN and motor nerve fire first as swimming starts. Records in E and F are to the same scale. a, Ascending; c, commissural; d, descending; dl, dorsolateral; IN, interneuron; L, left; mn, motoneuron; R, right; RB, Rohon-Beard.