Fig. 1.

Stimulation of the same input elicits two types of mutually antagonistic motor programs. (A) Both ingestive and egestive programs consist of two phases of activity: radula protraction (Prot) coincident with activity in the I2 nerve (39), and radula retraction (Ret) coincident with large-unit activity in buccal nerve 2 (Bn2) (18). In Figs. 1, 2, and 4, activity associated with protraction is shown in red and activity associated with retraction is shown in blue. Gray ovals are cluster boundaries (15) of the ingestive and egestive programs in the plane spanned by the firing frequencies of radula-closer motoneuron B8 (19) in protraction (abscissa) and retraction (ordinate). Representative recordings of ingestive and egestive programs are shown in the clusters. Both programs were elicited by stimulating the same identified CBI-2 neuron at 9 Hz. (B) CBI-2 was stimulated at 9 Hz for the duration of the protraction phase (19.4 ± 0.6 s). A 30-s rest was then allowed before the beginning of the next CBI-2 stimulation. Eight consecutive programs (orange circle, 1–8) were elicited in this way, then a single program was elicited 2, 4, 7, and 12 min later (black circle) (n = 7). EN was stimulated with 3-ms current pulses at 2 Hz for 2 min, with stimulation amplitude adjusted so that about five programs (orange triangle, 1–5) were elicited. Then EN was stimulated 1, 3, and 6 min later to elicit single programs (black triangle) (n = 5). (C) Same data as in B plotted against time. Repeated CBI-2 stimulation (Upper, buildup) elicited progressively increasing B8 firing in retraction but not in protraction; repeated EN stimulation (Lower, buildup) elicited the converse changes in the pattern of B8 firing. Throughout the figures, all group data are shown as mean ± SE. Statistical significance was tested with the two-tailed t test: ***, P < 0.001; **, P < 0.01; *, P < 0.05; n.s., P > 0.05.