Fig. 3.

Neuronal substrates for radula motor pattern selection and initiation. a Schematic of isolated bilateral buccal ganglia (B.g.) and sites for extracellular recordings of radula motor patterns (below) from peripheral motor nerves conveying protraction (Protr.), retraction (Retrac.) and closure (Clos.) phase motor discharge. During tonic electrical stimulation of sensory nerve 2,3 (Stim.) to mimic the inciting non-ingestible food stimulus applied in vivo (see Fig. 2a), two distinct motor patterns corresponding to ingestion (left) and egestion (right) can be generated. The patterns are distinguished by the relative durations of the protraction/retraction phases and the occurrence (or absence) of closure motor activity during the retraction phase. b Simplified representation of the buccal CPG that drives radula motor patterns. Protraction phase activity (Protr.) is triggered by electrically coupled neurons, including B30/B63/B65. Retraction activity (Retr.) is triggered by a different CPG subset that includes B64/B4/5. Closure motor activity (Clos.) is driven by neurons such as B30/B51, which discharge either uniquely during protraction (egestion) or during both the protraction and retraction phases of activity (ingestion). Resistor symbol electrical coupling, small black circles chemical inhibition, triangles chemical excitation. c Radula motor pattern selection. Ingestive (left) an egestive (right) motor patterns are generated by the same multifunctional CPG. In both patterns, bursting activity in B63 and B64 neurons generate the protraction (P) and retraction (R) phases, respectively. Bursting in neuron B51 occurs uniquely during ingestive motor pattern genesis (left) and elicits closure (C) activity during the prolonged retraction phase (R) of this pattern. In contrast, when B51 remains inactive it enables egestion motor pattern production (right). d, e Motor pattern initiation. The two electrically coupled B63 neurons in the bilateral B.g. are necessary and sufficient to initiate individual radula motor patterns. d Experimental depolarization of one B63 (horizontal bar) triggers bursts in both B63 neurons and elicits a fictive ingestive bite (left). However, motor pattern genesis fails to occur when the contralateral B63 is simultaneously hyperpolarized to prevent it from bursting (right). Note that burst discharge in the B30 and B65 neurons can activate the B63 cells through their electrical coupling (and thereby trigger a motor pattern) but neither are alone (or together) sufficient to trigger a bite cycle. Thus, any one of these cell types can be the first active cell in a radula bite cycle (e), although the actual pattern-initiating process is dependent on the activation of the bilateral B63 neurons