Motor program switching in Pleurobranchaea - I. Behavioural and electromyographic study of ingestion and egestion in intact specimens

1. The gastropod mollusk Pleurobranchaea performs at least two distinct cyclic behaviours using its buccal musculature, namely ingestion of food and egestion of unpalatable objects. The movements and motor programs underlying each of these behaviours have been characterized by cinematography and electromyography in intact specimens. A third buccal movement of unknown behavioural significance, is also described. 2. Both ingestion and egestion entail cyclic protraction and retraction of the radula within the buccal mass at slightly different cycle frequencies (Table 1). Cinematography has revealed three principal differences in the respective movements (Figs. 1, 2): 1) during egestion, the radula protraction phase of the movement cycle (the functional power stroke) is proportionately longer in duration than protraction during ingestion (the functional return stroke); 2) during the retraction phase of ingestion the radula is medially folded and rolls inward, while during the retraction phase of egestion the radula is flattened and does not roll inward; 3) during ingestion the jaws close near the end of retraction, while during egestion the jaws close near the beginning of retraction. 3. The functional morphology of the buccal musculature was studied anatomically (Fig. 3) and physiologically (Table 2). Attention was focussed on seven muscles, two radula retractors, two protractors, a buccal constrictor, jaw closer and lip retractor. 4. Electromyograms (EMG's) from five of these muscles were made during ingestion, egestion and the third, undefined rhythm. Ingestion and egestion are characterized by reliable differences in EMG activity which are consistent with observed differences in the movements (Figs. 4-6, Tables 3 and 4). The EMG differences during egestion include: 1) a relative increase in protractor muscle discharge (duration and intensity); 2) reduction or elimination of discharge in the buccal constrictor, a muscle which contributes to radular folding; 3) increase in the activity of lip retractor and jaw closer muscles; 4) a phase advance of the jaw closer muscle in the retraction phase of the cycle. The respective motor programs for the different behaviours are summarized in bar diagrams (Fig. 7). 5. The results are consistent with the hypothesis that ingestion and egestion employ the same basic central nervous oscillator and motor neurons, with distinctions between these behaviours caused by shifts in the balance of activity in different motor pools.