Motor program switching inPleurobranchaea

Abstract

1. Three distinct motor programs, namely ingestion, egestion and a third slower rhythm, have been identified in the buccal musculature of intact specimens of the gastropod molluscPleurobranchaea (Croll and Davis 1981). In the present study we have elicited each of these motor programs and characterized them in greater detail utilizing the technically more amenable reduced preparation, consisting of mouth, buccal mass, esophagus and attached central nervous system. 2. The ingestion motor program, rigorously identified by inward movement of objects placed in the buccal mass, is characterized by alternate bursts of activity in protractor and retractor muscles. The duration of the retraction phase of the cycle is longer than the duration of the protraction phase. The buccal constrictor muscle fires in phase with retractors. Salivary duct activity is in phase with protraction. In accordance with these muscle activities, the activity of buccal nerve root 3 is of longer duration and more intense than that of buccal root 1 during ingestion, and the salivary nerve is active in bursts during ingestion. The stomatogastric nerve fires cyclically in phase with retraction. 3. The egestion motor program, rigorously identified by outward movement of objects, is also characterized by alternate bursts of activity in protractor and retractor muscles. The egestion motor program differs from ingestion, however, in several specific respects. First, the duration of the protraction phase of the cycle is longer than the duration of the retraction phase. Second, the buccal constrictor muscle is silent during egestion. Third, the salivary duct is typically also silent during egestion. Fourth, the amplitude of recorded potentials from retractor muscles (m.1 and m.3) is significantly suppressed during egestion. Fifth, as would be expected, the activity of buccal nerve root 1 is longer and more intense than that of buccal root 3. Sixth, the salivary nerve is silent. Seventh, the stomatogastric nerve fires tonically rather than cyclically during egestion. 4. The ‘neutral’ or third buccal rhythm is generally significantly slower than ingestion or egestion, but is qualitatively similar in most respects to ingestion and dissimilar from egestion. 5. Stretching the esophagus induces the egestion motor program. This reflex is interpreted as a ‘clearance’ reflex which ensures egestion of objects too large to swallow. 6. Low-intensity stimulation of the stomatogastric nerve elicits a motor rhythm similar to ingestion but different from egestion. High-intensity stomatogastric nerve stimulation elicits the ingestion motor program, interrupted by stereotypic episodes of egestion.