THE MOTOR CONTROL OF FEEDING IN NAVANAX (MOLLUSCS, NEUROPHYSIOLOGY, CELLULAR)

Abstract

Towards the goal of understanding the neurophysiological basis of a behavior, the neural control of the movements occurring during feeding was studied in Navanax, an opisthobranch mollusc. Interesting questions of motor organization are raised by the capability of circumferential pharyngeal muscle to constrict sequentially during peristalsis and of expansion muscle to produce regional and generalized pharyngeal expansions during feeding.;The pharyngeal surface contains a gridwork formed by circumferential and longitudinal bands. Band number and position show little interanimal variability. Individual bands are identified by number in sequence. Circumferential bands have reproducible, regional morphologic specializations; the number of bands in each subgroup is characteristic.;Motoneuron fields were quantitatively determined and described with respect to this two dimensional reproducible pharyngeal reference system. Circumferential motor fields were directly described in terms of identified motoneurons innervating identified muscle bands. A population of circumferential motoneurons with small, nonoverlapping and annular motor fields underlies the capability of the Navanax pharynx to generate a fine peristaltic wave. The spatial distribution of these small fields corresponds to the regional morphologic specializations. Most small fields comprise an entire regional subgroup. Circumferential motor fields, like the band arrangement, show little interanimal variability; this may result from natural selection against variations with substantially impaired capability to perform peristalsis. Several circumferential motoneurons have giant motor fields which can produce a synchronous pharyngeal squeeze during feeding. A population of expansion motoneurons have small regional motor fields which provides a neurophysiological basis for the hypothesized role of expansion motoneurons in peristalsis. A regional expansion caudal to a circumferential constriction can augment the peristaltic wave. Expansion motor fields exhibit greater interanimal variability than do circumferential fields. The observed variability represents variations on a typical pattern. A population of expansion motoneurons have giant motor fields that cover the entire pharynx which can mediate generalized expansion.;Motoneuron populations with giant as well as small motor fields provide a second mechanism in addition to coupling and coupling reversal for circumferential muscle to constrict asynchronously in peristalsis and synchronously during a generalized pharyngeal constriction. Motoneurons with apparently more important or unique roles in feeding have less interanimal motor field variability.