ORAL MECHANORECEPTORS IN TRITONIA-DIOMEDEA .1. ELECTRO-PHYSIOLOGICAL PROPERTIES AND LOCATION OF RECEPTIVE-FIELDS

1. Central somata of oral mechanoreceptor neurons in Tritonia diomedea are located in two bilaterally symmetrical clusters, each in a small lobe of the cerebral ganglion (Fig. 3). Each lobe contains 40-60 cells, each of which sends one axon through the fourth cerebral nerve to the oral tube, and one axon through the cerebro-buccal connective to the buccal ganglia (Fig. 9). From the buccal ganglia, mechanosensory axons pass through one or more of the 6 major buccal nerves innervating the buccal mass. 2. Mechanosensory cell bodies respond to current injection with a rapid, phasic discharge followed, in some cases, by a more prolonged tonic discharge. No electrical coupling nor chemical synaptic interaction was observed between the cells. Mechanosensory neurons are typically silent unless stimulated. Evidence is presented that these are primary sensory cells. 3. Oral mechanoreceptors produce a rapidly accommodating discharge in response to pressure on receptive fields in the oral tube, jaw closer muscles, and internal buccal mass. This response was only elicited by pressure or stretch, with rapid application most effective. 4. A mechanoreceptor typically has a single receptive field in the ipsilateral oral tube, and one or more fields in the muscles of the buccal mass, both ipsi-and contralateral (Fig. 8). Fields tend to be small (1 to 5 mm2) and to overlap in some areas. 5. On rare occasions, naturally induced firing was recorded in mechanoreceptors during buccal mass movements with an object in the buccal mass. Firing was not recorded with the buccal mass empty. It is hypothesized that these receptors are activated by the resistance caused by the presence of food in the buccal mass during normal feeding movements. © 1979 Springer-Verlag.