Input-output relationships of identified buccal neurones involved in feeding control in Aplysia

Optical methods can be utilized to map the location and activity of putative motor neurons and interneurons during rhythmic patterns of activity in the buccal ganglion of Aplysia

We sought to develop a map of the locations of neurons that are active during patterned activity in the buccal ganglion of Aplysia using optical techniques. Staining ganglia with a voltage-sensitive absorbance dye (JPW 1124) did not prevent them from generating patterned activity similar to that observed before staining, in response to shock of the esophageal nerve. Absorbance changes were monitored with a 124-element photodiode array, while extracellular electrodes monitored activity of the 6 buccal nerves. Optical and extracellular spikes were grouped with the aid of a template matching program; a total of 120 distinct units were detected in one 15 s recording. Optical signals (83 units) were found in the region of the ganglion containing mainly large neurons. Of these, 13 were detected on both optical and extracellular electrode recordings, suggesting that they might be motor neurons, while 25 of the optically detected neurons appeared not to be correlated with extracellular activity, suggesting that they might be interneurons. It was not possible to determine whether the remaining 45 optically identified units did or did not have correlated nerve activity. The ganglionic locations of putative motor neurons corresponded to the locations of large neurons identified by backfilling nerves of other buccal ganglia, and were consistent with the locations of putative motor neurons found in two other ganglia studied using optical methods. Thus, optical methods have generated a map of the locations and activity patterns of putative motor neurons and interneurons in the buccal ganglion that may be involved in the generation of rhythmic patterns.

Neuromuscular organization of the buccal system in Aplysia californica

The intrinsic muscles and peripheral nerves in the buccal system of the sea hare Aplysia californica were studied to build a foundation on which to base future investigations of feeding in intact animals. A detailed description of the bilaterally paired intrinsic muscles is given identifying previously unreported muscles. Each of the six buccal nerves (n1-n6) and the cerebrobuccal connective (CBC) have been characterized in several respects. Cell bodies in the buccal ganglion with projections into each of the buccal nerves have been identified via the cobalt backfilling technique. All nerves contain axons of cell bodies in the ipsilateral as well as the contralateral ganglia. For each nerve, there is a consistent pattern in the distribution of cell bodies in the paired ganglia with the number of cell bodies in the contralateral ganglion being less than or equal to the number in the ipsilateral ganglion. Although the total number of backfilled cell bodies varies among the nerves, their size ranges are similar with the majority being small. Nerves 1, 2, 4, 5, and 6 provide motor innervation to the intrinsic buccal muscles in varying degrees with nerve 4 supplying all the intrinsic muscles; nerve 2 supplies only one. The axon composition of each nerve was scrutinized and revealed large numbers of axon profiles, the majority of which were less than 2 microns in diameter. The present study provides a framework for analysis of feeding behavior in Aplysia californica.

Cross-correlation-based evaluations of the impulse transmission in a nerve: simulation and experimental studies

Bidirectional impulse transmission can be evaluated by cross-correlation analysis when two recording points are simultaneously available on a nerve. This method was tested here on digitized experimental recordings--from the cerebro-buccal connective of Aplysia--and on computer-simulated recordings with predetermined signal and noise content. The data were processed as such, or after being subjected to one of two preliminary treatments aiming at improving the sensitivity and discrimination power of the method. In the first treatment--Positive Value Saving, PVS--digitized values that were larger than the mean level were left unmodified, while the others were replaced by the mean value itself; in the second treatment--Positive Peak Saving, PPS--the values left unmodified were those which were larger than the mean level and represented a relative maximum. PVS tended to eliminate the negative deflections of the extracellular spikes; PPS tended to transform each spike into a single value equal to the spike amplitude. The cross-correlation histograms obtained yielded a clear separation of the impulses travelling in one and the opposite direction of propagation, and provided their subdivision and quantitative estimation according to propagation velocity. In the conditions adopted, spikes comparable in size to the noise range could be revealed. PVS improved sensitivity and discrimination power; PPS provided a very sharp discrimination between impulses with similar propagation velocity, at the expense of a loss of sensitivity.

Relationships between a neuronal buccal population and the peribuccal regions in Aplysia: an electrophysiological study

1. The relationships between Aplysia buccal neurons projecting the cerebral ganglion (L cells) and peribuccal regions were studied by electrophysiological techniques. 2. Stimulation of the cerebral upper labial (UL) and anterior tentacular (AT) nerves produced excitatory postsynaptic potentials in L cells. 3. Sixteen cells out of 24 were found possess an axonal branch in the labial branch of the AT nerve, 1 out of 8 in the UL nerve. 4. These axonal branches did not show any direct motor or sensory function in "reduced" preparations. 5. A modulatory function for the axonal projections and a sensory role for the synaptic relationships are hypothesized.