Cross-correlation study of synaptic input distribution inside the buccal ganglion of a gastropod mollusk

In isolated 'nonperturbed' neural preparations of the gastropod mollusk Aplysia, synaptic inputs of undetermined origin ('spontaneous' synaptic activity) converge upon the buccal ganglion neurones. Activity simultaneously recorded from two neurones may include synchronous postsynaptic potentials (PSPs), due to firing of shared presynaptic cells. The distribution of spontaneous input to buccal neurones of known types (A, B, D, L and s cells) was quantitatively examined by cross-correlating intracellular recordings obtained from cells belonging to the same (homonomous) or different (heteronomous) type. Homonomous pairs of B or D motoneurones shared much of their input in all examined cases, compared to 50% cases for L neurones. Shared input appeared capable of facilitating synchronous firing in B and L cells, and of driving D cells in various phase relationships. Heteronomous pairs showed a high proportion of shared synaptic input when composed by A, B, D and s neurones, but no clear correlation was observed when pairing any of those four cell types with an L neurone. Shared synaptic input outlined therefore two buccal neuronal subsystems, internally coordinated and well separate from each other. The first one includes the A, B, D and s neurones, the second one, which is less compact, the L neurones. This agrees with an involvement of the first in patterned motor output production and of the second in coordination and control functions.