Connectivity patterns of crayfish giant interneurons: visualization of synaptic regions with cobalt dye

Intracellular injection of cobalt dye was used to visualize electrical synapses between two pairs of central giant interneurons and giant motoneurons in the crayfish central nervous system. A pair of giant motoneurons in each ganglion contacts the interneurons, but not all contact points are functional synapses. Cobalt dye reveals numerous fine projections that are present at synaptic contact points and absent at nonsynaptic contacts; intracellular recording confirms this correlation. The different connectivity patterns of the two pairs of interneurons are consistent with the different behavior patterns which they evoke.