The fast conducting system of the leech: a network of 93 dye-coupled interneurons

Summary

• 1. The two C cells and the single S cell in each segmental ganglion of the leech nerve cord are strongly electrically coupled, as are the S cells of adjacent ganglia. Impulses arising in any of these neurons propagate rapidly, via the fast conducting system formed by the axons of the S cells, to all other neurons in the network.

• 2. The dye Lucifer Yellow crosses the electrical junctions among the C and S cells. Exploiting this property of the dye, I traced the fast conducting system rostrally into the subesophageal ganglion, and caudally into the tail-brain.

• 3. The dye revealed 9 previously unidentified neurons in the subesophageal ganglion: 2 neurons resembling C cells in each subganglion, and a single asymmetrical neuron, designated S₀, in the most anterior subganglion.

• 4. Physiologically, the S₀ cell resembles the S cells of the segmental ganglia, and it may be homologous to them despite its unusual structure.

• 5. Strong electrical coupling (coefficient 40%) between the S₀ cell and the S₁ cell (the S cell in the first segmental ganglion) ensures that an impulse in one neuron is always matched by an impulse in the other.

• 6. Input from tactile and photic receptors on the head excites the S₀ and S₁ cells, eliciting impulses from initiation sites in the processes of the S₁ cell, within the subesophageal ganglion.

• 7. Lucifer Yellow revealed 21 previously unidentified neurons in the tail-brain; these evidently consist of an S cell and 2 C cells in each subganglion.

• 8. A recently-proposed model for the origin of the unpaired neurons of the leech nervous system explains the asymmetry of the S₀ cell.