Visual interneurons in the leech brain

Summary

• 1. The H cell is an unpaired neuron originating in subganglion 3 of the subesophageal ganglion. Its processes form an ‘H’ with long axis on the midline of the nervous system (Fig. 1). The soma is off-center, originating from the junction of the main neurite (the crossbar of the ‘H’) and one or the other axon (upright of the ‘H’).

• 2. Illuminating any eye excites the H cell, producing a volley of impulses riding on a compound synaptic potential. The synaptic potential is graded with light intensity and it persists in 40 mmol/l Mg saline (Fig. 3).

• 3. Touching the skin inhibits the H cell by a polysynaptic pathway that includes the mechanosensory T cells (Fig. 4).

• 4. The H cell has two impulse initiation sites, one in each axon. Impulses in the axon ipsilateral to the soma cause large spikes in the soma; those in the other axon cause small spikes (Fig. 2).

• 5. Large spikes match synaptic potentials in the AV (anterior visual) cell ipsilateral to the soma of the H cell (Figs. 5, 6); small spikes match synaptic potentials in the contralateral AV cell.

• 6. This synaptic potential has a large inhibitory component and a small excitatory component (Fig. 7). The inhibitory component is caused by an increase in Cl conductance (Figs. 7, 8); the excitatory component may be electrical (Fig. 9).

• 7. The connection between the H cell and the AV cell may be polysynaptic since it is blocked by high Ca/Mg saline (Fig. 9).