The nervous system of the Glossiphoniid leechHaementeria ghilianii

Abstract

Synaptic pathways linking sensory neurons, interneurons and motoneurons participating in local bending and whole-body shortening reflexes inHaementeria ghilianii were found to be generally similar in both connectivity pattern and physiology to those ofHirudo medicinalis, although some difference in detail was observed. In both species the local bending reflex is accounted for in part by direct connections from the mechanosensory neurons to the segmental body wall shortening motoneuron cell L: rectifying electrical synapses from cells T, P and N and excitatory chemical synapses from cells P and N. Moreover, in both species the whole-body shortening reflex involves the giant intersegmental nerve fiber formed by longitudinal articulation of the axon of serial homologs of interneuron S; it is excited by T, P and N mechanosensory neurons and in turn excites cell L along the whole body. However, impulses evoked in cell S by intracellular stimulation reliably elicit whole-body shortening only inHaementeria, and not inHirudo. Thus theHaementeria cell S, but not theHirudo cell S, may be considered a putative command neuron, although both may play a similar role in the control of whole-body shortening. This neural difference in the cell S circuitry may account for the difference in behavioral response ofHirudo andHaementeria to sensory stimuli that excite cell S. And, in contrast to its homolog inHirudo, the Retzius cell ofHaementeria was found to elicit inhibitory junctional potentials in longitudinal muscle fibers. The pathway of Retzius cell-mediated inhibition appears to be indirect, passing via inhibitory motoneurons.