Neuronal control of heartbeat in the medicinal leech
An analysis has been made of the synaptic connections linking the heart interneurons, or HN cells, of the leech ventral nerve cord. The activity of the HN cells controls the rhythm of the heart excitor motor neurons, or HE cells. (1) The antiphasically active right and left members of the HN cell pairs in ganglion 3 [HN(3)] and in ganglion 4 [HN(4)] are linked via reciprocal inhibitory synapses. (2) Two additional pairs of heart interneurons, namely cells HN(2) and HN(5), serve to interconnect other HN cells. They do not appear to provide synaptic input to the HE cells. (3) Each HN cell inhibits other ipsilateral HN cells in posterior ganglia. Thus, cell HN(2) inhibits the ipsilateral cell HN(3). Cells HN(3) and HN(4) inhibit the ipsilateral cell HN(5). Cell HN(5) in turn inhibits bilaterally the HN(7) cell pair. (4) The pair of HN(7) cells receive periodic inhibitory input only from the HN(5) cell located on the body side coordinated in the non-peristaltic mode, since the HN(5) cell located on the side coordinated in the peristaltic mode is inactive, or dormant, throughout the heartbeat cycle. (5) The effects of systematic ablation of ganglia on the heartbeat rhythm of the remaining ganglia of the ventral cord confirm the inferences based on the electrophysiological analysis of these synaptic links. (6) The inhibitory synapses which link the HN cells to the HE cells are subject to presynaptic modulation, in that the amplitude of inhibitory synaptic potentials caused by HN cell impulses in an HE cell decreases with increasing polarization of the HN cell. This presynaptic modulation can occur not only intra- but also intercellularly, since the processes of different HN cells are linked via electrical junctions. In particular, the level of polarization of an HN cell which does not itself provide direct inhibitory synaptic input to an HE cell can nevertheless reduce the direct inhibitory influence exerted by another HN cell on that HE cell.
KeywordsInhibitory Synapse Ventral Nerve Cord Cell Pair Medicinal Leech Ventral Cord
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