Activity pattern of an oscillatory network after synaptic block in the leech central nervous system.

The activity of heart interneurons (HN cells) and heart motor neurons in the central nervous system of the medicinal leech was recorded intracellularly from their cell bodies in the third and fourth segmental ganglion (G3 and G4, respectively). Reciprocal inhibitory synaptic transmission between HN cells in the G3 was blocked by photoinactivation of neuropil glial cells in the same ganglion. The block disrupted the alternating rhythmic spike activity of HN cells in the G3 in isolated G3s but not in chains of G3 and G4. In the latter case, the rhythmic spike pattern of HN cells in the G3 appears to originate in the G4 because, for example, severing one connective between the G3 and G4 silenced the ipsilateral heart interneuron in the G3, whereas its contralateral homologue remained rhythmically active. Simultaneous recordings from HN cells in the G3 and G4 suggest that the latter may serve to coordinate the rhythmic activity of HN cells in the G3, when synaptic interaction between HN cells in the G3 is blocked. This study reveals a considerable capacity of the neural network controlling the heart beat to compensate for the impairment of synapses within one ganglion.