Oscillatory behaviors in pharmacologically isolated heart interneurons from the medicinal leech

The central motor pattern for heartbeat in the medicinal leech is based upon the alternating bursting activity of mutually inhibitory pairs of heart interneurons (HNs). When pharmacologically isolated, these neurons spike tonically. Using a canonical model of an HN cell (Nadim et al., J. Comput. Neurosci. 2 (1995) 215}235) as a starting point, we generated three models, possessing di!erent subsets of the currents exhibited by HN cells, that are capable of autonomous bursting behavior. These three models represent three di!erent experimentally testable mechanisms that make HN cells potentially capable of bursting or generating slow oscillations under di!erent ionic and pharmacological conditions. The three di!erent mechanisms for producing bursting involve the interplay of: (1) fast Na` current, leakage current and outward currents, (2) rapidly inactivating low-threshold Ca2` current, leakage current and outward currents, and (3) slowly inactivating low-threshold Ca2` current, leakage current and outward currents. ( 2000 Elsevier Science B.V. All rights reserved.