Abstract

The most regular biological rhythm known is the electric organ discharge (EOD) of wave-type gymnotiforms, South American weakly electric fish. These fish produce continuously, day and night, the EOD, an oscillating electric dipole field that they use to electro-locate and electro-communicate. The fish's ability to electrolocate depends critically on measurements of the electric field phase and amplitude made at electroreceptors in its skin, and the precision of electrosensory information can be no better than that of the EOD. The extreme precision of the EOD and its command center, the medullary pacemaker nucleus (Pn), is the topic of this thesis.

The first chapter of the thesis motivates my research, and reviews related work that others have done. Chapter 2 focuses on the electric fish's EOD and changes in its regularity. Multiple species of gymnotiforms produce EODs with a precision, measured by the coefficient of variation (CV = standard deviation/mean period), of 0.0002 (standard deviation as low as 0.1 microsecond). Likewise, individual cells in the in vivo Pn can have nearly the same CV: ca. 0.0006. The CV changes spontaneously and in response to some sensory stimuli, indicating that the regularity of the EOD is under active central control.

In Chapter 3, the focus shifts to the in vitro Pn, which demonstrates the same remarkable precision as in vivo. Experiments explore the relative roles of network coupling and individual cell properties in setting the Pn's temporal precision. The network coupling, entirely gap-junction mediated, is found to be sparse but strong enough to make cells' firing frequency highly robust to current injections. When gap junction conductances are decreased, the precision of the Pn cells remains constant.

The effects of different network parameters on the Pn's precision are dissociated in a computer model (Chapter 4). This `compartmental model' incorporates the known Pn physiology and anatomy. Gap junctions can increase the precision of a model neuron and are most effective at the post-synaptic axon, but individual neurons must have atypically high precision to account for the observed network precision.

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Katherine T. Moortgat

Last modified: Fri Oct 8 12:23:53 PDT 1999