The mormyromast region of the Mormyrid electrosensory lobe. I. Responses to corollary discharge and electrosensory stimuli

This is the first of two papers on the electrosensory lobe ( ELL) of mormyrid electric fish. The ELL is the first stage in the central processing of electrosensory information from electroreceptors. Cells of the mormyrid ELL are affected at the time of the electric organ discharge ( EOD) by two different inputs, EOD-evoked reafferent input from electroreceptors and corollary discharge input associated with the motor command that elicits the EOD. This first paper examines the intracellular responses of ELL cells to these two different inputs in the region of ELL that receives primary afferent fibers from mormyromast electroreceptors. Mormyromast electroreceptors are responsible for active electrolocation. The paper extends previous studies of the mormyrid ELL by describing the physiological responses of cell types, which had been previously identified only morphologically, including: the two types of Purkinje- like medium ganglionic cells, MG1 and MG2; the thick smooth dendrite cells; and the medium fusiform cells. In addition, two previously unrecognized cell types, the large thick smooth dendrite cell and the interzonal cell, are described both morphologically and physiologically for the first time. Finally, new information is provided on the two types of ELL efferent cells, the large ganglionic and large fusiform cells. All cell types, except for the medium fusiform cell, show nonlinear interactions between electrosensory and corollary discharge inputs. All cell types, except for the medium fusiform cell and the interzonal cell, also show plasticity of the corollary discharge response after pairing with electrosensory stimuli.