Abstract
Weakly electric fishes use electric organs to generate electric organ discharges (EODs). These EODs create an electric field which the fish can sense through electroreceptors on the skin. Perturbations of the field incite changes in the amplitude or timing of the EOD, which the fishes are able to detect and utilize for various processes through electroreception. The specialized time coding system of electric fishes detects changes in the timing of the EOD, and uses these temporal differences to encode spatial information about prey, among other functions. As part of the time coding system, Gymnotiform fishes have a time-comparator circuit in the midbrain which allows them to compare time differences experienced by electroreceptors from different parts of the body, similar to time difference detection circuits in the auditory systems of owls and bats.
In this dissertation, I utilize a combination of physiological and anatomical techniques to make determinations about this time coding system in the fish Apteronotus albifrons. My findings demonstrate that A. albifrons has numerous time-locked neurons in the midbrain, which fire 1:1 with the EOD at frequencies which surpass 1 kHz. I also found that these time-locked neurons are found in a midbrain structure, the magnocellular mesencephalic nucleus (MMN). MMN shares much of the same circuitry and cellular makeup as is found in other Gymnotiform fishes, but there are many anatomical features which are unique to A. albifrons. Briefly, the MMN contains the terminals of spherical cells whose somata are in the ELL, the somata, axons, and terminals of giant cells, and the somata, dendrites, and axons of small cells. Anatomically, MMN is a closed, unpaired nucleus with only two points of entry through bilateral horns (hMMN), differing from the laminar organization seen in many of its lower frequency counterparts. Finally, I document the receptive field organization of the time-locked neurons within MMN. My results show that the receptive fields for this time coding system are biased towards the head area, with very little receptive field distribution towards the tail.
My overall conclusions are that MMN is a temporal processing center in these high frequency fish, where the phase-comparator circuit is likely found. Despite EOD frequencies which surpass 1 kHz, phase-locked neurons are found within MMN and are phase-locked to the EOD at a 1:1 ratio. The absence of receptive field representations for the posterior 50% of the body in midbrain neurons may indicate unique behavioral adaptations in these fish, and future studies may further our understanding of the role EOD frequency plays in physiological and anatomical adaptations in weakly electric fishes.
