Schematic illustrating the problem of self-generated electrosensory input for the passive electrosensory system of mormyrid fish. Electric fish possess receptors (orange shading) that sense voltage across the skin, such as those generated by invertebrate prey (upper left) as well an electric organ in the tail. The electric organ generates a brief pulse of electricity known as an electric organ discharge (EOD) (lower right). EODs are used by the fish for communication and active electrolocation. However, the EOD also strongly activates electroreceptors tuned to low-frequency fields such as those due to prey (lower left). Neurons in the electrosensory lobe learn to predict and cancel out the unwanted effects of the EOD (image by Ann Kennedy).
Granule cells labeled after a dextran injection into the molecular layer of the electrosensory lobe (ELL) of a mormyrid fish. Like their counterparts in the cerebellum, these granule cells are small, highly numerous neurons which receive just a handful of input onto their short, claw-like dendrites (image by Kaleena Zhang).
In vivo whole-cell recording from a medium ganglion (MG) cell. These cells exhibit frequent, small amplitude narrow spikes and infrequent, large broad spikes. The broad spikes propagate into the dendrites and serve as the trigger for inducing synaptic plasticity at parallel fiber synapses. Parallel fibers are the axons of the granule cell and convey a wide variety of signals including corollary discharge and proprioception.
An artist’s rendition of negative images in mormyrid fish (by Ann Kennedy). In the 1950’s von Holst and Mittelstaedt proposed that corollary discharge signals might be used to form “negative images” of sensory input such that when added to the self-generated sensory input the sensory input would disappear. Work in mormyrid fish illustrates just such a function.
Mormyrid fish have amongst the largest brain to body mass ratio of any animals, larger even than humans. This is due largely to the hypertrophy of the cerebellum which covers the entire brain. Curtis Bell referred to the mormyrid brain as a “kind of cerebellar festival” because the cerebellum was so large and because of the interesting differences in the histology of different parts of the mormyrid cerebellum.
Dextran injection in the electrosensory lobe (ELL) showing labeled ELL principal cells and parallel fibers.
Detailed schematic illustration of the various cell types and input to the electrosensory lobe of mormyrid fish (ELL) (by Hans Meek).
Simplified schematics of the main circuits the lab studies. One goal of the lab’s work is to compare the function(s) of cerebellum-like circuits and the cerebellum proper in fish and mammals.