Bottlebrush dendritic endings and large dendritic fields: motion-detecting neurons in the mammalian tectum

The widefield vertical neurons of the lower stratum griseum superficiale (SGS3) and upper stratum opticum (SO) of the superior colliculus provide an extrageniculate route for visual information to reach the pulvinar. Previous physiological studies indicate that SGS3/SO neurons have large receptive fields and respond to small moving stimuli. We sought to better characterize the dendritic morphology of SGS3/SO neurons with intracellular filling in slice preparations of the ground squirrel superior colliculus. We found that dendrites of widefield vertical cells end in monostratified arrays of spiny terminal specializations called "bottlebrush" dendritic endings. Two major subtypes of neurons are described. Type I neurons have somata restricted to the SGS3 and bottlebrush endings in the most superficial sublayer of the SGS. Type II neurons are found at the base of the SGS and in the upper SO, and have bottlebrush endings arrayed within the middle sublayers of the SGS. Bottlebrush endings may sample and integrate laminated afferents to the superior colliculus, and cellular subtypes may underlie multiple information streams within the tectopulvinar pathway. A similar dendritic morphology and projection pattern can be found in cells of the avian optic tectum that project upon the nucleus rotundus, a thalamic nucleus homologous to the mammalian caudal/inferior pulvinar. Because motion processing is a dominant feature of the avian tectorotundal pathway, the current results suggest that both dendritic morphology and motion processing are conserved features of widefield vertical cells in the tectopulvinar pathway of vertebrates.