The pretectal connectome in lamprey.

In vertebrates, the pretectum and optic tectum (superior colliculus in mammals) are visuomotor areas that process sensory information and shape motor responses. Whereas the tectum has been investigated in great detail, the pretectum has received far less attention. The present study provides a detailed analysis of the connectivity and neuronal properties of lamprey pretectal cells. The pretectum can be subdivided roughly into three areas based on cellular location and projection pattern: superficial, central, and periventricular. Three different types of pretectal cells could be distinguished based on neuronal firing patterns. One type, the rapid spike-inactivation cells, preferentially lie within the periventricular zone; the other cell types are distributed more generally. In terms of afferentation, the pretectum receives electro- and mechanoreceptive inputs in addition to retinal input. Histological data reveal that a large number of pretectal cells in the superficial and central areas extend dendrites into the optic tract, suggesting a predominant retinal influence even outside of the normal retinal terminal areas. The pretectum receives inhibitory input from the basal ganglia, and input from the pallium (cortex in mammals) and torus semicircularis. In addition, the pretectum is reciprocally connected with the thalamus, tectum, octavolateral area, and habenula. The main pretectal output is to the reticulospinal nuclei, and thus the pretectum indirectly affects the control of movement. Efference copies of some of this output are relayed to the thalamus and tectum. Overall, its extensive circuitry-especially the reciprocal connectivity with other retinorecipient areas-underlines the importance of the pretectum for sensory integration and visuomotor functions. J. Comp. Neurol. 525:753-772, 2017.