Abstract
A fundamental question in the neurosciences is how central nervous system (CNS) space is allocated to different sensory inputs. Yet it is difficult to measure innervation density and corresponding representational areas in the CNS of most species. These measurements can be made in star-nosed moles (Condylura cristata) because the cortical representation of nasal rays is visible in flattened sections and afferents from each ray can be counted. Here we used electrophysiological recordings combined with sections of the brainstem to identify a large, visible star representation in the principal sensory nucleus (PrV). PrV was greatly expanded and bulged out of the brainstem rostrally to partially invade the trigeminal nerve. The star representation was a distinct PrV subnucleus containing 11 modules, each representing one of the nasal rays. The 11 PrV ray representations were reconstructed to obtain volumes and the largest module corresponded to ray 11, the mole's tactile fovea. These measures were compared to fiber counts and primary cortical areas from a previous investigation. PrV ray volumes were closely correlated with the number of afferents from each ray, but afferents from the behaviorally most important, 11th ray were preferentially over-represented. This over-representation at the brainstem level was much less than at the cortical level. Our results indicate that PrV provides the first step in magnifying CNS representations of important afferents, but additional magnification occurs at higher levels. The early development of the 11th, foveal appendage could provide a mechanism for the most important afferents to capture the most CNS space.
Highlights
The preferential allocation of cortical territory to behaviorally important sensory receptors is a hallmark feature of the mammalian brain
What determines how much cortical territory is allocated to a sensory surface? Is cortical magnification just a reflection of subcortical maps in the thalamus and brainstem, which are in turn reflecting the density of inputs from the sensory surface?
This comparison was chosen because hairy-tailed moles and star-nosed moles are close relatives that are similar in most ways, with the clear exception of the star
Summary
The preferential allocation of cortical territory to behaviorally important sensory receptors is a hallmark feature of the mammalian brain. A number of studies have attempted to address this question, sometimes with different results. This is in part because of the difficulty of accurately measuring innervation density and corresponding representational areas in the central nervous system of most species. Cortical magnification for mouse whiskers is a reflection of peripheral innervation density. A number of studies suggested that cortical magnification of the retinal fovea was a direct reflection of the number of corresponding ganglion cells providing output from the fovea [6,7,8]. The history of different results from different investigators attempting to address this question is a testament to the difficulty in making such measurements
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
