Chemoarchitecture of Layer 4 Isocortex in the American Water Shrew <i>(Sorex palustris)</i>

Abstract

We examined the chemoarchitecture of layer 4 isocortex and the number of myelinated nerve fibers of selected cranial nerves in the American water shrew (Sorex palustris). This study took advantage of the opportunity to examine juvenile brain tissue, which often reveals the most distinctive cortical modules related to different sensory representations. Flattened cortical sections were processed for the metabolic enzyme cytochrome oxidase, revealing a number of modules and septa. Subdivisions related to sensory representations were tentatively identified by performing microelectrode recordings in a single adult shrew in this study, combined with microelectrode recordings and anatomical findings from a previous investigation. Taken together, these results suggest that characteristic chemoarchitectonic borders in the shrew neocortex can be used to delineate and quantify cortical areas. The most obvious subdivisions in the water shrew include a relatively small primary visual cortex which responded to visual stimuli, a larger representation of vibrissae in the primary somatosensory cortex, and a prominent representation of oral structures apparent in the more rostral-lateral cortex. A presumptive auditory area was located in the far caudal cortex. These findings for the cortex are consistent with counts from optic, auditory and trigeminal nerves, suggesting that somatosensory inputs dominate the shrew’s senses whereas visual and auditory inputs play a small role in navigation and in finding prey. More generally, we find that shrews share unusual features of cortical organization with moles, supporting their close taxonomic relationship.

Brain Mass and Cranial Nerve Size in Shrews and Moles

“…At the cortical level, the star-nosed mole has an elaborate, modular representation of the star that takes up much of somatosensory cortex. Similarly, the whisker representation in water shrews is expansive40. Our results suggest that major changes and elaborations of sensory surfaces and fiber tracts are not necessarily accompanied by measurable, corresponding changes in brain size.…”

“…Both groups spend much of their lives in tunnels and grassy runs, apparently requiring less visual information than most terrestrial animals. Both behavioral2637 and physiological data from the central nervous system383940 suggest that the visual system of shrews is poorly developed. Often these species forage at night or in low light conditions41, situations in which visual information would be less useful.…”

Barrelettes without Barrels in the American Water Shrew

“…These abilities depend heavily on the shrew's elaborate whiskers (Figure 1). In contrast, the visual and auditory systems of the water shrew are poorly developed [4]. For example, only 6,300 and 7,000 myelinated fibers supply the shrew's small eyes and ears respectively, whereas the trigeminal nerve contains 27,500 myelinated fibers.…”

“…These sensory priorities are in turn reflected in the organization of the shrew's neocortex. The primary and secondary somatosensory representations (S1 and S2) are dominated by 2 large representations of the whiskers whereas visual and auditory cortex are both quite small [4].…”

“…The earliest investigations of cortical anatomy were conducted in 5 species of adult shrews, including the water shrew [7]. A later investigation of juvenile water shrew neocortex [4] revealed a distinctive cytochrome oxidase dark wedge of tissue corresponding to the S1 representation of the whiskers, but this region did not contain barrels. This latter result was most conclusive, because juvenile cortex processed for cytochrome oxidase reveals the clearest cortical subdivisions, barrels, and barrelettes whereas in adults such subdivisions may be less obvious [8], [9].…”