Cortical organization in shrews: Evidence from five species

Abstract

Cortical organization was examined in five shrew species. In three species, Blarina brevicauda, Cryptotis parva, and Sorex palustris, microelectrode recordings were made in cortex to determine the organization of sensory areas. Cortical recordings were then related to flattened sections of cortex processed for cytochrome oxidase or myelin to reveal architectural borders. An additional two species (Sorex cinereus and Sorex longirostris) with visible cortical subdivisions based on histology alone were analyzed without electrophysiological mapping. A single basic plan of cortical organization was found in shrews, consisting of a few clearly defined sensory areas located caudally in cortex. Two somatosensory areas contained complete representations of the contralateral body, corresponding to primary somatosensory cortex (S1) and secondary somatosensory cortex (S2). A small primary visual cortex (V1) was located closely adjacent to S1, whereas auditory cortex (A1) was located in extreme caudolateral cortex, partially encircled by S2. Areas did not overlap and had sharp, histochemically apparent and electrophysiologically defined borders. The adjacency of these areas suggests a complete absence of intervening higher level or association areas. Based on a previous study of corticospinal connections, a presumptive primary motor cortex (M1) was identified directly rostral to S1. Apparently, in shrews, the solution to having extremely little neocortex is to have only a few small cortical subdivisions. However, the small areas remain discrete, well organized, and functional. This cortical organization in shrews is likely a derived condition, because a wide range of extant mammals have a greater number of cortical subdivisions. J.

The evolution of brains from early mammals to humans

“…By comparing brain and especially cortical organization across current members of the six major branches, and seeing what features are common, we can infer that these features were likely retained from a common early mammal ancestor, and when some specializations were lost and others acquired [2,26]. Common features are perhaps easiest to discover in small-brained mammals, since there is less brain and cortex to explore, but small-brained present-day mammals could also have simpler brains than their ancestors as features were lost when smaller brains sometimes evolved from ancestors with larger brains [27] [28]. In addition, early mammals needed to have some parts of their nervous systems develop very early since they hatched from eggs, as in present-day monotremes, or were born early in development, as in present-day marsupials, so that they could grasp maternal hair and nurse [29].…”

Brain and cognitive evolution: Forms of modularity and functions of mind.

“…Whatever the balance, the clearest evidence for species-specific brain and cognitive specializations comes from studies of the cortical architecture of evolutionarily related species that have different morphological and behavioral specializations (e.g., Barton, 1996;Barton & Dean, 1993;Barton, Purvis, & Harvey, 1995;Catania, 2000;Catania, Lyon, Mock, & Kaas, 1999;Dukas, 1998;Dunbar, 1993;Hof, Glezer, Nimchinsky, & Erwin, 2000;Huffman, Nelson, Clarey, & Krubitzer, 1999;Moss & Shettleworth, 1996;Moss & Simmons, 1996). The comparison of species with a recent common ancestor is important because existing differences cannot be attributed to their distant evolutionary history.…”

A histologically derived stereotaxic atlas and substance P immunohistochemistry in the brain of the least shrew (Cryptotis parva) support its role as a model organism for behavioral and pharmacological research

“…Some baseline physiological norms [35] and electrophysiological cortical maps [7] have recently been described, but aside from this work the vast majority of data produced in the least shrew is pharmacologically driven [11,[13][14][15]. Although pharmacologically very similar to humans, the shrew's use as a model also depends on the shrew's neuroanatomy being similar enough to human neuroanatomy to allow comparisons to be made.…”