Even the Smallest Mammalian Brain Has Yet to Reveal Its Secrets

the size of the cortical fields as assessed by microelectrode mapping [Roth-Alpermann et al., 2010] and reflect the differences in size of the cranial nerves [Brecht et al., 2011]. Multiple staining methods revealed a map of about 15 cortical regions and areas with similar cytoarchitectonic and histochemical features as in larger mammals [Naumann et al., 2012]; a number of them are shown as a flat-map of the cortex in figure 1 b. Using this map, I counted up neuron numbers in all of the cortical regions and found that the visual cortex contains only about 40,000 neurons whereas the somatosensory cortex contains about 200,000; this is consistent with the Etruscan shrew’s heavy reliance on whisker touch for prey recognition. In total, there are about 1,000,000 neurons in each cortical hemisphere. The Etruscan shrew’s cortex is probably the smallest of all mammals – it is certainly unusually small. Thus, the brain of the last common ancestor of all mammals was likely significantly larger than that of the Etruscan shrew and, unlike most mammals, the Etruscan shrew’s cerebral cortex has decreased significantly in size from that of its ancestors. Even with such a small cortex, the Etruscan shrew retains several aspects of the unique complexity of mammalian cortical circuits, such as a The evolution and function of the cortex remain an enduring mystery in the neurosciences. Some animals, such as bats and star-nosed moles, exhibit remarkable specializations in their sensory organs, which are reflected in specialized cortical architecture. Thus, studying unusual species may reveal aspects of cortical circuit function and evolution. The Etruscan shrew is the smallest terrestrial mammal and, with a weight of only 64 mg, has one of the smallest brains of all mammals. Shrews show a wide range of social and exploratory behaviors as well as sophisticated prey-capture capabilities and unique adaptations of the cardiovascular and respiratory systems to small body size [Brecht et al., 2011]. I wanted to know whether such a small animal would still have the complex cortical architecture and multiple cortical areas found in larger mammals. As shown in a sagittal section through the shrew brain that had been stained for myelinated fibers ( fig. 1 a), histochemical staining methods revealed a number of well-defined regions and areas in the Etruscan shrew’s cerebral cortex. For example, the somatosensory cortex is a large, myelin-rich region in the center whereas the visual cortex is a much smaller area posterior to the somatosensory cortex. These proportions correlate with Published online: March 7, 2015