We all know that scientists declare the evolution and function of the cortex remains a lasting mystery in neuroscience since it has no explanation. There are certain animals, like bats and star-nosed moles, these animals present remarkable specializations in their sensory organs, which are reflected in the specialized cortical architecture of course. besides, the study of unusual species can reveal aspects of the function and evolution of the cortical circuit, now know that the Etruscan shrew is the smallest terrestrial mammal and with a weight of only 64 mg, very surprising, she has one of the smallest brains of all animals. Shrews exhibit 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 sizes. I wanted to know if such a small animal would still have complex cortical architecture and the multiple cortical areas present in large mammals. As shown by a sagittal section of the shrew’s brain which had been stained for myelinated fibers, histochemical staining methods have revealed several well-defined regions and areas in the cerebral cortex of the Etruscan shrew. For example, the somatosensory cortex is a large region rich in myelin in the center while the visual cortex is a much smaller area posterior to the somatosensory cortex. These proportions correlate with the size of the cortical fields evaluated by the mapping of microelectrodes and reflect the differences in the size of the cranial nerves. Multiple staining methods have revealed a map of fifteen cortical regions and areas with cytoarchitectonic and histochemical characteristics similar to those of large mammals, a number of them are represented as a flat map of the cortex. Using this map, I counted the number of neurons in all cortical regions and discovered that the visual cortex contains only about 40,000 neurons while the somatosensory cortex contains about 200,000; this is consistent with the strong dependence of the Etruscan shrew on the touch of the whiskers for recognition of prey. at the moment content with approximately 1,000,000 neurons in each cortical hemisphere, it is a very interesting number for creatures of small size.
Also remember that the Etruscan shrew cortex is probably the smallest of all mammals, it is undoubtedly very small. the brain is the last common ancestor of all mammals was probably much larger than that of the Etruscan shrew and, unlike most mammals, the cerebral cortex of the Etruscan shrew has decreased considerably in size compared to that of his ancestors. and Even with such a small cortex, the Etruscan shrew retains many aspects of the unique complexity of mammalian cortical circuits, such as a well-differentiated laminar structure and a relatively large number of cortical areas, but the cortical thickness is remarkably reduced. the cortex is, on average, less than 500 µm thick, which makes it possible to record the activity of entire cortical columns using 2-photon imaging. It may even be possible to record the activity of all neurons in one of the smallest areas such as the visual cortex. Thus, the use of a new animal model can offer advantages for the exhaustive recording of activity in the cortical circuits of mammals and the discovery of the evolution of structure-function relationships. However, mapping areas and regions are only the first steps towards understanding the mysterious machine that is the cortex. To reveal its secrets, scientists have used techniques to study the functioning of a single neuron and the circuit is necessary, something that concerns the entorhinal cortex of course.
however in terms of columns and modules, they are present in many cortical areas, however, their function has remained elusive. In layer 2 of the rat’s medial entorhinal cortex, calbindin-positive cells form modules. the surface of the medial entorhinal cortex and the adjacent parasubiculum; the green dots indicate the distribution of clusters of cells positive for calbindin which is called “patches” and the blue lines indicate the main direction of the myelinated fibers at the border of layers 1 and 2. as for layer 2 also has the highest density of grid cells, that is, cells that shoot into a hexagonal patterned tile space when an animal explores the environment. The grid cells have been described in the entorhinal cortex of rodents, bats, monkeys, and humans, and they probably function in orientation and navigation. Interestingly, the grid cells form functional modules in layer 2 of the medial entorhinal cortex, of course, this led our group to suggest that calbindin positive cells could be involved in the generation of cell firing patterns. Grid. There are two main types of cells in layer 2 of the rat’s medial entorhinal cortex, star cells, and calbindin positive pyramidal cells. Other anatomical studies have shown that calbindin plaques strongly overlap the cholinergic axons, which are involved in the generation of theta rhythm. Then, we recorded the activity of identified cells in animals in motion or anesthetized, followed by immunohistochemical detection of cell types. This allowed us to correlate the physiological properties with the morphology of neurons and cell type markers. We discovered that calbindin-positive cells like grid cells were more highly modulated in theta than calbindin negative cells. Other records of cells identified anatomically in free-moving animals suggest that the calbindin positive cells themselves are more often grid cells than the other layer 2 cells of the medial entorhinal cortex, something rather interesting.