Microcolumns in the Cerebral Cortex



The following is an excerpt from:
  • L. Cruz, S. V. Buldyrev, S. Peng, D. L. Roe, B. Urbanc, H. E. Stanley, and D. L. Rosene, ``A Statistically Based Density Map Method for Identification and Quantification of Regional Differences in Microcolumnarity in the Monkey Brain,'' Journal of Neuroscience Methods, 141/2, p. 321-332 (2005). [PDF]

The most prominent feature of cortex is the arrangement of neurons into layers with classical “neocortex” identified as having six layers. Because these layers differ in thickness, cell type, and cell density from one part of the cortex to another, these “laminar” differences have been used to subdivide the cortex into different regions (e.g. Brodmann, 1909; Vogt and Vogt, 1919; von Economo and Koskinas, 1925; Von Bonin and Bailey, 1947; Petrides and Pandya, 1994). It has also been noted that different cortical regions display a “vertical” organization of neurons grouped into columnar arrangements that take two forms: macrocolumns, approximately 0.4–0.5 mm in diameter (Mountcastle, 1957; Calvin, 1995), and microcolumns or minicolumns approximately 30 microns in diameter (Jones, 2000).

Macrocolumns were first identified functionally by Mountcastle (1957), who described groups of neurons in somatosensory cortex that respond to light touch alternating with laterally adjacent groups that respond to joint and/or muscle stimulation. These groups form a mosaic with a periodicity of about 0.5 mm. Similarly, Hubel and Wiesel (1963, 1969, 1977) using both monkeys and cats discovered alternating macrocolumns of neurons in the visual cortex that respond preferentially to the right or to the left eye. These “ocular dominance columns” have a spacing of about 0.4 mm. In addition, they discovered within the ocular dominance columns smaller micro- or minicolumns of neurons that respond preferentially to lines in a particular orientation.

Once these physiological minicolumns were recognized, it was noted that vertically organized columns of this approximate size are visible in many cortical areas under low magnification and are composed of perhaps 100 neurons stretching from layer V through layer II. To prove that the physiological and morphologically defined minicolumns or microcolumns are identical to the physiologically defined minicolumn would require directly measuring the response of a majority of the neurons in a single histologically identified microcolumn, but this has yet to be done. Nevertheless, current data on the microcolumn indicate that the neurons within the microcolumn receive common inputs, have common outputs, are interconnected, and may well constitute a fundamental computational unit of the cerebral cortex (e.g. Szentagothai, 1975; Swindale, 1990; Purves et al., 1992; Saleem et al., 1993; Van Hoesen and Solodkin, 1993; Buxhoeveden et al., 1996; Mountcastle, 1997; Buxhoeveden and Casanova, 2002a,b; Mountcastle, 2003). These microcolumns vary in spacing across the cortex and species, but are about 30 microns apart in human visual cortex (Calvin, 1995).

The microcolumn has recently been shown to be disrupted in a number of different conditions including Alzheimer’s Disease (AD) and Lewy Body dementia (LBD) (Buldyrev et al., 2000), autism (Casanova et al., 2002a), dyslexia (Casanova et al., 2002b), and schizophrenia (Buxhoeveden et al., 2000b). Interestingly, in normal aging monkeys where cortical neurons are largely preserved (e.g. Peters et al., 1998) there is evidence of age-related functional disruption of orientation selectivity in the visual cortex of aged monkeys (Schmolesky et al., 2000; Leventhal et al., 2003). In these studies, Leventhal and colleagues reported a loss of two functional properties of microcolumns—orientation and direction selectivity. Moreover, they demonstrated that administration of GABA agonists restored these functions. Since the small GABAergic interneurons are important components of the microcolumn, this suggests that there may well be a disruption of at least this or a related component of the microcolumn in normal aging.


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