Abstract
In functional neuroimaging, neurovascular coupling is used to generate maps of hemodynamic changes that are assumed to be surrogates of regional neural activation. The aim of this study was to characterize the microvascular system of the primate cortex as a basis for understanding the constraints imposed on a region's hemodynamic response by the vascular architecture, density, as well as area- and layer-specific variations. In the macaque visual cortex, an array of anatomical techniques has been applied, including corrosion casts, immunohistochemistry, and cytochrome oxidase (COX) staining. Detailed measurements of regional vascular length density, volume fraction, and surface density revealed a similar vascularization in different visual areas. Whereas the lower cortical layers showed a positive correlation between the vascular and cell density, this relationship was very weak in the upper layers. Synapse density values taken from the literature also displayed a very moderate correlation with the vascular density. However, the vascular density was strongly correlated with the steady-state metabolic demand as measured by COX activity. This observation suggests that although the number of neurons and synapses determines an upper bound on an area's integrative capacity, its vascularization reflects the neural activity of those subpopulations that represent a "default" mode of brain steady state.
Highlights
Despite its relatively small size the brain consumes roughly a quarter of the body’s total glucose and a fifth of the oxygen
A recent celebrated example is the so-called blood oxygenation level--dependent (BOLD) contrast (Ogawa et al 1990; Kwong et al 1992) of magnetic resonance imaging (MRI), which reflects a complicated interplay of changes in blood volume, blood flow, and oxygen consumption (Logothetis and Wandell 2004)
The brain of one of the animals was used for vascular corrosion casts, 3 brains were used for histochemical processing and 3 were stained for cytochrome oxidase (COX) activity
Summary
Despite its relatively small size the brain consumes roughly a quarter of the body’s total glucose and a fifth of the oxygen. The high energy demand in combination with the fact that brain tissues lack any substantial capacity to store energy requires a tight spatiotemporal control of the energy supply. Changes in neural activity are followed by precisely controlled changes in hemodynamics, as hypothesized more than a century ago (Mosso 1881; Roy and Sherrington 1890). This remarkable site- and time-specific neurovascular coupling has been systematically exploited to generate detailed maps of hemodynamic changes that are assumed to be surrogates of the actual regional neural activation. Because BOLD functional MRI (fMRI) is the mainstay of biomedical neuroimaging, a lot of research has been focused recently on the functional aspects of neurovascular coupling, including the underlying signaling mechanisms, and the biochemistry of the
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