Cerebral oxygen delivery and consumption during evoked neural activity

Vazquez Vazquez

doi:10.3389/fnene.2010.00011

Abstract

Increases in neural activity evoke increases in the delivery and consumption of oxygen. Beyond observations of cerebral tissue and blood oxygen, the role and properties of cerebral oxygen delivery and consumption during changes in brain function are not well understood. This work overviews the current knowledge of functional oxygen delivery and consumption and introduces recent and preliminary findings to explore the mechanisms by which oxygen is delivered to tissue as well as the temporal dynamics of oxygen metabolism. Vascular oxygen tension measurements have shown that a relatively large amount of oxygen exits pial arterioles prior to capillaries. Additionally, increases in cerebral blood flow (CBF) induced by evoked neural activation are accompanied by arterial vasodilation and also by increases in arteriolar oxygenation. This increase contributes not only to the down-stream delivery of oxygen to tissue, but also to delivery of additional oxygen to extra-vascular spaces surrounding the arterioles. On the other hand, the changes in tissue oxygen tension due to functional increases in oxygen consumption have been investigated using a method to suppress the evoked CBF response. The functional decreases in tissue oxygen tension induced by increases in oxygen consumption are slow to evoked changes in CBF under control conditions. Preliminary findings obtained using flavoprotein autofluorescence imaging suggest cellular oxidative metabolism changes at a faster rate than the average changes in tissue oxygen. These issues are important in the determination of the dynamic changes in tissue oxygen metabolism from hemoglobin-based imaging techniques such as blood oxygenation-level dependent functional magnetic resonance imaging (fMRI).

Highlights

The wide-spread use of imaging methods that are sensitive to the cerebral oxygenation level of blood, such as blood oxygenation-level dependent functional magnetic resonance imaging (BOLD fMRI), has sparked significant interest in the properties and role of oxygen delivery and consumption in the brain, during changes in brain function
The delivery of oxygen is driven by the increases in cerebral blood flow (CBF) and its associated increases in cerebral blood volume (CBV), but additional mechanisms are necessary to describe observations of tissue oxygen delivery
While measurements of the tissue oxygen tension with increases in neural activity indicate that the role of CBF is not to maintain a constant average tissue oxygen tension, it is possible that the transient increases in tissue oxygen are necessary to maintain a minimum intra-cellular oxygen tension

Summary

Introduction

The wide-spread use of imaging methods that are sensitive to the cerebral oxygenation level of blood, such as blood oxygenation-level dependent functional magnetic resonance imaging (BOLD fMRI), has sparked significant interest in the properties and role of oxygen delivery and consumption in the brain, during changes in brain function. The increase in CBF is produced at least in part by the dilation of feeding arteries, and increases in cerebral blood volume (CBV) have been observed (Berwick et al, 2005; Vanzetta et al, 2005; Hillman et al, 2007; Kim et al, 2007) This general picture appears to be coherent because it is expected that increases in neural activity (e.g., synaptic transmission and firing rate) require additional energy, which is supplied by increases in oxidative metabolism. The use of blood oxygenation methods to interpret and quantify brain function remains uncertain Because these processes are not simple and many important variables are not routinely measured, models have been employed to explore, interpret and quantify the dynamics of this process (Zheng et al, 2002; Valabregue et al, 2003; Huppert et al, 2007; Boas et al, 2008). Preliminary findings of the dynamic changes in cellular oxidative metabolism with evoked function obtained using flavoprotein autofluorescence imaging (FAI) are presented

Background

Findings

Materials and Methods