Appraising the brain's energy budget
- Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, MO 63110
In the average adult human, the brain represents about 2% of the body weight. Remarkably, despite its relatively small size, the brain accounts for about 20% of the oxygen and, hence, calories consumed by the body (1). This high rate of metabolism is remarkably constant despite widely varying mental and motoric activity (2).
The metabolic activity of the brain is remarkably constant over time.
Despite these well-known facts about the brain's large energy budget, a clear understanding of how it is apportioned among the many ongoing functional processes in neurons and glial cells has not been clearly spelled out. Understanding these relationships has assumed new importance because of the rapidly increasing use of modern imaging techniques such as positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) to study the functions of the living human brain in both health and disease. Both of these techniques and their derivatives [e.g., single photon emission tomography (SPECT) and various optical imaging techniques] use measurements related to the brain's metabolism and circulation to draw inferences about brain function in terms of its cellular activity (for review, see ref. 3).
In this issue of PNAS, two papers from investigators at Yale University (4, 5) provide important new information on the relationship between brain energy metabolism and cellular activity. This information, when understood in the context of other extant information, allows new insights into the manner in which we employ both neuroimaging and neurophysiological techniques to probe the functions of the human brain. Together with other work, it also lends considerable support to conceptualization of the instantiation of functional processes themselves.
The two reported studies in this issue of PNAS (4, 5) combined magnetic resonance spectroscopy (MRS) techniques with the extracellular recording of neuronal activity in the cerebral cortex of the anesthetized …
