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Scientific Report 2007-2009<br />
Condensed matter physics and biophysics<br />
C41. The human brain: connections between structure, function and<br />
metabolism assessed with in vivo NMR<br />
NMR has become the election technique for the in<br />
vivo study of brain structure and function, because of<br />
its exquisite multiparametrical properties. Our research<br />
focuses on human brain functional metabolism, both in<br />
healthy subjects and in some pathologies. The experimental<br />
work is performed on medium (1.5 T and 3 T)<br />
and high (7 T) field systems.<br />
We studied the brain metabolic response to short stimulations,<br />
thus contributing to the debate on the link<br />
between brain metabolism, activity as seen with functional<br />
MRI (fMRI), and electrophysiology (neurovascular<br />
and neurometabolic coupling). By means of MR<br />
spectroscopy, it was shown that brain metabolism is<br />
aerobic from the very beginning [1]. Findings about<br />
metabolic alterations in epilepsy were obtained, as well<br />
as the first in vivo evidence that temporally resolved MR<br />
spectroscopy is sensitive to neuronal spiking (while it is<br />
well known that fMRI is sensitive mainly to postsynaptic<br />
potentials). In the last period, the work focused on<br />
the kinetic and thermodynamic modeling of metabolic<br />
events. A theoretical model was built, able to reproduce<br />
the main experimental findings about brain metabolism,<br />
These theoretical calculation showed that the intercellular<br />
nutrients trafficking, namely the flux of lactate between<br />
astrocytes and neurons (that can’t be measured<br />
directly), is energetically negligible if compared to the<br />
direct uptake of glucose by cells, thus suggesting that the<br />
proposed metabolic partnership between neurons and astrocytes<br />
is not obligate [2].<br />
A second important field is the study of the spinal<br />
cord function. The functional response to impulsive<br />
stimulation and the temporal dynamics of the signal<br />
were <strong>report</strong>ed for the first time, thus assessing that the<br />
functional signal in the spinal cord is linear and time–<br />
invariant, similarly to what happens in the brain, but<br />
with different dynamics [3]. This study is essential for<br />
the knowledge of the biophysical mechanisms underlying<br />
the function of the the spinal cord.<br />
Some important improvements of quantitative approaches<br />
were introduced. These improvements enhance<br />
the processing and facilitate the integration of structural<br />
and functional data, in order to gain more insights<br />
from the integrate analysis of several NMR derived parameters.<br />
As an example, functional data (areas activated<br />
by a given task) were combined with the knowledge<br />
of structural connectivity between those areas, assessed<br />
with tractographic techniques that exploit the directional<br />
properties of water diffusion in white matter.<br />
In this regard, a new and really promising field is the<br />
network organization of the human brain. We recently<br />
highlighted that the large scale brain networks observed<br />
at rest are affected, but not suppressed, by the execution<br />
of demanding cognitive tasks.<br />
Finally, an exciting field is the direct observation<br />
Figure 1: Example of functional spectroscopic experiment.<br />
Spectra are acquired in the region highlighted in the inset<br />
(colors code the activation identified by fMRI). Spectra are<br />
acquired at rest and during stimulation. Difference spectrum<br />
between resting and stimulated conditions (top) shows the<br />
relevant, tiny changes of metabolites [1].<br />
of the magnetic effects of the tiny currents that flow<br />
across neurons during activity. We conducted some of<br />
the pioneering works aimed at observing these effects.<br />
We further investigated the issue by means of realistic<br />
simulations of neuronal networks. These theoretical<br />
calculations suggested that the neuronal currents are<br />
probably too tiny to be observable with the current<br />
technology. A possible improvement in this regard can<br />
be obtained by the use of ultra–low field MR, because<br />
with very low Larmor frequencies some spectral content<br />
of neuronal currents can be, in given conditions, on<br />
resonance, thus inducing direct excitation of the spin<br />
ensemble [4].<br />
References<br />
1. S. Mangia et al., J. Cereb. Blood Flow Metab. 27, 1055<br />
(2007).<br />
2. S. Mangia et al., J. Cereb. Blood Flow Metab. 29, 441<br />
(2009).<br />
3. G. Giulietti et al., Neuroimage 42, 626 (2008).<br />
4. A. M. Cassarà et al., Neuroimage 41, 1228 (2008).<br />
Authors<br />
M. Carnì 4 , A.M. Cassarà 4 , M. Di Nuzzo, G. Garreffa 4 , T.<br />
Gili 4 , F. Giove, G. Giulietti 4 , M. Moraschi 4 , S. Peca.<br />
http://lab-g1.phys.uniroma1.it/<br />
<strong>Sapienza</strong> Università di Roma 94 Dipartimento di Fisica