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