Biomedical Engineering Reference
In-Depth Information
parameters are not suitably adjusted and pCO
2
levels span out of
optimal range. The core temperature of the animal also influences
the acid-base balance of the blood, and must be maintained sta-
ble (
37
◦
C) despite the cool temperatures in the imaging bore
∼
18
◦
C). In the case of a drop in pH of metabolic origin, such as
acidosis caused by the anesthetic urethane
(114)
, periodic intra-
venous injections of sodium bicarbonate can be used to stabilize
the animal and obtain reliable hyperemic responses.
(
∼
Anesthetics have been valuable in neuroscience to understand
the effects of afferent inputs on localized regions of the brain
(115-117)
because most peripheral stimuli (i.e., auditory, visual,
olfactory, limbs, whiskers) still reach the brain with anesthe-
sia. Most anesthetics act on different levels of neuronal sig-
naling by depressed axonal conductance of action potentials,
modified dendritic and somatic integration, reduced presynap-
tic neurotransmitter release, and/or altered postsynaptic recep-
tors. The GABA
A
receptor is commonly considered to be an
important molecular target since it seems to be ubiquitously
affected by most anesthetics. A notable exception is urethane, the
effect of which appears to be mediated by the enhanced con-
ductance of a specific potassium-channel subtype
(118)
.Anes-
thetics, inhaled or injected, reduce brain energy consumption
rather uniformly across regions and cortical energy decline is
typically monotonic with anesthetic depth
(119)
. Consistent
with this observation, anesthetics also depress firing rates and
impede slow field oscillations of excitatory pyramidal cortical
neurons
(120)
.
Use of anesthesia during rodent fMRI and neurophysiology is
needed to ensure immobilization and comfort during long exper-
iments under physically constrained circumstances. Many reports
on the “confounding effects” of anesthesia exist in the literature
(121, 122)
, pointing to the inherent limitation of studying neu-
ral systems under abnormal (i.e., non-awake) conditions
(123)
.
Our belief is that although the full complexity of awake brain
responses to sensory stimuli clearly cannot be reached under
anesthesia, important insights into general functional properties
of a given sensory system can be made
(29)
. Combined with
the detailed knowledge of morphological, functional, vascular,
and neuronal properties of rat brain tissue, results obtained in
the anesthetized preparation can be extrapolated and generalized
towards the awake brain. The common response features obtained
under multiple different anesthetics to the same stimulation, in
our laboratory and others, offers indirect support to the general-
ity of the results.
4.2.2. Effect of
Anesthesia
Despite the high NMR sensitivity at 11.7 T, there are some chal-
lenges. Magnetic susceptibility is a property of matter indicating
4.2.3. Magnetic
Susceptibility Effects and
“Shimming”
