Biomedical Engineering Reference
In-Depth Information
with a cut-off wavelength of 560 nm). TPLSM revealed fluo-
rescent axons in the olfactory nerve layer, converging terminals
in precisely delineated glomeruli. This allowed us to show that,
in the rat, capillaries were exclusively located below the olfac-
tory nerve layer, i.e. in the glomerular layer and below
(56)
.
Their presence was tightly linked to the sites of synaptic inter-
actions and there was no clear organization within the glomerular
layer, capillaries often crossing the juxtaglomerular zones between
two glomeruli. The capillary network thus does not present an
obvious differentiation in close relationship with the glomerular
organization. In addition, because sensory axons converging on
a single glomerulus outnumber principal cells by a factor of sev-
eral hundred and because action potentials are predicted to con-
sume a major part of the energy budget during signaling
(57)
, our
result raises the question of how much energy is consumed in the
olfactory nerve layer and how it is distributed to olfactory nerve
axons. Glucose consumption is significant in the olfactory nerve
layer
(41, 42, 58)
. However, our preliminary data based on oxy-
gen measurements suggest that oxygen is much more consumed
in glomeruli than in the nerve layer during brief odor stimula-
tions. What is thus possible is that few axons are activated dur-
ing odor stimulation and that part of the oxygen is brought by
diffusion from arteries and arterioles
(59)
. Still, because in some
cases, few, if any, vessels were apparent in the nerve layer within
a 400-400
m column, it is probable that oxygen is supplied by
diffusion from the glomerular layer. Hence, the question of the
energy distribution and consumption in the nerve layer remains
open.
μ
5. Blood Flow in
Individual
Glomerular
Capillaries at Rest
and During Odor
Stimulation
Three blood flow parameters can be extracted from line scan mea-
surements in individual capillaries, instantaneous red blood cell
(inst.RBC) flow; inst.RBC velocity and inst.RBC linear density
(
see
Figs. 4.1 and 4.2
).
At rest, all parameters are variable and fluctuate in time.
Variations of inst.RBC flow result principally from changes in the
inst.RBC linear density rather than from changes in inst.RBC
velocity. In the rat, we usually have to screen 5-20 odorant
molecules in order to obtain a response in a given glomerulus,
even though the same region is targeted in the dorsal bulb.
Note that this “efficiency” depends on the anesthetics used.
In the majority of cases, odor induces an increase in RBC
flow that results from an increase in RBC velocity (
Fig. 4.1
right). In about a quarter of the cases, an increase in RBC
linear density is also observed. As expected from neuronal
