Neurovascular coupling
In gray matter, neurovascular and neurometabolic coupling correspond to a set of processes that allow controlling blood flow and energy metabolism according to neuronal activity.
In white matter, the energy cost of transmitting information along myelinated axons is reduced, and the need for neurovascular coupling remains unknown. Theoretical calculations suggest the hypothesis that, at least in myelinated nerves, characterized by their rarity of synapses, neurovascular coupling may not be necessary for the propagation of action potentials.
Optic nerve, a unique model of white matter
The optic nerve in rodents is a very good model of myelinated axonal tract.
A team from CIEL (BOABAB/NeuroSpin), in collaboration with a team from the
Institut de la Vision, used two complementary imaging techniques to study neurovascular
coupling along the entire length of the optic nerve:
- Two-photon imaging in mice via lens implantation to visualize red blood
cells and measure their veolicty in brain capillaries; ;
- High magnetic field functional BOLD MRI (17.2 T) in rats, which allows
establishing maps of transient local variations in blood flow.
The researchers observed that visual stimulations (flashing light or drifting grating) increase blood flow in the retina, the unmyelinated head of the optic nerve, and at the synaptic terminals of the nerve. However, these stimulations do not affect blood flow or oxygenation in the myelinated part of the optic nerve, i.e., the intracranial optic nerve and the optic tract.
The authors conclude that during natural visual stimulation, the propagation of action potentials in activated myelinated axons does not trigger neurovascular coupling.
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