Tag Archives: Wilder Penfield

The pericyte controls local cerebral blood flow

Actively firing neurons get all the blood flow they need. More in fact. And this is the entire basis of functional magnetic resonance imaging (fMRI). At long, long last we may be close to understanding exactly how this happens.

Almost 100 years ago Wilder Penfield operating on unanesthetized patients with epilepsy to find the epileptic focus and remove it, noted that when a patient had a seizure on the table, veins became red, because so much blood flowed to the active area that it couldn’t absorb all the oxygen contained in the hemoglobin of the red cells, so they stayed red. Penfield was not a sadist, the brain contains no pain fibers, and so the skull could be opened using just local anesthetics. 

Exactly the same thing happens locally when neurons become active firing lots of action potentials. The functional MRI signal is due to the difference in magnetic susceptibility of the iron atom in hemoglobin when it is binding oxygen and when it isn’t.

So how does a firing neuron tell blood vessels it needs more flow?  A superb paper [ Proc. Natl. Acad. Sci. vol. 117 pp. 27022 – 27033 ’20 ]–https://www.pnas.org/content/pnas/117/43/27022.full.pdf probably explains exactly how this happens.  

The pericyte is a cell which is found outside cerebral capillaries and very small arteries.  It isn’t like a rubber band around the vessel (that’s for smooth muscle).  It’s like our bony spine with ribs coming from it, so the spine lies on the long axis of the vessel with the ribs coming down and wrapping (partially) around the vessel.

Pericytes in the brain and the retina are found primarily where two capillaries join each other according to the paper (which provides a convincing picture).

Neurons firing impulses release potassium into the extracellular space.  The endothelial cells of brain capillaries sense this and open up the inwardly rectifying potassium channel KIR2.1, exposing the outside to the resting potential of potassium which is quite negative (e. g the endothelial cell hyperpolarizes in response to neuronal activity.  The signal propagates upstream THROUGH the endothelial cells (because they are coupled together by gap junctions). 

Enter the pericytes which are electrically coupled to the underlying capillary endothelium by gap junctions, so they can receive the endothelial hyperpolarizing signal directly.  This causes the pericyte process receiving the signal to relax opening up the capillary or small artery increasing blood flow.  The authors followed this by watching intracellular calcium changes in pericytes, and noted that individual processes (ribs in the analogy above) could respond individually.  This is how a pericyte straddling the junction of two capillaries will open just the one which is hyperpolarized by neural activity.  

An incredibly elegant mechanism.  Of course with something so dramatic the work needs to be repeated. 

It is a pleasure to write something not involving the pandemic virus and our response to it.