The tissues of the central nervous system
reside in a special environment separated from the general
body fluid compartment
by what is called a ‘blood-brain barrier’. This
interface regulates the composition of the fluids bathing the
structures of the central nerve tissues. All foreign material
placed inside this compartment has to be compatible with the
specialized cells which respond to this invasion.
Response to Surface Electrical Stimulation
The exposed surface of the brain is shown in the Figure
in an anesthetized experimental preparation. The arrow points
to an area where electrical stimulus was applied through
a spherical Platinum electrode placed on the brain surface.
Serum Albumin labeled with the dye Evan’s Blue, injected
into the veins prior to stimulation can be seen leaking out
into the tissue in the stimulation zone.
Around 1885, Paul Ehrlich found that an intra-venous injection
of Evan's Blue into an animal stained all the tissues blue
except for the brain. The specialized ‘blood-brain
barrier’ regulates transport of substances to and from
the fluid compartment bathing the brain tissues. The electrical
stimulation applied during this experiment resulted in breakdown
of the ‘Blood-brain barrier” and allowed the
dye to leak and stain the local tissue. This breakdown is
believed to be an early sign of neural injury.
Vascular Response to Stimulation
Two frames from a video during a surface stimulation experiment
are shown in the Figure. A spherical Platinum electrode can
be seen on the surface of the brain,
Within 60 seconds of application of a Cathodic, monophasic
stimulus train, the small vessels in the zone surrounding
the electrode are seen to close down, vasoconstriction.