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The fluid in the extracellular space around the neurons and glial cells is enclosed within the brain, kept separate from the circulation and the rest of the body-fluid. This brain interstitial fluid forms a distinct compartment; a sponge-like “inverse cell” that surrounds all the cells. During neuronal resting and action potentials, sodium and potassium ions shuttle into, and out of, this “Reciprocal Domain” within the brain. This localised flux of ions is the counterpart to all the neuronal electrochemical activity (having the same intensity and duration, at the same sites in the brain), so a complementary version of all that potential information is integrated into this space within the brain. This flux of cations in the Reciprocal Domain may indirectly influence neuronal activity in the brain, creating immensely complex feedback. This Reciprocal Domain is unified throughout the brain, and exists continuously throughout life. This model identifies which species have such Reciprocal Domains, and how many times similar systems evolved. This account of the Reciprocal Domain of the brain may have clinical implications; it could be vulnerable to disruption by chemical insult, traumatic injury or pathology. These are key characteristics of our core selves; this encourages the idea that this Reciprocal Domain makes a crucial contribution to the brain. This hypothesis is explored and developed here.
This is Version 3. The manuscript has been updated following feedback.