Understanding The Mechanism Underlying The Effects of tDCS

The paper, Calcium imaging reveals glial involvement in transcranial direct current stimulation-induced plasticity in mouse brain, is being lauded as a major discovery among tDCS researchers. It is however, extremely hard to follow. Fortunately RIKEN also issued a press release describing the study in a way most tDCS-curious will understand. Read the full press release here.

Researchers at the RIKEN Brain Science Institute in Japan have discovered that the benefits of stimulating the brain with direct current come from its effects on astrocytes — not neurons — in the mouse brain. Published in Nature Communications, the work shows that applying direct current to the head releases synchronized waves of calcium from astrocytes that can reduce depressive symptoms and lead to a general increase in neural plasticity — the ability of neuronal connections to change when we try to learn or form memories.

(top) Low spontaneous calcium activity in a normal mouse followed by tDCS-induced calcium surges. (bottom) tDCS-induced calcium surges are absent in IP3 Receptor 2 knockout mice, indicating that the calcium surges originate in astrocytes, not neurons.
Note: The upper. ‘normal’ mouse brain vs. modified mouse brain, bottom. Watch near ticking clock when ‘spontaneous’ switches to ‘tDCS’.

Let’s put this in some context by having a quick look at astrocytes and glial cells. From 2-Minute Neuroscience

Shockingly Smart: The Physics Behind Brain Stimulation | PhysicsCentral

Contrary to the popular notion of the brain as an organic computer, the signals responsible for most behavior aren’t transmitted purely by electricity. The motion of ions, atoms with either a deficit or surplus of electrons from their neutral state, is responsible for regulating the release of neurotransmitters, the “firing” of the neuron that leads to thought or sensation.

By applying a current in a certain direction, tDCS can effectively increase or decrease electrical polarization, affecting the chance that neurons in a given region of the brain will fire depending on the orientation of the subject’s synapses and whether a positive or negative current is applied. The potential to decrease neuronal activity seems just as important as the potential to increase it by way of medical and psychiatric applications of the technology. Disorders like schizophrenia, for instance, seem to be linked to over-excitability in certain parts of the brain.

via Physics Buzz: Shockingly Smart: The Physics Behind Brain Stimulation.