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F. Rattay:
"Intracellular vs. extracellular stimulation with microelectrodes";
Talk: Workshop, Uppsala; 2016-06-26.

The response of a neuron to electrical stimulation is quite different when the stimulation site is changed from inside to outside. External stimulation causes a plenty of additional phenomena which are explained using models of pyramidal cells, retinal cells and neural substructures. Examples are (1) block of spike conductance by hyperpolarized regions using cathodic pulses - this can cause one sided firing when the action potential is conducted into the axonal terminal but not to the soma, (2) reduction and block of neurotransmitter release by Ca current reversal for strong stimuli which seems to have impact on indirect (=synaptic) spiking, e.g. in the electrically stimulated cortex and retina, (3) chronaxie, which was assumed to be the same for intra- and extracellular stimulation as it is direct related to the time constant of the cell membrane, is up to 20 times shorter for external than internal stimulation with microelectrodes, (4) thin axons are easier to stimulate internally, external electrodes commonly stimulate the thick fibers first (known as reversed recruitment order for peripheral nerve stimulation). However, direct cathodic excitation of dendrites requires a minimum number of sodium channels and consequently a minimum electrode-fiber distance, which increases with dendrite diameter. Therefore thin dendrites can profit from the stronger electrical field close to the electrode but low current stimulation cannot activate large diameter dendrites, contrary to the reversed recruitment order known from axon stimulation.