Dear Thomas,

At present, the electrodes used for implantation in primates (incl. humans) do not look like the rendering that you attached (is that your own creation, btw?). Specifically, they do not "attach" to individual neurons. Rather, an electrode array is used - imagine a square pin cushion: e.g. 10 rows and 10 columns of fine spikes, extending from the surface of the brain down through the cortical layers, and terminating at some predetermined depth (e.g. 1 mm long). There are also some special arrays where the length of the electrodes varies, so that different depths (layers) can be sampled. In either case, only the tip of each electrode picks up signals.

This type of electrode is called "extracellular": it sits in the extracellular space, between neurons, and picks up tiny currents through this extracellular space. These currents result from ions flowing through the membrane of nearby neurons, both in and out of the neurons, at the soma, dendrites and axons. Typically these currents are only large enough to be detected when the currents result from an action potential ("spike"), which is a very brief (~1 ms) signal that starts at the axon hillock (where axon leaves the soma) and propagates along the axon, but also back through the dendrites. A complicated situation. The electrodes are designed in such a way that they pick up spikes from nearby neurons - it's difficult to say exactly what "nearby" means (in terms of distance, and in terms of cellular compartment) - but for your purpose, perhaps simplify this as: the electrode tip must be within ~25 um of the surface of the soma  (which is about 10-15 um in diameter). Note: this distance has probably be characterized experimentally, if you find this in literature please share the reference with us.

Perhaps you'll find more info with a manufacturer:

http://blackrockmicro.com/neuroscience-research-products/low-noise-ephys-electrodes/blackrock-utah-array/

http://blackrockmicro.com/neuroscience-research-products/low-noise-ephys-electrodes/blackrock-electrode-concepts/

There are many other types of electrodes (e.g. silicon probes with dozens of recording sites along a single shaft), but they are not (yet) used in human implants.

This paper has images of such an array implanted in human neocortex, with dimensions:

Fernández, E., B. Greger, P. A. House, I. Aranda, C. Botella, J. Albisua, C. Soto-Sánchez, A. Alfaro, and R. A. Normann (2014, July). Acute human brain responses to intracortical microelectrode arrays: challenges and future prospects. Frontiers in Neuroengineering 7.

Dear Thomas! You portrayed beautifully neurons on which fixed contacts are hypothetical devices. In reality, scientists and doctors are trying to preserve the structure of nerve cells. Usually dominated by extracellular recording activity (article attached). Other techniques (patch clamp, etc.) are accompanied by damage to neuronal membranes.

The ratio of the size of neurons (10-15 microns to 200 microns), and the electrodes can be found in the articles (in the appendix). Good luck to you.

Vladimir

Thank you Benjamin and Vladimir for your answers! I am astonished that the considerable progress in neural implants is possible with extracellular recording... I had a similar answer in the facebook neuroscience group, although with slightly different measures (40 - 50 micrometers for large neurocyte cell bodies). And I had information on the spacing of the Utah-array to be about 400 micrometers. From this I concluded that a single electrode might be about 50 micrometers or comparable to a neurocyte cell body and produced the image attached to this post. (Precise measures are not that much important here... And, yes, I am producing these images myself.) I am planning to produce some more specific images displaying neurocytes in the retina and cochlea (and a very similar tip) and composing this to a little news feature on my website (http://news.scivit.com). This should be available in a few weeks. Many thanks again and comments helping improve my visualizations are always welcome,

Thomas