Detection of neuronal activity using microwave energy

http://adsabs.harvard.edu/abs/2004APS..MARW10006Z

Abstract

Electromagnetic interactions with neurons can be used to monitor and detect their activities. When neurons fire, voltage-dependent ion channels open, allowing a transient influx of sodium ions. After a brief delay, this is followed by a transient efflux of potassium ions. The opening of the ion channels result in a small change in the average conductivity of the neuronal cell membrane. To study the feasibility of using polarized 80 – 120 GHz electromagnetic waves to detect neuronal activity, we first estimated the effective conductivity changes. More specifically, we used the buccal ganglion of the marine mollusk Aplysia californica as a model system. The model assumed that neurons were 100 microns in diameter, that there was a 2 mm thick layer of sea water, and that the total interaction area (roughly the size of one hemi-ganglion) was 1 mm by 2 mm. The neurons were assumed to be three layers deep. The three layers of cells were treated as six lipid layers. We assumed the conductance of a lipid layer at rest was 2 millisiemens/cm2, and that the conductance increased to 30 millisiemens/cm2 upon excitation of the neuron. We estimated that the conductance due to activation of a single neuron, averaged over the entire area of interaction, would be 42 microsiemens. If all the neurons were inactive, the conductance would be 40 microsiemens. Using the above conductance change, the complex permittivity and impedance of the buccal ganglion was calculated. The real part of the impedance was close to that of free space ( 377 ohms) and did not change when a single neuron fired. The imaginary part of the impedance was 1 microOhm larger when a single neuron was active. Although this change is below the limit of what can be detected using a simple microwave transmission measurement ( 0.1 milliOhms), it can be readily detected using a microwave resonant cavity method or interferometry. We have performed preliminary experimental measurements passing microwaves through an active buccal ganglion, and recorded some data that suggest that it may be possible to detect a change in microwave transmission when many neurons fire simultaneously, which occurs during feeding-like motor programs, which can be readily induced in the isolated buccal ganglion. We plan additional experiments to verify that changes can be reliably detected during motor programs, and then improve the sensitivity of the system so that the activity of individual nerve cells can be detected and imaged.