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Galvanic Skin Responses

Galvanic Skin Responses 

Description of the GSR measurements process:
Circuit of the DC current: steps of the current transfer from active electrode to passive electrode































Steps of the current transfer from anode to cathode and then, from cathode to anode for each pathway.


 Step 1 and 2 : Entrance of the Current from the active electrode

                Because voltage lower than 10 V cannot penetrate the stratum corneum1, the only way for the current to enter the body is  through the eccrine sweat ducts. This route represents the physiological pathway of the interstitial fluid, ie, the source of the eccrine sweat. 2

 


Step 3 : Pathway into the human body between the 2 electrodes:


1.According to the Fricke’s circuit, At DC current, the plasma membrane acts as an insulator and the current is not able to penetrate the cell, and most of the current flows around the cell and therefore in the interstitial fluid. 3, 4
2.In the interstitial fluid, the free ions are able to conduct electric current in the presence of an external electrical field. We can consider biological tissue electrically and macroscopically an ionic conductor. The total ionic conductance of a solution depends on the concentration, activity, charge and mobility of all free ions in the solution.3,4
3.The free ionic concentration and mobility are proportional to the ratio intensity/voltage; more the ionic concentration and mobility increase, more intensity and less voltage are observed. 3,4
4.Electrical dispersion: The cell membrane has the ability to store capacitive energy (dielectric or insulator properties). The cell membrane is the cellular structure with the major contribution to the dielectric behavior of living tissue. Living tissue is considered as a dispersive medium. 5-7
Cole (1940) introduced the first mathematical expression able to describe the ‘depressed semicircles’ found experimentally. It is known as the Cole equation. 5-7
The α value can also be regarded as a parameter denoting the derivation from Fricke–Morse model. That is, the Cole equation with α = 1 is equal to the Fricke–Morse model.In the case of living tissues, the spectral width of the electrical Bioimpedance dispersions (closely related with αparameter in the Cole equation) evolves during the ischemic periods. The simulations indicate that the dispersion width could be determined by the morphology of the extra-cellular spaces.8

 

Step 4, 5 and 6 : Process of the exit of the Current to the passive electrode

Electrical stimulation of the post sympathetic cholinergic fiber causes sweat rate response.9
The mechanical shear stress causes a phosphorylation cascade that removes phosphate groups from proteins and kinases activating endothelial nitric oxide (NO) synthase to synthesize NO. Nitric oxide is produced facilitating the release of cyclic guanosine monophosphate and a change in potassium permeability. The relaxation of the smooth muscle and vasodilatation of the vessels allows an exchange between vessels and sweat gland facilitates the production of sweat.10, 11
The release of acetylcholine (Ach) is also regulated by the hypothalamus and, the sweat response, acts as a response to an increase in blood and/or skin temperature.11
As regard to the EIS method , the mechanical response is not dependent on a temperature increase and it appears that the electrical stimulation acted as a mechanical shear stress activator, 10,11 describe above.


Step 7 : Electrochemical reactions on the bulk of the passive electrode (electrolysis)
-From Grimmes, S. & Martinsen, Ø. G. (2000).
Electrolytics. Bioimpedance & Bioelectricity.12
Analysis of the DC current in cathode and anode in electrolytic solution Na+ Cl- using metal electrodes: The electrochemical window is defined by both reduction and oxidation of water according to the following reactions: 12

At the Cathode
First of all, Na+ ions are not discharged at the cathode. Sodium has a very electro-negativity which means that it takes a large energy and a large negative voltage on the cathode to impose electrons on Na+ ions. At lower voltage there is reduction of dissolved oxygen and decomposition of water molecules. Both processes are linked with non-charged species which are transported to the electron transfer sites by diffusion not by migration.
At voltage supply adjusted (> 1V), the oxygen reduction current is sufficient. Na+ need not be considered, but Na+ ions are necessary for the conductance of the solution.
The water electrochemical reaction (reduction) at the cathode is:
2H2O + (2e -) = (H2) + (2 OH-)
 
At the Anode
Is the current at the anode due to the discharge of C1- ions? Yes. Chloride is highly electronegative, but less energy is necessary for taking electrons from the chloride ions than from water molecules. The water electrochemical reaction (oxidation) at the anode is:
2H2O = (O2) + (4H +) + (4e-)

In interstitial fluid the Na+ represents 96% of the positive free ions and Cl- and HCO3- ions represents also 96 % of the negative free ions.12
The interstitial fluid can be considered as electrolytic solution of Na+ and Cl-and because, HCO3-and Na+ ions may not discharged and they do not contribute very much to the DC current transfer. (3), the electrochemical model in vitro describe above could be applied only for dissolved oxygen diffusion at the cathode and Cl- ions at the anode.12

Analysis of the DC current in cathode and anode in electrolytic solution Na+ Cl- using Ag/Ag Cl: The Cotlove coulometric chloride titrator method.13
The technique measures the total chloride concentration. With this method, the passage of a constant direct current between Ag/AgCl electrodes produces silver ions. The free silver ions react with the chloride forming silver chloride as follow:
Ag => Ag+ Ag+ + (Cl-) => AgCl

After all the chloride combines with Ag+, free silver ions accumulate and precipitate , causing an increase in current across the electrodes and indicating the end point to the reaction..

References
Chizmadzhev YA, Andrey V. Indenbom, PI. Kuzmin, SV. Galichenko, JCW, Russell OP.  Electrical Properties of Skin at Moderate Voltages: Contribution of Appendageal Macropores. Biophysical Journal.1998.74 (2) : 843–856
Hashimoto K. Demonstration of the Intercellular Spaces of the Human Eccrine Sweat Gland by Lanthanum. J. Ultrastructure research.1971.36: 249--262
S. Gabriel, R. Lau W, Gabriel C. The dielectric properties of biological tissues: III. Parametric models for the dielectric spectrum of tissues, Physics in Medicine and Biology. 1996. 2271-2293
Foster K R, Schwan, H. P. Dielectric Properties of Tissues and Biological Materials: A Critical Review. CRC Critical Reviews in Biomedical Engineering.1989 17(1): 25-104.
Cole, K. S. Electric phase angle of cell membranes. Journal of General Physiology.1932. 15: 641-649.
Cole, KS., Li CL,Bak AF. Electrical analogues for tissues. Exp Neurol. 1969. 24(3): 459-473.
Cole K S, Cole R H. Dispersion and absorption in dielectrics. I. Alternating-current characteristics. Journal of Chemical Physics. 1941. (9): 341-351.
Ivorra A, Genesca M, Sola A, Palacios L, Villa R, Hotter G, Aguilo J. Bioimpedance dispersion width as a parameter to monitor living tissues. Physiol. Meas. 26.2005. 1(9).
Donadio V, Lenzi P, Montagna P, Falzone F, Baruzzi A, Liguori R. Habituation of sympathetic sudomotor and vasomotor skin responses: neural and non-neural components in healthy subjects. Clin Neurophysiol. 2005;116:2542-2549.
Soucy KG, Ryoo S, Benjo A, et al. Impaired shear stress-induced nitric oxide production through decreased NOS phosphorylation contributes to age-related vascular stiffness. J Appl Physiol. 2006;101:1751-1759.
Petrofsky J, Hinds CM, Batt J, Prowse M, Suh HJ.The interrelationships between electrical stimulation, the environment surrounding the vascular endothelial cells of the skin, and the role of nitric oxide in mediating the blood flow response to electrical stimulation. Med Sci Monit. 2007.13:CR1-CR7.
Grimmes S. ,Martinsen Ø G. Electrolytics. In Bioimpedance & Bioelectricity Basics: Academic Press.2000
Cotlove E, Nishi HH. Automatic titration with direct read out of chloride concentration. Clin Chem. 1961; 7:285–91.

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