Lecture on Cell Potential

BME 313 Bioinstrumentation Instructor: Wei Lin

BiopotentialBiopotentials are the results of electrochemical activities of excitable cells, which are the components of nervous, muscular or glandular tissue.

Na+: 10 times higher outside than insideK+: 40 times greater inside than outsideCl-: 30 times higher outside than inside

Biopotential


Cell membrane potential is the difference between intercellular potential and extracellular potential.


Biopotential

i

ok K

KnFRTE

][][

ln

Nernst Equation:

Goldman Equation:

oCliNaik

iCloNaok

ClPNaPkPClPNaPkP

FRTE

][][][][][][

ln


State of Excitable Cell

There are two states of the excitable cell:

Images from http://www.gregalo.com/action_potential.jpg


Propagation of Action Potential


Propagation of Action Potential

Myelinated transduction


Volume Conductor

http://www.cvphysiology.com/Arrhythmias/A014.htm

The three dimensional conducting medium composed of tissue is referred as volume conductor. The source of electrical signals are the excitable cells serving the current sources.


Electrocardiogram (ECG)

Sinoatrial node (SA)

Atrioventrical Node (AV)

Bundle of His

Rest of the heart

Common Bundle

Purkinje fiber


Electrocardiogram (ECG)


Electroencephalogram (EEG)

EEG is the electrical activity of the brain. EEG can be recorded using three

different types of electrodes Scalp electrodes Cortical electrodes (electrocorticogram) Depth electrodes


Types of EEG (Alpha waves)

Alpha waves are rhythmic waves in the frequency range of 8-12 Hz arising from synchronous and coherent (in phase / constructive) electrical activity of large groups of neurons in the human brain. They are found in EEGs when a normal person is in a quiet, resting state and are most intense in the occipital area.


Types of EEG (beta waves)

Beta waves normally occur in the frequency range of 14 to 30 Hz and could be as high as 50Hz during intense mental activity.


Types of EEG (theta waves)

Theta waves have frequency range of 4 to 7 Hz. They mainly occur in the parietal and temporal regions in children. They can also occur in adults during emotional stress.


Types of EEG (delta waves)Delta waves include all the waves in EEG below 3.5 Hz. They occur in deep sleep, in infancy and in serious organic brain disease.


Other Biopotentials

Electromyogram (EMG): electrical activities of skeletal muscle.

Electroretinogram (ERG): electrical activities of retina stimulated with light.


Biopotential Electrode

Electrode/Electrolyte interface Half cell potential Polarization of electrode Electrode circuit model.


Biopotential Electrodes

The electrode consists of metallic atoms C. The electrolyte is an aqueous solution containing cations of the electrode metal C+ and anions A-.

C

CC A-

A-C+

C+

e-

e-

Electrode Electrolyte


Electrode Electrolyte Interface

Assuming the electrode has same material as cation in the electrolyte, the electrode material becomes oxidized to form a cation and one or more electrons. The electrons remain as the charge carrier in the electrode. The anion can also be oxidized at the electrode to form a neutral atom and release one or two electrons to the electrode. The reverse of the above reaction is reduction. It controls the movement of electrons in the opposite direction.

C Cn+ + ne-

Am- A + me-


Half-cell Potential

When the electrode is inserted into the electrolyte, the concentration of cations and anions at the interface changes. As the result, there is an electric potential difference between the electrolyte surrounding the electrode and the electrolyte in other places. The difference is called half-cell potential, which cannot be measured.

+

+

+

+

+

+

+

Electrode

Electrolyte

Half-cell potential


Polarization

When current passes through the electrode-electrolyte interface, it changes the half-cell potential. The change is called overpotential and it has three components

Ohmic overpotential

Concentration overpotential

Activation overpotential

+ Total overpotential


Nernst Equation

BA

DC

aaaa

nFRTEE ln0

When two aqueous ionic solutions of different concentration are separated by an ion-selective semi-permeable membrane, an electric potential exists across the membrane and is given by Nernst equation.

For the general oxidation-reduction reaction: neDCBA

Where E : Half Cell PotentialE0 : Standard Half Cell Potential a : activity of cationsn : Number of electrons


Polarizable and Non-Polarizable Electrodes

Perfectly Polarizable Electrodes No actual charge crosses the electrode-electrolyte

interface when a current is applied. The current across the interface is a displacement current and the electrode behaves like a capacitor.

Perfectly Nonpolarizable Electrode Current passes freely across the electrode-

electrolyte interface, requiring no energy to make the transition. These electrodes see no overpotentials.


Silver/Silver Chloride Electrode

eAgAg AgClClAg

Cl

sAg a

KnFRTEE ln0


Equivalent Circuit Model for Electrode

Ehc

Rd

Cd

Rs

Ehc: half cell potentialRd and Cd: the impedance associated with the electrode-electrolyte interfaceRs: the series resistance associated with interface effects and the resistance in the electorlyte.


Electrode Skin Interface and Motion Artifact

Surface electrode


Micro electrode


Electrode Arrays


Electric stimulation of Tissue


Documents

Lecture on Cell Potential