Electrophysiology,

ELECTROPHYSIOLOGY

Relationship of current and capacitance for physiological system

CURRENT♦ Electric current is define as the rate of flow

of charges passing through a point

or Amount of charges (Q) passing per unit

time (t)

and is given as

Equation 1

CAPACITANCE• Is the ability of a capacitor to store charges• A capacitor is made up of two conductive

plate that are separated by small distance or a thin layer of dielectric (insulator) b/w two conducting plates.

• The surface area of plates increases the capacitance while the distance of dielectric decreases the capacitance.

• When positive and negative ions are separated by conducting substance a potential difference generate across them but when insulation is provided in between them charges store there and do not flow.

Equation 2

RELATIONSHIP B/W CURRENT AND CAPACITANCE

• Placing equation 2 (Q= CV)

in equation 1 (I= Q/t)

we get I = CV/t

• We can conclude that current is directly proportional to voltage and capacitance but inversely proportional to time.

• Nerve Conduction Velocity (NCV) is the speed of current passing through a nerve and is dependent upon capacitance.

ELECTRICAL PROPERTIES OF CELL MEMBRANE

• Compare the plasma membrane with a capacitor , charges are separated by lipid bilayer through electrogenic pump (Na+/K+ ATPase) generating a voltage across it

• The hydrophobic thin layer of lipids act as an insulator and membrane become able to store charges.

• Membrane of all cells in particular excitable cells have many ionic gates which allow flow of charges (current) through them after receiving certain stimuli.

• Thus membrane is also a good conductor of current.

• Depolarizing phase of Action potential is similar to the flow of ions from a capacitor.

• While repolarizing phase is similar to the restoring the charges for further impulses.

VARIOUS DIAGNOSTIC LAB TESTS TO RECORD ELECTRICAL ACTIVITY

OF DIFFERENT ORGANS

• EEG (Electroencephalogram of brain)

• EMG (Electromyography of muscles)

• ECG (Electrocardiogram of heart)

• ENoG (Electroneurograpghy of nerves)

• NCV (Nerve conduction velocity of nerves)

NERVE CONDUCTION VELOCITY

• Is the rate of propagation of electric signal through a nerve.

• Or the distance covered by AP per unit time.

• NCV = lambda /time

• From the equation we can see that NCV is directly proportional to the Rm.

• While inversely proportional to the Cm.

• MYELINATION provide insulation and thus decrease Capacitance although there is a decrease in amplitude of current but impulses are ensured to be very fast in ling nerves.

• Nodes of Ranvier after few millimeter provide conduction and propagation of impulse in forward direction

• Invertebrates have unmyelinated short neurons that is why AP are very rapid .

Commonly used method for Animal Model

1. Dissect the frog Schiatic nerve.2. Or the median and lateral giant axon of

earthworm.3. Set-up the instrument (stimulator,

electrodes, wires and oscillator and plates) and chemical (such as Ringer buffer).

4. Place the nerve over electrodes.5. Record the electrical activity. 6. Calculate NCV.

Procedure for Human NCV

• Locate the nerve to be studied.• Small electrode will be taped to the skin over the

nerve after applying conducting gel.• An stimulating electrode will be placed at a known

distance away from recording electrode.• The nerve will be stimulated by a mild and brief

electrical shock.• The patient may experience discomfort or pain due

to the shock.• The stimulation of the nerve and the electrical

impulse and current will be displayed on an oscilloscope (a monitor that display electrical activity).

Observations• Any decrease in the NCV will indicate:

1.Extent of demyelination of nerve

2. Any Conduction Block

3.Axonopathy (damage to the long

portion of a nerve cell)

SIGNIFICANCE•Diagnosis of various diseases

•Detection of source and location of conduction block

References• Neurophysiology:

3rdedition by R.H.Carpenter

Nerve Conduction studies:

• https://backyardbrains.com/experiments/comparingNerveSpeed

• https://Hmphysiology.blogspot.com

Presented By : SARA SIDDIQUI

Course No : 630 (Advances in Biophysics)

Thank you!