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School of Engineering

E338-BIOMEDICALINSTRUMENTATION

Problem 2

I am excited!

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Electrical Activity of Excitable Cells

Bioelectric potentials are producedas a result of electrochemicalactivity of excitable cells.

Four types of tissues are found ina human body, these are: Epithelial , Connective , Muscle and

Neural tissues.

Of the four tissue types, theMuscle and the Neural tissuesare excitable i.e. they generate and respond to

electrical potentials

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Bioelectric signals from Muscle andNeural cells

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Cells at their Resting State

An excitable cell maintain a steady electrical voltagebetween its internal and external environments.

The source of this voltage (a.k.a. potential) is strictlyionic in nature.

The principal ions involved with the mechanism ofgenerating cell potentials are Sodium ( Na+ ), Potassium(K+ ) and Chloride ( Cl- ) ions.

This resultant potential is called the Resting MembranePotential (RMP ) and a cell with such a potential is saidto be polarized .

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Resting Membrane Potential of anexcitable cell

+

V

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++

+ +

+

++

+

++

+

++

+

- -

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-

-

-

-

-

-

-

-

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Cell

membrane

-70mV

Extracellular

Intracellular

This static RMP remains constant until the cell isdisturbed or stimulated by some other action .

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Calculating the Membrane Potential So do we know the membrane

potential is approximately -70mV?

If only one type of ionpenetrates the membrane you

can use the Nerst equation

The Goldman equation is a littlemore complicated because theion permeability must be taken

into account, but is used whenmore then one type of ion isinvolved.

21

log58C

C mV E

C represents concentration levels of a particular ion.

P represents the permeability of a particular ion.

out K K inClClout Na Na

inK K out ClClin Na Na

C PC PC P

C PC PC PmV E

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

log58

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So what happens when nerve andmuscle cells are stimulated?

The nerve and muscle cells can be stimulated by avariety of stimuli such as heat flow, light, electricity,

mechanical deformation etc. which results in changes inthe cell membrane.

When a cell receives a stimulus of some kind, the RMP

changes and this change is called an ACTIONPOTENTIAL .

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The Action Potential

caused by an exchange of ions across the neuronmembrane.

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The Action Potential Explaineda) A stimulus first causes sodium channels to

open. Because there are many more sodiumions on the outside, and the inside of theneuron is negative relative to the outside,sodium ions rush into the neuron so theneuron becomes more positive and becomesdepolarized.

b) It takes longer for potassium channels toopen. When they do open, potassium rushesout of the cell, reversing the depolarization.

c) Also at about this time, sodium channelsstart to close. This causes the actionpotential to go back toward -70 mV (arepolarization).

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The Movement of Ions during theAction Potential

d) The action potentialactually goes past -70mV (a hyperpolarization )because the potassiumchannels stay open a bit

too long.

e) Gradually, the ionconcentrations go back

to resting levels and thecell returns to -70 mV.

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ALL or NOTHING Law

If a stimulus is sufficiently high, the action potential, interms of response time and amplitude is always thesame .

The minimum value of the stimulus required is called thethreshold of the cell.

For an action potential to be produced a stimulusgreater than the threshold value must be applied.

If not, action potential will not be generated at all.

The stimulus amplitude has no effect on the shape ofthe action potential generated .

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Conduction Of Action Potential

The propagation of an action potential through a group of cellsis due to the activity of local circuit currents. This current flows from the inactive to the active areas of the

cells. It is large enough to cause depolarization in the vicinity of the

action potential which in turn cause subsequentdepolarisations of adjacent cells.

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Since a typical organ such as the heart or the brain hasa number of excitable cells, there occurs a spatial-temporal (i.e. spread across space and varying withtime) summation of all the action potentials arising due toan internal or external stimulus.

This spatial-temporal summation leads to (surprisingly)consistent patterns of signals such as the: Electrocardiogram ( ECG ) from the heart Electroencephalogram ( EEG ) from the brain Electromyogram ( EMG ) from the skeletal muscles

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These signals can berecorded on the surface ofthe body and they form themost important sources ofinformation about the stateand health of not only thespecific organs but also thelife process itself.

To acquire these signals weuse special transducersknown as Electrodes .

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Characteristics of common Biosignals

Notice that these signals generally have Low Bandwidth and LowAmplitude.

What challenges would you face trying to acquire these signals?

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Learning Objectives

Understand the Electrochemical Activities of ExcitableCells.

Understand the term Resting Membrane Potential. Using Nernst and Goldman equations to calculate

Membrane Potential. Understand the term Action Potential and how it is

generated. Understand the process resulting in conduction of Action

Potential.

Principle of Biopotential Measurement.

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Resources

Webster , Medical Instrumentation: Applicationand Design (3rd edition) : John Wiley & Sons Inc

R. S. Khandpur, Biomedical Instrumentation:Technology And Applications : McGraw-Hill

Documents

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