elec4623/elec9734: semester 2 2009elec4623/elec9734: … · 2009-07-28 · electroneurogram (eng)...

ELEC4623/ELEC9734: Semester 2 2009ELEC4623/ELEC9734: Semester 2, 2009

Dr Stephen RedmondfSchool of Electrical Engineering & Telecommunications

Email: [email protected]: 9385 6101Rm: 458, ELECENG (G17)

Physiology Coloring Book:Panels 29, 32, 33, 98-100

Session 2, 2009 ELEC4623/ELEC9734 1

Biomedical Instrumentation, Measurement and DesignELEC4623/ELEC9734/

Lecture 2The Origin of Biopotentials

Overview

Background PhysiologyExcitable Cells

Phospholipid molecule

ElectrophysiologyAutonomic nervous systemCardiac potentials

Cell membraneEnergy gradientsPassive and active transportNernst equation

ElectrocardiogramElectroneurogramElectrooculogramElectromyogramNernst equation

Membrane potentialsNerve cell and impulsesIon channelsC ll it ti

ElectromyogramElectroencephelogram

Cell excitationAction potentialRefractorinessConduction and myelination


Measurement Modalities

We have come a long way since the first physiological measurementmeasurement


ECG recording

Phospholipid molecule

Polar portion (hydrophilic)Charged group (alcohols, phosphate glycerol)phosphate, glycerol)

Nonpolar portion (hydrophobic)Fatty acid chain


Phospholipids in water

Polar head groups remain in waterNonpolar tails are

l d dexcludedMicelles – fat absorption in liverLi id bil th ‘ llLipid bilayer – the ‘cell membrane’


Cell membrane

Membrane proteins folded so polar parts exposedReceptors for hormonesReceptors for hormonesCatalyse specific chemical reactionsLinks between cells

Some proteins traverse entire membranepTransport


Energy gradients

Energy gradients are forces that generate movementsSubstances flow down energy gradients

h h f d ( h hThe steeper the free energy gradient (the greater the energy differences), the faster the flow (flux)Concentration gradient – diffusionOsmotic gradient – osmosisVoltage gradient – ionic currentPressure gradient - bulk flowPressure gradient bulk flow


Concentration gradient

Solutes flow (diffuse) down concentration gradientsStops when concentrations inStops when concentrations in compartments are equalProcess of transporting oxygen and nutrients from capillaryand nutrients from capillary blood vessels to tissue cells


Osmotic gradient

Semipermeable membrane prevents solute from passing but allows water movementWater flows down free energy gradientWater flows down free energy gradient toward the soluteOsmotic flow can be prevented by applying a pressure


Responsible for swelling/shrinkage of tissue

Voltage gradient

Ions are solutes that carry electrical chargeIons of like charge repel andIons of like charge repel and unlike attract


Transport


Passive transportS l t i l dSolutes move passively down concentration gradient


Active transport (against concentration gradient)(also co-transport and counter-transport)


Energy from phosphorylation of ATP to ADP

Sodium potassium pumpNa pumped out K pumped in (bothNa pumped out, K pumped in (both against concentration gradient)Energy from phosphorylation of ATP to ADPProvides osmotic stabilityProvides co-transport (glucose in gut cells)Provides voltage gradient (maintains low Na+ concentration inside cell)


Membrane potentials

I bl b t K+ d Cl t diff t t tiA. Impermeable membrane separates K+ and Cl- at different concentrationsB. K+ channels introduced into membrane (not Cl- channels though)

K+ diffuses from left to right down concentration gradientC Voltage gradient grows until it is able to balance the concentrationC. Voltage gradient grows until it is able to balance the concentration

gradient. K+ movement ceases and cell is at Equilibrium (Nernst) potential


Nernst equation

Nernst equation: Eion = - RT ln [ion]izF [ion]o

R = universal gas constant (8.314 J mol-1K-1), T = absolute temperature (in K), z = valence of ion (i.e. Cl- = -1), F = Faraday's constant (96500 C mol-1 valence-1)

Applies when membrane is totally permeable to specific ion species aloneA b d bAt rest membrane tends to be permeable mainly to K+

Membrane potential (Vm) is therefore negative and near E


therefore negative and near EK

Goldman-Hodgkin-Katz equation

Vm = -RT ln (PK[K+]o + PNa[Na+]o + PCl[Cl-]i)

F (P [K+] P [N +] P [Cl ] )F (PK[K+]i + PNa[Na+]i + PCl[Cl-]o)

where PK : PNa : PCl are the relative permeabilities of

the ion species e.g. 1.0 : 0.01: 0.1


Nerve cell and impulses

Nerve cells have short dendritic processes extending from cell body, and a long cylindrical y, g yaxonAxons transmit signals (nerve impulses to other nerve cells or

ff ( lto effector organs (muscles or glands)Impulses consist of a wave of electrical negativity (aselectrical negativity (as measured on cell surface) that moves along axon


Nerve impulseI l ll d tiImpulses are called action potentials (AP)To produce an AP, need a stimulus that brings cell voltagestimulus that brings cell voltage to a threshold i.e. depolarises membrane (makes voltage inside cell more positive,

l ti t t id )relative to outside)Occurs under a negative (cathodal) electrodeAre all or none events onceAre all-or-none events – once initiated they are always the same sizeThe more impulses per second


p p(higher frequency), the ‘larger’ the signal

Ion channelsCell membrane contains separate channels for different ionsMany channels contain voltage sensitive ‘gates’sensitive gatesNa+ channel also has a time dependent inactivation gate

A. normal resting potential:leaky K+ channel and Na-K pump workingp p g

B. depolarisation: fast Na+ gate opens

C. repolarisation:


C. repolarisation: slow Na+ gate closes and a slow K+ gate opens

Refractory periodWeak stimulus (1, 2) not enough gNa+ flows in to overcome outflow of K+ (caused by the stimulus induced reduction in Vm). NB: as membrane depolarises the K+ drive increases pas we are further away from EKWith stronger stimulus (3, 4, 5), this is overcome and Na+ gates open, depolarising membrane moredepolarising membrane more, causing more Na+ gates to openAbsolute refractory period, no stimulus can cause AP (Na+ gates still closed)still closed)Relative refractory, can cause another AP but threshold is higher (as voltage sensitive K+ gates still

)open)


Transmission of impulses

Most axons encased in fatty, myelin sheath, broken at Nodes of RanvierConserves energy and result in faster conduction as impulse jumps from nodeconduction as impulse jumps from node to node (saltatory conduction)Without myelin a 1mm diameter nerve would need to be 38 mm to achieve


same conduction speed

Transmission of impulses

Current can flow through external mediummedium

Electrodes can be used to ‘collect’ currentto ‘collect’ current


http://www.youtube.com/watch?v=DJe3_3XsBOg

Cardiac electrophysiology

Autonomic nervous systemCardiac muscle

d dAnatomy and conductionElectrocardiogramHome telecare


Autonomic nervous system


Cardiac muscle

D ti f di tiDuration of cardiac action potential can be 100 times more prolonged than that of skeletal muscleskeletal muscleLong refractory periodPlateau sustained by slow C ++ t d l K+Ca++ entry and slow K+

efflux


Anatomy and conductiony


The electrocardiogram (ECG)


Some typical ECGs

Normal sinus rhythmTachycardia and bradycardiaHeart blockArial fibrillation/flutterfibrillation/flutterHeart blockPremature ventricular

icontractionsVentricular fibrillationAsystoleyPacing


Electroneurogram (ENG)

The figure shows a spinal reflex generated by stimulating the posteriorstimulating the posterior tibial nerve (a mixed nerve)

Later evoked response (H

Measure potentials in or near axons

Later evoked response (H wave) is from spinal reflex

lCan test propagation velocities and reflex arcs

As stimulus increases H wave decreases but M wave increases


Electromyogram (EMG)We can measure AP from a single motor unit (SMU) of a group of muscle fibresOr can measure it at the skin surface


Electrooculogram (EOG)

There is a steady state potential difference between the cornea and retina

The eye acts like a dipoleThe eye acts like a dipole

This can be used to track the position/gaze of the eyeUsed in sleep science to determine ‘rapid eye movement’ (REM) sleep phase

This is achieved by placing electrodes above or lateral to the eye


Electroencephelogram (EEG)Different areas of the brain govern different functionsDifferent areas of the brain govern different functionsCan measure single neurons invasivelyOr superposition of large groups from the scalp


Electroencephelogram (EEG)

When measured on the scalp the EEG is seen to occupy bands from 0 1 Hz to 30 Hzbands from 0.1 Hz to 30 Hz (approx.)

Four sub bands have beenFour sub-bands have been arbitrarily defined

Delta (<3.5 Hz)Theta (4-7 Hz)Theta (4-7 Hz)Alpha (8-13 Hz)Beta (14-30 Hz)


elec4623/elec9734: semester 2 2009elec4623/elec9734: … · 2009-07-28 · electroneurogram (eng)...

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