1.0 the Origin of Biopotentials (New)

EU501-BIOMEDICAL SIGNAL

MEASUREMENT

IDA MARIA BINTI MOHD YUSOFF

TOPICS

1. THE ORIGIN OF BIOPOTENTIAL

2. BIOPOTENTIAL ELECTRODES

3. BIOPOTENTIAL AMPLIFIERS

4. CARDIOVASCULAR MEASUREMENT

5. RESPIRATORY MEASUREMENT

Course Assessment

Test - 1 (20%)

Quiz - 4 (10%)

Practical Work - 6 (50%)

Practical Test - 1 (15%)

End of Chapter - 2 (5%)

Essay Question - 1 (5%)

CHAPTER 1:

THE ORIGIN OF BIOPOTENTIAL

Define sources of bioelectric potentials

Explain resting and action potentials

resting potentials

action potentials

waveform of action potential

Determine propagation of action potential

Determine bioelectric potentials

ECG

EEG

EMG

ENG

ERG

EOG

Construct signals of bioelectric potentials

Cell Membrane Potentials

Cell Membrane

• very thin (7-15 nm) lipid-protein complex

◦ transmembrane ion channels (pores) allow flow of ions

across the membrane

◦ like a leaky capacitor: a thin dielectric material acts as a

charge separator

impermeable to intracellular protein and other organic anions

selectively permeable to sodium (Na+) potassium(K+)and

chlorine(Cl-)ions

ion concentration difference across membrane creates a

diffusion gradient

ions flow, creating an electric field that opposes flow, until an

equilibriumis established

◦ similar to p-n junction, ions flow by diffusion and create a

potential difference which inhibits further flow of charged

ions

Extracellular

Domain

Intracellular

domain

Structure of Cell Membrane

Electrical Activity of Excitable Cells

Biopotentials are produced as a result of electrochemical

activity of excitable cells: i.e., nervous, muscular (cardiac and

smooth) and glandular cells

Electrical states of excitable cells

resting state

action state

Factors influencing the flow of ions across the cell membrane

1. Diffusion gradients

2. Inwardly directed electric field (inside negative, outside

positive)

3. Membrane structure (availability of pores; K+, Na+ and

permeability of membrane to different ions)

4. Active transport of ions across membrane against

established electrochemical gradients

Resting Membrane Potential

● Higher concentration of potassium (K+) inside than outside the cell and a higher concentration of chloride (Cl-) and sodium (Na+) outside the cell than inside.

● The negatively charged protein molecules (A-) inside the neuron cannot cross the membrane

● Use sodium/potassium pump to expel 3 sodium ions from the cell, each time 2 potassium ions are moved inside the cell

● The cell membrane is fairly permeable to potassium ions, so potassium ions diffuse out of the cell, leaving behind them a negative chargeat the interior side of the cell membrane

● Resting membrane potential of a membrane is about –70 mV (mV=millivolt )

● At rest, potassium ions (K+) can cross through the membrane easily. ● Chloride ions (Cl-)and sodium ions (Na+) have a more difficult time crossing.

Resting Potentials

Nerve and muscle cells are encased in a semi-permeable

membrane that permits selected substances to pass through while

others are kept out.

Body fluids surrounding cells are conductive solutions containing

charged atoms known as ions.

In their resting state, membranes of excitable cells readily permit

the entry of K+ and Cl~ ions, but effectively block the entry of

Na+ ions (the permeability for K+ is 50-100 times that for Na+).

Various ions seek to establish a balance between the inside and

the outside of a cell according to charge and concentration

The inability of Na+ to penetrate a cell membrane results in the

following:

Na+ concentration inside the cell is far less than that outside.

The outside of the cell is more positive than the inside of the cell.

To balance the charge, additional K+ ions enter the cell,

causing higher K+ concentration inside the cell than outside.

Charge balance cannot be reached due to differences in

membrane permeability for the various ions.

A state of equilibrium is established with a potential difference,

with the inside of the cell being negative with respect to the

outside

A cell in its resting state is said to be polarized. Most cells maintain

a resting potential of the order of - 70 to -90 mV until some

disturbance or stimulus upsets the equilibrium

Electrically these cells exhibit a resting membrane potential

[Nernst (1) and Goldman/Hogkin and Katz equation (2)]

When appropriately stimulated, they generate an action

potential (flow of ions across the cell membrane and

generation of a propagating wave of depolarization along

the membrane)

Model-generated transmembrane potential vm and membrane

ionic conductance changes for sodium (gNa) and potassium

(gK) during the action potential.

These waveforms are obtained by solving the differential

equations developed by Hodgkin and Huxley for the giant axon

of the squid at a bathing medium temperature of 18:5 C. ENa

and EK are the Nernst equilibrium potentials for Na and K across

the membrane.

Action Potential

The action potential (AP) is the electrical signal that

accompanies the mechanical contraction of a single cell when

stimulated by an electrical current (neural or external)

It is caused by the flow of sodium (Na+), potassium (K+),chloride

(Cl~), and other ions across the cell membrane.

The action potential is the basic component of all bioelectrical

signals.

the sodium–potassium pump

actively transports Na+out of cell and K+into cell in the ratio

3Na+: 2K+

associated pump current iNaK is a net outward current that

tends to increase the negativity of the intracellular potential

energy for the pump is provided by a common source of

cellular energy, adenosine triphosphate(ATP) produced by

mitochondria in the cell

Na-K pump

Vm

Action potential

● as a "spike" or an "impulse" for the action

potential

● When it is stimulated, protein channels in the

membrane open, and let sodium into the cell,

making the membrane potential more

positive.

● Action potential is an explosion of electrical

activity that is created by a depolarizing

current

● A stimulus causes the resting potential to

move toward 0 mV

● When the depolarization reaches about -55

mV a it will fire an action potential.

● This is the threshold

RESTING MEMBRANE POTENTIAL

♦ Na+ channels and

K+ channels are

closed.

♦ The outside of the plasma membrane is positively charged compared to the inside.

DEPOLARIZATION

♣ Na+ channels open.

♣ K+ channels begin to open.

♣ Depolarization results because the inward movement of Na+ makes the inside of the membrane more positive

REPOLARIZATION

♠ Na+ channels close and additional

♠ K+ channels open.

♠ Na+ movement into the cell stops and K+

movement out of the cell increases, causing repolarization

Membrane Current

membran current im

im

t

time / ms

distance / mm

Action Potential = ALL x NOTHING

Action Potential

Action Potential = opening of sodium and potassium channels

Action Potential

K+ -channels

Na+ -channels

Vm

excitable cell

time

resting potential

Waveform of Action Potential

Transmembrane potential (v) and membrane

ionic conductance changes for sodium (gNa) and

potassium (gK) during the action potential

◦ Absolute refractory period

membrane cannot respond to any stimulus

no matter how intense

Relative refractory period

action potential can be elicited by an intense

superthreshold stimulus

◦ Set upper limit action potential frequency EX: for nerve axon with absolute refractory period of 1 ms

max action potential frequency is 1000 impulses/s

but, typical neuron firing rate is ~30 Hz

Electrical Recording from a Nerve

Fiber

Recording of cell activity can be made via a

penetrating micropipet

•Recording of action potential of an

invertebrate nerve axon

•cell membrane potential vs. time

•resting potential

•action potential

ACTION POTENTIAL

Propagation of Action Potential

An action potential propagates along a muscle fiber or an unmyelinated nerve fiber as follows

Once initiated by a stimulus, the action potential propagates along the whole length of a fiber without decrease in amplitude by progressive depolarization of the membrane

Current flows from a depolarized region through the intra-cellular fluid to adjacent inactive regions, thereby depolarizing them

Current also flows through the extra-cellular fluids, through the depolarized membrane, and back into the intra-cellular space, completing the local circuit

The energy to maintain conduction is supplied by the fiber itself

Myelinated nerve fibers are covered by an insulating sheath of myelin

The sheath is interrupted every few millimeters by spaces known as the nodes of Ranvier,where the fiber is exposed to the interstitial fluid

Sites of excitation and changes of membrane permeability exist only at the nodes, and current flows by jumping from one node to the next in a process known as saltatory' conduction

How Electrical Activity Allows Neurons to

Communicate in The Action Potential?

http://brainu.org/files/movies/action_potential_cartoon.swf

(a) Charge distribution in the vicinity of the active region of an

unmyelinated fiber conducting an impulse. (b) Local circuit current flow

in the myelinated nerve fiber.

Resting vs. Active State

•Resting State

Steady electrical potential of difference between

internal and external environments

Typically between -70 to -90mV, relative to the

external medium

•Active State

Electrical response to adequate stimulation

Consists of “all-or-none” action potential after

the cell threshold potential has been reached

ECG

Electrocardiograph

ElectroCardioGram

Change of electric potential

heart muscle activation

atrium depolarization

3 diff. recording schemes:

Einthoven, Goldberger, Wilson

Frequency = 1-2 Hz !

Heart

Eindhoven’s triangle

36

Heart

EEG

The EEG (popularly known as brain waves)

represents the electrical activity of the brain

• Delta (): 0.5 - 4 Hz;

• Theta (): 4 - 8 Hz;

• Alpha (): 8 - 13 Hz; and

• Beta (): > 13 Hz.

ElectroEncefaloGram

Waves:

•Delta: < 4 Hz ... sleeping, in awakeness pathological

•Theta: 4.5 -8 Hz ... drowsiness in children, pathological in aduls

(hyperventilation, hypnosis, ...)

•Alfa: 8.5 -12 Hz ... relaxation physical / mental

•Beta: 12 - 30 Hz ... wakefulness, active concentration

•Gama: 30–80 Hz …higher mental activity including perception and

consciousness

Brain

EMG

Skeletal muscle is organized functionally

on the basis of the motor unit which

consists of a single motor nerve fiber and

the bundle of muscle fibers to which it is

attached

Electromyograph

Measures electrical activity of the muscles

Electromyogram (EMG)

Skeletal muscle is organized

functionally on the basis of the

single motor unit (SMU).

SMU is the smallest unit that can be

activated by a volitional effort

where all muscle fibers are activated

synchronously.

SMU may contain 10 to 2000 muscle

fibers, depending on the location

of the muscle.

Factors for muscle varying strength

1. Number of muscle fibers

contracting within a muscle

2. Tension developed by each

contracting fiber

Muscle Fiber (Cell)

http://www.blackwellpublishing.com/matthews/myosin.html

Figure 4.10 Diagram of a single motor unit (SMU), which consists of a single motoneuron and

the group of skeletal muscle fibers that it innervates. Length transducers [muscle spindles, Figure

4.6(a)] in the muscle activate sensory nerve fibers whose cell bodies are located in the dorsal root

ganglion. These bipolar neurons send axonal projections to the spinal cord that divide into a

descending and an ascending branch. The descending branch enters into a simple reflex arc with

the motor neuron, while the ascending branch conveys information regarding current muscle

length to higher centers in the CNS via ascending nerve fiber tracts in the spinal cord and brain

stem. These ascending pathways are discussed in Section 4.8.

Field potential of the active fibers of an SMU

1- triphasic form

2- duration 3-15 msec

3- discharge rate varies from 6 to 30 per second

4- Amplitude range from 20 to 2000 V

Surface electrode record field potential of surface muscles and over a

wide area.

Monopolar and bipolar insertion-type needle electrode can be used to

record SMU field potentials at different locations.

The shape of SMU potential is considerably modified by disease such

as partial denervation.

Electromyogram (EMG)

Figure 4.11 Motor unit

action potentials from

normal dorsal interosseus

muscle during progressively

more powerful contractions.

In the interference pattern (c

), individual units can no

longer be clearly

distinguished. (d)

Interference pattern during

very strong muscular

contraction. Time scale is 10

ms per dot.

ENG

The ENG is an electrical signal observed as a

stimulus and the associated nerve action

potential propagate over the length of a nerve.

It may be used to measure the velocity of

propagation (or conduction velocity) of a

stimulus or action potential in a nerve

ENGs may be recorded using concentric needle

electrodes or silver -silver-chloride electrodes

(Ag - AgCl) at the surface of the body.

Electroneurogram (ENG)

Recording the field potential of an excited nerve.

Neural field potential is generated by

- Sensory component

- Motor component

Parameters for diagnosing peripheral nerve disorder

- Conduction velocity

- Latency

- Characteristic of field potentials evoked in muscle supplied by the

stimulated nerve (temporal dispersion)

Amplitude of field potentials of nerve fibers < extracellular potentials

from muscle fibers.

Figure 4.7 Measurement of neural conduction velocity via measurement of

latency of evoked electrical response in muscle. The nerve was stimulated at

two different sites a known distance D apart.

Reference

Velocity = u =

2 ms

V°(t)S2

S2

S1

S1

Muscle

+ - + -

D

R

L2

L1- L2

L1

t DV°(t)

V°(t)

1 m

V

Conduction Velocity of a Nerve

Field Potential of Sensory Nerves

Extracellular field response from the sensory nerves of the median or

ulnar nerves

Figure 4.8 Sensory nerve action potentials evoked from median nerve of a healthy subject at elbow and

wrist after stimulation of index finger with ring electrodes. The potential at the wrist is triphasic and of much

larger magnitude than the delayed potential recorded at the elbow. Considering the median nerve to be of the

same size and shape at the elbow as at the wrist, we find that the difference in magnitude and waveshape of

the potentials is due to the size of the volume conductor at each location and the radial distance of the

measurement point from the neural source.

To excite the large, rapidly conducting

sensory nerve fibers but not small pain

fibers or surrounding muscle, apply brief,

intense stimulus ( square pulse with

amplitude 100-V and duration 100-300

sec). To prevent artifact signal from

muscle movement position the limb in a

comfortable posture.

Reflexly Evoked Field Potentials

Some times when a peripheral nerve is stimulated, a two evoked

potentials are recorded in the muscle the nerve supplies. The time

difference between the two potentials determined by the distance

between the stimulus and the muscle.

Stimulated nerve: posterior tibial nerve

Muscle: gastrocnemius

Figure 4.9 The H reflex The four traces show potentials evoked by stimulation of the

medial popliteal nerve with pulses of increasing magnitude (the stimulus artifact increases

with stimulus magnitude). The later potential or H wave is a low-threshold response,

maximally evoked by a stimulus too weak to evoke the muscular response (M wave). As the

M wave increases in magnitude, the H wave diminishes.

Reflexly Evoked Field Potentials

Low intensity stimulus stimulate only the

large sensory fibers that conduct toward the

CNS. No M wave

Medium intensity stimulus stimulate

smaller motor fibers in addition to

the large sensory fibers. Motor

fibers produce a direct muscle

response the M wave.

With strong stimuli, the excited motor

fibers are in their refractory period so

only the M wave is produced.

Electroretinogram (ERG)

A transparent contact lens contains one electrode and the reference electrode can be

placed on the right temple.

ERG is a recording of the temporal sequence of changes in potential in

the retina when stimulated with a brief flash of light.

Glaucoma

High pressure

Aqueous humor

Electroretinogram (ERG)Ag/AgCl electrode impeded in a special contact lens.

Source of Retinal PotentialThere are more photoreceptors than ganglion cells so there is a convergence pattern.

Many photoreceptors terminate into one bipolar cell and many bipolar cells terminate into one

ganglion cell. The convergence rate is greater at peripheral parts of the retina than at the fovea.

Rod (10 million) is for vision in dim light and cone (3 million) is for color vision in brighter light.

Electroretinogram (ERG)The a-wave, sometimes called the "late receptor potential," reflects the general physiological

health of the photoreceptors in the outer retina. In contrast, the b-wave reflects the health of

the inner layers of the retina, including the ON bipolar cells and the Muller cells (Miller and

Dowling, 1970). Two other waveforms that are sometimes recorded in the clinic are the c-wave

originating in the pigment epithelium (Marmor and Hock, 1982) and the d-wave indicating

activity of the OFF bipolar cells (see Figure 3).

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http://webvision.med.utah.edu/ClinicalERG.html

Electro-Oculogram (EOG)

EOG is the recording of the corneal-retinal potential to determine the

eye movement.

By placing two electrodes to the left and the right of the eye or above

and below the eye one can measure the potential between the two

electrode to determine the horizontal or vertical movement of the eye.

The potential is zero when the gaze is straight ahead.

Applications1- Sleep and dream research,

2- Evaluating reading ability and visual fatigue.

Bionic Eyes