1. Bio Electricity

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BIOELECTRICITY

Denmor Israel M. Villanueva/BSECE4


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Neuroelectric Principles

Natural bioelectric processes are responsible

for nerve and muscle function

These processes can be affected by externallyapplied electric currents that are intentionallyintroduced through medical devices or

unintentionally introduced through

accidental exposure (electric shock).


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Externally applied electric currents can excite

nerve and muscle cells.

Muscle can be stimulated directly orindirectly through the nerves that enervatethe muscle.

Thresholds of stimulation of nerve aregenerally well below thresholds for direct

stimulation of muscle


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An understanding of neuroelectric principles

is a valuable foundation for investigation into

both sensory and muscular responses toelectrical stimulation.


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This illustratesfunctional componentsof sensory and motor(muscle) neurons. Theillustrated nerve fibersare myelinated, i.e.,

covered with a fattylayer of insulationcalled myelin andhaving nodes of Ranvierwhere the myelin isabsent. The conductingportion of the nerve

fiber is a long, hollowstructure known as anaxon. The axon plusmyelin sheath isfrequently referred toas a nerve fiber, orneuron. Bundles ofneurons are callednerves.


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The body is equipped with a vast array ofsensors (receptors) for monitoring its internaland external environment. Electricalstimulation generally involves thesomatosensory system, i.e., the system ofreceptors found in the skin and internalorgans. Other specialized receptors includethose in the visual and auditory systems andchemical receptors by which neuronscommunicate with one another.


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When a sensory receptor is stimulated, itproduces a voltage change called a generatorpotential. The generator potential is graded:if you squeeze a pacinian corpuscle, forexample, it produces a voltage; if yousqueeze it harder, it produces a greatervoltage. The generator potential initiates asequence of events that leads to apropagating action potential (a nerveimpulse in common parlance).


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The functional boundary of the biological cell is athin (about 10 nm) bimolecular lipid and proteinstructure called a membrane. Electrochemical

forces across the membrane regulate chemicalexchange across the cell.

The medium within the cell (the plasm) andoutside the cell (the interstitial fluid) is

composed largely of water containing variousions. The difference in the concentration of ionsinside and outside the cell causes anelectrochemical force across the cell membrane.


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Nerst Equation

where

[S]i and [S]o: represent the concentrations of ionicsubstance S inside and outside the cell,

R: is the universal gas constant,

T :is absolute temperature,F: is the Faraday constant (number of coulombs per mole of

charge), and

Z: is the valence of substance S.


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It consists of nonlinearconductances for Na+and K+ and a linearleakage element.

The potential sourcesshown in the diagram arethe Nernst potentials for

the particular ions.The capacitance termCm is formed by thedielectric membraneseparating theconductive media oneither side.

The conductances gNaand gK apply to Na+ andK+ channels; theconductance gL is ageneral leakagechannel that is notspecific to any particularion.


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Electrical Model for Nerve

Excitation Myelinated fibers have much lower

thresholds of excitation than unmyelinated

fibers. Accordingly, the myelinated fiber is an

appropriate choice for electrical stimulation

studies.


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Excitation

This illustrates anelectrical model formyelinated nerve asoriginally formulated byMcNeal [1976].

The myelin internodes

are treated as perfectinsulators and the nodesas individual circuitsconsisting of capacitanceCm and an ionicconductance term.

The nodes are

interconnected throughthe internal axon mediumby conductances Ga. Thecurrent flowing in thebiological medium createsvoltage disturbances Ve,nat the exterior of thenodes.


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ExcitationThis illustrates theresponse of the

myelinated nervemodel of the last figure

to a rectangular current

stimulus [Reilly et al.,1985].

The example is for asmall cathodalelectrode that is 2 mmradially distant from a

20-mm fiber anddirectly above a central

node.


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ExcitationThis illustratesstrength-duration

curves derived from themyelinated nerve

model described previ-

ously under the sameconditions applying tothe figure above.

Three types of stimuluscurrent apply to thisfigure: a monophasic

constant current pulse,a symmetric biphasic

rectangular current,and a single cycle of asine wave.


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ExcitationModel response tocontinuous sinusoidal

stimulation at 500 Hz.The lower panel depicts

the response to a

stimulus current set atthreshold level (IT) for asingle-cycle stimulus.

Upper panels showresponses for

stimulation 20 and 50%above the single-cycle

threshold.


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Bioelectricity

Refers to the electrical, magnetic or

electromagnetic fields produced by living

cells, tissues or organisms. Examples includethe cell membrane potential and the electric

currents that flow in nerves and muscles, as a

result ofaction potentials.


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Bioelectricity

Studied primarily through the techniques ofelectrophysiology. In the late eighteenthcentury, the Italian physician and physicist Luigi

Galvani first recorded the phenomenon whiledissecting a frog at a table where he had beenconducting experiments with static electricity.Galvani coined the term animal electricity todescribe the phenomenon, while contemporaries

labeled it galvanism. Galvani andcontemporaries regarded muscle activation asresulting from an electrical fluid or substance inthe nerves.


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Bioelectricity

is an aspect of all living things, including allplants and animals. Some animals have acutebioelectric sensors, and others, such asmigratory birds, are believed to navigate in partby orienteering with respect to the Earth'smagnetic field. Also, sharks are more sensitive to

local interaction in electromagnetic fields thanmost humans. Other animals, such as the electriceel, are able to generate large electric fieldsoutside their bodies.


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Bioelectricity

In the life sciences, biomedical engineeringuses concepts of circuit theory, molecularbiology, pharmacology, and bioelectricity.

is associated with biorhythms andchronobiology. Biofeedback is used inphysiology and psychology to monitorrhythmic cycles of physical, mental, andemotional characteristics and as a techniquefor teaching the control of bioelectricfunctions.


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Bioelectricity

is also used in certain touch screen

technologies that don't actually rely on

"touch" but rather on recognizing theelectromagnetic waves of body (e.g. the

finger) when it comes close to the screen.


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Bioelectric Principles

Bioelectric signals are exploited for the

diagnostic information that they contain.

Such signals are often used to monitor andguide therapy. Although all living cells exhibit

bioelectric phenomena, a small variety

produce potential changes that reveal their

physiological function.


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Bioelectric Principles

The most familiar bioelectric recordings are:

Electrocardiogram, ECG Electromyogram, EMG

Electroencephalogram, EEG


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Electrocardiogram, ECG

Is a transthoracic interpretation of the

electrical activity of the heart over time

captured and externally recorded by skinelectrodes. It is a noninvasive recording

produced by an electrocardiographic device.


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Works mostly by detecting and amplifying the tiny electricalchanges on the skin that are caused when the heart muscle"depolarises" during each heart beat. At rest, each heart musclecell has a charge across its outer wall, or cell membrane.Reducing this charge towards zero is called de-polarisation,which activates the mechanisms in the cell that cause it tocontract. During each heartbeat a healthy heart will have anorderly progression of a wave of depolarisation that is triggeredby the cells in the sinoatrial node, spreads out through theatrium, passes through "intrinsic conduction pathways" and thenspreads all over the ventricles. This is detected as tiny rises and

falls in the voltage between two electrodes placed either side ofthe heart, which is displayed as a wavy line either on a screen oron paper. This display indicates the overall rhythm of the heartand weaknesses in different parts of the heart muscle.


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It is the best way to measure and diagnose abnormalrhythms of the heart, particularly abnormal rhythmscaused by damage to the conductive tissue thatcarries electrical signals, or abnormal rhythmscaused by electrolyte imbalances. In a myocardialinfarction (MI), the ECG can identify if the heartmuscle has been damaged in specific areas, thoughnot all areas of the heart are covered. The ECGcannot reliably measure the pumping ability of the

heart, for which ultrasound-based(echocardiography) or nuclear medicine tests areused. It is possible to be in cardiac arrest with anormal ECG signal (a condition known as pulselesselectrical activity).


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Electromyogram, EMG

is a technique for evaluating and recording theelectrical activity produced by skeletal muscles.EMG is performed using an instrument called an

electromyograph, to produce a record called anelectromyogram. An electromyograph detectsthe electrical potential generated by muscle cellswhen these cells are electrically or neurologically

activated. The signals can be analyzed to detectmedical abnormalities, activation level,recruitment order or to analyze thebiomechanics of human or animal movement.


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Electromyogram, EMG

The electrical source is the muscle membranepotential of about -90 mV. Measured EMGpotentials range between less than 50 V and up

to 20 to 30 mV, depending on the muscle underobservation.

Typical repetition rate of muscle motor unitfiring is about 720 Hz, depending on the size of

the muscle (eye muscles versus seat (gluteal)muscles), previous axonal damage and otherfactors. Damage to motor units can be expectedat ranges between 450 and 780 mV.


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is the recording ofelectrical activity along the

scalp produced by the firing ofneurons within

the brain. In clinical contexts, EEG refers tothe recording of the brain's spontaneous

electrical activity over a short period of time,

usually 2040 minutes, as recorded from

multiple electrodes placed on the scalp.


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In neurology, the main diagnostic application ofEEG is in the case ofepilepsy, as epileptic activitycan create clear abnormalities on a standard EEG

study. A secondary clinical use of EEG is in the

diagnosis of coma, encephalopathies, and braindeath. EEG used to be a first-line method for the

diagnosis oftumors, stroke and other focal braindisorders, but this use has decreased with theadvent of anatomical imaging techniques suchas MRI and CT.


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Derivatives of the EEG technique include evokedpotentials (EP), which involves averaging theEEG activity time-locked to the presentation of astimulus of some sort (visual, somatosensory, orauditory). Event-related potentials refer toaveraged EEG responses that are time-locked tomore complex processing of stimuli; thistechnique is used in cognitive science, cognitivepsychology,and psychophysiological research.


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Magnetic (Eddy-Current)

Stimulation When scalp electrodes are used to stimulate

the brain, there is considerable skin sensation

under the electrodes owing to the highperimeter current density.

It has been found that sufficient eddy current

can be induced in living tissue by discharging

an energy-storage capacitor into an air-coredcoil placed on the skin.


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Stimulation This mode of stimulation is almost without

skin sensation; by some it is called ouchlessstimulation and it can be used to stimulate

the brain, peripheral nerves, and the heart. The parameters associated with eddy-current

stimulation are kiloamperes, Teslas/sec,milliohms, microhenries, microseconds, andlow damp- ing. Because the forces on the coilconductors are very large, special care isrequired in fabricating such coils.


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StimulationSimplest typeof magnetic(eddy-current)

stimulator andcoil andinducedcurrent

waveforms.


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Stimulation

Magnetic(eddy-current)stimulator

that recoversenergy storedin themagnetic field.


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Summary and Conclusion

Although eddy-current stimulation of

excitable tissue is quite popular now, the first

report was by dA

rsonval [1896], whoreported seeing bright flashes in the visual

field (phosphenes) when the head was placed

in a coil carrying 30 amperes of 42 Hz current.

It is now known that stimulation of the retinalreceptors in the eye produces such

phosphenes.


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Magnetic stimulation is largely used to excite

nerve cells in the brain and spinal cord. The

diagnostic information is contained in thetime between the stimulus and the response

(action potential). The same measure is used

when peripheral nerve is stimulated.


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We have focused only on the three most

prominent bioelectric events, those of the

heart, skeletal muscle, and brain. The eye,ear, sweat glands, and many types of smooth

muscle produce action potentials that are

used for their diagnostic value, as well as

being the subject of on-going research.


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References

http://en.wikipedia.org/wiki/Electroencephalography

http://en.wikipedia.org/wiki/Electromyography

http://en.wikipedia.org/wiki/Electrocardiography http://en.wikipedia.org/wiki/Bioelecetricity

www.docstoc.com/.../Microsoft-PowerPoint---Bioelectricity-3-class-notesppt

Reilly, J.P., Geddes, L.A., Polk, C.BioelectricityThe Electrical EngineeringHandbookEd. Richard C. Dorf Boca Raton: CRCPress LLC, 2000

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1. Bio Electricity

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