Part 3 Biosignals Origin and Measurement

7/26/2019 Part 3 Biosignals Origin and Measurement

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Bio-signals


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Origin of Bio-potentials


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Bioelectric phenomena


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Goals

Monitoring and Recording many forms of

bioelectric phenomena

ECG(Electrocardiography)

EMG(Electromyography)

EEG(Electroencephalography)

ENG(electroneurography)


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Bio-potentialsCertain systems of the body create their own

monitoring signals! which con"ey usefulinformation regarding the functions they represent#

$hese signals are the Bio-potentials %B&'associated with the conduction along the sensory

and motor ner"ous system! muscular contractions!brain acti"ity! heart contractions! etc#

$hese potentialsare a result of theelectrochemical

acti"ity occurring in certain classes of cells within thebodyExcitable Cells#

Measurements of these Bio-potentials can pro"ide

clinicians with in"aluablediagnostic information


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Cell Membrane &otentialsCell membranesin general! and membranes of ner"e cells

in particular! maintain a small "oltage or potential across

the membrane in its normal or resting state#In the rest state! the inside of the ner"e cell membrane is

negati"e with respect to the outside (typically about -milli"olts)#

$he "oltage arises fromdifferences in concentrationof the

electrolyte ions *+ and ,a+#$here is a process which utilies ATPto pump out three ,a+

ions and pump in two *+ ions# $he collecti"e action of thesemechanisms lea"es the interior of the membrane about -

m. with respect to the outside#

/f the e0uilibrium of the ner"e cell is disturbed by the arri"alof a suitable stimulus dynamic changesin themembrane potential in response to the stimulus is called an

Action Potential#1fter the action potential the mechanisms described abo"e

bring the cell membrane bac2 to its resting state#


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E3citable CellsE3citable cells are a class of cells that produce

bioelectric potentials as a result of electrochemicalactivity.

1t any gi"en time! these cells can e3ist in one oftwo states! restingandactive#

Chemical and electricalstimuli can force ane3citable cell from the resting to the acti"e state#

4hile there are numerous ionic species presentboth inside and outside the cell! onlythree ions (for

which the cell membrane in its resting state ispermeable)play a 2ey role in the beha"ior of these

cells5K+, Na+and Cl-#


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1cti"e 6tate/f ade0uately stimulated! either electrically or

chemically! the e3citable cell will enter into the acti"estate#

$he transmembrane potential "aries with time and

position within the cell in this state! and is called anaction potential#

$he following se0uence of e"ents occurs when the

cell enters the acti"e state5$he chemical or electrical stimuli increases the

permeability of the membrane to ,a#

,a rushes into the cell due to the large concentration

gradient#


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1cti"e 6tate (cont#)$hese positi"ely charged ions entering the cell cause the

transmembrane potential to become less negati"e! and

e"entually slightly positi"e# $his change is often referredto as a depolarization#

1 short time ( tenths of microseconds) later themembranes permeability to * increases! which results in

an outflow of *#$he outflow of * causes the transmembrane potential to

decrease# $his decrease in potential causes themembranes permeability to both ,a! and e"entually *!

to decrease to their resting le"els$here is only a relati"ely small (immeasurable) net flow

of ions across the membrane during an action potential#$he ,a-* pump restores the concentrations (pumps ,a

out and * in) of the ions to their resting le"els#


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resting to the acti"e state is the 1ction

&otential

/n response to the appropriate stimulus! the cellmembrane of a ner"e cell goes through a se0uenceof

depolariation from its rest state to the acti"e state

followed byRepolariation to the rest state once again#

$he cell membrane actually re"erses its normalpolarity for a brief period before reestablishing the

rest potential#$he action potential se0uence is essential for neural

communication# $he simplest action in response tothought re0uires many such action potentials for its

communication and performance


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$he different phases a cell

membrane


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$he process in"ol"es se"eral steps5 #71 stimlsis recei"ed by the dendrites of a ner"e cell# $hiscauses the ,a+ channels to open# /f the opening is sufficient to

dri"e the interior potential from - m. up to -88 m.! the processcontinues#

#9:a"ing reached the action threshold! more ,a+ channels(sometimes called "oltage-gated channels) open $he ,a+influ3 dri"es the interior of the cell membrane up to about +;

m.# $he process to this point is called!EP"#A$I%ATI"N#

#;$he ,a+ channels close and the *+ channels open# :a"ing both,a+ and *+ channels open at the same time would dri"e thesystem toward neutrality and pre"ent the creation of the action

potential#

#1fter hyperpolariation! the ,a+?*+ pumps e"entually bring the

membrane bac2 to itsresting stateof - m.#


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1b l t @ R l ti R f t & i d


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1bsolute @ Relati"e Refractory &eriod

1R& @ RR&

Auring the initial portion of the 1ction potential

membrane does not respond Absolte

refractory 'eriod

Auring the$elati)e $efractory Period%RR&' theaction potential ta2es action

$he refractory periodlimits the fre0uency of arepetiti"e e3citation procedure

e#g# 1R&7ms *upper limit of repetiti"e discharge 7 impulses?s

1b l t @ R l ti R f t & i d


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1bsolute @ Relati"e Refractory &eriod

1R& @ RR& (cont#)

v5 action pot#

Nernst eil Pot for Na

Nernst eil Pot for K


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:ow the action is recordedD

$he tip ismo"ed to until

the resting pot#

is recorded

1 short timelater an

electrical

stimulus is

deli"ered forthe period

until recording


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Bioelectric 6ignal Measurement


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Bioelectric measurements

Part 3 Biosignals Origin and Measurement

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Transcript of Part 3 Biosignals Origin and Measurement