Part 3 Biosignals Origin and Measurement
Transcript of 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