exam bioelectricity tudeft 10april2015_answers

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Exam Bioelectricity, TUDelft , code ET 4130

Friday april 10, 2015

Dr A.C. i!!e!"a!#

$ectio! Bioelectro!ic%

- Nederlandse studenten: alleen antwoorden in, correct, Nederlands !

- there is no need for ‘long stories’ in your answers, but of course, adequate

explanations should show the insight you have. So, not too long, not too short !

- no calculators or obile phones are allowed on your des. "eep the in your bag.

$t&de!t !&m"er'

(ame'


2/12

)&e%tio! 1. *i%tory. Scores:

#an Swaerda did the experient shown in the

figure in $%%& 'by coparison (uigi )alvani did his

faous bioelectricity experients in the last two decades

of the $*th century+

a+ y pullng the silver wire c the nerve connecting to the

frog uscle b could be pressed against the brass support

d . -hat could result in a contraction of the frog uscle.

hat is e and what is it used for/

e is a droplet that would ove if the volue of the

uscle would change.

b+-here are two possible explanations for the

contraction. 0or one it is essential that the support and

the wire are ade fro different aterials. 1xplain why

that is essential and why the uscle would contract in

this case.

-wo etals with two different half2potentials for a

battery if in contact with fluid. 3f that is large enough

'silver2brass is+ this can start a depolari4ation of the

nerve and subsequently of the uscle.

c+ #aao 5alivuo recently repeated the experient and showed that it is indeed the

ore liely explanation. -hat would iply that a sall part of scientific history needs to

be rewritten or at least updated with a footnote. rite that footnote:

)alvani is generally credited for being the first to use bietal stiulation of excitable

tissue. -his experient shows that Swaerda probably preceeded hi by ore than acentury, although it is unliely that he understood the significance of this finding.

Experiment by Jan Swammerdam


3/12

)&e%tio! 2' Actio! +ote!tial

3n a living cell there is a potential difference between the inside and the outside of the cell.

Nerve, uscle and heart cells can quicly change the value of that potential.

1xplain the overall shape of the ventricular uscle action potential, the lower trace in the

right colun, 6ust above the surface 17). hy does this ventricular uscle cell rather

suddenly go fro about 2*89 to around 8 9, stays soe tie at that level and then

ore gradually return to 2*8 9. 'ou don;t need to explain the overshoot or discuss

ore than two types of ions+

hat 3 wanted to hear is soething about changing pereability of the ebrane for

different ions. -hat the upstroe is sodiu and the repolari4ation is potassiu.5entioning positive feedbac is a plus.


4/12

)&e%tio! 3' $olid A!le T-eorem

a+ -he solid angle theore says that the potential that you record depends on the su

of the strengths p of the dipoles at a surface S and how uch of that dipoles you can ;see;

fro that electrode.

)ive the forula for the solid angle theore that allows you to copute the potential at

an electrode.

0orula $$.%

b+ explain why a closed surface with a constant dipole strength will result in a 4ero

potential at any point outside that surface.

0igure $$.


5/12

)&e%tio! 4' %oelectrical !ter/al%

a+ 3n the 17) the intervals between two beats 'the -2> interval+, that between the atrial

and ventricular actiavtion 'the >2? interval+ and the period 6ust after the ?@S coplex

'the S2- segent+ are in healthy huans 'alost+ isoelectrical. 1xplain why there are no

large potentials in these intervals.At all three intervals all cells in the atriu and in the ventricle have approxiately the

sae potential, hence they for a closed double layer and no potential is recorded. -2> all

cells at resting voltage. >2? all ventricular cells at resting voltage, all atrial cells

approxiately the sae potentials, not really a plateau, but close enough for very little to

reain. At S2- all atrial cells repolari4ed and all ventricular cells in plateau phase, so also

the sae ebrane voltage

b+ Bne of the three tie periods entioned in a+ depends on the shape of the action

potential. 5ouse ventricular action potentials are ore triangular shaped. hich

isoelectrical interval is therefor absent in ouse and why/

-here is no plateau so 6ust after the last ventricular cell depolari4ed cells that were

activated earlier have lower potentials. =ence the assuption that it is a double layer with

the sae strength everywhere is invalid. 'Also: you can not see the end of the ?@S for

ice on the 17)+.

c+ hen there is a local in6ury of the heart, cells in that area can have a higher resting

potential 'say 2&8 in stead of 2*8 9+ while the depolari4ed potentials are still all about

the sae 'around 8 9+. 3n the S2- segent the cells are all depolari4ed. et, therecorded 17) will show S2- segent changes, why/

S2- changes are easured as difference with the -2> interval. 3t is the latter that changes.

P-QT-P S-T


6/12

)&e%tio! 5. (er!%t e&atio!.

a+ hat can you tell about the sodiu current through the ebrane of the cell, when

the transebrane potential is equal to the Nernst potential for sodiu/ 'N 3 will be

quite picy about the wording+

-hen there flows no net current.

-o derive the Nernst equation we have to use the expressions for the two relevant

currents

b+ one ingredient in our derivation is:

hat quantities do these sybols represent:

C current of ion as a result of diffusion

C 0ic;s constant of ion

C concentration of ion

c+ another ingredient is:

hat quantities do these sybols represent:

C current of ion as a resilt of the electric field

C valence of ion

C ionic obilityof ion


7/12

d+ has the diension of . Soeties in the boo it is referred to as a

velocity. -hat is not entirely correct, but what has it to do with velocities/

3n an electric field ions will reach a speed of

e+ cobine equations in b+ and c+ and possibly the equation derived by 1instein in $D8E:

to find an expression for the total current

C

forula


8/12

)&e%tio! . EC i!terpretatio!

a+ (abel each ?@S coplexes by a ;?; at the top

b+ 7an you also label all > waves/

no

c+ hat is the average ventricular rhyth in beats per inutethere are * beats in % seconds so *8 a inute

or you easure fro the first coplex to the last and divide by G

d+ hy is the ventricular rhyth not constant

this is atrial fibrillation not all atrial coplexes are passed on to the ventricle

e+ hat is the clinical nae of this rhyth

atrial fibrillation, afib, A0

f+ hat role does the A9 node play in eeping the ventricular rhyth low enough togenerate enough cardiac output for this patient to not collapse 'though it ay be tiring+/

-he A9 node can not pass on an activation when it coes too soon after the previous. So

it provides a echanis to ensure the ventricles have had enough tie to contract and

recover before the next beat,

Figure 3: ECG trace. Standard axes: 2.5 cm per second and 1 m per cm


9/12

)&e%tio! ' Arr-yt-mia%

a+ Hraw the noral activation sequence in the scheatic

drawing of the heart below. Ise arrows to indicate the

direction of the sequence.

Now assue that the 17) of question % is of a GE year

old an and this rhyth is only now and then present.

0ro an earlier visit to the cardiologist we have the 17)

of figure &. Soeties the ?@S begins earlier than at

other ties 'here two early ones+, although the overall

shape and tiing of the ?@S is still ore or less the

sae. )ive an explanation for the short >? interval and

draw arrows to indicate the activation sequence of the

abnoral activation in the scheatic drawing on the right below.

Noral conduction is SA2node J atrial yocardiu J A92node J his2bundle J

puri6e fibers J ventricular yocardiu. -he early activation coes fro an extraconnection between atriu and ventricle 'ent bundle+. -hat will result in preature

activation of an are of the yocardiu close to the A9 ring. As soon as the noral

activation over his and purin6e taes place that will doinate the 17). -hat is why the

?@S is siilar to a noral beat except for the early phase. So on the right draw a vector

fro atriu to ventricle at any place other than the A92node.

b+ -he cobination of these arrhythias will ae a cardiologist want to do an ablation

procedure 'overheating a sall part of the heart that is causing trouble+ as soon as

possible. hy is this cobination so potentially dangerous/

See question %f: the extra bundle is not an A92node with build2in rate control. -hat

connection ight conduct all ipulses fro the atriu to the ventricle. -hen Afib could

turn into ventricular fibrillation.

ECG wit! s!ort "#$ inter%a&

norma& ear&y $'S


10/12

c+ Now assue that this last 17) with a short >2? interval 'and the underlying

physiological proble+ belongs not to an old an but to a young woan. She does not

suffer fro the irregular rhyth of question %, but soeties she suffers fro fast

rhyths that aes it ipossible to do siple exercises, lie clibing stairs. Hraw in the

picture at least on of the two activation sequences that ay explain the fast rhyth.

-here are two connections between atriu and ventricle. -he noral way via the A9

node and the ent bundle. -wo possibilities for a reentrant tachycardia: over the A9 node

to the ventricle and bac to the atriu via the ent bundle and then A92node etc. Br the

other way around. 3n both cases atriu and ventricle are part of the circle.

d+ She responds positive to anti2arrhythic drugs. i.e. with the drugs she is free fro

the fast rhyth and has no other cardiac related coplaints. Still, there can also in this

case be a strong reason to do an ablation, while that would be less pressing for a ale.

hy/

-hat can be the case if the drugs are not tested in pregnant woen or nown to cause

probles to get pregnant or increase ris of birth defects. ?uality of life and side effects

of drugs can be a reason to treat in a non2pharocological way even if the drugs wor.

spare


11/12

)&e%tio! . Tor%o ta!#.

a+ 3n Salt (ae 7ity for soe

experients a torso2tan was used.

-his was ade fro a cast of a child,

because dog hearts are saller than

adult huan hearts. Hoes it ae a

difference in the recorded values of the

potentials if one uses this set or a set

where all coponents are twice as

large/ hy/

3t does not ae a difference. Solid

angle theore again. 3t also why the

?@S of an elephant and an elephant

shrew will have an aplitude in the

order of $ 9, 6ust as we do

b+ this setup can be used to easure at each ;surface; electrode the potential resulting

fro a source inside the tan that can be positioned in three orthogonal directions. =ow

do we call those quantities/

(ead vectors

c+ this setup can be used to in6ect current at one tan2electrode and record the electric

field at all points within the tan. =ow do we call those quantities.

(ead field


12/12

)&e%tio! . 6odel%.

a+ when we have built a

exam bioelectricity tudeft 10april2015_answers

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