bachelor2013_neuronmodel-jacobhkold_

8/12/2019 Bachelor2013_NeuronModel-JacobHKold_

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Collision Between Nerve Signals

Jacob Hamfeldt Kold

March 15th 201

Bachelor !ro"ect

Membrane Bio#h$sics %ro

Niels Bohr 'nstit&te

(niversit$ Co#enhagen

Servisors) *lfredo %on+ale+,!ere+ and -homas Heimb&rg


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Abstract

'n this #ro"ect ' will investigate what ha##ens when two nerve signals collide. -he

collision will be created b$ stim&lating a nerve from the right side and the left side

sim&ltaneo&sl$. 'n this wa$ two nerve signals will traverse in o##osite directions

along the nerve and meet in the middle. B$ com#aring #ro#erties of a traveling signal

from right and from left res#ectivel$/ to the #ro#erties of two traveling signals/ ' will

investigate the o&tcome of a collision between two nerve signals. -he general view is

that when the two signals meet the$ will annihilate each other so that both waves

disa##ear after the$ meet. ' find that this is in fact not tr&e. M$ res&lt s&ggest that the

two signals dont affect each other so that each wave goes thro&gh the other and has

the eact same #ro#erties before and after collision has taen #lace.


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Contents

1 'ntrod&ction............................................................................................................................

1.1 3i#ids and membrane #roteins........................................................................................

1.2 -he cell membrane.........................................................................................................

1. -he Nervo&s S$stem.......................................................................................................4

1.4 ertebrates and 'nvertebrates.........................................................................................5

1.5 Nerve cells6Ne&rons.......................................................................................................5

1.7 'on channels....................................................................................................................8

1.8 Signal transmission.........................................................................................................9

1.9 Motivation :...................................................................................................................;

2 -heoretical Models...............................................................................................................11

2.1 -he Hodgin,H&le$ Model.........................................................................................11

2.2 -he Soliton Model........................................................................................................12

Methods and Materials.........................................................................................................15

.1 Materials.......................................................................................................................15

.1.1


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2


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1 Introduction

1.1 Lipids and membrane proteins

* cell membrane mainl$ consist of li#ids and #roteins. 3i#ids are long h$drocarbon

chains with a charged head gro in one end>see fig&re 1.1?. -he h$drocarbon chains

are similar to fatt$ acids and will not mi with a@&eo&s sol&tions. -he head gro is

#olar and lie ionic salts this #art of the molec&le will mi well with a@&eo&s

sol&tions. -herefore a li#id is said to be an am#hilic molec&le/ which refers to the fact

that one #art of the li#id>the headgro? mi well with water while the other #art >the

carbon chains? dontA1.

!roteins are chains made of amino acids. -here are 20 different amino acids that mae

#roteins. * membrane #rotein is a #rotein fo&nd in a biological membrane. -here

are two t$#es of membrane #roteins. 'ntegral #roteins that have a strong interaction

with the membrane and #eri#heral #roteins that have a m&ch weaer interaction with

the membrane>see fig&re 1.2 and 1.?.

Figure 1.1 Schematic andchemical structure of a

phospholipid [2]

1.2 The cell membrane

-he effect of the am#hilic #ro#ert$ of li#ids is that in the cell membrane the li#idsform a so called two dimensional li#id bila$er as seen on fig&re 1.4. -wo la$ers of

li#ids are connected to each other. 'n each of the two la$ers the head gros form a

wall against the water inside and o&tside the cell res#ectivel$. Beca&se of this

str&ct&re the h$drocarbon chains is shielded from water in the cell membrane. !roteins

are embedded in the membrane. -he membrane contains l&m#s and cl&sters of

#roteins and li#ids so the #roteins and li#ids are not evenl$ distrib&ted in the

membrane.

3

Figure 1.2 Peripheralproteins [3]

Figure 1.3 Integralproteins [3]


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Figure 1.4 Example of a cell memrane [4]

* cell membrane ee#s the interior of the cell with all the organelles together. -he

#ro#erties of a cell membrane allows the cell to maintain and reg&late the

concentration if ions and other com#onents in the cell. * li#id bila$er is a ver$ good

electric ins&lator. -his means that since ions are charged ions cant go thro&gh a li#id

bila$er. B&t the #rotein str&ct&res embedded within the li#id bila$er form so called ion

channels in the membrane. 'on channels are certain areas of the membrane where the

#resence of a s#ecific #rotein maes it #ossible for a s#ecific ion to diff&se thro&gh

the membrane a that s#ecific area. -hro&gh forming of ion channels the #roteins

within the membrane allows for reg&lation of ion concentration within the membrane.

1.3 The Nervous System

-he nervo&s s$stem is what allows h&mans and animal to react to o&r s&rro&ndings

and gives &s conscio&sness. -he nervo&s s$stem is basicall$ a networ of nerves that

can send and receive nerve signals from and to one another. -he nervo&s s$stem of

vertebrate animals/ which incl&de h&mans/ can be divided into two #arts. -he central

nervo&s s$stem and the #eri#heral nervo&s s$stem. -he central nervo&s s$stem consist

of the brain and the s#iral chord. -he s#iral chord is connected to the brain and signals

to and from the brain goes thro&gh the s#iral chord. -he central nervo&s s$stem is

connected to the #eri#heral nervo&s s$stem. -he #eri#heral nervo&s s$stem consist of

sensor$ ne&ron that detects sensor$ in#&ts from the o&tside. -he #eri#heral nervo&s

s$stem also consists of nerves connecting these sensor$ ne&rons to the central nervo&s

s$stem. 't also consist of motor ne&rons and connections between motor ne&rons and

the s#inal chord. Motor ne&rons are ne&rons that can send signals to m&scles to

#rod&ce m&scle movement. -hro&gh the d$namic between these s$stems the bod$ can

ad"&sts and react to eternal s&rro&ndings and in#&t.

Since the nervo&s s$stem mainl$ consists of nerves/ the #ro#erties of nerves and the

wa$ signals are transmitted in nerves are a ver$ im#ortant and interesting scientific

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s&b"ect.A5

1.4 ertebrates! and invertebrates

ertebrates are animals with a s#inal chord and a bacbone. or eam#le re#tiles/

mammals and birds. 'nvertebrates are animals witho&t a bacbone. or eam#le

insects/ worms and crabs>see fig&re 1.5?. -he nervo&s s$stem in vertebrates are &ni@&e

to vertebrates and vertebrates are the onl$ gro of animal that have a #ro#er brain. ,

4 D of all animal s#ecies are vertebrates. -he vast ma"orit$ of animal s#ecies are

invertebratesA7.

1." Nerve cells!Neurons

* nerve cell or ne&ron can receive a nerve signal from other nerve cells and #ass it on

to other nerves. * nerve consist of a bod$ called the soma/ dendrites and the aon.

-he soma contain a n&cle&s and a lot of other molec&les needed to ee# the nerve cell

alive and f&nctioning. -he dendrites can be considered as branches which reach o&t

and connects to other nerve cells. 'f a nerve cell is stim&lated s&fficientl$ a nerve

signal will #ro#agate down the aon. Ehen the nerve signal reaches the end of the

aon/ dendrites from other nerve cells connected to the aon can interce#t the signal

and the signal can be #assed on to aons of other nerve cells thro&gh the dendrites.

* nerve is a so called ecitable media. 'f the total signal or stim&lation sensed from all

dendrites in a nerve eceeds a certain threshold val&e/ the nerve will be activated and

send a signal thro&gh its aon. 'f the total stim&lation sensed from the dendrites is

below the threshold val&e/ then nothing ha##ens. *s long as the stim&lation is above

the threshold val&e the signal that the ne&ron is sending is inde#endent of the strength

of the stim&lation received. 'mmediatel$ after a ne&ron has send a signal the ne&ron

5

Figure 1.! "ifference et#een $erterates and In%erterates[&]


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s#ends a certain #eriod in refractor$ state where it cant send another signal. -he time

that the ne&ron s#ends in this state is called the refractor$ time.

Ne&rons are often classified into three main t$#es de#ending on their role in the

nervo&s s$stem. Sensor$ ne&rons and motor ne&rons which was mentioned earlier and

inter ne&rons. 'nter ne&rons are ne&rons that from a connection between other

ne&rons. 'nter ne&rons are neither sensor$ or motor ne&rons.

B&t ne&rons can also be classified into fo&r main categories according to their

str&ct&re. (ni#olar/ !se&do&ni#olar/ m&lti#olar and bi#olar>see fig&re 1.8?. * &ni#olar

ne&ron onl$ have one #ro"ection from the soma. * #se&do&ni#olar ne&ron is a sensor$ne&ron where the aon has s#lit into two branches where the one branch goes to the

s#inal chord and the other branch goes to sin m&scle or "oint. * m&lti#olar ne&ron

#ossesses one aon and a lot of dendrites. M&lti#olar ne&rons incl&de motor and

interne&ron and the ma"orit$ of ne&rons in the brain are m&lti#olar ne&rons. * bi#olar

ne&ron is a ne&ron where the dendrites and the aon are on o##osite sites of the soma.

Bi#olar ne&rons are s#eciali+ed sensor$ ne&rons.

6

Figure 1.' Schematic picture of a ner%e cell [(]


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M$elination)

'n this #ro"ect ' have wored with earthworms and recordings has been made of nerve

signals #ro#agating thro&gh the so called median and giant fibers in earthworms.

-hese fibers can/ for all #ractical #&r#oses/ be regarded as long aons r&nning thro&gh

the entire length of the worms. *n im#ortant as#ect of the -he median and giant fibers

in relation to this e#eriment/ is that the$ are covered b$ a m$elin sheat.

* m$elin sheath is one or more la$ers of m$elin covering a ne&ron. M$elin is an

ins&lating material and consist of water/ li#ids and #roteins. -he m$elin sheat maes

nerve signals #ro#agate faster thro&gh the nerve. -he m$elin la$er also increases

electrical resistance and thereb$ hel#s to maintain electrical c&rrent within the ne&ron.

1.# Ion channels

'on channels are membrane #roteins that mediate the trans#ort of different ionic

s#ecies between the inside and the o&tside of the cell A10. * n&mber of ions eist in

one ionic concentration inside the cell and another ionic concentration o&tside the cell.

-hese concentration differences give rise to a different amo&nt of charge on each side

of the membrane or an electrical gradient across the cell membrane. -he membrane

#otential is the difference between the electric #otential inside and o&tside the cell

ca&sed b$ these ions.

-he li#id bila$er acts as an ins&lator so the ions cannot #ass thro&gh the li#id #art of

the membrane. B&t #roteins are embedded in the membrane and some #rotein can

creates a channel where a given t$#e of ion can #ass thro&gh. S&ch channels are called

7

Figure 1.& )he four main

structural t*pes of ner%es. 1+,nipolar neuron 2+ -ipolarneuron 3+ ultipolar neuron 4+

Pseudoinupolar neuron [/]


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ion channels. 'f an ion channel is o#en the related t$#e of ions can #ass thro&gh the

ion channel and if the ion channel is closed the$ cant. Some ion channels called

voltage gated ion channels are o#en with a #robabilit$ given b$ the membrane

#otential.

-he combination of the ins&lating #ro#ert$ of the li#id bila$er and the ion channels in

the membrane means that the cell membrane is selectivel$ #ermeable. -his means that

for some ions the membrane is im#ermeable and acts lie a strong ins&lator while for

other ions the membrane is #ermeable beca&se of some o#en ion channels.

'f the membrane is #ermeable for an ion two forces acts on the ion. Fne force tr$ing to

even o&t the concentration on each side of the cell and another stemming from the

membrane #otential >charge gradient? and the fact that an ion is charged.


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* cell membrane has a &niform resting #otential of abo&t ,70m. *t this #otential the

membrane is #ermeable for #otassi&m. 'f the infl& rate of sodi&m is not great eno&gh

the effl& of #otassi&mions flowing o&t of the cell will offset the effects of sodi&m

flowing into the cell. -his can e#lain the all or nothing behavior of nerve signals.

'f the de#olari+ation sensed b$ nearb$ sodi&m channels is s&fficientl$ great/ a new

networ of #rocesses will be initiated f&rther down the membrane/ and as a res&lt of

that a signal will #ro#agate down the membrane. Ftherwise no signal will be seen.

1.& 'otivation

'n the #revio&s cha#ters it is described how nerve cells transmit nerve signals

according to an all or nothing res#onse to a stim&li. 't is described how a nerve signal

travels thro&gh the membrane and the #art ion channels #la$ in signal transmission.B&t there has been no mathematical descri#tion of how a nerve,signal #ro#agate.

-here eist two mathematical models for signal #ro#agation in nerve cells/ which

deserves to be taen serio&s. -he first one is the Hodgin,H&le$ Model>HH? and the

second one is the ver$ new Soliton Model. -he$ both have differential e@&ations

which can be &sed to mae sim&lations and thereb$ mae #redictions for the

#ro#agation of a nerve signal. -his e#eriment investigates what ha##ens when two

nerve #&lses collide. -he HH and the Soliton Model #redict two different o&tcomes of

this e#eriment. -herefore this e#eriment can #otentiall$ falsif$ one of or both of the

models.

-he collision between two nerve signals have been investigated #revio&sl$ b$ -asai

A12. He concl&ded in a collision e#eriment that the transmission of #&lses is

bloced. -herefore according to this reference the nerve signals annihilate each other.

*ccording to the soliton model sim&lations the signals sho&ld go thro&gh each other

witho&t annihilation A1. 'n this bachelor #ro"ect ' will #erform new e#eriments to

investigate the collision between two nerve signals.

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2 Theoretical 'odels

'n this section ' will introd&ce the two theoretical models for signal #ro#agation in

nerve cells A14.

2.1 The (od%)in*(u+ley model

'n the Hodgin,H&le$ model the cell membrane is treated as a ca#acitor and the

#rotein or ion channels are treated as voltage de#endent resistors. Based on their

e#eriments on nerve aons Hodgin and H&le$ concl&ded that the membrane

contained com#onents acting as inde#endent voltage and time de#endent cond&ctors

of G and NaG ions A14. -he$ treated the membrane as an ins&lator that acts as a

ca#acitor and constr&cted a model of the electrical circ&it in a membrane. Based on

this circ&it the$ came with a differential e@&ation for the flow of c&rrent thro&gh

the membrane. -he Hodgin,H&le$ model onl$ contains electric variables &nlie the

soliton model which is based on thermod$namical variables and considerations.

Besides the sodi&m and #otassi&m channels the circ&it also contains a lea c&rrent

mainl$ of chloride >cl,? ions.

-wo c&rrents mae the total c&rrent. *n Fhmic c&rrent and a c&rrent charging

the ca#acitor or membrane. 'n normal condition where there is no stim&li or eternal

voltage/ there is still a c&rrent thro&gh the membrane beca&se of the differentconcentrations of ions on each side of the membrane. -his give rise to diff&sion. -he

tendenc$ of a molec&le to travel from a high to low concentration area. *ccording to

the Hodgin,H&le$ model the total c&rrent thro&gh the membrane is given b$)

I=CMdVm

dt +gK(VmEk)+gNa(VmENa)+gl(VmEl) eq. 2.1


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o&ter concentration of their res#ective ion.

'n the Hodgin,H&le$ model the membrane is regarded as a cable and it is ass&med

that the voltage follows the wave e@&ation. -he model combines e@ 2.1 an e@&ationfrom cable theor$ and the wave e@&ation and end with the final e@&ation for

#ro#agation of an electric #&lse)

a

2=i 2=CM

dVm

dt +gK(VmEk)+gNa (VmENa)+gl(VmEl) eq. 2.2

gKand gNaare the cond&ctance val&e of the ali&m and natri&m channels. !h$sicall$

the$ re#resent the amo&nt of o#en #rotein channels in the membrane. Hodgin and

H&le$ s&ggested a ver$ com#licated method for deciding the val&e of gNaand gK. 't

involves three differential e@&ations and over 20 fit #arameters that sho&ld be fitted so

that gKand gNaand the overall model fits e#erimental data. -his is a weaness for the

Hodgin,H&le$ model since a model sho&ld ideall$ be as sim#le as #ossible.

Ftherwise one co&ld s&s#ect that a model can onl$ be fitted to a certain ind of data

beca&se the model can be fitted to a wide range of data in general.

-he HH model can re#rod&ce the sha#e and #ro#agation of a nerve signal. However it

cant e#lain wh$ the signal comes with a vol&me or densit$ #&lse traveling down the

aon &nder signal transmission. *lso when c&rrent r&n thro&gh a resistor energ$ is lost

to the release of heat in the transistor. -herefore the Hodgin,H&le$ model #redicts

that energ$ is lost and that heat will be released &nder the transmission of a nerve

signal. -his is not the case. (nder signal transmission there is a release followed b$ are,tae of heat. -he total heat echange &nder a signal event is +ero. -he activation

and the #ro#agation of a nerve signal is an adiabatic #rocess. Since the HH model

cant e#lain these #ro#erties of the nerve signal/ the Hodgin,H&le$ model cannot

be regarded as a com#lete theor$.

-he HH model doesnt #redict an$thing abo&t thermod$namical #ro#erties s&ch as

#ress&re and tem#erat&re. -his maes it harder to falsif$. *n ideal scientific theor$

sho&ld be ver$ eas$ to falsif$.

2.2 The soliton model

-he carbon chains in a li#id can have different conformations/ beca&se the carbon

atoms maing the chains are free to rotate aro&nd the bond between them. *t low

tem#erat&re the li#ids chains eist in a so called trans conformation and at higher

tem#erat&res the li#id chain begin to eist in a ga&che conformation. 'n gel state the

li#id chains in the membrane mainl$ eist in trans conformation and in fl&id state the

li#id chains mainl$ eist in a ga&che conformation. -he #hase transition of a cell

membrane is the #oint where the amo&nt of chains in trans conformation is e@&al to

the amo&nt of chains in ga&che conformation.

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'n the soliton model the nerve #&lse is regarded as a mechanical wave >soliton?

traveling down the aon. -his wave is coled to the li#id transition in membranes.

-he soliton #&lse is a #ro#agating densit$ #&lse in the fl&id membrane. *t the #osition

of the #&lse the membrane is locall$ in gell state.

-he behavior of thermod$namical #ro#erties aro&nd #hase transition means that the

com#ressibilit$ of the membrane is nonlinear. -his means that the com#ressibilit$ of

the membrane changes when it is com#ressed. Nonlinearit$ is necessar$ for the

#ro#agation of a soliton. -his s&ggest that the f&rther awa$ the membrane is from

#hase transition state the harder it is to create a soliton. 'n the bod$ the #hase

transition tem#erat&re is slightl$ below bod$ tem#erat&re so the cell membrane is in

the fl&id state in the bod$. B&t the membrane state is close eno&gh to #hase transition

state to allow for #ro#agation of a soliton. B$ changing the s&rro&ndings for eam#le

b$ increase the #ress&re/ the membrane state can be altered to state f&rther awa$ from

#hase transition state and thereb$ creation of a soliton will be more &nliel$ andre@&ire a greater stim&li.

-he soliton model is based on the wave e@&ation for area densit$ change)

2

t2 A=

z(c2

z A)

where t is time/+ is the #osition along the nerve aon and c is the so&nd velocit$.

Close to melting transition so&nd is a f&nction of densit$ so c is e#anded aro&nd its

val&e in fl&id #hase.

c2=c0

2+ A+q ( A)+ ...

-he s#eed of so&nd is fre@&enc$ de#endent. -herefore a term is added to the e@&ation)

h z4

A, h=constant

Combining these e@&ations gives the final #artial differential e@&ation)

2 t2

A= z

[( c02+ A+q( A)2+ ...)

z A]h

z4 A

-his e@&ation can be sim#lified mathematicall$ and &sed to sim&late and #lot the

variable u=A

0A as a f&nction of #osition + and time t.

-he soliton model is based on thermod$namics while the HH model is #&rel$ electric.

-he soliton model is consistent with the thicening of the nerve aon and the release

and re,tae of heat &nder a signal event. -he soliton model can e#lain the voltage

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com#onent b$ the electrostatic feat&res of the membrane.

-he soliton model #redict that when two waves collide the$ will go thro&gh each other

and have the same sha#e and am#lit&de as before collision. Fnl$ if the waves travelwith a velocit$ ver$ close to the minim&m #ossible velocit$ the waves will change to

a significant etend &nder collision.

Eith a few ece#tions the HH model generall$ #redicts that the waves will bloc each

other and that no waves will eist after collision. -he ece#tions are soliton,lie

regimes where which are certain velocit$ intervals where the end res&lt end being

the same as in the soliton model.

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3 'ethods and 'aterials

3.1 'aterials

10D of ethanol in ta# water?

* little #iece of metal.

3.1.1 ,arth-orms

'n an ideal e#eriment one sho&ld dissect a living animal for eam#le a worm/ tae

o&t the ventral cord and then mae recordings on the #&re nerve. B&t doing that is

ver$ diffic&lt and taes a lot of time. -herefore ' mainl$ made recording of nerve

signals etracell&lar on an anestheti+ed earthworm. -hat means that the nerve signal

from the nerve inside the worm was meas&red thro&gh the sin of the worm. -hat

&nfort&natel$ means that the signal was ver$ wea com#ared to meas&rements on a

#&re nerve. Eith a #&re nerve from an earthworm the signal $o& can meas&re is abo&t

a factor 100 stronger than the meas&rements ' made etracell&lar on an earthworm.

-he im#ortant thing abo&t an earthworm in this contet is that an earthworm contains

15

Figure 3.1 Earth#orm $entral ord [1']


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a medial giant fiber and two lateral giant fibers >see fig&re .2?. -hese fibers consist of

an arra$ of overla##ing giant ne&rons and the$ behave lie a long aon that r&ns the

entire length of the worm and the$ can be stim&lated from both ends. *n earthworm

also contains a b&nch of ver$ small ne&rons b&t the$ dont r&n the length of the wormand the$ are so small that the signal from these ne&rons can be disregarded when

meas&ring the signal from the giant fibers. Normall$ a signal onl$ r&ns in one

direction thro&gh a nerve in a biological s$stem b&t that is beca&se of the wa$ the

nerve is connected to other cells. * stim&li ever$where on the fibers in an earthworm

can start a signal.

'n o&r e#eriments we &sed two t$#es of earthworms. -heLumbricus terrestrisand

theEisenia foetida. -hese worms are anatomicall$ e@&al at the level of the entral

Cord. Lumbricus terrestris is generall$ bigger thanEisenia foetida>see fig&res . and

.4?

16

Figure 3.2 Earth#orm arra* of neurons [1']


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3.1.2 Nerve Chamber

'n m$ e#eriment we &sed two different nerve chambers. * big one which was

commercial and a small one which was home made >see fig&res .5 and .7?.

17

Figure 3.4 Eisenia fetida differentscale5 [1(]

Figure 3.! 6er%e hamer and cales [1/]

Figure 3.3 7umricus terrestris [1&]


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-he M3-F176 nerve chamber &sed in this e#eriments consists of 18 stainless steel

wire electrodes . Some electrodes are s#aced 0.5 cm a#art and others 1 cm a#art. 'n

this e#eriment cables where connected to the electrodes on the nerve chamber and to

the M3*2540 5,3ead Shielded Bio *m# Cable shown below. -he M3*2540 cable

was connected to the Bio *m#lification Channel in the M3957 !ower3ab 27-

described below. -he Bio *m#lification Channels where &sed beca&se the signal had

to be am#lified to the order of microvolt to be detected. 'n this wa$ a voltage

difference between two electrodes of o&r choice co&ld be created from the cables

connected to o&t#&t channels and meas&red thro&gh the cables connected to in#&t

channels.

18

Figure 3.' Small home made ner%echamer

Figure 3.& )he 782!49 !7eadShielded -io 8mp ale [29]


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3.1.3 'L&"# o-erLab 2#T

M3957 !ower3ab 27- is a a data ac@&isition s$stem &sed to #erform meas&rementson biological s$stems. -he s$stem incl&des hardware with in#&t and o&t#&t channels/

high and low #ass filters/ a bio am#lifier among other things. 't also incl&des the

software #rogram labchart and it comes with stim&lator and recording cables. -he

hardware &nit is &sed to stim&late the nerve/am#lif$ and record the signal while at the

same time &sing filters and bio am#lification to remove &nwanted fre@&encies to

im#rove signal to noise ratio. -he hardware is connected to a com#&ter which have the

software #rogram labchart installed. -he integration of hardware and software allows

for hardware f&nctions to be controlled from the software #rogram labchart on the

com#&ter.

3.1.4 Labchart

rom labchart ' co&ld initiate the stim&li and the recording of the nerve signal b$

#ressing a b&tton on the screen. rom labchart ' co&ld control #ro#erties lie the

am#lit&de of the stim&lation/ t$#e of signal filtering/ recording time and n&mber of

stim&lations. 'n 3abchart the data is dis#la$ed and saved as gra#hs in a diagram with

time on the ,ais and voltage difference on the $,ais. B$ com#aring these gra#hswith meas&rements of distance between the stim&lating #oints and the recording

#oints on the worm/ #ro#erties lie s#eed and am#lit&de of the nerve signals can be

calc&lated. -he strength of the stim&li is #resent on each set of gra#hs6data so the

threshold val&e of the nerve co&ld be read from the gra#h which had the lowest

stim&lation val&e of the gra#hs showing a nerve signal. 3abchart can tae the average

of an arbitrar$ n&mber of meas&rements with the same stim&li. -his was a ver$

im#ortant tool for im#roving signal to noise ratio.

19

Figure 3.( 7(!' Po#erla 2') [21]


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3.2 'ethods

'n order to meas&re the nerve,signal the worm is #laced on a nerve chamber. * nerve

chamber is a row of electrodes that in this e#eriment was s#aced at intervals of 0.5cm

and 1cm. -he worm is #laced along the row of electrodes. 't m&st be #laced so that the

ventral side to&ches the electrodes beca&se otherwise the signal cant be seen. -his is

beca&se the nerve that creates the signal lies ver$ close to the ventral side of the worm.

-he worm m&st also be stretched and #er#endic&lar to the electrodes/ so that the

distance between two electrodes is the same as the length of the section of the worm

that is between the two electrodes.


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the worm is e#osed to a constant stim&li for a given amo&nt of time >see fig&re .10?.

'f the worm moves while recordings tae #lace it creates a h&ge amo&nt of noise.

-herefore/ in order to get &sef&l data/ the worm m&st almost lie com#letel$ still when

recording the nerve,signals. -herefore the worm m&st be anestheti+ed before it is

#laced in the Nerve Chamber. -hat is done b$ #re#aring a 1)10 alcohol6water sol&tion

and #lace the worm in this sol&tion for abo&t 5 min&tes. 'f this is not eno&gh to

anestheti+e the worm the sol&tion can be made 1)5. 'f the worm is left in the sol&tion

for abo&t 10 min&tes or more it is #ossible that the worm will die. *s a general r&le/

when the worm sto# moving/ the worm is read$ for e#erimenting and can be

removed from the anesthetic sol&tion and cleared with ta# water.

irst the am#lit&de and the width of the stim&lating #&lse is fied. o& then begin

with a wea stim&li >low voltage? and slowl$ increase the amo&nt of voltage while

ever$ time recording the voltage difference meas&red from the recording cables. *t

some #oint the threshold voltage is reached and a nerve signal will a##ear in the data

ac@&ired. Now the threshold val&e can be ded&ced from the data. *ccording to theor$

a nerve signal also called s#ie sho&ld have a wave lie sha#e. 'f the signal does not

have this sha#e it is #robabl$ not a nerve,signal.

o& now contin&e to increase the voltage and chec if $o& almost get the eact same

signal ever$ time. *s mentioned earlier the signal transmitted thro&gh a nerve is

inde#endent of the strength of the stim&lation. 'f the signal is noise coming from anelectrical device the signal often change with time and changes when the stim&li

changes. -herefore if the same signal is recorded for a long time over a large range of

stim&lation and if the signal has the right sha#e it is ver$ liel$ that the signal comes

from a transmitting nerve.

'n this wa$ data is now recorded with man$ different worms and for each worm nerve

signals are meas&red with res#ect to two different config&rations. Fne set where the

stim&lation comes from the tail and another where stim&lation comes from the head.

rom this data the #ro#erties of nerve,signals from the tail and nerve,signals from

head are determined.

21

Figure 3.19 -asic principle of the experimental setup [1']


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-hings lie tem#erat&re/ the dr$ness of the worm and how m&ch the worm is stretched

infl&ences the signal a lot. -herefore/ as m&ch as it is #ossible/ the worm m&st be in

the same state at all time when recording is being done. or eam#le if the wormstarts moving after data recording for one set ' finished $o& have anestheti+e it

again. *fter that the #ro#erties of the worm has changed which means that $o& have to

start all over again with the s#ecific worm.

Now for a cole of worms the same is done b&t where a third config&ration is added

where the worm is stim&lated from both the tail and the head.

-he distance between the #oint or #oints of stim&lation and the #oint or #oints of

recording on the worm is meas&red and noted. -his information together with the data

re@&ired can be &sed to describe certain #ro#erties of nerve,signals lie s#eed and

am#lit&de of the signal. A22.

B$ com#aring data from sets where the worm is stim&lated from either right or left

or both right and left the e#eriment can give information abo&t what ha##ens when

two nerve,signals collide in a nerve.

Noise minimi+ation)

'n most rooms a sensitive instr&ment meas&ring electrical signals can easil$ meas&re

electrical signals or noise coming from com#&ters/ lab instr&ments/ #hones and other

electrical devises.


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Fne ver$ im#ortant method &sed to minimi+e noise/ was to #&t the nerve chamber in a

arada$ cage. * arada$ cage is a bo or a cage that shields ever$thing inside the

cage from all eternal electrical signals. 'n this #artic&lar case the arada$ Cage was ametallic cooie bo. * screwdriver was &sed to mae a little hole/ so a gro&nded cable

co&ld be connected to the cooie bo. 'n this wa$ ever$ electric signal that hit the

cooie bo wo&ld r&n to the gro&nding cable and from the gro&nding cable to the

gro&nd and awa$ from the nerve chamber. * bigger hole &nfort&natel$ also had to be

made beca&se there had to be a hole for the cables connecting the nerve chamber and

the recording hardware. -herefore the the set was not totall$ shielded from eternal

electric signals.

*nother thing that had to be done was that all cables which was a #art of thee#erimental set was covered with al&min&m foil and all this al&min&m foil

covering the cables was also connected to a gro&nded cable.

4 /esults and discussion

4.1 ,+periment -ith a livin% earth-orm

' managed to collect a high amo&nt of data showing that a nerve signal can be initiated

from both ends of the giant fibers r&nning along the entire length of an earthworm. '

started with a low stim&li and increased it slowl$ &ntil a s#ie s&ddenl$ started to

a##ear on the screen. -he lowest stim&li able to create a s#ie then had to be the

threshold val&e. -o reass&re m$self f&rther that the s#ie seen was in fact a nerve

signal/ and not "&st something that accidentall$ looed lie a nerve signal/ ' checed

that the s#ie wasnt there when the worm was taen o&t of the set and that the

s#ie was alwa$s there when the worm was there and the stim&li was above the

threshold val&e. 'f this was the case ' new the s#ie had to come from the worm.

*lso ' new from theor$ that the worm had to have a threshold val&e beca&se of the

fibers in the worm. Ehen we contin&ed to increase the stim&li we didnt get an$ new

s#ies. -he fact that no other s#ie was seen when stim&li was increased strongl$indicated that the s#ie had to come from the nerve inside the worm.

*nother good indication that the s#ie was a nerve signal/ was that the width and the

am#lit&de of the s#ie didnt change when stim&li was changed. -his is beca&se a

nerve signal #ro#agating down a nerve is inde#endent of the strength of the stim&li

initiating the signal. -his is not the case with s#ies stemming from lab e@&i#ment.

s#ies stemming from activit$ of the lab e@&i#ment changes significantl$ when

stim&li is changed.

ig&re 4.1 show one set of data where the stim&li on the worm was 1.2 volt and no

23


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signal a##eared. ig&re 4.2 is data from the same worm. Ehen the stim&li was

increased to 1.25 volt a nerve signal a##eared in the data. -his means that for the this

#artic&lar worm the threshold voltage was between 1.2 v and 1.25v. -he big s#ie

after abo&t .5 ms comes from the lab e@&i#ment itself and is called the stim&l&sartifact.

-he res&lts show that a nerve signal can be created ever$where on the worm. -he

threshold val&e of the worms varied from worm to worm b&t alwa$s lied in the

interval from 0.5 v to .2 v. -he variation of threshold val&es are d&e to differences in

the thicness of the worms and their nerves. Iifferences in tem#erat&re between

e#eriments and the dr$ness and anestheti+ation of the worm also affect the threshold

val&e.

Fn one worm of the t$#eEisenia foetida the nerve signal s#eed was 5.7 m6s when

stim&li came from one side and 7.1 m6s when stim&li came from the o##osite side. Fnone worm of the t$#e 3&mbric&s terrestris ' got a nerve signal s#eed of 2.19 m6s. -he

signal s#eed and am#lit&de var$ according to the si+e of the worm/ how dr$ the worm

is/ the sha#e of the worm and how anestheti+ed it is.

24

Figure 4.1 "ata from a recording #here stimuli is elo# the threshold %alue andtherefore no ner%e signal appear in the data. )he ig spie after 3.! ms is created

* the la e;uipment itself and is called the stimulus artifact.


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' managed to collect one set of data where the same worm was first stim&lated from

right/ then from left and then finall$ from both sides and where the recordings of thenerve signal wasnt s#oiled b$ noise or movements of the worm etc. Eith this worm

recordings were onl$ done on one #oint on the worm.

-his dataset shows that the worm had a threshold val&e on 1v when the worm was

stim&lated from the left side onl$. -his signal a##eared after 9ms of meas&ring.

Ehen the worm was stim&lated from the right side onl$ there was a nerve signal with

a threshold val&e of 1.7v. -he signal a##eared after 11.5 ms of meas&ring.

Ehen the worm was e#osed to stim&li from both sides one nerve signal a##eared

after the stim&li was increased from 1 to 1.05 volt. -his signal a##eared after 9/45 msof meas&ring. Ehen the stim&li increased from 1.20v to 1.25v another signal a##eared

after abo&t 12 seconds of meas&ring.

25

Figure 4.2 0ere stimuli is ao%e the #orms threshold %alue and therefore a ner%esignal is recorded


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*n earthworm has one median giant fiber that can be regarded as an aon and twolateral giant fibers that together can be regarded as an aon. Sometimes a nerve signal

from the median giant fiber is followed b$ a nerve signal from the lateral giant fibers.

Ehen two nerve signal are seen the one can be from the median giant fiber and the

other can be from the lateral giant fibers. B&t in that case the sha#e of the two s#ies

are the same. *n eam#le of that can be seen on fig&re 4.2. -he s#ie from the median

giant fiber first goes and then down. -he same is the case for the s#ie from the

lateral giant fibers. 'n fig&re 4. the s#ies have different sha#es. -his indicates that

the s#ies has traveled in o##osite directions and th&s comes from o##osite sides of

the worm.

-he fact that two different nerve signals can be meas&red at the same #oint at twodifferent times is an indicator that the signals doesnt annihilate each other when the$

collide. 'f the$ did/ the signal reaching the #oint of meas&ring on the worm as the first

one of the two wo&ld annihilate the other signal before that signal co&ld reach the

meas&ring #oint. 'f the$ reached the #oint at the same time and annihilated each other

at the #oint of meas&rement one wo&ldnt get two f&ll signals either.

-herefore m$ res&lts strongl$ indicate that the nerve signals has gone thro&gh each

other and that nothing significant has ha##ened to the nerve signals &nder the

collision.


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val&e also a##eared latest. -his is was what one wo&ld e#ect if the signal go thro&gh

each other. -his is beca&se when the worm was stim&lated from right side onl$ both

the threshold val&e and the time &ntil the signal a##eared was greater than when

stim&li was from left side onl$.

-he threshold val&es and the a##earance time of the signals recorded when stim&li

came from both sides are different than when stim&li came from onl$ one side. -his

was e#ected beca&se it was inevitable that the #ro#erties of the worm changed d&ring

the time recordings too #lace. -hese were #ro#erties lie dr$ness and how m&ch the

worm was anestheti+ed. -hese changes meant that the #ro#erties of the nerve signals

changed d&ring the time the recordings too #lace.

M$ res&lts thereb$ s#orts the #redictions of the soliton model b&t the$ dont fit with

the #redictions of the Hodgin,H&le$ model.

ig&re 4. shows three gra#hs #lotted in the same #ict&re. Fne gra#h shows data from

when the stim&li were below the threshold val&e for both the tail and the head. -his

gra#h shows no sign of a nerve signal. -he other gra#h shows data from when stim&li

was below the threshold voltage of the right side b&t above the threshold voltage of

the left side. Fne nerve signal a##ear in this gra#h. -he last gra#h shows data from

where the stim&li was above both threshold voltages and two nerve signals a##ear in

this gra#h. -he two nerve signals starts and ends with a decrease or a valle$ in the last

gra#h. -herefore the$ m&st have different sha#es.

4.2 ,+periment -ith an e+tracted entral Cord

' also made meas&rement on a #&re nerve where recordings were done on

config&rations where stim&li came from right side/ left side and both left and right

side res#ectivel$. -he res&lts from these meas&rement shows that a nerve signal can

be initiated from both sides. Both the signal seen when stim&li comes from right side

and the signal seen when stim&li comes from left side a##ear in the data when the

stim&li comes from both sides. -here fore the data from the #&re nerve also indicate

that the signals go thro&gh each other witho&t annihilation when the stim&lation

comes from both sides.

ig&re 4.4 shows that a nerve signal a##ear when the nerve is stim&lated from oneside. ig&re 4.5 shows that another nerve signal a##ears when the nerve is stim&lated

from the other side. -his signal is a lot weaer and a bit f&rther awa$ from the

stim&l&s artifact. 'n ig&re 4.7 the stim&li came from both sides and both the signal

from fig&re 4.4 and fig&re 4.5 a##ear in the data.

27


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28

Figure 4.4 Stimulation from one side initiates a signal that appear in the data.

Figure 4.! Stimulation from the other side initiates a small signal that appear inthe data.


31/34

29

Figure 4.'


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" Conclusion

-he #&r#ose of this #ro"ect was to investigate what ha##ens when two nerve signals in

an earthworm #ro#agates in o##osite directions then reach the same #oint in the nerveand collide with each other. -here was two realistic scenarios. -he first scenario was

the #rediction of the soliton model and the other scenario was the #rediction of the

Hodgin,H&le$ model. -he soliton model #redict that the signals #ass thro&gh each

other &naltered and the Hodgin,H&le$ model #redict that the signal annihilate each

other when the$ collide. -he res&lts of this e#eriment strongl$ s#orts the idea that

the two nerve signals go thro&gh each other &naltered b$ the collision ece#t for

ma$be for a decrease in am#lit&de. -hereb$ the res&lts of this e#eriment is in line

with the soliton model.

30


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Bibliogra#h$

A1 Fle %. Mo&ritsen/ Fle Mo&ritsen/>2004? 3ife as a matter of fat/ 2005 edition.

A2 htt#)66bioweb.w&.ed&6co&rses6biol1156w$att6biochem6li#id63i#id2.as# date)

096061

A htt#)66en.wiiboos.org6wii6Str&ct&ralBiochemistr$6Membrane!roteinsdate)

126061

A4 htt#)66en.wii#edia.org6wii6ile)Cellmembranedetaileddiagramedit2.svg

date)096061

A5 2000? !rinci#les of Ne&ral Science/

4 2002? 'nvertebrate Nervo&s S$stem/ 2001? 'on Channels of


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