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The Lorentz force on ions in membrane channels of neurons as a mechanism for transcranial static magnetic stimulation

Abstract

Transcranial static magnetic stimulation is a novel noninvasive method of reduction of the cortical excitability in certain neurological diseases that makes use of static magnetic fields generated by permanent magnets. By contrast, ordinary transcranial magnetic stimulation makes use of pulsed magnetic fields generated by strong currents. Whereas the physical principle underlying ordinary transcranial magnetic stimulation is well known, that is, the Faraday´s law, the physical mechanism that explains the interaction between neurons and static magnetic fields in transcranial static magnetic stimulation remains unclear. In the present work, it is discussed the possibility that this mechanism might be the Lorentz force exerted on the ions flowing along the membrane channels of neurons. The overall effect of the static magnetic field would be to introduce an additional friction between the ions and the walls of the membrane channels, thus reducing its conductance. Calculations performed by using a Hodgkin–Huxley model demonstrate that even a slight reduction of the conductance of the membrane channels can lead to the suppression of the action potential, thus inhibiting neuronal activity.

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The Lorentz force on ions in membrane channels of neurons as a mechanism for transcranial static magnetic stimulation

Author: Freire Rosales, Manuel José; Bernal Méndez, Joaquín; Pérez Izquierdo, Alberto Tomás
Publisher: Taylor & Francis
Year: 2020
DOI: 10.1080/15368378.2020.1793172
Source: https://idus.us.es/bitstreams/85f4eb68-61c3-432d-aa09-ad8c3e4df94c/download
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
Abs ac — Goal: T ansc anial s a ic magne ic s imula ion is
a no el nonin asi e me hod o educ ion o he co ical
exci abili y in ce ain neu ological diseases ha , unlike
o dina y ansc anial magne ic s imula ion, makes use o
s a ic magne ic ields gene a ed by pe manen magne s. The
physical p inciple unde lying ansc anial magne ic
s imula ion is well known, ha is, he Fa aday´s law. By
con as , he physical mechanism ha explains he in e ac ion
be ween neu ons and s a ic magne ic ields in ansc anial
s a ic magne ic s imula ion emains unclea , which makes i
di icul o imp o e and ine une he ea men . In he p esen
wo k i is discussed he possibili y ha his mechanism migh
be he Lo en z o ce exe ed on he ions lowing along he
memb ane channels o neu ons. Me hods: To suppo his
hypo hesis, a dimensional analysis i is ca ied ou o compa e
he La mo adius o he ions in he p esence o a s a ic
magne ic ield wi h he dimensions o he c oss sec ion o
human axons and memb ane channels in neu ons. Resul s:
This analysis shows ha whe eas a mode a e s a ic magne ic
ield is no expec ed o a ec he ion lux h ough axons,
ne e heless i can a ec he ion lux along memb ane
channels. Conclusion: The o e all e ec o he s a ic magne ic
ield would be o in oduce an addi ional ic ion be ween he
ions and he walls o he memb ane channels, hus educing i s
conduc ance. Signi icance: Calcula ions pe o med by using a
Hodgkin-Huxley model demons a e ha e en a sligh
educ ion o he conduc ance o he memb ane channels can
lead o he supp ession o he ac ion po en ial, hus inhibi ing
neu onal ac i i y.
Index Te ms— T ansc anial s a ic magne ic s imula ion,
S a ic magne ic ield, Lo en z o ce, b ain s imula ion
I. INTRODUCTION
RANSCRANIAL magne ic s imula ion (TMS) is a
well-es ablished nonin asi e me hod o b ain
s imula ion o diagnosis and ea men o neu ological
diseases ha is based on he applica ion o s ong and sho
pulses o magne ic ield ( ypically 1T o ampli ude and
300𝜇𝑠 o du a ion) gene a ed by cu en - ed coils [1]. The
Submi ed o e iew o IEEE T ansac ions on Biomedical Enginee ing
on Janua y 23 h, 2020.
Manuel J. F ei e is wi h he Depa men o Elec onics and
Elec omagne ísm, Uni e si y o Se ille, Spain (e-mail: [email p o ec ed]).
Joaquín Be nal-Méndez is wi h he Depa men o Applied Physics,
Uni e si y o Se ille, Spain.
physics unde lying TMS is well known and i is based on
he induc ion o cu en s in neu ons by i ue o he
Fa aday´s law. P o ocols o TMS he apy a e well
es ablished, being he he a-bu s p o ocol he mos
ex ended o induce long-las ing neu al changes [2].
T ansc anial s a ic magne ic s imula ion ( SMS) is a no el
nonin asi e o m o b ain s imula ion, ha makes use o
s a ic magne ic ields (SMFs) c ea ed by pe manen
magne s o educe co ical exci abili y in humans
[3][4][5][6]. Expe imen al e idences show ha SMFs o
mode a e alues ( ens o hund eds o mT) can in e e e wi h
physiological b ain unc ions [3][4][5][6]. The e is also
expe imen al e idence o e ec p oduced by e en g ea e
SMFs in Magne ic Field Resonance (MRI) exams [7].
Mo eo e , he in e ac ion o mode a e SMFs wi h exci able
memb anes o di e en biological sys ems has been
ex ensi ely epo ed [8][9][10][11][12]. Despi e hese
e idences, a physical mechanism p o iding a clea
explana ion o he in e ac ion o mode a e SMFs wi h
neu ons has no been iden i ied ye . A be e unde s anding
o he physic phenomena unde lying his in e ac ion would
help o inc ease he e iciency o he SMS. A a
undamen al le el, wo kinds o physical mechanisms seem
o be easible candida es o p o ide his explana ion: he
magne ic beha io o he cons i uen molecules o exci able
memb anes in he p esence o a SMF, and he in e ac ion
be ween a SMF and mo ing ions in neu ons h ough he
Lo en z o ce. Wi hin he i s pe spec i e, i has been
sugges ed ha he eo ien a ion o diamagne ic aniso opic
molecules in he cell memb ane can be esponsible o he
in luence o mode a e SMF on he cell memb ane [8][9].
The second hypo hesis has been used o in es iga e, om a
heo e ical poin o iew, he in luence o SMFs on he ion
cu en ha lows along he axon and is associa ed wi h he
p opaga ion o he ac ion po en ial (AP) in ne es [13] [14].
F om he analysis ca ied ou in [13] [14], i ollows ha he
Lo en z o ce exe ed by mode a e SMFs on he ions
lowing along ne es canno app eciably a ec he
p opaga ion o he AP. Ne e heless, he AP is associa ed
no only wi h he ion lux along axons bu also wi h he ion
lux along memb ane channels. Rega ding his, i is
in e es ing o no e ha i has been sugges ed ha ion
channels o neu ons can be modelled as FET ansis o s
[15]. Also, i is well known ha SMFs can a ec he
The Lo en z Fo ce on Ions in Memb ane
Channels o Neu ons as a Mechanism o
T ansc anial S a ic Magne ic S imula ion
Manuel J. F ei e, Senio Membe , IEEE, and Joaquín Be nal-Méndez, Senio Membe , IEEE
T
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pe o mance o FET ansis o s in MRI p eampli ie s due o
he Lo en z o ce in cha ge ca ie s [16]. Thus, in he
p esen wo k, i is discussed he possibili y ha he AP can
be a ec ed by mode a e SMF h ough he Lo en z o ce
exe ed on he ions lowing along he memb ane channels
in neu ons. To suppo his hypo hesis, a dimensional
analysis is ca ied ou o es ima e he a io be ween he
La mo adius o he ions in he p esence o a SMF wi h a
alue ypical o he SMS [3], and he dimensions o he
c oss sec ion o human axons and memb ane channels.
Based on his analysis, i is sugges ed ha , al hough
mode a e SMFs canno a ec he ion lux h ough axons, i
may a ec he ion lux along memb ane channels. I is also
sugges ed ha he e ec o he Lo en z o ce is o in oduce
an addi ional ic ion be ween he ions and he walls o he
memb ane channels. Since he con en ional ic ion
be ween he ions and he walls accoun s o almos 2/3 o
he conduc ance alue o he channels [17], we conclude
ha he ul ima e e ec o he Lo en z o ce is o educe
signi ican ly he conduc ance o channels. Resul s o he
AP ob ained wi h a Hodgkin-Huxley (HH) model [18]
e eal ha a sligh educ ion o he conduc ance o he Na
channel can lead o he supp ession o he AP.
Sec ion II p esen s an analysis ha ules ou he e ec o
Lo en z o ce associa ed wi h mode a e SMFs on ions
lowing along axons as a cause o neu on inhibi ion. Also,
he a io be ween La mo adius and he diame e o he
egion o conduc ion is p esen ed as a sui able benchma k
o de e mine whe he Lo en z o ce can al e he low o
ions. This c i e ion is employed in sec ion III o show ha
memb ane channels migh see i s conduc ance dec eased by
a Lo en z o ce such as ha c ea ed by a mode a e SMF,
and ha he expec ed dec ease can ac ually supp ess he
AP. Finally, conclusions a e p esen ed in sec ion IV.
II. ANALYSIS
As i is well known, he Lo en z o ce is he o ce exe ed
on a cha ged pa icle mo ing in he p esence o a SMF.
Because his o ce is pe pendicula o bo h he eloci y o
he pa icle and he di ec ion o he SMF, i makes he
pa icle o desc ibe a ci cula ajec o y in a plane
pe pendicula o he SMF. The adius o his ajec o y is
e e ed o as he cyclo on adius o La mo adius, 𝑅𝐿, and
i is gi en by 𝑅𝐿=𝑚𝑣/𝑞𝐵, whe e 𝑚, 𝑣 and 𝑞 a e he mass,
eloci y and cha ge o he pa icle, espec i ely, and 𝐵 is
he ampli ude o he SMF.
The AP p opaga ing h ough he axon o neu ons is
associa ed wi h a longi udinal ion cu en lowing along he
axon. In he p esence o a SMF, due o he Lo en z o ce he
ions lowing along he axon expe ience a de lec ion o hei
ajec o y which p oduces a ans e se cu en . In [13] i is
heo e ically analyzed o he i s ime he o de o
magni ude o he SMF necessa y o p oduce an app eciable
de lec ion in he longi udinal cu en associa ed wi h he
p opaga ion o he AP in he axons o human neu ons. The
calcula ions in [13] show ha a magne ic ield on he o de
o 25T is necessa y o p oduce a de lec ion o educ ion o
10% in he ion cu en along he axon. Such a ield is
se e al o de s o magni ude g ea e han mode a e SMF and
e en an o de o magni ude g ea e han ypical SMF in
MRI sys ems. Mo eo e , in [14] a deepe analysis es ima es
he e ec o his de lec ion in he AP by means o a HH
model whe e a e m ha accoun s o he ans e se cu en
ha appea s as a consequence o he de lec ion is added in
he di e en ial equa ions, his e m being p opo ional o
he alue o he SMF. In [14] i is de ined a a io 𝛼
be ween he ans e se cu en and he longi udinal cu en ,
and i is exp essed as a ela ion be ween he alue o he
SMF, 𝐵, and he ans e se mobili y o he ions, 𝜇, as 𝐵 =
𝛼/𝜇. The calcula ions in [14] show ha , in pa icula , a
mode a e alue o he SMF o 𝐵=11 mT will p oduce a
educ ion o 5% (co esponding o 𝛼=0.05 in [14]) in he
longi udinal cu en in he axon. In [14] i is shown ha
aking his in o accoun in he HH model, his will cause a
supp ession o he AP. This esul en i ely disag ees wi h
he conclusion in [13]. This appa en pa adox can be sol ed
by no ing ha he analysis ca ied ou in [14] assumes an
ion mobili y o 5 m2/Vs, which is h ee o de s o magni ude
la ge han alues expe imen ally epo ed [13]. Fo
example, in [13] he peak axial elec ic ield du ing he
passage o he AP is epo ed o be 𝐸=8 V/m and he ion
eloci y 𝑣𝑑=3.3 × 10−2 m/s. The e o e he ion mobili y is
𝜇 = 𝑣𝑑/𝐸 = 0.004125 m2/Vs. Assuming his much mo e
ealis ic alue o 𝜇, he equi ed SMF o a educ ion o
5% in he longi udinal cu en in [14] will be 14.7 T, which
is close o he o de o magni ude es ima ed in [13] (i.e.,
25 T).
F om he abo e discussion i can be concluded ha
mode a e SMFs canno a ec he p opaga ion o he AP in
human axons. This same conclusion can be also d awn om
a simple al e na i e analysis based on he compa ison o he
La mo adius wi h he diame e o he axons. Conside , o
example, a sodium (Na) ion, whose mass and cha ge a e:
𝑚 = 3.8 × 10−26 kg and 𝑞 = 1.67 ×10−19 C. To es ima e
he La mo adius we can assume an ion eloci y in he
axon o 𝑣𝑑= 3.3 × 10−2 m/s, (i.e., he same alue as in
[13]) and a SMF o alue 𝐵 = 164 mT. This is he alue
measu ed by he au ho s o he same magne used in SMS
in [3], a a dis ance o 2 cm om he su ace o he magne ,
which is he dis ance be ween he scalp and he mo o
co ex. Wi h hose assump ions, he La mo adius is 𝑅𝐿=
𝑚𝑣/𝑞𝐵 =478Å. This is wo o de s o magni ude smalle
han he ypical diame e o he human axon which is 1𝜇𝑚.
The e o e, in he p esence o a mode a e SMF o 164 mT
he ionic cu en is expec ed o low wi hou signi ican
de lec ion h ough he axon. Summing up, i can be
concluded ha due o he di e en o de s o magni ude o
he c oss sec ion o he axon and he La mo adius o
mode a e SMFs, mode a e SMFs canno a ec he anspo
o ions h ough he axon, in acco dance wi h [13].
The discussion p esen ed abo e sugges s ha he
compa ison be ween he size o he c oss sec ion o he
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axon and he La mo adius can be conside ed as a
benchma k o asce ain whe he he Lo en z o ce
associa ed wi h a gi en alue o SMF a ec s he ion ne e
conduc ion. In ac , we ha e jus shown ha his c i e ion
allows o ule ou Lo en z o ce due o a mode a e SMF as
he cause o he AP sup ession in axons. In iew o his, in
his wo k we p opose an al e na i e explana ion o he
e ec on he AP o a mode a e SMF based on he e ec o
Lo en z o ce on he conduc ance o memb ane channels.
To unde pin his hypo hesis, we will use he benchma k
index desc ibed abo e o de e mine whe he a mode a e
SMF can a ec he ion lux along memb ane channels. In
his ega d, a key poin o be aken in o accoun is ha he
size o he c oss sec ion o ion channels o exci able
memb anes is se e al o de s o magni ude smalle han he
diame e o he axon.
III. RESULTS AND DISCUSSION
In his sec ion a dimensional analysis is ca ied ou o
compa e he size o he po assium (K+) channel wi h he
La mo adius o K+ ions o mode a e SMFs. To his end,
an es ima ion o he d i eloci y o he ions h ough he
channel is equi ed as a i s s ep. Rega ding his poin , i is
impo an i s o de e mine whe he he low o he ions
h ough he channel can be conside ed an ohmic p ocess (o
ions should be conside ed ballis ic cha ges ins ead).
Scien i ic e idence poin s ou ha ic ion caused by he
po e shape and wall o uosi y play an impo an ole in he
conduc ance [19] [20]. The e o e, i is easonable o
conside he low o ions h ough he channel as an ohmic
p ocess. Unde his assump ion, he ampli ude o he cu en
can be w i en as 𝐼 = 𝐽 ⋅ 𝑆, whe e 𝑆 is he a e age c oss
sec ion o he channel, and he cu en densi y 𝐽 can be
w i en as 𝐽 = 𝑞𝑛𝑣𝑑, whe e 𝑛 is he numbe o ions pe uni
olume and 𝑣𝑑 he d i eloci y o ions. Mo eo e , 𝑛 can
be w i en as 𝑛 = 𝑁/𝑉, whe e 𝑁 is he numbe o ions ha
can occupy simul aneously he channel and 𝑉 is he olume
o he channel, ha can in u n be exp essed as 𝑉 = 𝑆𝐿,
whe e L is he leng h o he channel. The e o e, he d i
eloci y can be exp essed as:
𝑣𝑑=𝐼𝐿
𝑁𝑞.
(1)
The K+ channel ex ends 45Å, wi h a wide segmen o
leng h 23Å and a na owe selec i i y il e o adius 1.5 Å
and leng h 12Å whe e he ions would ha e o shed i s
hyd a ing wa e s o en e [17] [21]. The selec i i y il e
con ains wo K+ ions [19] [21], ha is, he numbe o ions
ha can occupy simul aneously he selec i i y il e is N=2.
Since he ampli ude o he cu en is o he o de o
picoampe es [17], assuming 𝐼 = 1pA and L=12Å, 𝑣𝑑 can be
es ima ed om (1) as 𝑣𝑑= 3.75 ×10−3 m/s. F om his
es ima ion o he d i eloci y, and aking in o accoun ha
he mass o K+ ion is 39.0983 uma = 6.49 ×10−26 kg, he
co esponding La mo adius o a SMF o alue B=164mT
can be calcula ed as: 𝑅𝐿= 𝑚𝑣𝑑/𝑞𝐵 =93Å. This alue is
o he same o de o magni ude as he leng h o he channel,
and wha i is mo e impo an , i is no negligible in
compa ison wi h he wid h o he channel. The e o e, he
componen o he SMF pe pendicula o he axis o he
channel will gi e ise o a Lo en z o ce ac ing on he ions
which will cu e he ajec o y o he ions inside he na ow
channel. This si ua ion is ske ched in Fig. 1.
Fig. 1: Ske ch o memb ane channel and he de lec ed ajec o y o an ion.
The La mo adius is app oxima ely wice he lengh o he channel.
Inside he na ow channels he ions a e o ced o ollow a
na ow and s aigh pa h. The e o e, he Lo en z o ce ac s
pushing he ions agains he walls o he channel, which
imposes a ic ion wi h he walls o he channel. This esul s
in a dec ease o he conduc ance o he ions h ough he
channel.
To es ima e o wha ex en he e ec desc ibed abo e can
ac ually dec ease he conduc i i y o he channel i is
in e es ing o e ise he ela ionship be ween ic ion,
di usion and conduc ance. In he B ownian mo emen , he
Eins ein ela ion ela es he ic ion o ce wi h he di usion
coe icien 𝐷 as 𝐷 = 𝐾𝑇/𝑚𝛾, K and T being he
Bol zmann´s cons an and empe a u e, espec i ely, and
𝑚𝛾𝑣 being he ic ion o ce in he Lange in’s equa ion
[22]. In [17] he di usion coe icien o K+ in he selec i i y
il e o he memb ane channels is calcula ed and i is on
a e age 1/3 o he bulk alue, whe eas in he wide
segmen o he channel is nea ly he same as he bulk alue.
In he same sense, in [20] i is also epo ed ha he ic ion
is esponsible o he di usion coe icien o K+ o be 3 o 5
imes lowe han in bulk wa e (𝐷 = 0.46 ×10−9 m2/s in
he channel and 2.2 × 10−9m2/s in bulk wa e egion).
Mo eo e , in [19] i is poin ed ou ha he di e en
conduc ance o K+ channels migh ha e di e en causes,
he ic ion among hem. Thus, in [17] i is shown ha he
educ ion o he di usion coe icien in he selec i i y il e
( he na owe pa o he channel) in luences he o e all
channel conduc ance. Those e idences sugges ha he
ic ion in oduced by he Lo en z o ce in he dynamics o
ions h ough memb ane channels can esul in he educ ion
o he conduc ance o he channels. The analysis was
ca ied ou o he K+ channel bu he conclusion can be
gene alized o he es o channels.
Al hough he expec ed educ ion o conduc ance caused
by ic ion due o Lo en z Fo ce is only a ac o o 2 o 3, as
men ioned abo e, his educ ion migh be enough o
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comple ely supp ess he AP. This is due he ac ha he
AP gene a ion is qui e sensi i e o small a ia ions o he
conduc ance alues. To illus a e his, Fig. 2 shows changes
unde gone by he ansmemb ane po en ial o a neu onal
cell segmen in esponse o h ee consecu i e equal s imuli
o h ee di e en alues o he conduc ance o he as Na
channel, which is g ea ly in ol ed in he onse o he AP.
These esul s ha e been calcula ed by sol ing he
di e en ial equa ions o he HH model o AP gene a ion by
means o he HHSim so wa e [18], a ee g aphical
simula o ha p o ides access o he pa ame e s o he HH
model. Fig. 2 shows h ee spikes gene a ed unde s imuli
o h ee di e en alues o he conduc ance o he as Na
channel. The i s spike co esponds o a conduc ance o
120 𝜇𝑆, he second spike co esponds o 80 𝜇𝑆 and he las
spike is o a conduc ance o 60 𝜇𝑆. Fo his las alue, i
can be obse ed ha , e en hough he change in
conduc ance is only a 25% wi h espec o he p e ious
alue, he AP is almos en i ely supp essed.
IV. CONCLUSION
In his wo k i is demons a ed ha whe eas Lo en z
o ce p oduced by mode a e SMF is no expec ed o
p oduce app eciable e ec s on he ions lowing along he
axon o neu ons, i migh well a ec he lux o he ions
along he memb ane channels in neu ons. This is due o he
di e en a ios o he c oss sec ions o axons and memb ane
channels wi h espec o he co esponding La mo adius.
I has been shown ha in he memb ane channels he
Lo en z o ce can e ec i ely p oduce a ic ion o he ions
wi h he walls o he channel, and ha his addi ional
ic ion migh educe he conduc ance o he channels.
Calcula ions o neu on esponses by using a Hodgkin-
Huxley (HH) model ha e illus a ed ha educ ions o
conduc ance o he same o de as hose expec ed can
e ec i ely supp ess he AP in neu ons. The e idences
p o ided by his analysis make o he Lo en z o ce a
easible candida e o be he main physical mechanism
explaining he educ ion o he exci abili y o he mo o
co ex achie ed by he SMS echnique.
Fig 2. Response o he ansmemb ane po en ial (con inuous line) o a neu onal cell segmen o h ee consecu i e equal s imuli (dashed line). Pa ame e s in he
HHSIm so wa e: conduc ances o Na, K, and Cl a e se , espec i ely, o 0.0265𝜇𝑆, 0.07𝜇𝑆 and 0.1𝜇𝑆. The conduc ance o he as Na channel is 120 𝜇𝑆 o he
i s s imulus, 80 𝜇𝑆 o he second s imulus and 60 𝜇𝑆 o he las s imulus. No e ha he AP is almos supp essed in he la e case.
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