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7/23/2019 Neuromuscular Junction &Synapses
1/26
CHAPTER
8
'.::__.rifi'.1,
a.,:t:..
'.
,:..--
):.
t
t
-';;t't'.
Synaptic
Transmission
and
the Neuromuscular
f
unction
Edward
G. Moczydlowski
The
ionic
gradients
hat cells maintatl
lcLoss hrn mcmhranes rovide
a
lbrm of stored
electrochemical nergy
cells can use lor electrical
signalling.
The
combination
of a
resting membrane
porential
of
-60
to
g0
mV and
a
diverse array
of voltage
gated on
channelsallorvsexcitable
cells to gener
ate
action
potentials
that propagate
over long clistances long
the surlace
membrane
o[ a singlenerve axon or musc]e iber. However,
another
classol
mechanisms s
necessary
o transmit
such electrical nfomation
from cell to
cell
throughout
the
myriad of neuronal
netu'orks that link
the brain lrrrh
sensoryand effector organs. Electrical signals must pass across he special
. \ . . ^
, h 1 ^ 5 : * p
. . 1 1 6 n 6 1 1 2 r e ,
r
. r r -
,
, l l c . l
a
" l
l b
sy-napse.The
process underifing this
cell to-cell transfer of electr.ical
ig-
nals is tenned
synaptic transmission.
Communication betr,veen
ells at a
synapse can
be erLherelecuical or chemical.
Electrical synapscspror.icle
direct electrical
continuity beti,r,eencells
by means of gap
lunctions,
u,hereaschemicaL
slnapses link two cells
iogether by a chemical neuro-
transmitter thaL
s released
rom
or.re eLland diffuses
o another.
ln this chapter
tve discr-rsshe generalpropertjes
ol synaptic transnissioli
and Lhen focus mainly on s1'napttc ransmissionbenveen a notor neuron
and
a skeletal muscle fiber. This interface
betg'een the motor neuron
ancl
the muscLe
ell is called Lheneuromuscular
uncljon.
In Chaprer12,
Lhe
locus is on
synaplic t ransmission between neulons in
the central nen'ous
s)'stem
CNS).
s3@
w
MEC}IANISMS
OF
SYNAPTICTRANSMISSION
ElectricalContinuity BetweenCells s Established ither by
DirectFIow
of
CurrentThrough
Gap
unct ion
Channels
t
an Electrical
Synapse
r by Diffusion
of a Neurotransmitter
acrossa Chemical
Synapse
Once the cor.rcept
l bioelectrlcitl' hacl taken l.iold among physiologists
of
l h e
J I L
e - r .
r r . b e , a m c ,
, r ' h . r r e
q u c < l . o n
" l
h o u
c l . " r r ' . a l
i g r a .
7/23/2019 Neuromuscular Junction &Synapses
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TABLE
A-I
CHEMI(AL
ELICTRICAL l onotropi( M etabotropic
Agonist
Membrane
proteln
speed
Effect
ACh,
acetylcholine;
-, membrane
potential.
A simiLar
calculation based
on the geometry and cable
propeflies
of a typical nenre-muscle ).napse uggests
hat
an
action
polential arriving al a
nerve terminal could
depolarize
he
posts)'naptic
membrane by
only 1
pV
after
crossing
he s).naptic
gap-an attenuation of 105. ClearLy,
Lhe evolution
of complex
multicellular organisms
equired
the
development
of specialsplaptic mechanisms
or
elec-
trical
signalling
to seNe as a
workable means of interceL-
Lular
communication.
lwo
.ompel inghyoothesesnerged
-
the loh cen-
tury to
explain
how closely apposed cells could commu-
nicate
electrically. One
schooLof thought proposed that
ce
15 are
dirpcll) l i nled b; micro,coDic cornecL 8
bridges
thar
enable electrical signals to
flow directly.
Other
pioneering
physiologists
used
pharmacologicobser-
vations to infer that cell -lo-ceLl ransmissionwas chemicaL
in nature.
Ultimate resolution of this
question awaited
both the
developmentof
electron microscopic echniques,
which
permitted
yisualization
of
the intimate contact re
gion
between cells,
and further studies n neurochemistry,
which
identifred
the small, organic molecules that are
responsible
or neurotransmission.
By 1960,
accumulated
evidence
ed to the
general recognition that cells use
borh
direct electricaland indirect chemical modes of transmis-
sion
lo
communicate
with
one another.
The
essential tructural
element o[ interceiLuLarommu-
nicarion,
the
slrrapse, is a speciaiized
point
of
contact
between
the
membranesof two different, but connected,
cells.
Electrical
and chemical
rynapses
have unique mor-
phologies,
distinguishable by electron
microscopy.
One
major
distinction is the dismnceof
separationbetween he
two apposingcell membranes.At electrical s)mapses, he
acljacent
cell
membranes are
separaled
by about 3 nm
and
appear
to be nearly sealed together by a
plate-1ike
structure
lhat is a lraction of a micrometer
in diameter.
Freeze-fracture
mages of the intramembraneplane
in
this
region
reveal a cLusterof closely packed
ntramembranous
particles
that
represent a gap
junction.
As described n
Svnaptic
ransmissionnd the
NeuromuscLrlar
unction
8
is
as large as 50
nm
at lhe
vertebratenewe-muscle syn-
apse.An additional
characteristicof a chemtcal
$mapse
s
the
presenceof numerous s)'naptic
vesicles
on
the side
of
the slnapse that
initiates the signal transmission,
ermed
the
pres).napticside. These vesicles
are
sealed,
sphedcal
membranebound structures that mnge in diameter tiom
40
to 200 nm and contain a
high concentrationof chemi-
cal
lreurotransmitter.
The contrasting
morphologiesof electricaland chemical
s1-napsesnderline
the contrasting
mechanisrnsby which
they
function
(Table
8 l). Electrical
slnapses pass volt
age
changesdirectly from one
cell to another across he
low-resistance continuity that
is provided by the con
nexon channels. On the other hand, chemical syrapses
link
two ceLlsby the diffusion
of a chemica l transmitter
across he large
gap
separating
hem. The neurotransmlt
ter rhat
is
stored
in the
ry.naptic
vesicles
s
released nto
the synaptlc
space,diffusesacross
he cleft of the slrrapse,
and
activates he
posts)'naptic
ell by blnding
to a specifrc
receptor
protein
on the
posts).naptic ell membrane.
Direct evidence
or the existenceof chemical transrnis-
sion
predated the experimental confirmalion of
electrical
slnapses.
The foundations of
s),Traptic hysioLogycan be
traced back io
early studies of the aulonomic
newous
system.
Early in the 1900s,
researchers oted that adrenal
gland extracts,which contain
epinephrine, elicited
physio
Logicaleffects
(e.g.,
an
increase n heart rate) that were
similar to those
elicired by stimulation of sl.mpathetic
nerve
hbers. ln 1904, ElLiot
proposed
that
sympathelic
nenes
might release a subsmnce
thal is analogous to
epinephrine that would functlon in chemicaL ransmission
beLueen
ner reand rs
r , i rget
"gar . ' r 'a r
< lLd.es
ug-
gested that the vagus nerve,
which is parasyrpathetic,
produces re ld ted ub5lan\e
hat
L re 'oons ib 'e r de-
nrp
7/23/2019 Neuromuscular Junction &Synapses
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8
/
Synaptic ransmissionnd
he Neuromuscular
unction
/
I
(eleclrotonic
urrent)
Ce
l-cel l
gap
juncl ion
FICURE
8
1 An eleclr ical
ryrpse
All e lecrr ic. l l
\
n ipsc
consinsol
cules.
Electrical nd ChemicalSynapsesBoth
Convey
Signals romOne
Cell o Another,
but
Differ Greatly n the Particulars
ELECTRI(AI"
YNAPSES.
hereas
overwhelnT
ng
sullporl
lo" . l r ,
-n . . l l
. y r .
l . .
Jn . n s r to t r
ac \unu la
eJ
in h \
lirst hall ol the
20th
century. Lhe irst direct evidence or
p l c . l . . , .
r ' : ] nm r - - . o
r d n
r u . h
l : r . r f r o r
c l . . r r o . l l ) ' l
ologic ecorclingsf a crayfish ervepreparation.n 1959,
t u , - h 1 " r
r d P o e r u . e J r r r . '
D ' r i . o '
. t
n
. r
r g : n C
recording
electrodeso shor,vhat
depolarization f a pre-
- ) n J p , r .
n r , e
f b e r '
t l t
' . l )
r - , o J o r ' r
- . - \ r
r r -
sulted
n excjtation l a
posts),naptic
erve
ce]]
(the
mo-
tor
nene to fie tail
nruscle)
wlth virtually no
time clelay.
In contrast,
chemrcal
s1'napses xhibiL a characLerjstic
e-
J \ o
df f ro \ lnd l r l .
r r -
r
L l r .
- . r ,
-1n" rur .o l r :g ,
gn. r l [ te" er ,
t - .
r
o l
1r . pre ,
n . rp t r ' " L T l re
c le . ror
stration
of an e lectrical s1-napse
elrveen L\\,onervc mem-
branes
highlighted an Lmportant unctional
differetlcebe
r \ ^ c c n
l ( l r
r r l
r d . \ e r ' . 1 1
y r
- c -
r r c ' r J . -
r g
.
propagation
electricaL)
ersus
rieilv clelayeclommunlca
t ion
(chemrcal)
hrough he
lunction.
An electrical
synapse rs a true
strucLrLral onnecLion
Linearll,rvith the translunctional voltage
(i.e.,
the
y,,,
dif-
Ierence
between
he tu'o cells). However, the crayfish
syn
apse
described b1. Furshpan
and Potter allorvs depolariz
. l
E . L r
n l
t o
. r , ,
r e a d
' o n l ) - o n , d r r
r t o
f f o | . h e
'
. . , - - i .
r
I . h . n ^ - r . .
- i .
c e l . ) L . n . e L l - d l
synapsesare called rectifying synapses to inclicate thar
the underl,vir'rg
ur.ictional
conducLance s voltage
depen-
d . r r
- r
, - ^
. 1 , . . r
' , 1
" " r r r q , . q 1 l
. T n . r r n .
- a v e
shown that the voltagedependence f electrical
ynapses
arises
rom uniqrie gating properties
o[ dillerenLconnexjn
)u
or rs
.o
n. . -o lo , r
-
r t r lLage
eper rJ rnr
.
, r ,
others are
loltage
independent. lntdr-rsic -ectilicaLion
an
also be altereclbl Lhe brmation of a gap
uncLion
Lhat s
c o n 1 ' o . e d 1 1 1 n" p i . l ; - - c l - . . l c h m a l eu 1 ' t r . l e r
' '
\ l ,
L
L _ i d
c " r n e r t n s a 1 L - J - L o
heterotyprcchannels.
CHEMICAL YNAPSES.y
thejr
\,ery nalure,
chemical
synapses
re inherently rectif,ving
or
polarized.
They prop-
J 3 , e , - l
e n l
i n o n r d r "
l t o n
" r '
l r e
f - c . ) n , r l r l i .
" l l
Lhat
releases
he transnirter ro the posrs).naptic
ell that
. r
' i n .
l - r -
. r r ^ . ,
h r ' r .
. o - i
e
o T J
. t n .
t
.
L d 1 5
_ l r ' '
Fo r ' r . r h r r ' - -e
'
a \ \ .
i o t
, , r
' a t l l -
t l l ch .m- -
. " 1
- v , f
i .
r n - r r
. "
n
h p l
. t h p p o - - b i l
r 1
r l - a . h e
p . - t : 1 n L p , , . l l
, . r n
r r r f r e r . e
r ) n ,
p . .
. . , r J o n o r
transmittel
release
b1' the preslnaptrc cell.
Studies ol s1n
apse clevelopment ancl regulation hale sholltr
that postsy
naptic
ceLls also p1a1
an actrve role in s,r'napse ormation
n rh r fNS no< t *n rnn r r c l l< m,
. l \ ' l lSO p roc lU(e r ' o
grade signall ing molecules,
such as
nitdc
oxide
(NO),
that
d1ffuse back into the presvnaptic Lerminal and modulate
l c
'
,
e l
o h ,
. ) n . r t .
c o
- e c r i o
r f . 1 2 2 .
F r r f , c r -
- , . ' " i l .
"
hanh . . - ,
. , - , , .
. r l
) n m e
) n d l
e
. O T
rains receptor-s hat lnay eiLher inhibiL
or
facilltate
the
le [ - :e
o
l r
n , r r i . ie
b] b .o , -emi . np. h . r r
>m' .
T
u , .
. h ' c ; l
- r
; - e ,
" u l , l
h e
,
^
, 1 .
,
J , u n r d . r e . L i u r r
pathu.a). or signal propagation
that can be modulated
by
bidirectional chemical commur-rication
etween two inler-
actingcells.
The process o[ chemical transmissic]n
an be summa-
rizeclby
the l,rl lowing
eries f steps
Frg.
8 2):
\ (e
,
.
\ c r . to t rdn,n
le
- ro lecJ
e- ._ ' r 'e cL. rgcd r to
\ )
d p t .
\ ( . t
h -
' l r , ( i 5 .
- d
l o f l l f , . e i n .
I n
I n c
r \ < t -
cle membrane use the energl of an H gradient
Lo
energizeuptake o[ lhe neurotransniLLern the vesicle.
\ lcp
2 . A r
, .
on LrJr (
"
r r l r i .h
in ro
rc ,
ro l t "ge
B. r r '
N r
. r
d
(
c h , - n e l ,
. )
1 8 2 1 .r r , . . - , r
r h c
p - c s y - a p
Ltc
ne nre Le mtnal.
Step
3r Depoladzation opens voltage-gated
Ca2t channels,
* h . . l - - l lo ' r
\
J '
lo
c
le .
t
.e prP)yr l . , rpL
lermin l l .
SLep4;
The increase
n intracellular
Cart concentration
([Ca2-1,)
riggers he fusionof s] 'napric esicles ith
Lhe
7/23/2019 Neuromuscular Junction &Synapses
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Synaptic ransmissionnd he Neuromuscular
unction
8
Extfacellular
space
i(electrotonic
current)
Presynaptic
nerve erminal
of he nerve
cell
\
str.ucrion
f t l.re ransmitler
e.g..
hydro\,siso[ ACh b,v
acetyLcholinesterase
AChEl),
(2)
uptake of transniuer
lnLo the
presynaptic erve
erminal or
into
other celLs
by
Nr*-dependent ranspor-t) 'stems, r
(3)
diffusionoI
r l - , . r : n < m , r r n r m . l . . r r l . < t r \ \ , \ 1 r
O l L n ( \ n r P \
FICUREB
2. A chcmic al )nrpsc SlnapLic mnsmission1 r rhcmicalstnapsc an
be rh)rghtu[ . ]soccLrnnsrrs| r . . t .Lr.
A neurotransmitter
reaksdown,
is takenup by the presynaptic
terminal or oiher cel1s, i
diffuses
away ftom the syr'upse.
h
.
t
u . : .
, .
: r - r rd 'd l
r ln ^b
l )
|
,
. , '
, l
i
' . \B 1 .
e l y
.
. d F .
. . p r ,
-
' r . h . - . n J u r ' p l
a J e r
\ c o h . ' l r n ,
The
Transmitter at a ChemicalSynapseCan
Activate Either an lonotropic Receptor hat
ls
. , ^ 2 +vorrage gareoLa
channels oDen.
Postsynapticell
7/23/2019 Neuromuscular Junction &Synapses
5/26
IONOTROPIC ECEPTOR
. , :
Axol
t\;
Electr ical
I l ; : l : i : l
srmulus
I |
'1 ,
\, ' ,,/ \ . N"./
. l l
";
-'--"
@ ,
. .
. . . . . .
:
,
i l , ,
: . '
. , , ,
|
.
. . : . . , . . . . . , , .
. : .
. . . , . : t . .
. : . .
^
' ' .
- r i
. . . _ ) , i . , 1 t : 1 , . , , . , .
, / t
"
2Oa 8 Sylapt iL
-tdn 'Ti. . 'on
a1d
t l-" \eL-omJ)culat
,LtL
01
B METABOTROPICBECEPTOR
Acetylcholine
Skeletalmuscle
fibermembrane
mate.
Ellutamete
receptors
thaL are ton
chilt-tr-rels rc
knorvnas
ionotropic
receptors,
and
gluLanate
ecepLors
cor,4t1ed
o C proLcinsare
calleclmetabotropic
receptors.
fhis nonenclature is berng increasingly
used ro
clescribe
thc Lwo
malor l)rpes
)1
rcccptols of
Lfansmitters
ther
llun
llutamate.
lonotroprc
ancl metabotropic
eceptors
determine he
ult imate
unctionll response
o tftnsmiter re]ease.
ctiva-
Atrialmuscle
cellmembrane
FICURE 3. id lotropic
xnd n.rr
botrof ic . rc. t \ ( l r {r lmr
fe.eprors A,
This e\mplc i l lusrf :urs
nicor ir l ic '
acel) lchol inc cccptor
shrch is
r
tLg.Lml-gaLcdhannel
on Lhe
possr'-
naptic nembr.rnc In
r sktleral mLrs
cle,
Ihe fnd
result
s mrLsclc
rr
Lf lcLion B, This
eramptc l luslratcs
I rruscrf jr r ic
; l .er) lchol inc
crcp
1of.uhrrh is couplcd o e Lcrcrour
mcric
a;
t rotern.
ln I crr . l i rc rrLrs-
clc. Lhccml Lcsul t s. lecreis. . l
rcart
rr te. Noie
thrt rhc
frcr\nair t rc
r. ,
l .1se l r \ ( lh is \ . r \
s irr i l r r l1ef .
. rnd LLr - \Ch.
arcr\ l .hohnri
( ,1P.
3LLrnonne
f
phosphxre
a a
95). B) lreir YeD nalure, onotropic
receptors tecliaLe
fast ionic slnaptic responses hat occur elt it nillisccond
t r c
- . . ' 1 , .
. l t . r , . - , c . '
u -
\ .
r . . . p r
r -
,
.
/ r . . , \ \ .
biochenricalll '
edirted
sy'napticesponses
n rhe range
of
seconds
ominuteS.
Flgurc B-3 compares
Lhe basic processes
mecliare.lb) '
t\vo prototyPicACh receprors
AChRs): 1)
rhe ACh acLi
vatecl
on
channcl lt the neuromuscLLlar
unction
of skele
Nico
ch
I
l
,
L
: inicACh rece
annelactivatir
lr
IM".b"r*
lepolarization
lr-
"tb"
p"t""tb
exctaiion
ptor
ln
IVIusc
contraction
[,4uscarinicCh eceptor
I
acrvar n
]
lr
R"b"r"
"l
"TP
-
P./
from he
helerotrimeric
protein
-
]r
Activation
oi
lnward
I
rectifier
K- channelby
py
----r-
t-r.,r"-.t
*.
l
I
nyperporararon
]
t-
Decrease n
nean TaIe
7/23/2019 Neuromuscular Junction &Synapses
6/26
tinic)
receptor, opening
of the AChR channel
results n
a
transient
increase
n permeability
to Na* and K*, which
directly
produces
a brief depolarization
hat activates
he
muscle
fiber. In
the caseof the
metabotropic
(muscarinic)
receptor,
activation
of the G protein-coupLed
recepior
opens an inward rectiFer K+ channel, or GIRK (p. I97),
via
p7
subunits released rom
an activatedheterotrimedc
G
protein. Enhanced
opening of
these GIRKs produces
membrane hyperpoLarization
and leads to inhibiiion
of
cardiac excitarion
(p.
488). These
wo funcLionallydistinct
mechanisms
are the
molecular basis for
the seemingly
conflictrng
observations
of early physiologisrs
rhar ACh
(VagusstofJ)
ctivates skeletal
muscle bur inhibirs
hearr
muscle.
SYNAPTIC
RANSMISSION
T
THE NEUROMUSCULAR
UNCTTON
Neuromuscular
unctions
re Specialized
Synapses
ith Active
Zones
of Synaptic
Vesicles n PresynapticNeuronal)
Membranes
nd Highly
Amplified
unctional
Folds n
Postsynaptic
Muscle)
Membrane
The chemical
ry.napse
etween
peripheral nerve
terminals
and skeletal muscle fibers is
the most intensely
studied
synaptic connection in Lhe neryous
system. Even rhough
the detailed
morphology
and the
specific molecular com-
ponents
(e.g.,
neurotransmitters
and receptors)
differ con-
siderably among different iypes of slnapses, ihe basic
electrophysiologic
principles
of the neuromuscular
unc-
tion
are applicable o many
other
qpes
of chemical slrr-
apses,
including neuronal
sl.naptic connections
in the
brain, to
which
we wiLl retum
in Chaprer 12. In
this
chapter, we focus on the neuromuscular
unction
in
dis-
cu5s inSheba>rc r in . ip les f
: lnapLic rdnsmi5s ,on.
Motor
neurons with
cell bodies in rhe
spinal cord have
long axons that branch extensive lynear the point of con-
tact
wirh the target muscle
(Fig.
B-4). These axon proc-
esses
each inneruate a
separate iber of skeletal muscle.
The whole assembly of muscle
fibers innervated
by the
axon from one
motor
neuron is
called a motor unit.
T1pically, an axon makes
a single
point
of synapdc
contact
with a skeletal
muscle hber,
midway along
the
lengt\
oi rhe muscle 6oer.
thrs
7/23/2019 Neuromuscular Junction &Synapses
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210 8
/
Synaptic
ransmissionnd
the Neuromuscular
unction
Spinacord
Nerve ellbody
Axon/
Muscle ell
or
ber
Postjunctional
folds
Synaptic
vestcles
'o
I
o
I
Acetylchollne
receprors
Postjunctional
folds
Postsynaptic
membrane
FICURE -4.
Ihc
\efrcbr: r te
b o u 1 o n s ' i 1 s s ' c L l l s t h c s p c c i a l i : . r L i o l s o | l 1 r c p o s l s , v n a p l L
.onL:rLLrirg
he
tionrl loltls).
Depol^iz.rLiorl
Act ve zone
Acetylcholine
(re
eased rom
vesrcles
Presynaptic
memorane
,t
laT.rna
@k'
\@
7/23/2019 Neuromuscular Junction &Synapses
8/26
FICURI 8-5.
End pla ie poterhals
el ic iredar rhc lrog
nuro-
muscular
lunclion
by sLmulaturg the mot,:lr neuron. The mag-
r i r ' ' " o f t l . . . c r . ' r
\
I o . w
1 i .
r . l I D P
.
s , . . . .
near thc end
plxre
and deca)s farther awry
(De e
fonr
Fati I,
Kelz B: An anal)sis of the end-p1ate otenLial
ecorddwirh an
inLracel lu larlectrode. l ,hysio l l5 :120
370, 195i. )
cle cell) of
lhe neuromuscular
unction.
Normally,
nen'e
stimulatron
rvouid
drir.e the V,,, of the muscle
above
threshold
and elici t an action
potentlal
p.
172). How-
ever,
Falt and Katz were interestecl
not in seeing
the
Synaptic ransmission
nd he Neuromuscular
unction
/
8
0
Excitatorv
1o
postsynaptic
(or
end
plate)
0
potenlial
(mv)
1 0
0
1 0
0
1 0
0
r h , m r n m . ' ' r r . r n v , . " h o p . i n \
' . ' ' ' ' , /
spoto[ themuscle
ell.
\ V h e n
J l
" i
d h a t -
e l e c t r L " l l r \ ( i r e d h , ' m o r o r
neNe axon, they' obsen'ed a transient
depolarizaLion n
211
Stimulusf
motor
erye
1 0
Voltageecordiqg
\\ llJ-
MorotneNe
The muscle s treatedwith
curare o limit
ACh
receptorachvahon
o
subthreshold esponses.
The delay in
response is a
function of
acetylcholi11e
release,
diffusion, and
activation ol
Pos6ynaPtlc
aecepto$,
The delay
in
response
tlme
increases s
a
function
of the
distance
hom the
end
plate.
1 .0
mm)
7/23/2019 Neuromuscular Junction &Synapses
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212
8
/
Synaptic
ransmissionnd the Neuromusculaf
unction
A EXPERII\,lENTAL
REPARATION
END-PLATE
URRENTSOBTAINED
I
VARIOUS
OLDINGOTENTIALS
400
200
End-plate
current 0
(nA)
-200
-400
0 1 2 3 4 5 6 7 8
Time
msec)
C I-V
F]ELATIONSHIP
ORPEAKEND.PLATE URRENT
Clamped
membrane
potential
(mv)
FICURE
8 6.
End-plate currents obtained at dillerenr
membrane
poten
tials in a vohage-clamp expenmen . A, Two electrode vokage cLamp s
used to
measure
he end-plate cunent in a frog
muscle 6ber. The tips of
rhe rwo
microelectrods
are in the muscle fiber.
B, The
six
records
represenr
end-plaLe
currents that \'r'reobiaind while the motor
nerve
rvas stimulated
and the
poslslnaptic membrane was clamped to
V,,,
values
of
-I20,
91, 68, 37,
+24, +38 mV Not ice that ihe
peak
cunem
reveres
from inward to out\rard as the
holding polential shifts
from
-37
to
+14 mV.
C,
The rvelsaL
potential
is near 0 mV because
atory
postslrnaptic potential. lt
is produced
by
the tran-
s ienr
open inB l
AChR
thannel . .
nht ,h
are >e rL ' \e l )
permeable
o monovalent
calionssuch as Na*and
K-.
The
increase n Na* conductance
drives V* to a more
p o c r
e v d l L e n l \ e
\ r c r n I )
o ' t h e e - d - p J a L e
e B o n .
n
rhis expenment, curare blockade al1ows only a small
number
o[ AChR channels o open,
so that the EPP does
nor reach the
threshold to produce an action
polential. If
the experiment
s repeatedby inserting
the microelectrode
at
various distances
rom the end
plate,
the amplitude of
the
potential change
is successivelydiminished
and its
peak
is increasingly deLayed.This decrement
with dis-
lance
o. .ur ( becausehe EPPorg
rate . a t
the
ond-p l " te
region and spreads away from this site according to the
^ , . < ' . p
r h p
n . o n e r e q
n
' C 0 r
o [ L h e
r . u ' . e f b e r .
Thus,
the EPP n Figure B 5
is an example o[ a
propa-
gared.
g raded
responce.
ouerer . wrhout t l ' e curd re
blockade,
more AChR channels would open and
a larger
EPP rvould
ensue,which would drive
V,,,
above
threshold
and consequently
trigger a regenerating
action potentiai
(p.
172).
What
ions pass through the AChR channels
during
generation
of rhe EPP?This question can be answeredby
using
the same
voltage-clamp technique that was
also
. sed
ro , t r d1 the
bac iso f l he dc l
o ' po ler t l
r 5ee
F.8.
7 5B).
Figure 8-64 illustrates he
experimenlal prepara
tion
for a two-electrode voltage-clamp
expedment
in
which the motor
nerve is stimulated
while
the
muscle
fiber in the
region
of
i|s end plate is voltage-clamped
o a
chosen
V-. The recorded current, which
is proportional
to the conductance change at the muscLeend plate, is
called
the end-plate
current
(EPC).
The
EPC has a char-
acrerisric
rime course that rises to
a peak within 2 ms
after
stimulation of the motor
newe and falls exponen-
L i a l l ;
a c r
o
e r e
q f ' g
B - o B ' .
l h e l i m e c o u - " e f
, h e
EPC
corresponds o lhe opening
and closing of a
popula-
tion
of
AChR
channels,
governed by the
rapid
binding
and d i
,ppearan,e [ ACh as r t d i l lu 'e .
o
the
po. tsynap-
tic membrane and is hydrolyzed by AChE.
Aegn -11. .he , :gon. 'L-
binding site 1or ACh is in
the extracelLularN-terminal
domain o1 the a subunit. For
each of the subunits,
the
M2 transmembrane egment ines
the aqueouspore
of the
7/23/2019 Neuromuscular Junction &Synapses
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214
8
/
Synaptic
ransmissionnd the Neuromuscur
unction
A SINGLE-CHANNEL
URRENTS
Single-channel
currents
PA)
Embryonic
ozJltS
0
Single-channel
^
currents
pA)
-z
0 20 40 60 B0 100
120 140
Time msec)
B
t-v RELATTONSHIPS
2
-100
Embryonic
crz0t6
\naut,
0rP.6
have
clilferent
functional properties. The uniLary conduct
ence of
nonjunctionaL
receptors 1s approximately 50%
larger
and
the single channel
iletime is longer in duraLion
rhan thar
of junciionaL receptors.The basis for this phe-
nomenon
is
a difference n subunit composilion.
The non-
y t r ' r ,: . r r tc i
o
cn b r r
onrc l
ecPFlo15
c r Den l . r r - rc r ' .o ln
plex with
a subunit composition of
arp76 1n mammals,
just
as
in rhe eLectric
organ of the TolTedo ay. For the
.luncriondl
AChR
in adult skeletal muscle, subsiitution
ol
an
e subunit
for the leta]
7
subunit
results n a compLex
ivlth tl.re
composition
arPe6.
T e l u n . ' r o n " lr o p e r t e s l t . e t u o t ; p . o ' e e p t o - s
l.ravebeen
studied by coexpressrng
he cloned subunits
r.t
) , , ' . ,pu ,
oocvre ,
rgLrc
B-84
-Los
pach c lamo eco .d
ings of
single
ACh-activatedchannels n oocytes hat
had
been
injected
u'ith nRNA encoding either
a,
B. 7,
6
or
a,
B,
e, 6.
Measurements f currents at different
vohages
yreLded single-cl.rannel
-V cun'es
(Fig.
8-BB) showing
FICURE
8 8. Properties ol embrlonic and
adrlt acetylcholine
(ACh)
receptors
fron skeletal muscle A, T he resulLsol
pelch
cLamp
expe
-
rnenrs,
sith
lhe
p:rlch pipeues in th o utside out configutation
end the
parch exposed o 0.5
pNl
ACh, are sumnarizcd ln thc rpper
pdncl, rhe
ir estigatoE
expresscciLhc
emb$onic AChR. $'hich has the subuni
compositjon
d:87.5, in Xdrril]rrls oc]tes. ln thc ldv.f
ldnul,
thc
invesli
garors expresscd hc
.ldrlt,A.Chlt, $,hich has the subunii co]nposlion
arB5.
No(ice thlr rhe
meen open times ar
great,rr
urr he errbfyonic
forn,
s'hereas Lhc unitary currens are
greater lor the adult lornl. B,
Ihe lrfo
lires summarize d:ia that are simihr
to those obtarned rn A.
The single channel conductance
of the aduLt lorm
(i9
pS) is hiilher
rhan
rhaLol Lhe
eLnbry'tnlicorm
(40
pS)
(Data
{rori \,lishina M, Takai
T,
Imoro K. ei a]: lvloleclrlardisrincrion
between feLaland aduLt forns
ol
muscle ceLl lchol ine
eccpLofNaLure 21:406-411, 1986.)
" h l "
. . f l D r , l i D i r . n D . i r l i ? a ; r ^ 1 . < r r ' ) ' l *PL
Lrdnrm
>ro l r
ier>
-
cle\eofrl.c.l i
a d
,1nrp-e
9r1n"11"n
MolecuLar
cloning of
genes
hat
encode AChR subuniLs
of the TorTedo ay electric organ and mammalian skeletal
r
. .1 , r
cd ro rh " rd r t . f ' o t .on o [
. . r r rge
-umbcr
o [
relaLed
genes for AChR channel
proteins. For example,
mammals
have a family ol at
least eight genes hat encode
homologous
a subunits of
nicotrnic ACh activatecl
ecep-
L o
.
, l | e
o
s ,b r , n . r
f
t h ,
- L r l . t "
n u r c e
e c e p l o - , -
h p
procluct
of a gene called al. Seven
additional genes des-
iElnatecl
2
through
aB encode a subur-rilsLl-rat
re ex-
pressed n neuronal Lissues.Only the proLein producLsol
genes
n1, a7, and aB bind the
snake venom
protein
called
a bungarotoxin.
In
addiLion,
at least lour
B
sub-
units
exist. Besides he
B
subunit
of the skeletal muscle
AChR
which
is
calLed
B]-there
are three
neuronaL
homologs
(82.
83. P+).
Heteromeric associatron
f differ
ent combinalions
oI these subunrts
could potentially
pro
7/23/2019 Neuromuscular Junction &Synapses
12/26
(5-HT-
receptor), lycine
GlyR),
and GABA
(GABA,
re -
, e f . u i . A q m n t . n c d p c \
o u r ) .
\ r h l
r n d
5
H l r e
ceptor channels are boLh
permeable
o cations and
thus
produce
exciLaLory urrents, rl''hereas lycine
acti\,ated
ancl
GAB{,
chrnnels are permeable
o anions such as Cl and
produce rnhibitory crirrenls. Fillure
8-9 shorvs exanples
o - r r J \ r o r . n p c
. r r d
L r r r i t a r
l
.
r
c n L -
n e d
a L e q
_
5 1 ,
- e
r .
r v r t e d
d u { B A . . h . r
. l -
C l " n e d
e , c >
e n -
'
od i -g
.u lun t
-
o
hc- .
a t to r ' r p lu r . rnn , r
r .oJe
t ) t r ' . c i r -
h . r '
1 , o n o l o o . u . .
q r
h R > L r b - '
.
f h . r
primary amino acid sequences harc a common arrangc-
ment of ML, N {2. M3, and M4 transmembraneegmenLs,
as
described arlier or the mcotinicACl.rR
see
Fjg. 8-7).
T L
- e
p r o L e L h . s l b . l o n g
r o
- J E .
d c r e
r . r r y r L J l
is knoi,vn as the
ligand-gated
ion channel
superfamily
f p
L 0 \ .
. , o r . ,
. e r l r l , . i .
o l l - c . c
I
n c -
. u t L , . r - . L J l
' h p .
' ^ ' "
. , I n , a , r c e - l o
h , b
- j .
r o r
. r . L r T
\ c
- .
r r r i , ,
. ' e . r ' \
\
a D D c r o c - i d e
' l '
1
rvjthin
the tr42 segmenr. 4utatlon
of only three resLdues
ri.ithin Lhe
M2
segment of a cation-selective subunit of
a neuronal
nicotinic
AChR is sufFcLent o conven it
lo an
anion-selectrve hannel actlr.ated y ACl.r.
AcetylcholineReceptorChannels
Cannot
Open
Until Two Acetylcholine
Synaptic
ransmission
nd he NeuaomLrscular
unction
8
0
1
2
3
Equation 8- I
ln the case of an cgoni-st ctivateclchannel, such as the
AChR channel, ar least
one additional state must be
p r ee n l b ( . l J - \
t h ,
. o ' c d
. r ; n n . c :
'
e i t h e r
' r n d
l B
nisl or not:
Equation 8-2
a - la . r.-]
Closed hannel Closed hannel
Open
channel
No aElonist Agonist lound Aplomstbound
I t h i . t r ' . . p - h , ' c
L h c l o : c d L , . L c
, I
o t . ' ec . r o
nel must brnd one molecule of the agonist ACh
Lo lorm
J r d s ^ r i - l " u n C \ n n " l
L . r r
-
. l o , " d
l r
' - 1 e
. ;
r
r ,
. , . - -
, l _ , n " , I r l - ,
(AO).
Llon'evcr. even this scheme s oi'er1y
simplistic be-
cause
u.e knolr,
that each of the two cv subunits of the
AChR channel must bir.rd ACh srmultaneously for the
cl-rannel o open:
215
A GLYCINE
Macro-
scopic
current
(nA)
2
0
S ngle-
"
hannel
'
current
-4
(pA) -6
-8
0
_,1
0
_,1
B GABA
2 3 4 5 6
Tinre
(sec)
2 3 4 5 6
Tirne
sec)
Macro-
scopic
cutrent
(nA)
5
ngre-
-
channel u
curreni
-2
(pA)
4
1000 1500
Tme
msec)
FICURE B
9.
(:ltfrcnrs
acttrrred by glycrne and 1aminobut).ric Acid
(cAB,\)
,A These e\pernrcnts Nere performed on culLurcd mousc splnaLcord
neurofs usjng
palch
clamp lechniques The lcli pnncl
sho*,s
rhe
macroscopic Cl clrrrcnl,
*hich
s rneasured n the rvhole ce11
on[iguration and
c a r r i c d b 1 ' g l 1 c L n e | e c c P l o ] ( G \ ' R ) c h a n n c l s t v h e n e \ p o s e d 1 o g ] l c i l 1 ' T h e l l g h t p d n . l s h o r i ' s s l g l c . c h a n n c l
ouL
patch
conngufallof
In
both scenariLrs, he hol.ting porenlial \\'rs
-70
m\r n, The L/t pancl shoNs rhe mrcros.opic Cl currenr
ihai
is
carried bv
a;ABA,
re.eptor chan ncls when exposed
to CABA. The lighl
pdncl
shors single-chamreL
urrents
(Det:r
lroln Bofmann.l,
Hamill OP. Salinann B:
\ ' lechxnjsnofaniLrnpernreet iL lL, rLhloughchanrre
Equation
8- 3
2o
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216 8
/
Synapticransmission
nd heNeuromoscular
unction
charLnel,
ncluding some local
anesthetics, ct
by enteing
rhe lumen
of rhe channel and
blocking the flow of ionic
current.
Figure B-l0A shorvs he results
of a patch-clamp
experiment
in
which
a single AChR
channel opened and
closed n
response o its
agonist, ACh. After adding
QX-
222, an analogof the loca1anestheticagent idocaine (p.
189), to
the extracellular slde,
the channel exhibits a
rapidly flickering behavior. This flickering represents
a
series of
brief interuptions of the
open state by numer-
ous
closures
Fig.
B-
10B).
This tlpe of flickering
biock is
caused
by rapid binding and unbinding of the
anesthetic
drug to a site
in the mouth
of the open channel. When
the drug binds,
it block the channel
to the flow of ioru
(ArB).
Conversely,when the drug dissociates,
he channeL
becomes
unblocked
(ArO):
Equaron 8-4
-
d / O - A r B
Blocked
Channel
blockers have proved
to be effective tools to
study the mechanism of ion pemeation. For example,
QX-222
helped ln locating
amino acid residues on the
M2 transmembrane
egment hat form part
of the blocker
binding
site, thus identifiring residues hat line
the aque-
ous
pore.
Miniature
End-Plate otentials
eveal
he
Quantal
Natureof Transmitter
Releaserom
the Presynapticerminals
Under
physiological conditions,
an action
potential
in a
presynaptic
motor newe axon produces
a depolarizing
pastsynaptic
PP rhat
peaks
at approximately
40 mV more
positive than the
resting
V.. This large signal results
rom
the releaseof
ACh from
only about 200 sp.Laptic esicles,
each containing
6000 to 10,000
molecules of ACh. The
neuromuscular
unction
is clearly designed or
excessca-
pacity inasmuch as a single end plate is composed of
numerous
s)'napric contacts
(-1000
at the frog muscle
end
plate), each with an active zone that is lined with
dozenr
oI
malure
Synr'ptrc
esicles.
hus, a large
-rer
tory
of ready
vesicles
)l0a),
together with the
ability to
s)'nthesize
ACh and package t
into new vesicles,allows
the
neuromuscular
unction
to maintain
a high rate of
successful ransmlssion
without
significant oss of function
as a result of presy.rapticdepletionof vesiclesor ACh.
The originaL
notion
of a vesicular mode
of transmitter
delivery
is
based
on
classic observationsof EPPs
under
conditions
of reduced ACh release.
1n 1950, Fait
and
Katz observed
an interesling kind
of electrophysioLogic
"noise"
in their continuous, high-resolution
recordings of
V- with a microelectrode nserted at the
end-plate region
normal EPP, they were named miniature
end-plate po-
tentials
(also
known
as
"MEPPs"
or "minis").
These ob-
servations
suggested
hat even in the absence
of
nerve
stimulation,
there is
a certain low probability
of transmit-
ter release at the presl'naptic
terminal, resulting in rhe
opening ol a small number of AChRs n the postslnaptic
membrane.
An
examination of the size
of
individual
MEPPs suggested hat they
occur
in
discrete muLtiplesof
a unitary amplitude. This findlng led to the
notion that
ACh release s quantized,
with the
quantum
event corre-
sponding
to ACh release rom
one slnaptic vesicle.
Another way of studying
the
quantal
releaseof ACh is
lo stimulate the preslnaptlc motor neuron
and monitor
V-
at the end plate
under conditions when the probabil-
ity of ACh release s greatly
decreased.How can we de-
crea5p
he
probability
o' ACh release? he a-nplitude
ol
the EPP hat is evoked n response
o nerve stimulation is
de.reased 1 lowering
[Ca2
1.
ard
-creasinS
[tt4g'
. A
low
[,-;2 l.
decreases
a
'
errD
nro
Lhe
pre'ynapLic
re r r
a l
rF ig .
B
2.
s t ry 3 ) . A h igh
IVg, ] .
pan ia l ly
blocks
the pres)-naptic
Ca'z* channels and thus also
de-
creases
Ca2+ entry. Therefore,
the consequenceof either
decreasedCa'z*1" r increased Mgz*J. is a falL n [Ca'zt],
in the
presFaptic
terminal, which reduces transmitter
releaseand thus the amplirude
of the
EPP
(Fig.
I 11).
Del e"-tr l lo
and Kat-
explorted hrs suppressior
f trans
miter
reLease
nder conditiors of 1ow
[Ca,*].
and high
fMg'?*1"
o obsewe the
V- changescausedby the quantal
releaseof transmitter. Figure
B-12A shows seven super-
imposed
records
of MEPPs hat were recorded
rom a frog
muscle hber during seven repetitive trials of newe stirnu-
lation under conditions of reduced
[Ca'z*].
and elevated
l \ , 4 o r I T h e r c , a r d < z r c r l i o n p r l r r r h p n n < r i n n n f r h e
nerye
stimulus
artifact. The amplitudes of the peak re-
sponses
occur in
dlscrete multiples of approximately
0.4
mV. Among the
seyen records were one
"nonfe-
sponse,"
two responses
of approximately 0.4 mV, three
response) l approximatell0.8 rV. ard
o-e re5ponse [
appro \ lma le l )
1 .2 mV.
One o [ the record ing< l .o
-e -
vealed a spontaneousMEPP with a quantal amplitude of
approxlmately 0.4
mV
that appeared ater in the
trace.
Del Castillo and Katz proposed
that the macroscopicEPP
is the sum of many unitary
events,each having a magni-
tude of approximately 0.4 mV. Microscopic
observationo[
numerous vesicles n the synaplic terminal naturally led
to the
supposition
that a single vesicle eleases relarively
fixed amount of ACh and therebv
Droduces a unitary
MFDP. c.o rd ing o tn jsv iew. he qu in r izedVEPP< nus
cor
espond o lhe f l..or of
d'screte
urrbers
of svnapti.
*
ves iL .e>
, l , 2 .
l ,
and so on .
@
For elucidating the mechanismof
spraptic transmission
at the
neuromuscular
unction,
Bemard Kaiz
shared the
.
1970
Nobel Prize
n Physiologyor Medicine.
@
7/23/2019 Neuromuscular Junction &Synapses
14/26
Synaptic ransmissionnd he Neuromusculaf
unction
8 217
CONTROL
FIGURE
8-10. The ef fectof a local
anestheric n
rhe acerylcholLLle
eceplor channeL
AChR)
A, Sin
gle-charnel
recording ol nicotinic ACh receplor
ex
pressd n a
.Xenopfts ocyre The patch
rvas n rh
ou[side
out conllguralion, and the holding potentia]
rvas
-150
mV. The continuous presence
of
L
pM
A r
' . u * d
b
"
\ , r r e o p r g B . l l - ,
e ' l
i
ment is similar
to
lhai
in A,
excepr hat in addition
Lo rhe ACh,
the lidocaine
analog
QX
222
(20
/rM)
was
preseni
at
the
exrracellularsur{acof th recp-
1 . . -
- ,
| , c l
\ o
p
' r r
e
t r "
r r " l
o p n . r g . - d ,
^ r p , n r
o b .
r . T , .
l i
r " r
" g
a . : d L , r n a n o r c f
channel
closures
The
dme scale of th lower
panel
is expanded
10 fold.
(Data
rom Leonard RJ, r-abarca
CG. Charnet
P, et al: Il,rdence that
lhe M2
mem
brane'spanning
region lines rLle on
channel
pore
ol
rhe nicot in ic
recepror.
Science
242:1578-1581,
l9BB.)
500
750
Time
msec)
B LIDOCAINE
NALOG
close to the
pres).naptic
erminal membmne o[ a leech
neuron lhat uses serolonin as its only neurotransmitler.
The carbon liber is an electrochemica]detector of sero-
ronin
(see
Flg. B-13A), the currenl measured
by this
electrode
corresponds
o
four
eLectrons
er
serotonin
mol
ecule
oxidized at the
tip. Stimulating the
leech neuron
to
produce an action potential aLsoelicits an
oxidation cur-
rent, as measuredby the carbon hber, that corresponds
o
rhe releaseo[ serotonln. At a
[Ca'?*].
of 5 mM, rhe cur-
enr
J arge nd
r
omposed 'man, 'mal l
7/23/2019 Neuromuscular Junction &Synapses
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218
8
/
Synaptic
ransmission
nd he NeLrromuscular
unction
A MINIATURE
ND-PLATE OTENTIALS
MEPPS)
0.9
(rnv)
0.3
molecules.
Thus, the
amount of serotonin releasedby the
small
slnaptic
vesicles of the leech
neuron is about half
l.JLrfiber of
q',jania
the str.englh
f a particular
slnapse and thereby
give
rise
ro
dn l l t c rar .on bchauor . hree
ype-
o
5) r r , : rpr
moo
-0.3
22
20
1 8
1 4
1 2
1 0
B
6
4
2
0
0 5 1 0 1 t
Time
msec)
DISTRIBUTION
F
\,1EPPN4PLITUDES
Number f
observations
0.8 1 .2 1 .6 2 .0 2 .4
Amplitudef EPP
mV)
Number
of
quanta
released
x)
FICURE 8-12. Evoked and sponlaneousminiature end-plate potentiaLs MEPPS)A, The rnvstrgators ecorded y,,, in trog skeleral nuscle ibcrs LhaL
rvere exposed o
extracellular
solulions having a
tca'z'l
of 0.5 mM and a
IMg,'l
of 5
mN4.These alues mininrize
lransrnifter
release,
nd
(herefore
t
rvas possible o
resolve he srnallesrpossible
MEPP, whrch correspondsLo
the
reLease f a single synaptic vesicle
(i
e
,
1 quantum) The invenjgators
stimulatd
Lhe molor
neuron
seven consecutive imes and recordd the
evohed MEPPS
ln
one irial, ihe stimulus eloked no
response
0
quanta).
In
rwo lrials,
the
peak MEPP was
ebout 0.4 mV
(1
quantum). In LhreeoLhers,
he
peek responsewas
abour 0.8 mV
(2
quanla)
Finally, in one, rhe
peak
was about
l.2mV
(l
quanta)
ln once
case,e MEPP ol Lhe smallest magnirude appeardspontaneously.B, The histogram summarizes
claLa
rom 198
trials
on a ca
neuromuscular
unctioLl
in
Lhe
presencc
of 12 5-mM extracellularMgz . The data are in bins $'r h a width of
0.1
mV The
disiribution
has ight
peaks The first represenls
limuli that evoked no responses. he
olher seven
epresentslimuli that eYokecL EPPS haLwere
rcLlghlyLntcgral
muhiples
of the smallesr
MEPP
Thc cur.,'eoverlying each clusrer ol bins
rs a
gaussianor'normal" luncuon
and facjliralescalculaiion of the average
MEPP for each cluster
of
bins
The peak
r,rlues of these
gaussians
ollow
a PojssoLl
istribution.
(Data
trom Magleby KL: Neuromuscular
ransmission.
In Engel AG, Franzini-Armsuong C (eds):Myolo$/, Basicand Clinical, 2nd ed r.welv orL. Mccraw-Hil1, pp 442 ,163,1994.)
3.2
.4
Gaussian
urve
for5
quanta
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A EXPERIMENTALPREPARATION
Synaptic ransmissionnd the Neuromuscur
Junction
8 219
SEROTONIN
ELEASE
(mv)
Current
orcatoon
fiber
pA )
80
./Serotonin
Postsynaptic
membrane
/iled
and
pronour]ced
ncrease n transmitter release
hal
, . . - r ' :
I
. t r
i ' , , -
1 -
' d o l r i g l rf c . . u c r r c )
\ ' .
i r - r - u -
larion. This ellect can lasL br minuLcsafter the conclition
lng stimulus.
Petentiatior-l av
be causecl y' a
ll.rtoc,
ol
1 r ,
r e
e
\ L
F i r e . ^ , h I . - L a . e .
(
"
i n
L c
I ' r . - . -
naptic erminaL
nd thus rncreases
he
probability
of exo-
cytosis.
Sl-naplic
depression Ls a lr.l sicnt decrease n the
elfr-
L c r ' . )
o L
J . . r ' r . 1 . . -e l . J
- r r , l .
o r . n , l .
n l
) .
J . , o r \
It ' luoo
L i
"
10msec
Sn-a clea.,a., .a'n,""*ne.h ra,+'v,'*.y-.*.-^
+
vesces
\ l
r.^,,*"*-.-**"*****"*.4
Curreri
\
I
o ' carbon
'
\ /
fioer
pAr
[*.' t
",*"*,,."*.
****,
I
l-*'
{ywdl"airp.n****r,r,,r+,,r,.r*r.y,
' /
: : i : " " "1" ,J: :
. ' ' * * , ' t *u ' t , i . . , t t t - ; , , , - .n^* t /
I
--_1
u
oo
.
10msec
Stimulus
artilact
F | c U R E 8 ] 3 ' D c t e c 1 i o ] r 0 [ s c r o L o n l n d r a L L s l c l c a s c d | r o n s Y n t 1 p t i . v e s i c l e s 1 , . I h c s c r o ( D L n
n c u r n . l n b c d e t e c t e d e l e c t r o c h e n l i c e l l 1 ' u s L n g a c l t b o n | r b e r n r l c r o c L c c L r o c l c I h e c u [ e n | c l r r ] e d b 1 l h e
rccordccl
ront
( b l g h l e \ ' e ] o | 5 c r o t o n i n r e L e a s c ) a n d a [ ( - ^ : - ] . , ' o |
rd illLlsrLiLes
h.rt the Ielease
\ ' e s i C 1 c s l n d l a L g c d c n s c c o l e R s i c I e s ' b o t h o | w h i c h c 1 n l l . l l 1 ) s e r \ c d
.-ensmilrcf
fele.rse rom single
s)'napric
csiclcs
N.{urc 377:62 65. l9g5
)
' C . - . '
r , . h . '
- r r .
f
r r , . d o J - rt
l n ,
- . a n d . n g
o , r r ,
dir iclual
nene
ternrinalsmay
learn"
p.
318)
Synaptic
Vesicles
Package,Store, and Deliver
Neurotransmitters
f n e D h . . o . " , o l - \ n
r . . r
r . r . i . r ' r o nr
h . - r r ,
. r l
t h , n c ' r - . b )
. n J o .
, n , .
7/23/2019 Neuromuscular Junction &Synapses
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22O 8
/
Synaptic
ransmission
nd he Neuromuscular
unction
A B IOGENES]S
Endoplasmic
reticulum
,/+s.
N,4yelin
heath
W)
. Y Y
cts. trans
Golgi
Golgi
---.-.
Endosomes
(veslcles)
ucleus
CELLBODY
Nerve
Terminal
B EXOCYTOSIS
FIGURE 14. Slnihesisrnd ,eclcJmg
f sy'naprLr:
resrcLes
nd hcif
onLcnL
. . ,
q
o r .
p , r L . \ , .
L .
.
i J . r [ ' e J . r
o
.
f r ,
. l L ,
,
LhaL are
homologous
Lo those associatecl \ritlt
s).naptic
vesiclesol higl-tervertebrates.Thus, the processesuncler-
I ) n -
. l u . i o n | c t c t
1 L ,
r J r
' , a
lular exocl,Losis nd encLocytosis.
Synaptic
r.esrcles
are sphelical
organelles
wtrh
a cliame
Ler of
'10
to 200 nm. ,\s
shou,n rn Flgure
8 14A, syrap,
r ic
vesicles are procluced n
rhe neuronal
cel1 boc\, by a
process similar ro Lhc secretory pathway
(p.
36).
- lhus,
port
(p.
26) mediated
1.the microtubule
system, 'hich
alsocarrLes itochondria
o the ten-ninal.
VesiclesdestinedLo c ontain pepti,le eurotransmjtters
travcl
down
the axon with Lhe presynthesizecl eprLdes
r
pcplide
precursors
already .rside. On arrival
at the ner\.c
terninal
(Fig.
8-148). the vesicles-now
calledsynaprlc
vcsicles Lhat carry pepticle
-reurotransmrtters
ecome aL
tached o the acrinbasecL
yLoskeleLaLet\\'orh.
Other ves
. . 1 , . . ' c .
' l c J
, r i h
r o n f . . J n ,
u o d r - n
r ,
r -
c . g . .
Vesicleand peptide
neurotransmitter
recursors
nd
enzymes
rc synlhesizedn ihe
cell and are released
rom Colgi.
Vesicles
ravel through the
axon
on
rnicrotubr e tracks
via
fast axonal ransport.
Peptidc
ncurotransmitters
are
already n somevesicles.
Nonpeptide
neurotransmitters
rc
sy1'Ithesized
nd transported nto
vcsicles he n nerue ern]inal.
A nonpeptide
neurotransmitters
svnihesizedn the nerve erminal
and transported
nto a
vesicle.
**:;oii
a. ) - f l
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OUTSIDE YNAPTIC
ESICLE
INSIDE YNAPTIC ESICLE
Synapticransmission
nd heNeuromuscular
unction
8 221
cles
p..12)
recolers
membrane ompenents
nd recycles
them to an endosorne
omparLmentn Lhc erminal.
Syn-
ap1lc
vesicles
may then be reformed
within rh e rerminal
for
reuse in
neurotransmission,
or the)' ma)' be
trans-
ported
back ro Lhe ccll bod).for tu].nover
nd degrada
Lton
,
i r .
r
r L .
b . u r . ,
d . .
. L u
,
n , n l l , " I ' e n o u .
) s -
tem zrnd heir- elatively'
niform sizeand
molecular om
posit ion,
ynaptic esjcles
an be obrained n largequantr-
t ies
trom
ya
ous sources
uch
as
the rat bra in and
the
eleciricorganol the loipf, lo ray. The puril ication
of
syn,
aptrc
veslcleshas
n.rade t possible
to anal)'zeLheir com,
position, r,vhichhas
hciLitaLedgene cloning
ancl the mo-
lecularcharacterizatjonl nra[y proteins hat are ntrinsic
to
s)'napticvesicle
function. Figure
8 15 sunmarizes
a number ol the n-rajor
lasses f s)'naptic esiciepro-
te ins .
l
.
, L e
,
n l ,
l t . J (
t . , l o 1 a '
r .
1 e
- .
r , ,
u r . .
phshedb1.the combinaLion
l a vacuolar
ype
H' ATPase
' t
l
t .
r
' r r o
, I
r L
t . . n
- o t .
r l r o . c t n .
p
\ a c u o l a r .
type
H+ pump
is a large, n.rulLjsultur.rir
omplex
thar
cat;r1,vzeshe inr'varclmovemenl of H- into
the vesicle,
coupled Lo the hvdrol l,sis
[ cyrosolicATP
to adenosine
diphosphrte
ADP)
and inorgar.riclosphaLc
p.
64). The
resulting pH
ancl
voltage gradients
across
tlte vesicle
' ' _
"
,
. . , . t o l r
) n t L e r J i n L J
the \esicle by a unique fami\'
ol neurotransmitter
trans-
por t pro te ln< l
\ . h"
I
Pur^ l r .n-
r ' l '
-
In
|
.
c )
tosol
for Hf in
the r.esicLe.hls family'
of transporrers
includes members
specilic br ACh, monoanines
(e.g..
serotonin).catccholaminese.g., norepineplrrine), lr,rta
male, i1nd
GABA,/glycine
Another cl onccl sYnaltt ic esicle protein
named
SV2
(Ior
synaptic vesiclcproteln2)
structurally esembles
transpor-t
rotein:
hou'er.er,
transpor-t ubsLrateor
SV2
l.urs
not
been icLentillccl ncl is function
is unknor,r'n.
Synaptobrevin is an lg-kDa
s).naptjc vesicle
prorcin
contalning one transnembrane scgntent.
SlmapLobrevin,
which is a v-SNARE p. 39), ]s essentialor transmitter
ielease.As
discussecln the next section,
slnapLobrei' in
nn
tL.
, t
i , n r
, r ' ,
-
I
' - r ' .
. - r
. r '1p | .1
\ '1
. \ \
. -s
' p r T
o n
r , ,
. . . I 1 ; .
, .
r 1 L r .
, n C
" ,
p - d t \ .
. c .
.
,
l . r .
n l f . , c . ' .
1
r , 1
1 t
1 , " 1 , 1 i . 1 , 1 1 . '
) : r .
B , D , l , . n J
{ .
are
encLoproteinases
hrt digestsynaptobrevin
nd a re po-
, . n t r n h h r n ' - n l - . n , ' r r i . . - ' . 1 ,
' | \ o
) ' o
Rab3 is a mcmber
ol a
large anrily
of lou-molecular-
rveightGTP
binding
proreins
har appears o be unl\,er-
sally invoLvecl n cellular membrane
rafllcking
(p.
39)
via
the blnding and hydroLysisf
GTP.Synaptotagmin s he
slnaptic
:esicle
az' recepLor, proLein
'ith two external
repetiti\'e
domains tl.iaL re homologous
o the C2 clonuin
of
protein l
7/23/2019 Neuromuscular Junction &Synapses
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@
222
8
/
Synapticransmissionnd heNeuromuscular
unction
s).naptic
Yeslcle
proteins that are phosphorylatedby both
cyclic adenosine
monophosphate
(cAMPldependent
and
calmoduLin-dependent
protein kinases. Interactions
of
sl,napsins
wlth
cytoskeletalproteins
and their
inhibition
by
phosphorylation
have led to the notion thar s)'napsins
normally mediate the attachment o[ synaptic vesicles o
lhc ac
n cytos l ,e le to l
W' t l ^ r , rL -edsen
[ (
a
I
; r rd
subsequent
phosphorylation, the synapsin detaches and
permi ts
e . lc
e ' to
ro \e
io a t . i re ' i tes a r ,he ' y .nao- i ,
membrane.
Neurotransmitter
ReleaseOccursby
Exocytosis f SynapticVesicles
Al houg.
rhe
necha-
sn by
wn.L
s1-ap t ic
es ' .
e ,
fu ,e
with the plasma
membrane and release
heir
contents is
far from
fuLLy nderstood, ve have working models
(Fig.
8-16)
for the function of various ke)' co-Oon"rlrr und
steps
involved in s)'naptic
vesicle
release.These models
are based
on a
variety
of
in vitro
experimenls.
The
use of
speciflc
toxins
that acr at nerve
slnapses
and elegant
functional studies of genetic mutanls in Drosphiha, C.
elegans,
nd
gene knockout mice have provided important
information
on the roLeo[ various components.
We
have already
ntroduced
the key proteins located n
the s)'naptic
vesicle. Of these,
we
now focus on the
v
SNARE
yrap tobrer -
and the
La sen)o- )mdPlo lag
min.
ln addition,
several other protelns-Located in the
rarget area
o[ the presynaptic membrane of the nerve
terminal-play
an
important role
in the fusion
process.
S).ntaxin
is anchored n lhe preslnaptic membrane by a
single
membrane-spanningsegment. SNAP-25
(slnapto-
come-eqsocir ' led
rote1
-21
kDa)
i. tethe-ed o tle
pr e
- ) r - ldpr r
memhran(
ia
pc r
to )
,de cLa in ,
BoLh
yr-
taxin
and SNAP-25 are I-SNARES
(p.
39).
Borulinunr
ro^ ; rsA ar rd
t . r r \ch a -c e dop loLea 'e ' .
' pecf i , r1 ly
cleave
SNAP-25,
whereas another endoproteinase,botuli-
num toxin
C],
specifically
cleavessyntaxin.
These
toxins
b lock he us ion l synapt rcestc1c. .
According
to the
model
shown in Figure 8 16, dock
ing of
the vesicle
o the pres)-naptic
membrane occurs as
n Secl
dissoclates
rom slntaxin. The free
ends of s1'nap-
tobrevin,
s)'ntaxin,
and SNAP-25
begin
to coil around
each other.
The result is a ternary compLex,an extraordi-
narily
stable
rod-shaped
structure o[ a helices. As the
energeticaliy
avorablecoiling of the three
SNARES
ontin-
ues, th vesicle membrane is pulled ever closer to the
pres),'naptic
membrane.
Car+
enters
through
voltage-gated
Ca2*
channels
ocated in register with the active zone
of
Lhe
pre .w. rp t ic
membrane.
'o . : l
in . e rse n
[Cr ]
triggers
he
final event, usion and exocytosis.The synap-
ric vesicle
protein synaptotagmin is believed to be the
actual
sensor
of increased
[Ca2+],
because
knockout n-rice
whereas he , l ,n ra^ inand SNAP
5
on lhe p res)Tapl ic
membrane are availabLe
or
the
next round
of
vesicle
fusion.
The
modeL
ust
presented eavesunanswered
some
im-
portant queslions. For example, whal is the struclure of
the fusion pore detectedby electrophysiologicalmeasure-
menLsas a
primary
event
in membrane fusion? Also,
the
model
does not fuily explain the basis for the rapid catal-
ysis of lusion by Ca'z*.Neuroscientists re very interested
in the
deLailso[ synaptic vesicle usion because his exo-
cytotic
process might be a target lbr controlling synapllc
srrengrh
and may thus play a role in the synaptic plasric-
ity that is responsible or changes n animal behavior.
Re-Uptake r Cleavageof the
NeurotransmitterTerminates
Its Action
Eflective transmission across chemical syeapses equires
not only
releaseof the neurotransmitterand
aciivaiion of
the
receptor on the postsynapticmembranebut aLso apid
and e lh . ien tmechdn i ,msor removing he ran .m.L te r t
synapses
where ACh is released, hls removal is
accom-
plished by enzymatic destruction of the neurotransmitter.
However, the more
general mechanism in
lhe nervous
- r . re r
r ro l res
-e- r . t rLe
n f
the
neJroL-dnsr ' t
Ler
ned. -
ated by
specific, high-affinity transporl systems ocated n
the
presynaptic plasma membrane and surrounding glial
cel
.
fl-ese secordaryaclive r-d->po-L ys .emsu5e lhe
normal
ionic gradients
of
Na+, K*, H*, or
Cl to
achieve
concentralive
uptake of transmilter. Vertebrateshave two
ots nct
[a r ' res
o f
neL-oL_a
n
t re r
I rdn5porL roLe >
The first
family is
characterized y
a
common
motif
of
12
membrane-spanning
segments and incLudes
ransporte$
wi ln
>pec
c i t l fo cr rec l -o lan ines.
e roron ,n .
ABA.
1-
c ine .
and ,ho l ine . rnergy coup l ingo I l ranspo l in lh ic
class
o[ 'v,rems is generally asedon tor"ansporto[ the
substrate
with Na* and
Cl
. The second farnily is repre-
sentedby transporters or the excitatory amino acids glu
lamaLe
nd asparLaLe.n
i l -e 'e
a t te '
.1 te ' r ' .
sub ' t -a te
t r rnspor t
Bener r l l )
o
rp l " ,
o
co l ra rcLdrunct ,o r . ALhF acl i l ) can be de-
tected throughout
the nervous system.The enz).meoccurs
in a
variety of physical forms. The globular or G forms
exist
as monomers, din-rers,or tetramers of a common,
approximately
72 kDa glycoprotein catalytic subunit.
These molecules can be lound either
in
soLuble
orm
or
bound
to cer m.nb-a-e .v ia a CPI Ln lage
p.
t5 ) in
7/23/2019 Neuromuscular Junction &Synapses
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E
Vesicles
with svnaptotagrrinand
synaptobrevin
a
V-SNARE)
move
to
the nerve
erminal membrane,
h.hich contains
yntaxinand
SNAP-25
both
SNAREs).
Synaptic
ransmlssionnd he Neuromuscular
unction
8
INITIALSTATE
FORI\,1ATION
F
TEBNARY
COIVIPLEXF SNARES
q-SNAP
and the ATPaseNSI bind
to
the ternary SNAREcomplexand
use
the energyof
ATP
hydrolysis
o
disassemblehe SNARES.
TIGHTENING F TERNARY
SNARECOI\,4PLEX
The entry
of Ca2*and ts binding
to synaptoiagmin
riggers usion.
Synaplotagmin
Synaptic
I
vesrcle_
|
.
,,,.,,,,
,,,
l(
. - t
. ,F
"
' ' a " /
' '
. . t . .
- \ l
.
- : -
)
.::
:.
. .,.. ::
::.:
-
;hravin
-
'
n-Sec-1
l*
?
n'Sec
1
memorane
RECYCLING
FSNARES
0-SNAP\
Zt
n sec 1 dissociatesrom syntaxin,
allowing
the
syntaxin
ard
SNAP-25
o
Iorm
a
complex.The distal end of synaptobrevin
begins o
l\rind
around
the syntaxin/SNAP-
25 complex, onning a temary conplex.
It
The three SNARE5 synaptobrevin,
syntaxinand SNAP-25-continue
to form a tight bundle of o helices,
draliring
he vesicleand presynaptic
membranes nto closeapposition.
Syntaxin
DISASSEI\,lBLY F TERNARY
SNARECOMPLEX
FUSIONNDEXOCYTOSIS
fr
,-il
]
With
the endocytosis f the
vesicle,
he synaptobrevin
is
effectivel)'
ecycled.The
syntaxinand SNAP-25are
now ftee
for an additional
cycleof
vesicle usion.
F | c U R E 8 - 1 6 ' N l o d e ] o | s ] n a ] r r i c ' e s i c ] e l u s i o n a n c | e r o c \ ) S i s , { D P .
scnsiri|r
lacLori sN.{l'-25.
slnaptosone-associared
rorein
2i
l{Da: d
S\AP. solublc
NSF rt(dchlnenr
pforern:
SN-\RF. SNAP
r.LtPio
o-SNAP
D
&
NSF
I
I
. )
7/23/2019 Neuromuscular Junction &Synapses
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8
/
Synaptic ransmission
nd
the Neuromuscular
unction
DISEASES
FTHE
HUMANACETYLCHOLINE
ECEPTOR:
MYASTHENIA
RAVIS
ND A
CONGENITAL
YASTHENIC
YNDROME
The term "myasthenia"means
muscle
weakness
from
the Creek
mys
and asthenia)
nd is
used clinically
o
usually
mean
weaknessn
the absence
f
primary
muscle
disease, europaLhy,
r central ervous
yitem
d'isorder.
Myasthenia
gravls,
one specific
ype of myasthenia
and the mostcommon
adult orm,
afflicts 5
to 125 of
every1 mill ion
people.
t
can occur
at any age
but hasa
bimodaldistribution, ith peak
ncidences
ccurring
among
people
n th eir 20s
and
60s.Those lf l icted
t an
early age tend to be women
with
hyperplasia
f the
thymus, whereas hosewho are older are more
likely o
be
men wilh coexisting
ancer
t the
thymus
gland:
Th e
cellsof the thymus possess
icotinic
acetylcholine
ecep-
tors
(AChRt,
and the
disease
rises sa result
f antibod-
iesdirected gainst
hese eceptors.
he
antibodieshen
lead o skeletalmuscle
weakness
ausedn
Dart
bv com-
petit ive
nLagonism
f AChRs.
ymptomsnclude'fatigue
and weakness f skeletal
muscle.
Two maior
forms of the
disease re recognized:
ne hat
involves eakness
f
only the extraocular uscles nd another hat resultsn
generalized eakness
f
all skeletal
muscles.n
either
(ase,
myastheniaravis
s
ypilied
by
flucLuating
ymp-
toms,
with weaknessrealesl
oward
he end ot the
day
or after exertion. n severe
ases,paralysis
f the respira-
tory muscles an lead to
death. Treatment
directed
at
enhancing holinergicransmission,
loneor combined
with thymectomy r immunosuppression,
s highly
effec-
tive
in most
patients.
Progressowardachieving n understandingf the
causeof myasthenia
gravis
was
first made
when electro-
physiologic
analysis
f involved muscle
evealed
hat the
amplitude f the miniature
nd-plate otential
was de-
creased,although the
frequency
of
quantal
events
was
normal.This inding
suggested
ither
a defect n the
postsynaptic
esponse
o ACh
or a reduced
concentration
of ACh n the synaptic
esicles.
major
breakthrough
occurred n 1973,when
Patrick
nd Lindstrom
ound hat
symptoms imilar o th oseof humanswith myasthenia
developed
n rabbits
mmunized
with
AChR
piotein puri-
l ied rom the eleclric
el.This indinq
was
shortlv ol-
lowed by the demon stration
f anti-AchR
ntibodiesn
human
patients ith
myastheniaravis
nd a severe
e-
duction
n the
surface ensity
f AChR
n the
iunctional
folds.
These nti-AChR
ntibodies
re
directed
gainst
one or
moresubunits
f
the receptor, here
hey
bind
and activate ompl ement
nd
accelerate
esLruction
f
the receptors,The most common target of theseantibod-
ies s a reg ionof the AChR
d subunit
calledMIR
main
immunogenic egion).
Myasthenia
ravis
s now
recognized
o be
an acquired
autoimmune isordern which
he
spontaneous
roduc-
Lionol anti-AchR ntibodies
esulGn
proqressive
oss f
muscleAChRs nd degeneration
f poatju;ctional
olds.
patient's erum).Somepatients ith mya sthenia ravis
havea thymoma
a
tumor of
the thymus
gland)
hat is
often readily eenon routine
hest
adiographs.n
these
patients,
emoval f the
thymoma eads
o clinical
m-
provement
n
nearly75o/o
f the cases.Enhancement
f
cholinergic ctivity
s achieved
hrough he
useof AChE
inhibitors,
ith
pyridostigmine
eing he
mostwidely
usedagent.The dosage
f thesedrugs
must be
carefully
monitored
to
prevent
oyerexposure
f
the remaining
AChRs o ACh.
Overexposure
an lead o overstimuiation
of the
postsynaptic
eceptors,prolonged
depolarization
f
the
postsynaptic
embrane,nactivation
f
neighboring
Nat
channels, nd thus
synaptic
lockade.
Another
condition characterized
y progressive
muscle
weakness
nd fatigue s
the Lambert-Eaton
syndrome
(see
box on
p.
193).Lambert-Eaton
yndromes
caused
by antibodieshat attack
he
presynaptic
a2"
channel
and can be distinguished
rom myasthenia ravis
n sev-
eral
ways.First,
t
primarily
ttackshe limb
muscles,
ot
the ocularand bulbarmuscles. econd, epetit ive
timula,
t ion of a
particular
muscle eads
o enhanied
amplitude
of the
postsynaptic
ction
potential,
whereas n
patients
with myasthenia, epetitive
stimulation
eads
o
progres-
sive essening f
the action potential.
Thus, repeated
muscle t imulationeads
o increasinq
ontractile
trenqth
in
patients
ith Lambert-Eaton
yndr-omend
to decreis-
ing strength n
patients
with
myasthenia.
The term congenital
myasthenic
syndromes
(CMS)
refers o a variety of inheriteddisorders,presentat birth,
lhal aflectneuromuscular
ransmissionn
a
varietv
f
ways.Because
pecif ic ases
an nvolve cetylcholinester-
asedeficienc, abnormal presynaptic
elease
f ACh,
or
defective
AChR
unction
(without
the
presence
f antire-
ceptorantibodies),he signs
and symptoms
an alsovary
widely. n 1995,an
unusual xample
f a CMS
d;sorder
was raced o a mutation
n the
o
subunit
of the human
AChR.Single-channel
ecordings
rom
biopsiedmuscle
i-
bers of a young myasthenicpatient revealeda profound
alterationn AChR
inetics. he
burstduration
of AChR
openingswas
greatly
prolonged
n
comparison
ith that
of normalhumanAChR
hannels. he
molecular
efect s
a
point
mutation
of Thr to Pro
at
posit ion
64
in the
adult subunit
of the AChR. his
bminoacid
esidue
correspondso an
evolutionarily
onserved
osit ion
n the
M2 membrane-spanning
egment,which
is nvolved
n
Jormation
f the channel ore.
Thus,a human
mutation
in the pore egionof the AChRprotein esultsn failure f
the channe l o
closenormall,
thereby ausjng
xcessive
depolarizationnd pathologic
onsequencest
the mus-
cle end
plate.
The aforementioned
utation
s only one
of at least
3
mutationsn
55 different
inshipshat have
been denti-
f ied n the AChR.
Someof
the othe r mutations
esult n
7/23/2019 Neuromuscular Junction &Synapses
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FICURE
B 17. Pharma.oLogr of
rhe rcrlebrate
neuronuscular
jun.
lion N1:uy
of thc pnneins rhal are
jnvohed in slrapl ic lransmissron
at
rhe mammalian
neuromuscLrlar
lLrncrion
a.c
thf 1.rr8ers
l
narLrnll)
occurnng or
slnLLleLi.
drugs
The
anugonEts rfe
sbown
:rs
'
signs
highlithtd
jn
fcd
Thc agonisLs re
shown
as
+'
signs hiilhlighted in
gr..n
. e r n \ d , r c
" h , .
I
\
d \ e . a L e
l f o u p
r
. o . - r e n L
)
. . r .
. l r l , o ' - r i n e u r n r r n n n , h n n n ; . , n f h F - F ^ . d - ' ,
is the
hydrolysLs and release
ol this acetate.
as
\\,eLl
- - r h e t c p
p n / . n .
\ r h
" h - . f F .
r r \
- . o
n , , i
uplake
syslem, he nerve lerminal recovers he chollne
formed in Lhrs eactionand uses t lor the svnthesis f
ACh.
TOXINS
AND DRUGS FFECTING
SYNAPTIC
RANSMISSION
Much ol our
knou'ledge
of the s)'naptic
physiology
of
the
neuromuscularunction and the identltres f its various
molecular
components have
been derived from experi
, r l f . . F , m , n l n o r . o p n , , . , 1 , \ n - L - i
l l e - r
f r .
i " n ; l
d i
- '
o r o h ,
)
p - r
- g . e 8 . 7
illustrates he reLative yl.Iap lic ocation and corresponding
pharmacology
of AChE, as well as several on channeis
irnd
pfoteins
involved 1n
exocytosis.
Synaptic
ransmissionnd the Neuromuscular
onction
8
entire
process.
s cliscussecln Chapter7, LhedepoLarlz-
ing
phase
ol LheacLion
otential s mecliated
y voltage
dependent
Na'
channels hat are specif ically Jocked ,v
nanomolar oncentratiolrsl the small
guanidiniun
neu
rotorins tetrodotoxin
(TTX)
and saxitoxin
(STX)
(see
F ig.7 5C) .
The mamba snaLe to)iin dendrotoxin
(p.
19.1)has
an
effccL haLs precisely pposi Le hatol TTX: it facilitates
tl ' le
release
f
ACh
that
is
evoked by
nerve
stimulation.
1pr ' .
^ r^ 1 \ / ' / ' |
" f
,
) o \ rnr te \ , r . l
e r . tLr -
proteinsuJiih
hreedisulf idebonds that block certain
so
f . m . , f , k . h ' n n l - h . n r n
, r r - . . ] I ' r l i , r
.
. n . h P r . . , J . , t r n
" t \
L 6
. l [
-
'
" h . . .
n r
-
- e
r
-
- . -
I n
- c l .
. , 1
. .
. l ' a . e ,
in terminating he process [ transmltter elease. lockadc
ol
preslnapticK
clrannels l dendrotoxin
nhibiLs epo
l a r z , t n o l
i \ ,
1 - , - 1
, f l . r r ' n ' - h , r c L
; 1 r " l " r r g
I n g l . J r ' i o n o ,
c . , t o
p t (
r .
, r d f u . r t - t . g
L l r c
releasc o[ transmittel in responsc to the entry of extra
Caz-
nLo
he nen'e e rmLnaL.
Acetylcholine
Nicotine
E
d-Tubocurarine
E.r-Bungarotox
unction
7/23/2019 Neuromuscular Junction &Synapses
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226
8
/
SynapticTransmission nd the NeuromLrsculaf
TABLE
A_2
TARGET
"
h r n ,
r
i n l p , i n n
L r
h e c e
l o q 1 q 1 , c d n l e d d t o
death
because he toxins that they slnthesize are potent
inhibitors of neurotransmitter release.This inhibition oc-
cum
becauseboth tetanus and botulinum toxin proteins
harez ' - t -depedenL ndopro re ina.e- r i \ i l y
lab le
8 2)
These toxins enter nene terminals and specificallycleave
three
different proteins required for slnaptic
vesicle exo-
cytosis.
Tetanus
toxin and botulinum toxils B, D, F,
and G
cleave s1-naptobrer.in, n inlegral membrane pro-
tein of the s)'naptic vesicle membrane. Botulinum toxins
( 1
" - A
a r r
. " . - . . , , , . " t . . . , , . c y r t d y r n
n J S \ A l 2 5 .
two
proteins associated
with the
preslrraptic
membrane.
These
neurotoxins can have
uselul
medical
applications.
For example, botuLinum toxin is used Lo treat cenain
disorders characterizedby muscle spasms. njection of a
smal1
amount of botulinum toxin into the eye muscles
of
a patient
with
strabismus
a
condition in which both eyes
cannot
focus
on the same object because of abnormal
hyperactivity of
particular
eye
muscLes)s
able to suppress
aberrant
muscLe pasmsand restorenormal vision-
Both Agonistsand Antagonistsof the
Nicotinic
Acetylcholine
ReceptorCan Prevent
Synaptic
Transmission
The ionotropic
(nicotinic)
AChR channeL ocated in the
posts)'napticmuscle membrane
(see
Fig. 8- 17) aLsohas a
rich and diverse
pharmacology
har can be expLoi ted or
clinical applications,as
weLl
as
for
elucidating many func-
to-a l a .pecr . f t he neuro. rus ,u lar1 . - rn ,.o . . F igu reB-
lB shows the chemical structures of two classes f agents
that
act on the nicotinic AChR. Theseagentsare
cLassifred
as
agonistsor antagonistsaccording to whether they acti-
\ a le
open ingo1 the .hannel or prevent rc ac l iva l ion.
Many agonists have a structure similar to that of the
' , lu rJ l
neuroLrdn) r r . tLereh. In
genera l .
uch agonis t '
activate
the
opening of
AChR
channels
with
the same
u r i t a r y
o n d u , t a n c e.
t h o . e
a . n v a t e d1 A ( h . b u r
^ h
d. f ferenL e l ' c , o l chan ne ope-
rg
a-d
co, ng rhe
synthe