Synaptic Transmission at the
Skeletal Neuromuscular
Junction
The synapse.
synapse
y p . Transmitter release ((Ca2+-dependent
p
exocytosis
y
of acetylcholine)
acetylcholine).
y
). Postsynaptic
y p
potentials.. Acetylcholine receptors (ligand
potentials
(ligand-- gated ion channel)
channel).. Termination of the signal
(acetylcholine esterase)
esterase)..
Dr. Sergey Kasparov – room E9
The main text for this lecture is:
Vander’s Human Physiology
NOT well covered in Stanfield&Germann
From the previous lecture:
• Action potentials rely on concerted action of
voltage--gated Na+ and K+ channels:
voltage
– Positive feedback depolarisation and opening of
rapidly inactivating Na+ channels
– Repolarisation supported by depolarisation
depolarisation--gated
K+ channels
• Action potentials travel down from the soma to
axon terminals
The chemical synapse
… mediates signal propagation from neurone to neurone or from
neurone to skeletal muscle
1
The neuromuscular junction (NMJ)
The neuromuscular junction (NMJ)
mitochondria
synaptic vesicles
(ACh
ACh))
synaptic cleft
postsynaptic
membrane
presynaptic
membrane
active zone
voltage-gated
voltageCa2+ channels
…is a synapse between a presynaptic neurone and a
postsynaptic muscle cell
Two types of ion channels which play key
roles in synaptic transmission
Gated by
by::
• Membrane
potential
((voltage
(voltageg -g
gated
channels)
no
o
anions
•Signalling
moleucles
(ligandligand-gated
channels)
2
Voltage--gated cation channels
Voltage
+
depolarisation
+
out
in
refractory
time at
repolarised
membrane
potential
short time at
depolarised
membrane
potential
+
Na+ channels - very rapid inactivation … Ca++ channels – a bit slower..
Crucial for synaptic transmission:
Voltage--gated Ca2+ channels
Voltage
outside
inside
[mM]
[mM]
Na+
145
15
Cl-
115
4
K+
4
140
1.8
0.0001
Ca2+
A-
(to balance)
(organic anions)
Ca2+
The concentration gradient for
is very steep
and intracellular Ca2+ is kept very low
Transmitter release by
Ca++-dependent exocytosis
vesicles
filled with
neurotransmitter
+
Ca2+
Voltage-Voltage
dependent
Ca2+ channels
Ca2+
SNARE
proteins
t i
Ca2+
Ca2+
SNARE:
(Soluble NSF Attachment Protein Receptors) – a
family of proteins crucial for vesicular fusion and
vesicular transport in mammalian (and not only) cells
3
Transmitter release by
Ca++-dependent exocytosis
vesicles
filled with
neurotransmitter
+
Ca2+
SNARE
proteins
t i
neuroneurotrans-trans
mitter
Ca2+
Ca2+
Ca2+
Voltage-Voltage
dependent
Ca2+ channels
Exocytosis!!
Exocytosis
neuro-neuro
transtransmitter
Ca2+
Transmitter release by
Ca++-dependent exocytosis
vesicles
filled with
neurotransmitter
+
Ca2+
SNARE
proteins
t i
Ca2+
Voltage-Voltage
dependent
Ca2+ channels
Ca2+
Acetylcholine (ACh
(ACh))
- the transmitter at the NMJ
CH3
C O
O
CH2
CH2
H3C N+ CH3
CH3
acetyl
choline
acetyl
transferase
AcetylAcetyl
-CoA
ACh
CoA
ACh
ACh
choline
carrier
choline
CAT
choline
ACh
choline
ACh vesicular
uptake
.
4
Acetylcholine (ACh
(ACh)) is a
transmitter in:
- in the somatic nervous system (NMJ)
- in the autonomic nervous system
• in autonomic pre
pre--ganglionic neurones
• in parasympathetic post
post--ganglionic neurones
- in the central nervous system
Ligand--gated ion channels - molecular structures
Ligand
5x
(from: Goodman & Gilman 1996; Hille 2001)
The nicotinic acetylcholine receptor ((nAChR
nAChR))
– a ligandligand-gated ion channel
+
out
in
+
prolonged
agonist
exposure
nAChR is permeable to Na+ and K+ ions.
longer--term process than inactivation.
Desensitisation is a longer
5
The postsynaptic response: endplate potential
out
in
Na+ 145
K+
4
Em = 61.5 x log
ACh
ACh
[mM]
155
PK+ [K+]out + PNa+ [Na+]out
PK+ [K+]in + PNa+ [Na+]in
action potential
50
Em (mV)
12
endplate potential
(e.p.p
e.p.p.)
.)
0
Na+
channel
threshold
-50
nAChR
Muscle fibre
-100
0
2
4
6
8 time (ms)
Miniature and graded end plate potentials
Neuromuscular transmission –
Katz and coco-workers (60’s):
V
intracellular
microelectrode
Muscle fibre
0 Ca2+
‘more’
‘
’C
Ca2+
ACh
ACh ACh
ACh
Graded e.p.p
e.p.p:
p p:
multiple vesicles fuse after
presynaptic action potential
(8--10,000 ACh molecules ~100mM)
(8
‘little’ Ca2+
Miniature e.p.p:
e.p.p:
unitary signal “quantal “transmitter release
when single vesicles fuse
ACh
0 Ca2+
Concept of exocytotic
transmitter release
Termination of the transtrans-synaptic signal by acetylcholinesterase (AChE
AChE))
in the synaptic cleft
esteratic
anionic
CH3 O H2 CH3 CH3
O C
N+
C
C
CH3
OOH H
2
OH
COOOH
His
ACh
Glu
Ser
ACh
up to 600 000
molecules/min!
Em (mV)
50
0
-50
nAChR
e.p.p.
-100
0
Muscle fibre
2
4
6
8 time (ms)
…
6
Function of the skeletal NMJ
MOVIE
-1
MOVIE
-2
Chemical_synapse
NeuroMuscular
Diseases of NMJ
Myasthenia Gravis - (usually) autoimmune destruction of ACh receptors at
NMJ
Lambert--Eaton syndrome - autoimmune destruction of Ca2+ channels at the
Lambert
motor nerve endings
Numerous toxins affect NMJ
Latrotoxin (black widow spider) – triggers ACh release (muscle spasm )
Crotoxin (rattlesnake) – inhibits ACh release (flaccid paralysis)
Botulinus toxin (bacterial) – inhibits release of ACh
ACh..
Curare (plant poison) – blocks ACh receptors on skeletal muscles
War gases – block AChE – cause ACh buildbuild-up, spasm and them paralysis
Summary:
•The NMJ is a chemical synapse between a motor neurone neurone and
skeletal muscle fibre
•Communication between these two cells is carried out by Ach
• Release of Ach is initiated by the arrival of an action potential propagating
along the axon of the motor neurone.
• Depolarisation of the nerve endings leads to opening of presynaptic
voltage--gated Ca2+ channels and transmitter release by Ca2+-dependent
voltage
vesicle exocytosis
• Postsynaptic ligand
ligand--gated ion channels (“nicotinic” acetylcholine receptors)
open and let Na+ ions into the cell, thus causing depolarisation.
•Action potential is then generated on the membrane of the skeletal muscle
cell, this allows Ca2+ entry into the muscle cell.
•Ca2+ leads to muscle contraction.
•Action of ACh is terminated by ACh esterase
7
Test yourself (optional)
What would happen to the muscle fibre membrane
potential (and function) if AChE were blocked by
a drug and could not break down ACh
ACh?
? Consider
function of postsynaptic ligand
ligand-- and voltage
voltage-gated ion channels.
Test yourself (optional)
What would happen to the muscle fibre membrane potential (and
function) if AChE were blocked by a drug and could not break
down ACh
ACh?
? Consider function of postsynaptic ligand
ligand-- and voltagevoltage-
gated ion channels.
Initially, muscle contraction will be prolonged – spasm!
After a few milliseconds
milliseconds, due to the persistent depolarisation,
depolarisation
voltage--gated Na+ channels will inactivate – the epp signal will no
voltage
longer be transmitted over the entire muscle fibre – flaccid
muscle paralysis.
Eventually, nAChRs will also desensitise – no more epp signals will be
generated upon presynaptic ACh release – prolonged flaccid
paralysis.
Some organo
organo--phosphate pesticides and nerve gases work this way, but
also some clinically used drugs to improve muscle function in
myasthenia gravis, a progressive neuromuscular disease.
8