Download - Neuromuscular transmission
Anatomy
Critical to function
10m 10m
NMJ on the muscle fiber
Synaptic selectivity at developing NMJ
Synapse from a frog sartorius neuromuscular junction showing vesicles clustered in the active zone, some docked at the membrane (arrows). (from Heuser, 1977)
Synaptic Transmission
The Steps
• Precursor transport• NT synthesis• Storage• Release• Activation• Termination ~diffusion, degradation,
uptake, autoreceptors
Synaptic Transmission Model
PresynapticAxon Terminal
PostsynapticMembrane
Terminal Button
(1) Precursor Transport
_ _ _
NT
(2) Synthesis
enzymes/cofactors
(3) Storage
in vesicles
A quantum is the number of transmitters released from a single synaptic vesicle
Vesicles have a fairly uniform size and diameter ≈ 40- 50 nm
Individual vesicles contain 8000 - 10,000 phospholipid molecules and several proteins. The vesicle molecular weight is approx. 3-5 x 106
Proteins associated with synaptic vesicles(identified through sequencing and cloning of cDNA’s)
Membrane proteinsA. Synaptophysin (~ 36 kD)B. Synaptotagmin (~ 61 kD; the Ca2+ sensor)C. Snares (residents of either the vesicle [v-snare] or the target membrane [t-snare])
1. VAMP (also called synaptobrevin), a v-snare (~18 kD)2. Syntaxin, a t-snare that also associates with Ca2+
channels (~32 kD; technically not a vesicle protein)3. SNAP-25, a t-snare (~25 kD; also technically not a
vesicle protein)D. Electrogenic proton ATPase -creates emf that drives
neurotransmitter uptake against a concentration gradient
Proteins associated with synaptic vesicles(identified through sequencing and cloning of cDNA’s)
Membrane proteinsA. Synaptophysin (~ 36 kD)B. Synaptotagmin (~ 61 kD; the Ca2+ sensor)C. Snares (residents of either the vesicle [v-snare] or the target membrane [t-snare])
1. VAMP (also called synaptobrevin), a v-snare (~18 kD)2. Syntaxin, a t-snare that also associates with Ca2+
channels (~32 kD; technically not a vesicle protein)3. SNAP-25, a t-snare (~25 kD; also technically not a
vesicle protein)D. Electrogenic proton ATPase -creates emf that drives
neurotransmitter uptake against a concentration gradient
An alternative form of Ca2+-dependent vesicle fusion, termed fast tracking, or “kiss and run” predominates at low frequency stimulation.
Life cycle of a synaptic vesicle
Synapse
Terminal Button
Dendritic Spine
Synapse
Terminal Button
Dendritic Spine
(4) Release
Receptors
Synapse
Terminal Button
Dendritic Spine
AP
Ca2+
Exocytosis
From Kristin Harris Lectures.http://synapses.mcg.edu/lab/harris/lectures.htm
1X
4X
2X
Stimulation
1 mV
1X
2X
3X
4X
mini Mini histogram.
Evoked amplitudes.
Squire Fund. Neurosci.
From Kristin Harris Lectures.http://synapses.mcg.edu/lab/harris/lectures.htm
Electron micrographs of “omega figures” (fusing synaptic vesicles) after slam freezing a firing synapse provided clinching evidence for the vesicle hypothesis.
No firing
Firing
Heuser and Reese, 1981
“docked”
“fast”“slow”
A cholinergic synapseNerve fiber (axon)
Action potential
Choline
Na+, Cl-
Acetyl-CoA
Acetyl-Choline
Acetyl-Choline
Ca + +
Ca + +
A cholinergic synapse (2): Rapid transmitter inactivation by cholinesterase
Acetyl-Choline
Cholineesterase
Acetate
Acetyl-CoA
Choline
Action potential
Ca + +
(5) Activation
(1) Ionotropic ChannelsneurotransmitterNTChannel
Ionotropic Channels
NT
Pore
Ionotropic Channels
NT
Ionotropic Channels
NT
Acetylcholine Receptor
(or )
Miyazawa, A., Y. Fujiyoshi, and N. Unwin. 2003. Structure and gating mechanism of the acetylcholine receptor pore. Nature 423:949-955.
ACh
ACh
45
End Plate Potential (EPP)
Outside
Inside
Muscle membrane
Presynapticterminal M
uscl
e M
embr
ane
Volta
ge (m
V)Time (msec)
-90 mV
VK
VNa
0
Threshold
Presynaptic AP
EPP
The movement of Na+ and K+
depolarizes muscle membranepotential (EPP)
ACh Receptor Channels Voltage-gatedNa Channels Inward Rectifier
K Channels
• Normally, the average EPP amplitude = 60 mV -In frog, ~150 vesicles
• Safety factor for transmission is therefore high (greater than 1) - Frog example: VEPP VAPthreshold
= 60 mV │-90 mV*- [-50 mV] │
= 60 mV 40 mV = 1.5
(*muscle resting VM = -90 mV)
Normal EPPs invariably evoke muscle action potentials
(6) Termination
(6.1) Termination by... Diffusion
(6.2) Termination by...Enzymatic degradation
Acetylcholine Metabolism
AChacetylcholine
esterase (AChE)choline + acetate
• AChE is located in the synaptic cleft• Choline is taken back up into the presynaptic terminal – active process• Acetate diffuses away to be utilized in other metabolic roles
(6.3) Termination by... Reuptake
(6.4) Termination by... Autoreceptors
A
The Safety Factor !!!
• Number of Quanta
• The receptor density on the post synaptic membrane
• The activity of ACH esterase
• The folds of the PS membrabe
• The presence of active zones
Voltage-gated channels
Na+ channelopathies
Gene Channel Disease
Muscle SCN4A subunit of NaV1.4 Hyperkalaemic periodic paralysisHypokalaemic periodic paralysisParamyotonia congenitaPotassium-aggravated myotoniaMyotonia fluctuansMyotonia permanensetc
Neuronal SCN1A subunit of NaV1.1
(somatic)
Generalised Epilepsy with Febrile Seizures + (GEFS+), Severe myoclonic epilepsy of infancy (SMEI)
SCN2A subunit of NaV1.2
(axonal)
GEFS+
SCN1B 1 subunit
Ca2+ channel structure
2
1
Ca2+ channelopathies
Gene Channel Disease
Muscle CACNA1S subunit of CaV1.1 HypoK periodic paralysisMalignant hyperthermia
RYR1 Ryanodine receptor (sarcoplasmic channel)
Malignant hyperthermiaCentral core disease
Neuronal CACNA1A subunit of CaV2.1
(P/Q-type channel)
Familial hemiplegic migraineEpisodic ataxia type 2Spinocerebellar ataxia type 6Absence epilepsy?
CACNA1H subunit of CaV3.2
(T-type channel)
Absence epilepsy
Nicotinic receptor channelopathies
Gene Channel Disease
Muscle CHRNA1 1 subunit Congenital myasthenic syndrome
CHRNB1 1 subunit
CHRND subunit
CHRNE subunit
Neuronal CHRNA2 4 subunit AD nocturnal frontal lobe epilepsy
CHRNB4 2 subunit
Slow channel syndrome
Sine et al (1995)
Fast channel syndrome can be associated with congenital joint deformities (arthrogryposis multiplex)
Brownlow et al (2001)