12. Synapse from a frog sartorius neuromuscular
junction showing vesicles clustered in the active
zone, some docked at the membrane (arrows).
(from
Heuser,
1977)
19. 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
20.
21. Proteins associated with synaptic vesicles
(identified through sequencing and cloning of cDNA’s)
Membrane proteins
A.
B.
C.
Synaptophysin (~ 36 kD)
Synaptotagmin (~ 61 kD; the Ca2+ sensor)
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 Ca 2+
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
22. Proteins associated with synaptic vesicles
(identified through sequencing and cloning of cDNA’s)
Membrane proteins
A.
B.
C.
Synaptophysin (~ 36 kD)
Synaptotagmin (~ 61 kD; the Ca2+ sensor)
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 Ca 2+
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
23. An alternative form of Ca2+-dependent vesicle fusion, termed fast tracking, or
“kiss and run” predominates at low frequency stimulation.
34. From Kristin Harris Lectures.
http://synapses.mcg.edu/lab/harris/lectures.htm
35. “docked”
No firing
“fast”
“slow”
Firing
Heuser and Reese,
1981
Electron micrographs of “omega figures” (fusing synaptic vesicles) after slam freezing a
firing synapse provided clinching evidence for the vesicle hypothesis.
45. End Plate Potential (EPP)
Presynaptic
terminal
VNa
Muscle Membrane
Voltage (mV)
The movement of Na+ and K+
depolarizes muscle membrane
potential (EPP)
0
EPP
Threshold
-90 mV
VK
Presynaptic
AP
Time (msec)
Outside
Muscle membrane
Inside
ACh Receptor Channels
Voltage-gated
Na Channels
Inward Rectifier
K Channels
45
46. Normal EPPs invariably evoke muscle action
potentials
• 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)
54. 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
63. Fast channel syndrome
can be associated with congenital joint
deformities (arthrogryposis multiplex)
Brownlow et al
(2001)
Editor's Notes
neuro4e-fig-08-11-0.jpg
Cholinesterase provides for a very rapid mechanism of inactivation. This makes acetylcholine a preferred transmitter where rapid modulation of the signal is desired. Example: Voluntary muscular action.
ACh: acetylcholine
AChR: acetylcholine receptor is the nicotinic hetero(and penta)meric muscle form, which has (1)2,,, fetal subunit composition and (1)2,,, adult subunit composition
Endplate potentials (EPPs) are larger, compound MEPPs that result from the activation of many of the terminals of an endplate at the same time. The average EPPs results from the summation of ~150 average MEPPs. The average EPP amplitude is therefore ~+60 mV (0.4 mV/quantum X 150 quanta). If the resting VM of the muscle fiber is -90 mV and the threshold for action potential initiation is -50 mV, a single average EPP will provide 1.5X the voltage required to change VM to the action potential threshold! The normal “safety factor” for neuromuscular excitation is therefore quite high, ~2. That means that under normal circumstances, a muscle action potential will always be generated, and the muscle will contract, as a result of a single EPP. If the fiber is repeatedly stimulated over a very long period of time, some failures will be observed, because the axon will start to run out of vesicles and/or the muscle will fatigue.