Notes

Transmitter Release is Quantal (Chemical signals can work as amplifiers)

• Chemical transmitter based synapses are highly diverse and modifiable (as opposed to electric coupling).
• Post-synaptic effect can be excitatory or inhibitory.
• Transmission has to be fast.
• A consequence of releasing transmitter by the exocytosis of vesicles is that synaptic transmission is quantal.

Transmission in the neuromuscular junction

• Profusion and docking of synaptic vesicles at approx. 1000 active zones.
• Action potential triggers release of transmitter of approx. 300 quanta into the synaptic cleft (approx. 100nm wide).
• Diffusion to postsynaptic receptors and reaching concentrations of 1mM within 2ms.
• Activation of up to 2000 receptors, resulting in ion channel opening.
• Action potential induced release of 300 quanta results in a peak of few 10mVs, enough to generate an action potential in the muscle fiber.

Transmission in Central Synapses (as opposed to the neuromuscular junctions)

• A typical central synapse contains 1-4 active zones.
• Action potential triggers release of transmitter 5 to 10 vesicles.
• Cleft concentration also goes up to approx. 1mM, but typically there are less receptors, each vesicle activating approx. 30 receptors.
• Single evoked responses are in the range of few millivolts at most, clearly sub-threshold for excitatory responses to evoke postsynaptic action potential.

Excitation - Secretion Coupling

• Centrality of Ca2+.
• Coupling is achieved by the use of Ca2+ as intracellular messenger.
• Normal intracellular Ca2+ is buffered very low. Extracellular Ca2+ is in the mM range large driving force after Ca2+ channels open up to 1000-fold increase in intracellular concentration (change intra/extra concentrations to test dependency).
• Probing with Ca2+ loading
• Probing with ca2+ buffering
• Where is the Ca2+ sensor?

Ca2+ Microdomains

• Single active zone may have 100 Ca2+ channels.
• A single vesicle may be <5-nm from as many as 10 Ca2+ channels.
• Most prevalent Ca2+ channels for release are N- and P/Q-type channels.
• The exocytosis trigger must be fast (=fast on/off rate for speed).
• Cooperative Ca2+ binding; multiple Ca2+ sites need to be occupied for efficiency.

Molecular Mechanisms at the Nerve Terminal

• Neurons need an extremely efficient mechanism to recycle and reload vesicles within the terminal. (Otherwise they run out of readily releasable pool of vesicles).
• For most neurotransmitters, vesicles are refilled in approx. 30sec.
• A CNS synapse have only 2-20 fusion ready vesicles. These need to be replaced within seconds.

Structure and Topology of Major Synaptic Vesicle Membrane Proteins

• Protein compositions of synaptic vesicles are remarkably similar, independent of the neurotransmitter. (Extreme richness of different proteins).

SNARE Proteins and Core Complex are Key to Membrane Fusions

Quantal Analysis: Probing Synaptic Physiology (example: Neuromuscular Junction) In the 60's they noticed that by recording synaptic signals in the neuromuscular junction, an histogram of the distribution of potential amplitudes showed multiple peaks. It depends on the number of vesicles released during a synaptic release.

Short-Term Synaptic Plasticity Repetitive activation of the synapse, depending on the interval between the two stimulations, we can see a larger or smaller action potential amplitude in the second event, which are called "facilitation" (depends on the already higher concentration of calcium) and "depression" (which could be caused by a desensibilization of the synaptic site or depletion of the neurotransmitters vesicles).