Speaker: Stefan Hallerman (Carl Ludwig Institute for Physiology, University of Leipzig)
Subject: Presynaptic Calcium Dynamics at Central Synapse
Author: Jasper Veerman
Location: Lecture Hall 4, Erasmus Medical Center Rotterdam
Date: Monday, March 2 17:00-18:00
The cerebellum is the part of the central nervous system, and is primarily responsible for the coordination of movement. Many neurons are connected to this region, whose input axons are called mossy fibers. The fast transmission of signals in synapses is important for processing information quickly. Using high frequencies to encode signals, mossy fibers can act at speeds of 800 Hz. In 3-6 week old mice the action potential is in the range of . Using artificial stimulation, the highest possible frequencies at which the signal is still properly passed on, is just above 1.6 kHz.
It is fascinating try to understand how calcium vesicles, transmitters of the signal, can fuse so synchronously in such small time intervals. In trying to do so, the vesicles were imaged using several calcium indicators. Of course, each of these indicators reduces the freedom of movement, so only a combination of several calcium indicators allows for back extrapolation of the endogenous – without indicators – situation. From this experiment, it was concluded that the cytosolic solution does not capture much of the calcium coming in.
In trying to explain this, further calcium transmission measurements had to be done at high frequency bursts. It was found that after each action potential, the calcium concentration increased, saturating after 20 action potentials. This saturation poses a problem for passing on additional signals, since after these 20 signals, an additional signal will not result in an increase of calcium concentration, making it hardly detectable. Therefore, at these high firing rates, the system approaches its limit. It can cope with the first few signals, but needs some time to recover afterwards.