Mechanics of Presynaptic Transmission

<br> A group of phosphotungstic acid based fixatives demonstrated chains of synaptic vesicles in the presynaptic bulb of the motor endplates of the rat diaphragm and produced precipitates in those vesicles. PTA fixatives containing trichloroacetic acid resulted in precipitate in the neurotubules. An hypothesis was offered that the neurotubules were like sausage casings manufactured in the body of the nerve, filled with bound Ach and pushed down the axon by accretion until they reached the membrane wall of the synaptic bulb. The environment of the synaptic bulb is such that the tubule walls are weakened and surface tension forces pinch off vesicles. There is a competition between the breakdown of tubules and their advancement to the presynaptic membrane such that not only vesicles are excytosed but on rare occasions also tubules. This results in the resting stage mini-potentials described by Fatt and Katz. A second form of transmitter release is non-exocytotic. In this case inflow of calcium ions through the axon wall as a result of the AP reversal of the membrane potential cause an osmotic bursting of those vesicles in the vicinity of the flow with expulsion of transmitter which can be seen as streaks running from the interior of burst, now horseshoe-shaped vesicles, which flow across the inter-membrane space and enter into the muscle. Fig. 263 Barrage An attempt is made to unify the various observations into a comprehensive theory of the mechanics of presynaptic transmission.