How does the signal in the presynaptic nerve terminal relate to the post-synaptic response?
Amplitude of presynaptic action potential varied directly with post-synaptic response: Can partially block with TTX, to get small APs Below a certain amplitude — nothing: above around 40 mV the bigger the AP, the bigger the PSP. Could block all Na channels with TTX and still get response — so it wasn’t dependent upon sodium influx; could block K channels with TEA and get response: So it’s not movement of Na or K IV. Where is calcium acting and what is it doing? For many years, people actually thought calcium was acting extracellularly But if you voltage clamp the nerve terminal in the presence of TTX and TEA, you get an inward current that’s carried by calcium There are some interesting implications of what you see under these conditions: First of all, calcium enters the terminal (in ward current) Release gets bigger as you depolarize, but then shuts off when you get to ECa– ie it varies with inward Ca current No inactivation Release very rapid as you turn off the pulse — implies a
