Synapse
The same presynaptic train delivered to three receptor types. A synapse contributes a CONDUCTANCE, not a current — which is why inhibition can work by opening a channel that barely moves the membrane at all.
Depolarise past −70 and NMDA wakes up. Past 0 and AMPA reverses.
Short-term plasticity
1st 1.00× → last 0.17×
Conductance
- AMPAτ 0.2/2 ms · E0 mV
- NMDAτ 2/100 ms · E0 mV
- GABA-Aτ 0.5/6 ms · E-70 mV
Current at -65 mV
Outward-positive. Below its reversal potential a conductance drives the membrane up; above it, down. GABA-A at −80 mV is DEPOLARISING — which is exactly what it is in immature neurons, where chloride is high.
The NMDA magnesium block
Jahr & Stevens (1990). At rest a magnesium ion plugs the pore, so glutamate opens almost nothing; depolarisation pushes it out. That makes NMDA a coincidence detector requiring transmitter AND depolarisation together — the molecular basis of Hebbian learning, in one line of algebra.