Crossing the Synaptic Cleft
Neurons never touch. They communicate across the synaptic cleft, a microscopic gap between cells. When an action potential arrives, voltage-gated calcium (Ca2+) channels open, driving Ca2+ influx. This causes synaptic vesicles to fuse with the membrane and release neurotransmitters.
The released molecules cross the cleft and bind postsynaptic receptors. Binding triggers an EPSP (excitatory postsynaptic potential, depolarizing via Na+ influx) or an IPSP (inhibitory postsynaptic potential, hyperpolarizing via Cl- influx or K+ efflux).
| Receptor Family | Mechanism | Speed & Duration |
|---|---|---|
| Ionotropic | Ligand-gated ion channels | Fast, brief |
| Metabotropic | G-protein-coupled | Slow, long-lasting |
Summation and Clearance
A single EPSP rarely reaches threshold. Summation solves this through two distinct mechanisms: temporal summation adds signals from one synapse firing rapidly, while spatial summation combines inputs from multiple synapses simultaneously at the axon hillock.
After signaling, neurotransmitters must be cleared immediately via reuptake (pumping molecules back into the presynaptic terminal) or enzymatic degradation (destroying them in the cleft, like acetylcholinesterase).
Common pitfall: Assuming every synaptic signal is excitatory. Roughly half of all synaptic traffic consists of inhibitory IPSPs. Cognition depends just as heavily on well-timed inhibition as on excitation.