Synaptic Transmission

The point where one neuron hands its signal to the next is called a synapse. The neuron delivering the signal is the presynaptic neuron, and the one receiving it is the postsynaptic neuron; between their membranes sits a narrow gap called the synaptic cleft, which is both a physical gap and a functional bridge — the message has to cross it chemically rather than electrically. The axon terminals of the presynaptic neuron are packed with synaptic vesicles full of neurotransmitter molecules.

When an action potential reaches the axon terminal, it depolarises the presynaptic membrane and this opens voltage-gated calcium channels there. The resulting influx of Ca++ ions is the actual trigger for release: it draws the synaptic vesicles toward the presynaptic membrane, where they fuse with it and dump their neurotransmitter into the synaptic cleft by exocytosis. The neurotransmitter then diffuses across the cleft and binds specific receptors on the postsynaptic membrane, generating a new potential in the receiving neuron. This new potential can go either way — an excitatory postsynaptic potential causes depolarization (making the neuron more likely to fire), while an inhibitory postsynaptic potential causes hyperpolarization (making it less likely to fire) — so a synapse is not just a rel …