The Synapse: Where Signals Become Decisions
The gap between neurons is where the computing happens — chemical votes, counted in millivolts, decide whether the next cell fires.
Builds on: 8.1 The Neuron: Electricity from a Battery of Ions
The signal changes vehicles
Between one neuron and the next is a gap of about 20 nanometres — far too wide for the voltage spike to jump. So the signal changes vehicles. An arriving spike makes the sending terminal release neurotransmitters — small molecules stored in bubbles — which drift across the gap and dock onto receptor proteins on the receiving side, opening ion channels there. Electrical → chemical → electrical, in about a millisecond.
Why bother with the detour? Because a gap can do things a solid wire cannot: it can be strengthened, weakened, blocked, and — crucially — it can vote no.
Excitation, inhibition, and the vote
Some synapses are excitatory: their transmitter opens channels that nudge the receiving neuron’s voltage up toward threshold. Others are inhibitory: they push the voltage down, away from it. A typical neuron collects thousands of both kinds, and their effects sum — close together in time and space, plus votes and minus votes all land on the same membrane. Cross −55 mV and the axon fires; fall short and nothing happens.
That summing-with-a-threshold is a genuine computation — engineers borrowed exactly this arithmetic for artificial neural networks. Everything your brain does is built from roughly 86 billion of these voting machines, wired by about a hundred trillion synapses.
Learning lives in the gap
Synapses are adjustable: use one repeatedly and it grows stronger — more transmitter released, more receptors listening. Neuroscientists compress this into “cells that fire together wire together.” Memories are not filed in some storage organ; they are patterns of strengthened and weakened synapses. When you finish this course, what has physically changed in you is synaptic weights.
Why drugs act here
A chemical gap is a chemical target, which is why nearly every mind-altering substance works at the synapse. Caffeine blocks the receptor for adenosine, an inhibitory “sleep pressure” signal — blocking an inhibitor feels like a stimulant. SSRIs slow the clean-up of serotonin, so each release lingers longer. Botox blocks transmitter release at nerve–muscle synapses: the vote never arrives, and the muscle relaxes. Same machinery, three different levers.
Axons are fast, but every synapse costs about a millisecond, and a stimulus-to-button-press chain crosses many of them — plus the brain’s own deliberation. Sprinters who leave the blocks under 100 ms after the gun are disqualified: no human chain of synapses is that short, so they must have jumped.
⚗️ Lab — The Voting Machine
Excitatory inputs vote +4 mV, inhibitory ones −3 mV. The threshold counts the ballots.
- With no inhibition, find the minimum excitatory votes that make the neuron fire.
- Add inhibition until it falls silent again — the sum is all that matters.
- Try the stimulant and sedative: neither touches excitation, they only re-weight the no-votes.
Work it by hand — 0 / 1
No multiple choice here: compute the value and type it. Suffixes like 2.5k or 20m are understood; answers within ±2% count.