Concept preview

What you will be able to do

1. 01Describe the structure of a synapse, including the synaptic cleft and the pre- and post-synaptic membranes.
2. 02Explain how an impulse crosses a synapse: Ca²⁺ influx, neurotransmitter release, and binding to receptors.
3. 03Distinguish excitatory from inhibitory signals and explain how neurotransmitter is removed.
4. 04Explain spatial and temporal summation and how a neuron 'decides' whether to fire.

In the last topic an impulse raced down an axon to the axon terminals — and then it hit a wall. Neurons do not actually touch. Where one neuron meets the next there is a junction called a synapse, and across the middle of it runs a fluid-filled gap about twenty nanometres wide called the synaptic cleft. An electrical impulse cannot simply jump this gap, so at almost every synapse in your body the message is handed over by an entirely different means: it is converted from an electrical signal into a chemical one. The neuron sending the message is called the presynaptic neuron, and the one receiving it is the postsynaptic neuron. Understanding what happens in this microscopic gap is understanding how thoughts are passed from cell to cell — and it is also where almost every drug that affects the mind, from caffeine to antidepressants, does its work. The numbers involved are staggering: a single neuron in your brain may form tens of thousands of these junctions, so the roughly eighty-six billion neurons of the human brain are woven together by something like a hundred trillion synapses. Each one is a controllable switch, and it is in this vast, adjustable web of tiny gaps — far more than in the neurons themselves — that the real power and subtlety of the nervous system lie.