Communicating and integrating

The synapse

The electrical message stops at the end of the axon. To reach the next neuron it must change nature: the synapse is a chemical relay.

The structure

   PRE-synaptic neuron
        |
        v
    [  vesicles ●●●  ]
    [                 ]
    ─────────────────── membrane
         ○  ○  ○          <- neurotransmitters in the CLEFT (20-40 nm)
    ═══[R]══[R]══[R]═══   <- receptors
   POST-synaptic neuron

The two cells do not touch: a gap of a few tens of nanometres separates them, the synaptic cleft.

The mechanism

1. the action potential reaches the terminal
2. vesicles release a NEUROTRANSMITTER into the cleft
3. it crosses by simple diffusion
4. it binds to RECEPTORS on the next cell
5. this changes the potential of the receiving cell
6. the neurotransmitter is then degraded or taken back up

Step 6 is essential: without rapid removal the signal would stay switched on. Transmission takes about a millisecond.

This chemical relay also explains why the message travels one way only: only the terminal has vesicles, only the next cell has receptors.

Excitatory or inhibitory

EXCITATORY SYNAPSE   brings the receiving cell closer to threshold -> helps
INHIBITORY SYNAPSE   moves it away                                 -> hinders

A neuron receives thousands of synapses, of both kinds. It constantly adds up these influences:

   sum of excitations - sum of inhibitions
              |
              +--> above threshold?  ->  action potential
              +--> below?            ->  nothing

This summation is the basic operation of nervous processing. A neuron is not a mere relay: it is a decision unit weighing thousands of inputs before producing a binary output. Artificial neural networks copy this scheme directly — weighted sum, then threshold.

Substances acting on synapses

The synapse is the point of action of almost every psychoactive substance, medicines and drugs alike.

CURARE           blocks muscle receptors  ->  paralysis
                 (hunting poison, later anaesthesia)

ANTIDEPRESSANTS  block serotonin reuptake
                 -> it stays longer in the cleft

COCAINE          blocks dopamine reuptake
                 -> intense stimulation, then depletion of reserves

NICOTINE         mimics acetylcholine at its receptors

ALCOHOL          strengthens inhibitory synapses -> general slowing

A principle emerges: these substances create nothing. They mimic a neurotransmitter, block a receptor, or prevent a removal. All hijack existing mechanisms — which also explains tolerance: the neuron adapts by changing its number of receptors.

Summary