synaptic transmission

• Synapse

  Each neuron is separated from the next by an extremely tiny gap called the synapse.
• Chemical transmission

  Signals within neurons are transmitted electrically, but signals between neurons are transmitted chemically across the synapse. When the electrical impulse reaches the end of the neuron (the presynaptic terminal) it triggers the release of neurotransmitter from tiny sacs called synaptic vesicles. Once a neurotransmitter crosses the gap, it is taken up by a postsynaptic receptor site on the next neuron, so the impulse only ever travels in one direction. The chemical message is converted back into an electrical impulse and the process of electrical transmission begins.
• Neurotransmitters

  Neurotransmitters are chemicals that diffuse across the synapse to the next neuron in the chain. Many neurotransmitters have been identified. Each has its own specific molecular structure that fits perfectly into a postsynaptic receptor site, like a lock and key. Each has specific functions e.g. serotonin affects mood and social behaviour which is why it has been implicated as a cause of depression.
• Excitation and inhibition

  Neurotransmitters generally have either an excitatory or inhibitory effect on neighbouring neuron.

  .Adrenaline - generally excitatory, increasing the positive charge of the postsynaptic neuron, making it more likely the postsynaptic neuron will fire .

  .Serotonin - generally inhibitory, increasing the negative charge of the postsynaptic neuron, making it less likely the postsynaptic neuron will fire.

  .Dopamine - unusual neurotransmitter as it is equally likely to have excitatory or inhibitory effects of the postsynaptic neuron.
• Summation

  Excitarory and inhibitory influences are summed and must reach a certain threshold in order for the action potential of the postsynaptic neuron is less likely to be triggered. If the net effect of the neurotransmitter is inhibitory then the postsynaptic neuron is less likely to fire. It is more likely to fire if the net effect is excitatory.