Synapses

• each neuron is separated from the next by a tiny gap called the synapse.
• chemical transmission

  signals within neurons are transmitted electrically; however 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 the neurotransmitter crosses the gap, it is taken up by the postsynaptic receptor site on the next neuron. the chemical message is converted back into an electrical impulse and the process of electric transmission begins.
• neurotransmitters

  neurotransmitters are chemicals that diffuse across the synapse to the next neuron in the chain. several dozen neurotransmitters have been identified. each has its own specific molecular structure that fits perfectly into a post-synaptic receptor sit, like a lock and key. each has specific functions, e.g:

  • acetylcholine (ACh) found where a motor neuron meets a muscle, causing muscles to contract.
  • serotonin affects mood and social behaviour (among other things) 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 the neighbouring neuron.
• is adrenaline excitatory or inhibitory?

  generally excitatory, increasing the positive charge of the postsynaptic neuron, making it more likely the neuron will fire.
• is serotonin excitatory or inhibitory?

  generally inhibitory, increasing the negative charge of the postsynaptic neuron, making it less likely the neuron will fire.
• is dopamine excitatory or inhibitory?

  it is an unusual neurotransmitter as it is equally likely to have excitatory or inhibitory effects on the next neuron in the chain.
• summation

  excitatory and inhibitory influences are summed and must reach a certain threshold in order for the action potential of the postsynaptic neuron 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.