synaptic transmission

• A synapse

  • Tiny gap that separates each neuron from the next
• Chemical transmission:

  • Signals within neurons are transmitted electrically, but signals better neurons are transmitted chemically across the synapse
  • When the electrical impulse reaches the end of the neuron (the pre synaptic 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 = chemicals that diffuse across the synapse to the next neuron in the chain
  • Each neurotransmitter has its own specific molecular structure that fits perfectly into a postsynaptic receptor site (like a lock and key)
  • Each neurotransmitter has specific functions, eg:
  • 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 or inhibition:

  • Neurotransmitters generally have either an excitatory or inhibitory effect on the 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 on the postsynaptic neuron
• summation:

  • Excitatory and inhibitory influences are summers 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 neurotransmitters is inhibitory then the postsynaptic neuron is less likely to fire (ie no electrical signal is transmitted)
  • It is more likely to fire if the net effect is excitatory
• Synaptic transmission diagram