Synaptic Transmission

• Synaptic Transmission:

  • The process by which neighbouring neurons communicate with each other by sending chemical messages across the gap (synapse) that separates them.
• Chemical Transmission: (1)

  • Neurons communicate with each other within groups called neural networks.
  • Each neuron is separated from the next by an extremely small gap called the synapse.
  • Signals within neurons are transmitted electrically.
• Chemical Transmission: (2)

  • 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.
• Neurotransmitters: (1)

  • Neurotransmitters are chemicals that diffuse across the synapse to the next neuron in the chain.
  • Once a neurotransmitter crosses the gap, it is taken up by a postsynaptic receptor site on the dendrites of the next neuron.
  • Here, the chemical message is converted back into an electrical impulse and the process of transmission begins again in the neuron.
• Neurotransmitters: (2)

  • The direction of travel only goes one way. This is because neurotransmitters are released from the presynaptic neuron terminal and recieved by the postsynaptic neuron (at the receptor sites).
  • Several dozen types of neurotransmitters have been identified in the brain (as well as the spinal cord and some glands). Each neurotransmitter has its own specific molecular structure that fits perfectly into a postsynaptic receptor site.
• Neurotransmitters: (3)

  • Neurotransmitters also have specialist functions.
  • E.g acetylcholine is found at each point where a motor neuron meets a muscle, and upon its release, it will cause muscles to contract.
• Excitation and Inhibition:

  • Neurotransmitters have either an excitatory or inhibitory effect on the neighbouring neuron.
• Excitation:

  • When a neurotransmitter increases the positive charge of the postsynaptic neuron.
  • Increases the chances that the postsynaptic neuron will pass on the electrical impulse.
  • E.g adrenaline.
• Inhibition:

  • When a neurotransmitter increases the negative charge of the postsynaptic neuron.
  • Decreases the chances that the postsynaptic neuron will pass on the electrical impulse.
  • E.g serotonin.
• Summation:

  • Whether a postsynaptic neuron fired is decided by the process of summation.
  • The excitatory and inhibitory influxes are summed. If the net effect excitatory it is more likely to fire vice versa. Once the electrical impulse is created it travels down the neuron.
  • Therefore, the action potential of the postsynaptic neuron is only triggered if the sum of the excitatory and inhibitory signals at any one time reaches the threshold.