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5.8.4 – The structure and function of synapses including the role of neurotransmitters (including acetylcholine)

synapse is  the  junction  between  two  nerves.  It  is  also  a  verb,  i.e.  one nerve

synapses with another (meaning, passes a message to another).

The neurotransmitter on your syllabus is Ach, but over 2000 other transmitters have been discovered.

  1. The wave of depolarisation arrives at the synaptic knob. The membrane in the presynaptic neuron is depolarised to –50mv (threshold potential) and the voltage-gated Na+ channels open, letting Na+ into the cell.
  2. The membrane is depolarised to +30mV and voltage-gated K+ channelsThe membrane potential falls to –90mV and the cell goes into its refractory period, where the 3Na+/2K+-ATPase restored the ion concentrations.
  3. Unlike axons, presynaptic nerves also contain a Voltage-gated Ca2+ channel. As the presynapstic membrane depolarises these channels open and let Ca2+ into the cell.
  4. The Ca2+ causes vesicles in the presynaptic nerve to migrate and fuse with the presynaptic membrane, where they spill neurotransmitter chemical into the synaptic cleft.
  5. The neurotransmitter (Acetyl Choline) diffuses across the cleft and binds to receptors on the postsynaptic membrane.
  6. The receptors let a little Na+ into the postsynaptic neuron, which is enough to initiate another action potential in the postsynaptic nerve.
  7. The ACh is broken down by an enzyme called Acetyl Choline Esterase (AchE), which allows the postsynaptic receptors to be freed ready for a second synapse.

In a neuromuscular junction the sequence of events in the synapse is exactly the same. The only difference is that the posysynaptic nerve is a muscle cell and, instead of being flat, the postsynaptic membrane has deep grooves (t tubules) which allow the depolarisation to spread quickly through the muscle so all parts of the muscle contract at the same time.

Some neurotransmitters can hyperpolarise postsynaptic nerves, which essentially switches them off. An example of this type of inhibitory neurotransmitter is GABA