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Synapses (Nerve Junctions (The influx of calcium ions causes vesicles…
Synapses
Nerve Junctions
- The influx of calcium ions causes vesicles containing acetylcholine within the neurone to move to the cell-surface membrane and fuse with the pre-synaptic membrane.
- Via exocytosis, the vesicles release the neurotransmitter acetylcholine into the synaptic cleft which diffuses across the gap, down the concentration gradient, to reach the post-synaptic membrane.
- The acetylcholine binds to receptors on sodium channels on the membrane which causes the channels to open. This means that sodium ions diffuse into the cell.
- An action potential arrives at the synaptic knob of the pre-synaptic neurone.
- The triggers voltage-gated calcium ion channels to open on the pre-synaptic membrane, and so calcium ions diffuse into the cell.
- Influx of sodium ions triggers an action potential as normal, via the depolarisation of the membrane, on the post-synaptic membrane, which travels along the membrane of this new neurone.
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The Role of the Synapse
Synapses keep the movement of the signal in the correct direction, as the neurotransmitter substances are only found at the synaptic knob, so a signal can not be transmitted from post-synaptic to pre-synaptic neurone.
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In some places, several pre-synaptic neurones converge to one post-synaptic neurone so that signals from different parts of the central nervous system can trigger the same response.
Synaptic Summation
Not all neurones will send excitatory post-synaptic potentials. Some will send inhibitory post-synaptic potentials. An IPSP (inhibits the production of an action potential) can override incoming ESPS (generate an impulse) signals from adjacent neurones.
Spatial summation occurs where multiple neurones can fire impulses to one receiving neurone, where one of those action potentials alone will not be sufficient to produce an action potential on the otherside.
Temporal summation occurs where one neurone fires multiple impulses from the same place, which with pauses in between will not be sufficient enough to generate an impulse on the receiving neurone, but when fired within rapid succession of each other, it will be enough to produce an action potential.