Two neurones are linked by a synapse, a gap between them for the impulse to cross.
- They play a vital role in information processing.
- They can amplify or dampen down a signal.
- They ensure that a nerve impulse travels in one direction only.
Every cell in the CNS is covered with synaptic knobs from other cells.
- An action potential arrives at the end of the presynaptic neurone (PrSN), causing Ca channels to open.
- Ca2+ ions flood in.
- Synaptic vesicles containing the neurotransmitter acetylcholine (ACH) to move towards the membrane of the PrSN.
- Vesicles fuse with membrane. ACH is released into synaptic cleft by exocytosis.
- ACH diffuses aross cleft, binds to receptors on Na+ ion channels on postsynaptic neurone (PoSN). Na channels open, Na+ ions flood in.
- Postsynaptic membrane depolarises and an action potential is generated.
- ACH is broken down into ethanoic acid + choline by enzyme acetylcholinesterase
- Choline + ethanoic acid diffuse back into the cell. ATP is used to recombine them into ACH. The synapse can now transmit another action potential.
Excitory postsynaptic potential (EPSP) - Neurotransmitters open Na channels in the postsynaptic membrane causing an inflow of Na+ ions and depolarisation of the membrane.
If there are sufficient EPSPs, the positive charge in the post-synaptic membrane will exceed the threshold level and an action potential is set up.
Inhibitory postsynaptic potential (IPSP) - neurotransmitters open Cl and K channels causing Cl- ions to flow into the cell and K+ ions to flow out, resulting in hyperpolarisation of the membrane. IPSPs make the postsynaptic membrane less receptive to incoming signals.
If the EPSPs outnumber the IPSPs and the threshold potential is reached a new action potential is generated.
Summation and Accommodation
Spatial summation – action potentials need to arrive at several synapses at once to release the amount of neurotransmitter required to trigger an action potential in the post-synaptic fibre.
Temporal summation – one action potential doesn't release enough transmitter substance itself to set up another action potential, but makes it easier for the next impulse which arrives to do so (facilitation).
Accommodation - If the synapse is used too often the rate of resynthesis cannot keep up and we can no longer respond to the stimulus. The synapse is “fatigued”.