The Synapse

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The Synapse:
The synapse is the functional junction between neurons – most are between the axon end of one neuron and the dendrites (axodendritic synapse) or cell body (axosomatic synapse) of another neuron.

Synapses between neurons and muscles  neuromuscular junction; between neurons and glands  neuroglandular junctions

Two types of synapses:
1) Electrical
less common; the cells are electrically coupled
have protein channels that interconnect neurons  impulses/ions flow directly from one neuron to the next
allows for a rapid response
usually have a large presynaptic fibre and small postsynaptic fibre (needed as presynaptic fibre has to produce an electrical charge large enough to make an action potential in postsynaptic fibre)
2) Chemical
rely on the release and reception of neurotransmitters that diffuse from the presynaptic cell to the postsynaptic cell
neurotransmitters open or close ion channels  influence membrane permeability  change membrane potential
slower transmission that electrical synapse

Transmission across chemical synapses:
When nerve impulse reached axon terminal  chain of events:
1) Prior to impulse arriving, transmitters are synthesised from precursor molecules  stored in vesicles in axon terminal
2) The calcium gates open in the presynaptic terminal part of axon
3) Calcium act as intracellular messenger  synaptic vesicles fuse with axonal membrane  the neurotransmitter released by exostosis into synaptic cleft/gap (only a very small number of all vesicles release their contents with each action potential)
4) Neurotransmitter diffuses across synaptic cleft  binds reversibly to protein receptors on postsynaptic cell (‘docking’)
5) Binding of neurotransmitter to receptor  shape changes  ion channels open  change in membrane potential  excitation or inhibition of postsynaptic neuron  initiates events in postsynaptic cell/neuron (exact response varies depending on the neurotransmitter)
6) Transmitter dissociates from receptor – transmitter is removed from synaptic cleft by either reuptake (active transport back into parasynaptic neuron), degradation by enzymes or diffusion

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