Transmission across a synapse
A cholinergic synapse is one in whichthe neurotransmitter is a chemical called acetycholine. Acetycholine is made up of tow parts: acetyl (more precisely ethanoic acid) and choline. Cholinergic synapses are common in vertebrates, where they occur in the central nervous system and at neuromuscular junction(junctions between neurones and muscles).
1) the arrival of an action potential at the end of the presynaptic neurone causes calcium ion channels to open and calcium ions enter the synaptic knob.
2) the influx of calcium ions into the presynaptic neurone causes synpatic vesicles to fuse with the presynaptic membrane, so releaseing acetylcholine into the synaptic cleft.
3) acetycholine molecules fuse with receptor sites on the sodiumion channel in the membrane of the postsynaptic neurone. This causes the sodium ion channels to open, allowing sodium ions to diffuse in rapidly along a concentration gradient.
4) the influxof sodium ions generates a new action potential in the postsynaptic in the postsynaptic neurone.
5) Acetylcholinesterase hydrolyses acetylcholine into choline and ethanoic acid(acetyl), which diffuse back across the synaptic cleft into the presynaptic neurone. In addition to recycling the choline and ethanoic acid, the breakdown ofacetylcholine also prevents it from continuously generating a new action potentialin the postsynaptic neurone.
6) ATP releasedby mitochondria is used to recombine choline and ethanoic acid into acetylcholine. This is stored in synaptic vesicles for future use. Sodium ion channels close in the absence of acetylcholine in the receptor sites.