Cholinergic synapses

• A cholinergic synapse is one in which the neurotransmitter is a chemical called acetylcholine.
• In a cholinergic synapse, firstly the arrival of an action potential at the end of the presynaptic neurone causes calcium ion protein channels to open, then calcium (Ca2+) ions enter the synaptic knob via facilitated diffusion.
• The influx of Ca2+ ions into the synaptic knob causes the synaptic vesicles to fuse with the presynaptic membrane, releasing acetylcholine into the synaptic cleft.
• Acetylcholine molecules rapidly diffuse across the synaptic cleft because there is a short diffusion pathway. Acetylcholine then binds to the receptor sites on the sodium ion protein channels in the membrane of the postsynaptic neurone.
• The binding of acetylcholine to the sodium ion protein channels causes them to open, allowing sodium ions to diffuse in rapidly along a concentration gradient. This influx of sodium ions generates a new action potential in the postsynaptic neurone.
• Acetylcholinesterase hydrolyses acetylcholine into choline and ethanoic acid (acetyl), which diffuses back across the synaptic cleft into the presynaptic neurone (recycling).
• In addition to recycling the choline and ethanoic acid, the rapid breakdown of acetylcholine also prevents it from continuously generating a new action potential in the post-synaptic neurone.
• ATP released by mitochondria is used to recombine choline and ethanoic acid into acetylcholine. This is stored in synaptic vesicles for future use. Sodium ion protein channels close in the absence of acetylcholine in the receptor sites.