- Neurons communicate at synapses between dendrites and the terminal boutons. Chemical synaptic communication predominates, and when the action potential reaches the terminal bouton, synaptic vesicles fuse with the membrane, releasing their neurotransmitters into the synaptic cleft through the process of exocytosis.
- Neurotransmitters can be excitatory or inhibitory and can also be classified based on size; small or large-molecule transmitters. Neurotransmitters are synthesised at the terminal bouton and stored in synaptic vesicles.
- Acetylcholine is the primary neurotransmitter at the neuromuscular junction and it is synthesised from choline and acetyl-CoA, with the enzyme choline acetyltransferase. It is then transported from the cytoplasm into synaptic vesicles by a vesicle-associated transporter.
- When the action potential reaches and depolarises the presynaptic membrane, voltage-gated Ca2+channels open, causing the influx of Ca2+ into the axon down its electrochemical gradient. Ca2influx causes the synaptic vesicles to fuse with the presynaptic membrane and release, through exocytosis, their neurotransmitter into the synaptic cleft.
- At the synaptic cleft, acetylcholinesterase metabolises acetylcholine, hydrolysing the neurotransmitter to choline and acetate.
Figure 3. Release of acetylcholine at the cholinergic synapse. Acetylcholine is synthesised from acetyl Co-A and choline and transported into synaptic vesicles by VAT. When the action potential reaches the terminal bouton, voltage-gated Ca2+ channels open and the influx of Ca2+ down its electrochemical gradient causes the vesicle to fuse with the pre-synaptic cell membrane and release its contents through exocytosis.