get my students to agree. Students going for a second (MSc) degree are better than my 1st Year Pharmacy Techs at working out that insulin must be smaller than the pancreas. Neurotransmitters NTs are small molecules which communicate between two neurons by crossing the synapse between them. One important aspect of the picture is not included because it is much too big and that is the axon width at right angles to the synapse width [that is 20nm as shown in the picture above] . The width of each face of synapse is about 2μm or ~100x the space between the neurons. That is why we call the gap between neurons a cleft - it is long and narrow and may take a while to escape from.
We know of about 60 chemicals that act as neurotransmitters . . . including
- Half of them simple molecules
- amino acids: glutamate, glycine, GABA [prev]
- monamines: dopamine [prev], adrenalin [prev], serotonin [prev], histamine, dopamine
- trace amines: phenethylamine, tyramine,
- The rest peptides:
- vasopressin and oxytocin [prevoboth]
In their neurotransmitter capacity, these molecules have to
- be manufactured
- be stored in a vesicle in the upstream axon until required
- have the vesicle dock with the cell membrane and blurf out the contents when required
- drift across the 20nm synapse gap
- dock with an appropriate receptor
- the receptor is a large protein embedded in the membrane of the downstream neuron
- it is responsive to a subset of all the molecules in the body
- if an appropriate molecule = "ligand" docks then a cascade of events will happen inside the downstream cell.
- get out of the way - your role is over
But back to the NTs; having completed their task they have to be processed lest they continue to stimulate their receptor until that cell runs out of energy or over-heats and blows up. There are four main ways in which this is achieved:
- enzymatic digestion. The cartoon at the top includes such an enzyme acetylcholinesterase which splits the NT into acetyl and choline, neither of which is neurologically active on their own
- reuptake. The NT, having drifted across the synapse to deliver its message then drifts back and is hoovered up by the upstream neuron for recycling
- dissipation. The NT drifts out of the synaptic cleft into an intercellular space where there are no receptors
- removal. Astrocytes are found in the central nervous system. They are cells which don't transmit signal like neurons but carry out numerous housekeeping exercises for neurons: these include sucking 'spent' neurotransmitters out of the synaptic cleft so that they are no long annoying the receptors.