Say what ? “This proportion will likely be higher in rat dopaminergic neurons, which have even larger axonal arbors with ~500,000 presynapses, or in human serotonergic neurons, which are estimated to extend axons for 350 meters” – from [ Science vol. 366 3aaw9997 p. 4 ’19 ]
I thought I was reasonably well informed but I found these numbers astounding, so I looked up the papers. Here is how such statement can be made with chapter and verse.
“The validity of the single-cell axon length measurements for dopaminergic and cholinergic neurons can be independently checked with calculations based on the total volume of the target territory, the density of the particular type of axon (axon length per volume of target territory), and the number of neuronal cell bodies giving rise to that type of axonThese population analyses are made possible by the availability of antibodies that localize to different types of axons: anti-ChAT for cholinergic axons (also visualized with acetylcholine esterase histochemistry), anti-tyrosine hydroxylase for striatal dopaminergic axons, and anti-serotonin for serotonergic axons.
The human data for axon density and neuron counts have been published for forebrain cholinergic neurons and for serotonergic neurons projecting from the dorsal raphe nucleus to the cortex, and cortical volume estimates for humans are available from MRI analyses; forebrain cholinergic neuron data is also available for chimpanzees. These calculations lead to axon length estimates of 107 m and 31 m, respectively, for human and chimpanzee forebrain cholinergic neurons, and an axon length estimate of 170–348 meters for human serotonergic neurons.”
H. Wu, J. Williams, J. Nathans, Complete morphologies of basal forebrain cholinergic neurons in the mouse. eLife 3, e02444 (2014). doi: 10.7554/eLife.02444; pmid: 24894464
How in the world can these neurons survive as long as they do?
Not all of them do– At birth there are 450,000 neurons in the substantia nigra (one side or both sides?), declining to 275 by age 60. Patients with Parkinsonism all had cell counts below 140,000 [ Ann. Neurol. vol. 24 pp. 574 – 576 ’88 ]. Catecholamines such as dopamine and norepinephrine are easily oxidized to quinones, and this may be the ‘black stuff’ in the substantia nigra (which is latin for black stuff).
Here are the numbers for serotonin neurons in the few brain nuclei (dorsal raphe nucleus) in which they are found. Less than dopamine. A mere 165,000 +/- 34,000 — https://www.ncbi.nlm.nih.gov › pubmed
So being too small to be seen with a total axon length of a football field, they appear to last as long as we do. Have we missed a neurological disease due to loss of serotonin neurons?
Why should the axons of dopamine, serotonin and norepinephrine neurons be so long and branch so widely? Because they release their transmitters diffusely in the brain, and diffusion is too slow, so the axonal apparatus must get it there and release it locally into the brain’s extracellular space, no postsynaptic specializations are present in volume neurotransmission — that’s the point. This is one of the reasons that a wiring diagram of the brain isn’t enough — https://luysii.wordpress.com/2011/04/10/would-a-wiring-diagram-of-the-brain-help-you-understand-it/.
Just think of that dopamine neuron with 500,000 presynapses. Synthesis and release must be general, as the neuron couldn’t possibly address an individual synapse.
The more we know the more remarkable the brain becomes.
Chemiotics II 