For about the past 20 years we’ve been able to observe dendritic spines forming synapses in the living (rodent) brain — for months ! ! In 1970, if you told me that, I’d have said you were smoking something. The surprising finding is that dendritic spines are a work in progress, being newly formed and removed all the time. The early literature (e.g. 10 years ago) is contentious about how long a given spine lasts, but most agree that spine plasticity is present every time it’s looked for. Here are a few references [ Neuron vol. 69 pp. 1039 – 1041 ’11, ibid vol. 49 pp. 780 – 783, 877 – 887 ’06 ].
It is yet another reason why a wiring diagram of the brain wouldn’t help you understand it. For much more on this please see — https://luysii.wordpress.com/2021/04/25/the-wiring-diagram-of-the-brain-takes-another hit
Not only that, but not all of these new synapses are functional, e.g. stimulating the presynaptic side doesn’t result in a response in the post-synaptic side. These are the silent synapses. This is thought to be due to a lack of postsynaptic ion channels which can respond to released neurotransmitter. In particular AMPAR ion channels which respond to glutamic acid are thought to be absent in the silent synapse. Only after stimulation of NMDAR ion channels (which are thought to be present) are AMPAR ion channels inserted into the postSynaptic membrane converting it to an active synapse.
Obviously, in the fetal brain most synapses are newly formed, hence likely to be silent. It was thought that silent synapses are few and far between in the adult brain.
Not so says Nature vol. 612 pp. 323 – 327 ’22. They used superResolution protein imaging to study some 2,234 synapses in layer V pyramidal neurons in the adult mouse primary visual cortex (probably the best studied piece of cortex in the brain). Amazingly some 25% of these synapses lacked AMPARs and were presumably silent. Most of them were found where you’d expect — at the tips of dendritic filopodia, which are moving around looking to form a new synapse.
If this is generally true of the cerebral cortex, it helps explain our ability to learn. In this sense the brain is both similar and not similar to neural nets, which learn by increasing or decreasing efficiency (weights) of connections between ‘neurons’ as they are exposed to stimuli with feedback. The connections (synapses) are fixed in neural nets, but the individual synapses are not fixed in the human brain. However, if you think of all the connections between two neurons in our brains as a ‘synapse’ then clearly efficiency is clearly being adjusted, as synapses form and die.
Chemiotics II 