Here is a biologically plausible hypothesis on how a reward transmitting a signal received by output axon synapses might be slowly (back)propagated to input synapses.
More exactly put,
If A → B → C is a chain of three connected neurons, this mechanism by which when neuron C fails to respond to activations of A followed B , it could result in weakening not only the B->C synapse but also the A->B one.
- The brain is on a tight energy budget while neuron activations are high energy consumers.
- There is some feedback mechanism that rewards a firing synapse resulting in activation of the postsynaptic membrane, vs penalizing it if it fails.
And here-s the logic following the above:
It makes sense to have the neurons compete for the limited energy budget which regulates not only individual synapse strength but the whole neuron strength too.
Assume there is some expensive spice available only immediately after and in the close proximity of the depolarized membrane which can enter back the axon synapses:
- only after it fired
- and only IF the connected dendrite reached activation potential
Probably (or likely) this isn’t the exact mechanism but it is fairly plausible that when the two conditions above met, a chemical change has to happen within the axon button in order to inform it has succeeded or not in transmitting the signal in order to alter synapse strength/permanence.
I called it spice since for now is science-fictional, it’s expensive and for that reason can be used as a biochemical currency which :
- can be diffused/transported back from the axon ending to the neuron body or dendrites where it can have important uses
- like being traded for “fuel” used to restore the membrane after activation. A neuron with low spice levels would spend more time in refractory period which decreases the chances of transmitting the signal
- or a lack of spice could make the membrane less responsive, requiring one more input signal to reach action potential.
- or it could be directly discharged by postsynaptic receptors during activation, and a low supply of spice within neuron B will result in lower reward for the upstream neuron A even if it results in activation of B.
There can be a more complex equilibrium because on average the neurons have as many output synapses as inputs, so a general threshold of downstream activations would keep the supplies of spice safe, while a response below the threshold resulting from persistent activation of upstream neurons could result depleting strength in specific dendrites that were most active in the past seconds or minutes.
I know, it is far fetched, as you noticed I don’t really know much about biochemistry or neurology (hence the "spice term) but here you have it.
I read about sodium/potassium channels all along the membrane, needed for action potential to trigger and travel, maybe, just maybe, some molecules can plug and/or other can unplug some of these channels, influencing locally how well or fast the postsynaptic membrane responds to input signals, and availability of these molecules could be influenced, with a delay, by what happened at the axon end of the cell.