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I am referring to Eric Kandel and his experiment on Aplysia where he shows that synapses between a pair of neurons can be modulated by means of a third neuron that synapses onto the terminals of the two other neurons. The third neuron doesn't cause post-synaptic firing, but it modulates the activity and strength of the synapse.

This is known as modulatory input-dependent plasticity as depicted in the following pictures. For more information you can refer to this

My question is whether such synapses exist in the human brain too? If yes, are they abundant, or they are rare in the human brain, and what is their duty in brain?

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  • $\begingroup$ This can be emulated in analog computers to show how ganged signals can be modulated with isolated stimulus. A control could be a group forward gain modulator or a cluster or common activation threshold bias potential or a feedback cumulator of repetitive stmulus. such as in "muscle memory". $\endgroup$ Sep 18, 2022 at 14:25

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A bunch of papers I was able to dig up after a cursory search on Google Scholar strongly indicates that heterosynaptic modulation does exist in man, at least in the motor cortex. In these papers the conclusion was reached by using indirect measures, namely by making use of specific stimulation strategies in intact brains in living humans. This as opposed to methods that more directly approach the question, like histology, or single-cell recordings.

References
- Hartwig et al., J Neurosci (2004); 24(13): 3379–85
- Jung & Ziemann, J Neurosci (2009); 29(17): 5597–604
- Nitsche et al., J Neurosci (2007); 27(14): 3807–12
- Zhen et al, J Neurosci (2014); 34(21): 7314–21

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  • $\begingroup$ I know that heterosynaptic plasticity exist and it works like this picture: link there are a lot of experimental tests show that but it's different with modulatory input-dependent plasticity $\endgroup$
    – alireza
    Aug 18, 2020 at 12:22

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