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?