Getting a specific voltage output depending on the difference of two voltages
The way to find differences between two voltages in the analog domain is with a differential amplifier. 


Source: https://www.electronics-tutorials.ws/opamp/opamp_5.html

There are plenty of other ways to mimic a neuron, however. If you want one with the traditional sigmoid output -1 to 1 then this circuit might be most valuable. 


Source: https://www.researchgate.net/publication/329207364_On-chip_learning_for_domain_wall_synapse_based_Fully_Connected_Neural_Network/figures?lo=1