[NEST Users] How does the synapse weight influence 'iaf_psc_alpha' neuron(or any other neuron you know) to send a spike?

Hello everyone, As the title says, can someone help me answer this? I am really confused about this. Because I need to know how to control the neuron spiking from the mathematical formula, and I check out the iaf_psc_alpha.cpp and its .h file, and I can't get a clue how does the synapse weight value influence the neuron spiking. I would be appreciate if you can tell me how to check out the code in nest_simualator. Thank you. Enguang