Résumé. The essential non-linear dynamics underlying synaptic integration and spike generation in neurons define these cells’ unique computational role within the neural net. Yet, in the context of the intact system and physiological function the quantitative and qualitative identification of specific biophysical mechanisms is still at an early stage. In this talk I will review our recent work in addressing these questions using in vivo dynamic-clamp recordings that are strongly tied to cellular and membrane models. We are characterizing the influence of shunting synaptic inhibition and the BK potassium current on the neuron’s transfer function, probed with both artificial and functional, visual stimuli. In comparison to previous experimental and theoretical studies, we find that realistic shunting inhibition has a significant divisive effect on firing gain, as well as important effects on threshold and saturation. Shunting inhibition also has a non-linear effect on visual responses, reducing response amplitude but as well tightening response timing. We confirm predictions that the BK current facilitates spike firing, despite being a hyperpolarizing current. This effect is demonstrated by an increase in the gain of the f/I curve and of visual responses.