Brain changes include a drop
in dopamine receptors
in the reward circuitry. Reducing the amount
of dopamine receptors decreases your pleasure response to "normal" stimuli. In other words, real-life sexual partners and previous porn won't excite your brain enough to get your penis going. Turning on dopamine receptors is how you get turned on.
Here are two excellent presentations, by a science teacher, on the effects of pornography on the brain.