Question

when we talk to ourselves in our heads, what is really happening in the biochem level?

Answer 1

The same as a normal conversation. APs fire in certain combinations, creating the thoughts, but instead of neural activity being passed to the mouth and vocal chords etc, it remains in the conscious mind.
I recommend you youtube a video called "God is in the neurones". It isn't specifically about thinking, but it is very interesting in regards to the neural processes and the way we project and perceive the world.

Answer 2

i watched the video, it was amazing..it totally changed my way of thinking and reasoning..

Answer 3

Glad you enjoyed it. But do not take everything that was said on face value. Towards the end they author incorporates metaphysics because, frankly, we don't yet know the answer to all of the questions that were asked.
But everything that they talked about on a biological level was sound.