Good question. Some others may have this question too.
The key is to understand the following:
1. Citta (“thoughts”) arise in the hadaya vatthu (seat of the mind). That is located in the manomaya kaya (gandhabba) overlapping the physical heart.
2. Mana indriya is in the brain. Of course, scientists are not aware of that. But mana indriya is like eyes or ears (or other three physical senses).
– Rupa rupa “come in” through the eyes.
– Sadda rupa “come in” through the ears.
– Dhamma rupa (memories, kamma vipaka, etc) “come in” through the mana indriya.
3. The brain processes all six types of “incoming” sensory signals. Then those signals are transmitted to the heart (hadaya vatthu) area.
– Signals from the five physical senses received by the five pasada rupa located around the hadaya vatthu and transferred to hadaya vatthu (one at a time). That is what we discussed briefly in recent posts.
– Signals from the mana indriya are transmitted directly to hadaya vatthu.
– More information at, “Brain – Interface between Mind and Body.”
4. When the physical body dies, the mana indriya dies.
– But the gandhabba comes out with the hadaya vatthu and the five pasada rupa. (That is if there is more kammic energy left in the human bhava.) In that case, the gandhabba may get into another womb and then a new brain will develop with a new mana indriya in the new physical body.