In intra-personal communication, the medium that carries the messages is

In intra-personal communication, the medium carrying the messages is the central nervous system.

Thinking, self-talk and internal signals travel along the body's own neural pathways.

The personal ego is a psychological construct, not the physical carrier of these signals.

The physical surroundings are the external environment, not the internal medium of self-communication.

The symbol system supplies the codes used, but it is not the channel that carries them within.

Since the exchange is inside one person, the nervous system is its natural medium.