The atman existing in our heart is the cosmic consciousness that runs the human body. Both atman and consciousness mean the same.
Human consciousness is the Prana, life energy that manifest human form to work out its karma, cleanse itself of dross impurities within. Of its own no atman could purify itself and needs a medium, a body to reach state of absolute purity.
When the atman manifests a human form for the first time, it prompted from within to indulge in spirituality to reach cosmic end of life, state of total purity. But dominated by a bloated ego, human beings failed to hear promptings of atman coming from within.
Travelers of spiritual path, seekers have to demolish their ego in totality before they could hear voice of atman coming from within.
The voice of atman coming from within allegorically is Lord Krishna trying to contact us from within. Lord Krishna is the sap around which rotates human body. In absence of atman within no human body could survive of its own, would decay and die.
From six years of age my ego within was zero. I was also 100% truthful. I could distinctly hear promptings of atman, Lord Krishna coming from within all the time. Until about two years I feared listening to this voice not knowing what it was. Then one day when I was about eight years of age I give a patient hearing to this voice and realized it had something to do with God Almighty.
And thus started my spiritual quest of life with Lord Krishna guiding me all the time from within!