The Atman, the Brihadaranyaka Upanishad tells us, is ‘identified with the intellect, the Manas [mind] and the vital force, with the eyes and ears’, and so on. ‘When this being full of consciousness is thus asleep, it absorbs at the time the functions of the organs through its own consciousness, and lies in the Akasha (Supreme Self) that is in the heart’ (Brihadaranyaka Upanishad 2.1.17). From there, on awakening, ‘it comes back along the seventy-two thousand nerves called Hita which extend from the heart to the whole body’ (Brihadaranyaka Upanishad 2.1.19).
‘The heart is the seat of the intellect (buddhi)’, says Acharya Shankara, and clarifies: ‘the internal organ (antahkarana), and the other or external organs are subject to that intellect. … Therefore in accordance with the individual’s past actions the intellect in the waking state extends, along those nerves interwoven like a fish-net, the functions of the organs such as the ear to their seats, the outer ear etc., and then directs them. The individual self pervades the intellect with a reflection of its own manifested consciousness. And when the intellect contracts, it too contracts. That is the sleep of this individual self. And when it perceives the expansion of the intellect, it is waking experience. It follows the nature of its limiting adjunct, the intellect, just as a reflection of the moon etc. follows the nature of the water and so forth.’
Source – excerpts from article titled ‘the Self and the Atman’ by Swami Satyamayananda in Prabuddha Bharata Magazine November 2006 issue.