The Mandukya, a short and terse Upanishad of just twelve verses, expounds on the meaning and nature of OM. It describes the states of Jagrat (wakeful), Swapna (dream), Sushupti (deep sleep), and Turiya (the Fourth state beyond deep sleep, the state of pure consciousness). The focus of this Upanishad on meditating upon OM in a way, forms some of the roots of the Yoga Darshana. Similarly, we find a reference to Nadis in the Chandogya Upanishad, which says:
A hundred and one are the arteries of the heart, one of them leads up to the crown of the head. Going upward through that, one becomes immortal. (8.6.6)
The “crown of the head” mentioned here is the precursor to the development of Kundalini Yoga. The whole of Brihadaranyaka Upanishad is indeed, the exposition of the Moksha Yoga or the Yoga of Liberation. The Aitareya Brahmana mentions the Brahmarandhra, or the Gateway of Bliss located at the center of the skull, which again has a parallel with the Sahasrara Chakra found in Kundalini Yoga.