Type of Spiritual Experience
The word nadi comes from the Sanskrit root nad meaning "channel", "stream", or "flow". The idea of nadis first appears in the earliest Upanishads (7th-8th century b.c.e.). The heart for example was said to be the centre of the 72,000 nadis. The concept was developed in the later Upanishads - from 2nd century b.c.e. to the 2nd century c.e. and later - and the nascent Yoga and Tantric schools. The Kshurika-Upanishad and later the Hathayogapradikpa mention the 72,000 nadis, especially the Ida, Pingala, and Sushumna channels. Other figures have been proposed - 80,000 (Trisikhibramanopanishad), 200,000, or 300,000 (e.g. Siva Samhita, Goraksha Sataka, Goraksha Paddhati)!
In addition to the various primary and minor nadis, the Shakta Tantra and Kundalini/Laya Yoga traditions placed especial importance on the central nadis located vertically along or in front of the spine or backbone, and along which are strung the seven chakras.
Vibrational Medicine – Richard Gerber
The nadis are formed by fine threads of subtle energetic matter. They are different from the meridians, which actually have a physical counterpart in the meridian duct system. The nadis represent an extensive network of fluid like energies that parallel the body nerves in their abundance.
In the Eastern Yogic literature, the chakras have been metaphorically visualised as flowers. The nadis are symbolic of the petals and fine roots of the flowerlike chakras that distribute the life force and energy of each chakra into the physical body. Various sources have described up to 72,000 nadis or etheric channels of energy in the subtle anatomy of humans. These unique channels are interwoven with the physical nervous system. Because of this intricate interconnection with the nervous system, the nadis affect the nature and quality of nerve transmission within the extensive network of the brain, spinal cord and peripheral nerves. Dysfunction at the level of the charkas and nadis can, therefore, be associated with pathological changes to the nervous system
I don't necessarily agree with him but I think it helps to have alternative views.
A description of the experience
A hundred and one are the nadis of the heart, one of them leads up to the crown of the head. Going upward through that, one becomes immortal. (CU 8.6.6)