Nadis are astral tubes made up of astral matter that carry Pranic currents. They can be seen by the astral eyes only. They are not the nerves. They are 72,000 in number. Ida, Pingala and Sushumna are the important ones. Sushumna is the most important of all.
Ida and Pingala
There are the two nerve-currents one on either side of the spinal column. The left one is called Ida and the right is known as Pingala. These are Nadis. Tentatively, some take these as the right and the left sympathetic cords, but they are subtle tubes that carry Prana. The Moon moves in the Ida and the Sun in the Pingala. Ida is cooling. Pingala is heating. Ida flows through the left nostril and the Pingala through the right nostril. The breath flows through the right nostril for one hour and then through the left nostril for one hour. Man is busily engaged in worldly activities, when the breath flows through Ida and Pingala. When Sushumna operates, he becomes dead to the world, and enters into Samadhi. A Yogi tries his level best to make the Prana run in the Sushumna
Nadi, which is known as the central Brahman Nadi also. On the left of Sushumna is situated Ida and on the right is Pingala. The moon is of the nature of Tamas and the sun is that of the Rajas. The poison share is of the sun and the nectar is of the moon. Ida and Pingala indicate time. Sushumna is the consumer of time.
Sushumna is the most important of all the Nadis. It is the sustainer of the universe and the path of the universe and the path of salvation. Situated at the back of the anus, it is attached to the spinal column and extends to the Brahmarandhra (Crown chakra) of the head and is invisible and subtle. The real work of a Yogi begins when Sushumna begins to function. Sushumna runs along the centre of the spinal cord or spinal column.
Above the genital organs and below the navel is the Kanda, of the shape of a bird’s egg. There arise from it all the Nadis 72,000 in number. Of these, seventy-two are common and generally known. Of those the chief ones are ten and they carry the Pranas. Ida, Pingala, Sushumna, Gandhari, Hastijihva, Pusa, Yusasvini, Alambusa, Kuhuh and Sankhini are said to be the ten important Nadis. The Yogis should have a knowledge of the Nadis and the Chakras. Ida, Pingala and Sushumna are said to carry Prana and have Moon, Sun and Agni as their Devatas. When Prana moves in Sushumna, sit for meditation. You will have deep Dhyana. If the coiled-up energy, Kundalini, passes up along the Sushumna Nadi and is taken up from Chakra to Chakra the Yogi gets different sorts of experiences, powers and Ananda.
The Science of Pranyama by Sri Swami Sivananda