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.
The Ida and Pingala are Nadis. 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.
Anyone that is busily engaged in worldly activities, the breath flows through both Ida and Pingala. When Sushumna operates, that person becomes dead to the material world and enters into Samadhi. A Yogi tries their level best to make the Prana run in the Sushumna Nadi, which is also known as the central Brahman Nadi.
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
The Sushumna is the most important of all the Nadis. It is the sustainer of the Universe, the path of the Universe and the path of salvation.
The Sushumna runs along the centre of the spinal cord or spinal column. The Sushumna is invisible and subtle.
The real work of a Yogi begins when Sushumna begins to function. Below the navel is the Kanda, the shape of a bird’s egg. From the Kanda arises 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.
From The Science of Pranayama by Swami Sivananda