Nadis are the astral tubes made up of astral matter that carry psychic currents. The Sanskrit term ‘Nadi’ comes from the root ‘Nad’ which means ‘motion’. It is through these Nadis (Sukshma, subtle passages), that the vital force or Pranic current moves or flows. Since they are made up of subtle matter they cannot be seen by the naked physical eyes and you cannot make any test-tube experiments in the physical plane. These Yoga Nadis are not the ordinary nerves, arteries and veins that are known to the Vaidya Shastra (Anatomy and Physiology). Yoga Nadis are quite different from these.
The body is filled with innumerable Nadis that cannot be counted. Different authors state the number of Nadis in different ways, i.e., from 72,000 to 3,50,000. When you turn your attention to the internal structure of the body, you are struck with awe and wonder. Because the architect is the Divine Lord Himself who is assisted by skilled engineers and masons—Maya, Prakriti, Visva , Karma, etc.
Nadis play a vital part in Kundalini Yoga. Kundalini when awakened will pass through Sushumna Nadi and this is possible only when the Nadis are pure. Therefore, the first step in Kundalini Yoga is the purification of Nadis. A detailed knowledge of the Nadis and Chakras, is absolutely essential. Their location, functions, nature, etc., should be thoroughly studied.
The subtle lines, Yoga Nadis, have influence in the physical body. All the subtle (Sukshma) Prana, Nadis and Chakras have gross manifestation and operation in the physical body. The gross nerves and plexuses have close relationship with the subtle ones. You should understand this point well. Since the physical centers have close relationship with the astral centers, the vibrations that are produced in the physical centers by prescribed methods, have the desired effects in the astral centers.
Whenever there is an interlacing of several nerves, arteries and veins, that center is called “Plexus.” The physical material plexuses that are known to the Vaidya Shastra are:— Pampiniform, Cervical, Brachial, Coccygeal, Lumbar, Sacral, Cardiac, Esophageal, Hepatic Pharyngeal, Pulmonary, Ligual Prostatic Plexus, etc. Similarly there are plexuses or centres of vital forces in the Sukshma Nadis. They are known as ‘Padma’ (lotus) or Chakras. Detailed instructions on all these centres are given elsewhere.
All the Nadis spring from the Kanda. It is in the junction where the Sushumna Nadi is connected with the Muladhara Chakra. Some say, that this Kanda is 12 inches above the anus. Out of the innumerable Nadis 14 are said to be important. They are:—
1. Sushumna
2. Ida
3. Pingala
4. Gandhari
5. Hastajihva
6. Kuhu
7. Saraswati
8. Pusha
9. Sankhini
10. Payasvini
11. Varuni
12. Alambusha
13. Vishvodhara
14. Yasasvini
Again Ida, Pingala and Sushumna are the most important of the above 14 Nadis, and Sushumna is the chief. It is the highest and most sought by the Yogins. Other Nadis are subordinate to this.
IN:
http://www.srichakra108.blogspot.in/2012/08/yoga-nadis.html