The Science of Chakras and Nadis
Prana is considered a form of cosmic energy and is meant to flow freely within the subtle, physical and causal bodies through energetic channels called nadis (flow, motion or vibration). These nadis which are considered from Tantric texts to populate the body with 72,000 paths and centralise at certain points in the body known as chakras. There are three main nadis that yogis refer to consisting of the sushumna, ida and pingala. The practice of yoga is a way to unblock the nadis and produce an open channel through the sushumna for kundalini (divine energy) to flow leading to moksha; liberation from suffering.
Swami Vivekananda describes Kundalini briefly in his book Raja Yoga as follows:c
According to the Yogis, there are two nerve currents in the spinal column, called Pingalâ and Idâ, and a hollow canal called Sushumnâ running through the spinal cord. At the lower end of the hollow canal is what the Yogis call the "Lotus of the Kundalini". They describe it as triangular in a form in which, in the symbolical language of the Yogis, there is a power called the Kundalini, coiled up. When that Kundalini awakens, it tries to force a passage through this hollow canal, and as it rises step by step, as it were, layer after layer of the mind becomes open and all the different visions and wonderful powers come to the Yogi. When it reaches the brain, the Yogi is perfectly detached from the body and mind; the soul finds itself free. We know that the spinal cord is composed in a peculiar manner. If we take the figure eight horizontally (∞), there are two parts which are connected in the middle. Suppose you add eight after eight, piled one on top of the other, that will represent the spinal cord. The left is the Ida, the right Pingala, and that hollow canal which runs through the center of the spinal cord is the Sushumna. Where the spinal cord ends in some of the lumbar vertebrae, a fine fiber issues downwards, and the canal runs up even within that fiber, only much finer. The canal is closed at the lower end, which is situated near what is called the sacral plexus, which, according to modern physiology, is triangular in form. The different plexuses that have their centers in the spinal canal can very well stand for the different "lotuses" of the Yogi.