Madhya means ‘the middle’ and marga is a way or path. Madhya marga is the middle channel for the low of vital air. In the human body there are two paths going downward through the nostrils, one on the left side and the other on the right. In Yoga, the left channel is called ida nadi, Chandra nadi or vama nadi, and the right channel is called the pingala, surya nadi or dakshina nadi. The vital air called apana is supposed to flow along the 2000 nadis in the human body. Through them, ten vital airs (vayus) flow and these supply the energy required for life.
But these channels are not fully open; they are clogged by accumulation of impurities. Pranayama is the means for removal of these impurities. By practicing pranayama, the nadis are said to become free for the flow of vayu through them. An important development that takes place by the prolonged and intense practice of pranayama is the opening of the third channel, called the Madhya marga (middle channel), which lies between the ida and pingala. It is called the susuhmna nadi or Brahma nadi. It extends from the base of spine, from the inside of a triangular bony structure, called kanda in Yoga texts, and goes up through the spine to the inside of the skull. It is also called paschima marga (the dorsal path), because it lies on the dorsal side of the human body. This path is almost completely clogged in all of us in the normal course. By the practice of hatha yoga, pranayama, and mudras, Madhya marga opens up due to cleansing of the inner path. It may be regarded as the most remarkable and fortunate happening brought about by the hatha yoga practices.
Satcakranirupana of Purnananda describes in great detail the sushumna nadi, its inner path, its three coverings, kundalini (or the power which remains dormant at the base of the sushumna), its arousal and its passage upward through the inner path (brahma nadi) that leads through the six lotuses.
It is said that after the arousal of the dormant kundalini, it becomes straight and enters the inner path inside the sushumna and opens the dormant lotuses one by one. When prana follows it and gets established in the three empty spaces inside the three lotuses, respectively called anahata chakra (heart lotus), kantha chakra (throat lotus), and ajnacakra (lotus between the eyebrows), it loosens three knots of ignorance located there. The three empty spaces are called sunya, atisunya and mahasunya. The knots of ignorance are called brahmagranthi, vishnugranthi and rudragranthi. Their loosening is called granthibheda (Hathayogapradipika IV 69-80).
Here the adept starts hearing subtle sounds (nad). He is advised to concentrate his mind on the nad. Finally, the kundalini reaches the place of the God inside the skull (sahasrara chakra), leading to emancipation (mukti).