The sushumna is a passage for prana or a nadi which connects all the main chakras. That is, sushumna is regarded as connecting all the different levels of consciousness. It is a symbolic passage through which the kundalini rises on its ascent from mooladhara to sahasrara. The etymology of the word sushumna is not certain. The sushumna is far more subtle than the physical body. However, for the purpose of yogic practices, it is regarded as being located in the center of the spine, from top to bottom. The sushumna is the central pathway between the ida and pingala nadis. The ida and pingala have vast implications. They represent the internal and external aspects of existence.
Sushumna implies the balance between them. That is, the kundalini arises when there is perfect harmony between the outside and inside worlds. The sushumna is regarded as a hollow tube in which there are three more concentric tubes. Each is progressively more subtle than the previous tubes within it. These represent the three aspects of nature: tamas, rajas and sattwa; and consciousness. The tubes or nadis are as follows 1. Sushumna signifying tamas 2. Vajrini-signifying rajas 3. Chitrini-signifying sattwa 4. Brahma-signifying consciousness. In short, tamas indicates ignorance, laziness and negativity; rajas indicates passion and knowledge; sattwa indicates virtuousness. The three aspects interplay throughout nature, in plant life, mineral life, animal life and the life of man. The sushumna is regarded as being closed at its base near the mooladhara chakra. This is called the Brahma dwara-the gate of Brahman. This is the first gate that must be opened in order to ascend through the chakras. Yoga practices open this door and allow the kundalini to rise upwards through sushumna.
Sushumna is the nadi which carries kundalini straight up to sahasrara chakra. It flows along the spinal column and is intimately linked to the pranic body, or pranayama kosha. Within sushumna the slender nadi known as brahma nadi which is described as being slender like a fiber of the lotus stem is equipped to carry the kundalini energy to the highest centers. The flow of breath is not equal in both nostrils, at any given time the flow of breath in one nostril is greater than the other nostril. Ida and pingala are associated with the left and right nostril flows respectively. They flow seperately in the swara, or the breath, throughout the day. Ida flows predominantly for about an hour and then the swara changes over to pingala which flows predominantly for about an hour. At the time of changeover sushumna flows for a few seconds. Sushumna becomes active when both ida and pingala nadis are balanced, that is when breath flows through both nostrils simultaneously, the flows being equal through both nostrils. Through practice the duration for which sushumna flows can be increased.