There’s a widespread belief that Sushumna Nadi (also called Susumna Nadi) is actually composed of three inner Nadis: Vajra Nadi, Chitrini Nadi and Brahma Nadi. These three Nadis are enveloped (i.e. encircled) by Sushumna Nadi.
Probably the most prominent source of this conception is the Sat Chakra Nirupana (Sat-Cakra-Nirupana), an Indian text from the 16th century CE about Tantra in general and the Chakras in particular.
Nevertheless, in the Shiva Samhita, another important Yoga classical text and believed to be composed sometime between the 14th and 17th century CE, the Chitrini Nadi is also explicitly mentioned.
At any rate, below a brief description of Vajra, Chitrini, and Brahma Nadi:
Vajra Nadi
Vajra Nadi, also known as Vajrini or Vajrana Nadi, starts from the Kanda starting point of Sushumna Nadi and goes upwards, lying within Sushumna Nadi.
It’s also stated that Vajra goes down to the genitals, apart from its trajectory to the head.
“In the space outside the Meru [spinal column], placed on the left and the right, are the two Siras [channels, streams, Nadis], Sasi [Ida Nadi] and Mihira [Pingala Nadi]. The Nadi Susumna, whose substance is the threefold Gunas [Sattva, Tamas, and Rajas], is in the middle. She is the form of Moon, Sun, and Fire; Her body, a string of blooming Dhatura flowers, extends to the middle of the Kanda to the Head, and the Vajra inside Her extends, shining, from the Medhra [penis] to the Head.” (Sat Chakra Nirupana).
Chitrini Nadi
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Chitrini Nadi, also known as Chittra, Chitra, or Citrini Nadi, begins at the starting point of Vajra Nadi, goes upward, lying within the Vajra Nadi.
“Inside her [Vajra] is Citrini, who is lustrous with the luster of the Pranava and attainable in Yoga by Yogis. She [Citrini] is subtle as a spider’s thread, and pierces all the Lotuses which are placed within the backbone, and is pure intelligence. She [Citrini] is beautiful by reason of these (Lotuses) which are strung on her.” (Sat Chakra Nirupana).
“The innermost of these three is Chitra; it is my beloved. In that there is the subtlest of all hollows called Brahmarandhra. Brilliant with five colors, pure, moving in the middle of Sushumna, this Chitra is the vital part of body and center of Sushumna.” (Shiva Samhita).
Chitrini Nadi is said to correspond to Sattva qualities, peace, harmony, pure being, dreams, visions, and pure intelligence.
Brahma Nadi
Within Chitrini Nadi one finds the most important, most subtle Nadi Channel, the Brahma Nadi, which is said to be the actual pathway for Kundalini Energy. Brahma Nadi then stretches up to the Brahmarandhra (the fontanel i.e. soft spot in the top middle of the skull).
Brahma Nadi is considered the most inner gateway to Brahma or Spiritual Enlightenment. It’s the channel of Supreme Consciousness.
“Inside her [Citrini] is the Brahma Nadi, which extends from the orifice of the mouth of Hara [Kanda] to the place beyond, where Adi-deva is.” (Sat Chakra Nirupana).
