A word about the three channels of energy. The left channel (blue) corresponds to our past, emotions, desires, affectivity. Its termination is the superego, which is the storehouse of all our memories, habits and conditionings. The right channel (yellow) corresponds to our actions and planning, to our physical and mental activity. Its termination is the ego, which gives us the idea of I-ness, the sense that we are separate from the world. The central channel is the channel of ascent, it is the power which sustains our evolution and guides us, consciously or unconsciously, towards the higher awareness of the Sahasrara (seventh chakra). 

The idea of nadis - variously translated as "nerves" or "veins" but more properly channels of subtle vital force (prana) is there in the earliest Upanishads (7th-8th century b.c.e.).  The heart was said to be the centre of the 72,000 nadis.

The concept was developed in the later Upanishads - from 2nd century b.c.e. to the 2nd century c.e. and later - and the nascent Yoga and Tantric schools.  The Kshurika-Upanishad mentions the 72,000 nadis, especially the Ida (lunar), Pingala (solar), and Sushumna (central) channels.

In the later Shakta Tantra and Kundalini/Laya Yoga traditions emphasis was placed on the central nadis which represented concentric (hence increasingly subtle) channels located vertically along or in front of the spine or backbone, and along which are strung seven (or however many) chakras.  These main nadis are:

Sushumna-nadi starts from the Kanda-mula, lying just below the Muladhara chakra, and goes upward centrally within the vertebral column.
Vajra-nadi  starts from the starting point of Sushumna and goes upward, lying within Sushumna.
Chitrini-nadi starts at the starting point of the Vajra nadi and goes upward, liying within the Vajra nadi.
Brahma nadi or Brahmarandhra nadi starts from the orifice of Swayambhu-linga in the Muladhara and goes upwards, lying within Chitrini.


A more indepth explination on chakras and energy channels is available on audio cassette from the Yoga Boutique.