The axis from the kanda at the base to the crown, with the chakras as the points where the channels meet.
Function
The relationship of the channels to the chakras. The three principal nadis run the length of the body and the chakras sit on that axis as points of intensity rather than as separate organs: ida and pingala coil around the sushumna and cross it at each centre, which is why the hatha texts picture the pair as a double helix and call the chakras lotuses strung on the central channel. Where the two side channels cross, the prana is said to be able to enter the centre; the practice of pranayama is the work of making that crossing happen deliberately, and the rise of kundalini is what happens when the prana stays in the sushumna rather than returning to the sides.