It is in the spinal chord that the sushumna nadi runs.The thin hollow fibrous passage is also known as 'Brahmarandra'.Just as the veena is ever associated with its strings, the brahmarandra is inseparably associated with the power of the mystic"Om" or Parasakthi, the Supreme Power.The 24 vertebral joints that together constitute the spinal column are aligned in the living organism on the model of the above 24 seed-letters(bheejaaksharaas) of the Gayathri.This clearly means that the macrocosm and microcosm are but a condensation of the exalted splendor of the great Mother,Gayathri Devi.
DARSHANOPANISHAD:-"Prushta madhyasthitha nadi veena dandena suvrathi shamasthakaparyantham sushumnasamprathishtitha."
"O,man of learning,the chord that rises from between the posteriors even like unto the trunk of veena,goes upto the very brain.Inside this chord you find firmly established the Sushumna nerve."
SANDILYOPANISHAD:-"Gudasya prushtabhagesminveena danda sadehabhruthmoordastha dehaparyantham brahmanaadeethi kathyathe.Uchchaarayethparam shaktim brahmarandra nivasineem."
"At the rear of the posteriors there rises a straight nervine chord even as the trunk of the veena which rises upto the head.Inside this nervine chord one must locate the Sushumna nadi,one must meditate upon the Infinite power infusing this nadi,in other words,on the omkara which is the mysterious primival sound."
DHYANABINDOPANISHAD:-"Mooladhara sushumnaacha padmathanthu nibhashubha amoorthovarthathesamuththithaha."
"The nervous tendril that uprises from the mooladhara as sushumna nadi is as fine as a lotus filament and full of bliss-giving potency.There vibrates in this mysterious chord the indefinite primeval sonanteven as gross sounds issue from the sound of the Veena."(click on images to enlarge.) Courtesy-Yedathore Subbarama Sharma