Swami Kriyananda explains in some detail that the chakras are psychic centers in the spine that have their corresponding aspects in the astral body.

A number of these centers are symbolically represented in the Bhagavad Gita by the legendary brothers - the Pandavas (Yudhishthira - throat chakra, Bhima – heart chakra, Arjuna – navel chakra, Nakula – solar plexus, and Sahadeva – Earth, root chakra).

When life begins, energy enters the body through the medulla oblongata, goes into the brain, and down into the spine. The energy then goes out to the body through different spinal nerve plexuses that have their corresponding aspects in the astral body.