The deep (astral or energetic, not physical) spine, through which Kundalini energy rises as a person advances spiritually.
Generally, energy is not rising in the sushumna, but going outward through the senses. However, when one redirects this energy through pranayama techniques, it rises us through the sushumna to the spiritual eye, giving enlightenment.
Kriya Yoga, a technique made popular by Paramhansa Yogananda, is a pranayama technique that draws the energy in from the senses and up the sushumna. [ref]The Art and Science of Raja Yoga, Swami Kriyananda. Step Thirteen, “The Anatomy of Yoga.”[/ref]