In some systems of Yoga and Tantra, three nadis are considered especially important among the many channels described in the subtle body:
Ida, Pingala, and Sushumna.
They can be imagined as three major routes on an internal map of the body.
What Does Each One Represent?
Ida is traditionally associated with the left side, the moon, coolness, and a calmer or more receptive state.
Pingala is associated with the right side, the sun, warmth, and activity.
Sushumna is the central channel. According to some systems of Tantric Yoga, it is through Sushumna that Kundalini is meant to rise.
What Does a Practitioner Actually Do?
During pranayama, for example, a practitioner may alternate breathing through the left and right nostrils while visualizing the movement of prana through Ida and Pingala.
Attention may then be directed toward Sushumna — the central pathway.
The basic idea is that human experience constantly moves between opposites: activity and rest, stimulation and relaxation.
The practice is intended to bring these opposing tendencies into balance and gather attention toward the center.
Is This the Nervous System? No.
Today, Ida is sometimes compared with the parasympathetic nervous system and Pingala with the sympathetic nervous system. Such comparisons can be useful as modern analogies, but they do not demonstrate that ancient yogic traditions were describing the nervous system in the modern anatomical sense.
Put very simply: Ida and Pingala represent two contrasting flows, while Sushumna is the central channel that becomes especially important in yogic practices associated with Kundalini.