the chakras are in our aura, our energy field. They enter the body at points known as kshtrams: these are the trigger points for the chakras. In the aura they are seen on a pranic level as funnels coming out of the body. And as very beautiful, radiant, colours, the external colours differing from the internal colours.
Chakras are intimately involved with the spine. The late Dr Swami Gitananda said at a talk he gave over 40 years ago, that he had dissected cadavers & found that at each plexus on the spine, when they were cut horizontally, or across, the nerves looked like the petals of the corresponding chakra.
A plexus is a branching network of intersecting nerves. Each chakra relates to one of these plexus networks. The spinal plexus carry sensory & motor (movement) information. They are sacral (base, mooladhara); lumbar (2nd chakra, swadhisthana) ; brachial (heart chakra, anahata), cervical (throat chakra, vishuddhi). The solar plexus, however, is located in the abdomen, & is related to manipura chakra. Now, it's important to clarify that I am not a medical person so I do not know the complete truth of the plexuses in relation to the chakras.
Physical yoga, pranayama (breathing practices), bandhas (energy locks) are used in yoga to improve the health of the plexuses. This in turn has an effect on the chakras.
Each chakra also corresponds to a gland: mooladhara, in the base of the torso, the g-spot for women, & in the perenium, inside the anus for men. Swadhisthana/lumbar: ovaries & testes. Manipura/navel chakra: adrenals. Anahata/heart: thymus. Vishuddhi/throat chakra: thyroid. Ajna/3rd eye: pineal. Sahasrara/crown: pituitary. Again, physical yoga is the yogic means to improve the health of these glands, they are endocrine glands, which means that they secrete hormones directly into the blood stream. It is these hormones which help keep the body & the person, young, in yoga. Not the actual performance of complicated poses, no, it's the effects of the poses on the glands, & there is also an effect on the chakras. Not an awakening but a strong effect which influences both the trigger points & awakening of the chakras proper.
At the talk that I attended, 40 odd years ago, he also said that 3 minutes in the 1/2 shoulderstand gives maximum physical effect, so I would deduce from that, that it would apply to any pose. When we do a pose, for example, the bow/dhanurasana, as we go up & down in the pose, we get a squirt from the relevant gland, of hormones, into the blood stream, which is why just simply doing the poses regularly, without holding them, has such a profound effect physically.
Dr Swami Gitananda also said that the inner spine, the central nervous system (CNS), can calcify, as he observed when dissecting cadavers. The CNS receives information & co-ordinates activity to the whole body. The CNS is related, esoterically, to the spiritual spine, known as sushumna. Sushumna actually starts in mooladhara, so it doesn't start at the bottom of the spine. It hooks into the tailbone & goes up the spinal cavity, ending quite high inside the head, in a direct line in from the eyebrow centre. The eyebrow centre is between the eyebrows but a bit above that centre point.
Sushumna, of course, needs to awaken, for prana, then later, kundalini, to travel up from mooladhara. When it awakens, it looks like a "tunnel" of white light tinged with yellow. This is not the kundalini awakening, it is the sushumna awakening.
So firstly, in yoga we improve the health of the spine, & get the spine able to move. The purification of sushumna comes from; the physical poses, they are important, the cleansing practices (shat karmas), spinal column meditation techniques; bandhas, movements which block then release energy; ujjayi pranayama. Ujjayi is the pranayama, breathing technique, which purifies the energy pathways, nadis, of which sushumna is the main one.
There are 2 other main energy pathways, which are the next most important energy pathways (nadis). So next we will look at the 3 main nadis, & others.