In yogic texts I read that life current (from Brahma) is introduced into the human body through the crown chakra (and the equivalent causal region) and from there it descends downwards in the body.
Other times I read (like in kriya yoga) that the life current starts from the sacrum and flows upwards through the spine/shushumna/brahmanadi and then through the third eye it goes to circulate around the body.
How do these two seemingly different life currents relate?
There are indeed differences according to the text one reads. Even among the masters of Self-realization and their chief disciples such things are expressed differently. It is not to be seen as conflicts so much as differing points of perspective.
For example: Yogananda taught that enlightenment is achieved when one’s consciousness is fully awakened at the positive pole of the ajna chakra (or kutastha, the point between the eyebrows). At that point, a subtle channel then, at last, opens to the sahasrara (crown chakra) from which one communes with Brahma.
At the same time, he also taught that one exits progressively through the three concentric circles of the spiritual eye, going through the astral and causal bodies and spheres into Spirit beyond creation.
Then also it is taught that the spiritual eye is but a reflection of the incoming prana (energy) at the negative pole of the ajna chakra, i.e. the medulla oblongata.
Yogananda taught that the human body is formed, at the time sperm unites with the ovum, from the medulla first, to the brain, and then down the spine (astral first, physical second). Further, the ego appears on the astral level while the soul exists on the causal.
The soul is said to exist and be the crown chakra. Like Krishna who took place in the battle but didn’t fight, the soul (or even akin to one’s guardian angel for all its lack of involvement), is not involved except to the extent we make the effort to seek its guidance and influence.
From the point of view of creation, macro (universe) or micro (human body): it is, as you know: causal, astral, physical. So, stating that we begin at the crown chakra has its validity also.
Each of these descriptions relates to different points on the spectrum of creation and of which of the three bodies is the beginning reference point.
As I am not Self-realized, I can only go a few steps but, generally, Yogananda taught and I have incorporated into my being the “mantra” (tongue-in-cheek!) both-and. Try to see differing points of view as aspects of the reality being described: like the famous 7 blind men and the elephant. Relative to our own soul’s omniscience we are all blind men.
Finally, our salvation (enlightenment, liberation, etc) will not depend on how well we can express such subtle realities but how much we have loved, given the fullness of our Being into Spirit (God, guru, etc).
So I suggest you reflect on these as all valid from the point of view they take. Interesting, I grant you, but of relatively little assistance in our spiritual growth. Again: a both-and!
Blessings to you in a new year begun!