Jewish mysticism teaches that the soul is comprised of five general levels. Their names are from lowest to highest:
1) Nefesh - a biological soul,
2) Ruach - an emotional or personality soul,
3) Neshama - a rational soul,
4) Chaya - an intuitive soul,
5) Yechida - will or core identity
Together these five levels string together to form a spiritual umbilical cord stretching from the soul's uppermost origin of identity down into her earthly body. Finally settling around the body, the soul extends her illuminations into the various organs of the body, providing them with animation and function. The main centers of illumination are the mind, heart and liver. The neshama extends illumination into the left hemisphere of the brain. The ruach extends illumination into the heart/lung area and the "nefesh" into the liver. These main centers directly control very important bodily systems. The neshama mostly animates the nervous system. The "ruach" mostly animates the circulatory and respiratory system and the "nefesh" is mostly concerned with the digestive system.
When it comes to the chaya, the body does not really provide a complete vessel in the body to receive it's illumination. However, the right hemisphere of the brain serves as a partial vessel. The vessel is incomplete because it can only partially accommodate the chaya's illumination. The outpouring and intensity is much greater than the right brain is capable of grasping. It's like trying to capture a gallon of water in a shot glass. Only small portion of the totality will be contained. This is why the signals from the right hemisphere are marked by spotty flashes of insight and creativity, unlike the steady rigorous thought pattern of the left brain. In fact, in order for those spotty flashes to transform into fully useful ideas, they need to be transferred over to the left hemisphere, into the domain of the neshama's illumination, for unpacking.
So what serves as the vessel for the illumination of yechida? Generally, yechida is described has having no biological organ or organ system serving as it's vessel of illumination. Early on in his work Tal Orot, Rabbi Yaakov Meir Shpielman explains that the yechida's illumination uses the chaya and neshama soul levels to serve as it's vessel, thereby, affecting the body indirectly by way of these soul levels. However, this can't be the only pathway for yechida to reach the body - though it's probably the usual norm. The Chabad Chassidic classic Kuntres Hitpa'alut discusses downloading the yechida into conscious awareness - referring to this state of consciousness as, "a revelation of the yechida". Furthermore, the most recent Lubavitcher Rebbe, Rabbi Menachem Mendel Schneerson, explains that while yechida does not have a specific organ in the body to claim as it's own, it is associated with the whole body (see the book "On the Essence of Chassidus").
Since yechida's illumination can't be expressed by a fragment of the human being, like through a single bodily organ, it seems likely that the whole person working as an integrated single unit can serve as the vessel for yechida. This soul level is so powerful that it requires the cooperation and coordination of a whole integrated person in order to receive the download of its illumination. So the whole person ends up becoming yechida's vessel.
This understanding helps explain why Judaism places tremendous emphasis on the physical side of human life. While there are world religions which heavily emphasize transcendence, i.e. a retreat away from the physical realm, Judaism usually includes the physical side of life in religious devotional service, allowing a whole person, body and soul, to participate in reaching out towards the Creator. Only when the whole person participates in the devotion, can the highest soul level, the yechida also join along. In certain cases, this can extend to including even a person's life interests - as the holy Baal Shem Tov was fond of quoting, "In all your ways know Him ..." (Proverbs 3:6).
Besides needing a whole person to serve as a vessel for the yechida, there is also another dynamic occurring when the participation includes the lowest levels of the person, the yechida actually feels more at home. This is because there's a unique relationship between the highest and lowest levels. In any well planned project the final outcome is usually contained in the most initial thought. Therefore, the first step and last step of the project most closely match, in perfect reflection, more closely than any of the other stages. All the middle stages, no matter how incredible, only serve as a bridge linking the first thought to the final outcome and therefore, aren't included in the special relationship shared by the very first and last stages. For example, if one wants to build a house, if well planned out, the very first notion of how the house should look will be reflected in the final product. The same idea applies to the soul/body relationship, the stages a human being which the Creator created first, i.e. the yechida, are best reflected in the body, which was created last.
The five soul levels merely reflect the placement of soul in relation to body. So the level, illuminating the liver, is called nefesh and the level illuminating the heart/lung area is called ruach, merely because of their placement in relationship to the body. However, placement settings are flexible and can change when a person spiritually grows. What was previously considered ruach can join the nefesh, becoming part of nefesh. When this happens, what was previously considered neshama will become the new ruach. This pattern will chain all the way up and as a result what was previously yechida will now become chaya and the person will attain a brand new yechida.
Such a pattern of continual download of yet higher soul levels can occur because soul levels are endlessly continuous upward into higher spiritual realms. With each stage of growth a person can change the position of the different soul levels in relation to the body, thereby, changing their names and functions. So what was yechida to fragmented vessels in an earlier state of a human being's spiritual development might become merely the nefesh to a more wholesome integrated human being in a more advanced stage of spiritual growth.