Your very top spinal bone is called the atlas vertebra. The atlas is donut-shaped and different from the other vertebrae because it is much smaller and does not have the interlocking joints found in the rest of the spinal column.
This is so you can move your head fully from side to side, up and down, and all around to see, hear, smell, and taste what is going on as easily as possible without moving your whole body. Interlocking pieces on the atlas would restrict this range of motion, but, because of this great mobility, the atlas vertebra is more susceptible to injury and misalignment.
You are also particularly vulnerable right here because it is through your atlas that your brain stem descends into your spine to become your spinal cord. The brain stem is that very sensitive part of your brain that regulates many basic life processes such as: breathing, heart rate, blood pressure, and levels of consciousness. Your balance with gravity is also coordinated here via the Vestibulospinal reflex. In fact, this critical nerve center is the conduit through which all messages between your brain and your body must pass.
Because of the lack of interlocking joints between your atlas, your skull, and the rest of your spine, it is very easy to become misaligned. Because of the sensitivity of your brain stem, such misalignment can very easily interfere with the messages traveling between brain and body. When these messages are interfered with, your body cannot respond properly to what your brain is trying to tell it to do and it cannot function properly. That is why the NUCCA method of chiropractic focuses its attention here.