They are shaped unlike any other vertebrae
Most vertebrae are blocks of bone stacked with a disc between each pair. The atlas (C1) is a ring with no body at all. The axis (C2) has a peg called the dens that rises through that ring. There is no disc between the skull and C1, or between C1 and C2.
They move more than any other part of the spine
The joint between the skull and the atlas does most of your nodding. The joint between the atlas and the axis provides roughly half of the neck’s turning. That freedom comes from the shape of the bones and from ligaments, not from interlocking joints, so the region depends heavily on soft tissue for stability.
Important structures pass through
The lower brainstem becomes the spinal cord at this level. The vertebral arteries, which supply the back of the brain, thread through openings in the atlas. Nerves that serve the back of the head exit here too.
The area is rich in position sensors
The small muscles under the skull contain a very high density of sensors that tell the brain where the head is. This is one reason neck problems are sometimes associated with dizziness and a sense of imbalance.
What that means for care
Because the region is so mobile and so close to sensitive structures, upper cervical techniques avoid forceful rotation. They rely on precise imaging, small corrections and rechecking. Whether a correction is appropriate for you is decided only after an evaluation.