Final answer:
The axis (C2 vertebra) articulates with the atlas (C1 vertebra) at the atlantoaxial joint to allow side-to-side head movement when saying “no”, thanks to a pivot joint mechanism.
Step-by-step explanation:
The vertebral bone that articulates with the bone above it to allow your head to move from side-to-side when you say “no” is the axis (C2 vertebra). The specific joint that facilitates this motion is called the atlantoaxial joint, which is formed between the first cervical vertebra (the atlas, or C1), and the second cervical vertebra (the axis, or C2) with its distinctive dens (odontoid process). This joint is a type of pivot joint that permits rotation, allowing the head to turn from left to right.
The atlantoaxial joint consists of three articulations: the paired superior articular processes of the axis that articulate with the inferior articular processes of the atlas, and the pivot joint formed between the dens of the axis and the inner aspect of the anterior arch of the atlas. A ligament holds the dens in place against the anterior arch, enabling the atlas to rotate on top of the axis, thus allowing the head to move side-to-side as in the motion of saying “no”.
The bone that articulates with the bone above it to allow your head to move from side-to-side when you say "no" is the atlas (C1 vertebra).