which part of the brain extends from the brain to the spinal cord?

The part of the brain that extends into the spinal cord is the brainstem, specifically beginning at the medulla oblongata where it transitions into the spinal cord and continues downward through the brainstem into the cervical region. In humans, the brainstem comprises the midbrain, pons, and medulla oblongata, and it serves as the critical conduit and control center linking the brain to the spinal cord.