What structure acts as a gateway for sensory impulses before they reach the cerebral cortex?

It is the thalamus. The thalamus is a midline symmetrical structure of two halves, within the vertebrate brain, situated between the cerebral cortex and the midbrain. Some of its functions are the relaying of sensory and motor signals to the cerebral cortex, and the regulation of consciousness, sleep, and alertness. The medial surface of the two halves constitute the upper lateral wall of the third ventricle. It is the main product of the embryonic diencephalon