During darkness, there is no input from the retinohypothalamic tract to the suprachiasmatic nucleus (SCN). Thus, darkness signals the pineal gland to produce melatonin. What part of the brain is the SCN located?

A. Corpus Callosum: The corpus callosum is a large bundle of nerve fibers connecting the left and right cerebral hemispheres. It does not contain the suprachiasmatic nucleus and is not involved in regulating circadian rhythms or melatonin production. B. Cerebral Cortex: The cerebral cortex is responsible for higher-order functions such as cognition, perception, and voluntary movement. The SCN is a subcortical structure and is not located within the cortex. C. Hypothalamus: The suprachiasmatic nucleus is located in the anterior hypothalamus, directly above the optic chiasm. It serves as the brain’s “pacemaker,” regulating circadian rhythms and signaling the pineal gland to produce melatonin in response to light and darkness cues. D. Central Sulcus: The central sulcus is a prominent groove separating the frontal and parietal lobes of the cerebral cortex. It is unrelated to the SCN or circadian rhythm regulation.