Final answer:
The brain knows you're upside down through the vestibular system, which detects head orientation and motion via hair cells. Neural impulses from the stimulated hair cells travel to the brain, where the vestibulocochlear nerve, brainstem, and cerebellum adjust posture for balance.
Step-by-step explanation:
The process of how your brain knows you are upside down, even with your eyes closed, involves the vestibular system located in the inner ear. This system includes the utricle and saccule, which respond to head orientation and motion.
When the head is tilted, the gravity pulls on calcium carbonate crystals causing a shift in the gelatinous layer where hair cells—similar mechanoreceptors to those found in the auditory system—are embedded.
This bending of stereocilia on the hair cells activates them, generating neural impulses that travel through the vestibulocochlear nerve to the brainstem and cerebellum. Here they connect to various brain regions that play a role in balance, motion perception, and coordination of movements.