What membrane responds to gravity when the head position is changed?

The otolithic membrane plays a crucial role in balancing and spatial orientation by responding to changes in head position relative to gravity. It contains tiny calcium carbonate crystals called otoliths, which are embedded in a gelatinous layer. When the head moves or tilts, gravity causes these otoliths to shift, leading to a deformation of the stereocilia (hair cells) that project into the otolithic membrane. This deformation generates electrical signals that are communicated to the brain, allowing it to interpret the position and motion of the head.

In contrast, other membranes such as the saccule membrane and the cochlear membrane are involved in other functions. The saccule membrane is part of the vestibular system concerning vertical acceleration rather than direct gravity response. The basilar membrane, meanwhile, is involved in the auditory process, where it vibrates in response to sound waves, not head position or gravity. Therefore, the otolithic membrane is specifically designed to detect gravitational changes and provide the necessary sensory input for balance and orientation.