Which sensory system senses head movement in response to gravity and relies on mechanoreceptors?

• A. Mechanoreceptors
• B. Photoreceptors
• C. Nociceptors
• D. Chemoreceptors

Answer: (A)

Head movement and orientation relative to gravity are detected by the vestibular system in the inner ear, which relies on mechanoreceptors called hair cells. In the otolith organs (utricle and saccule), gravity causes the otolith crystals to shift with head position, bending the stereocilia of hair cells. This mechanical bend opens ion channels, altering neurotransmitter release and sending signals via the vestibular nerve to brain areas that control balance and gaze. The semicircular canals also use hair cells to sense rotational movement, but gravity-related sensing specifically involves the otolith organs. The other receptor types—photoreceptors for light, nociceptors for pain, and chemoreceptors for chemicals—do not mediate sensing head movement due to gravity.