Final answer:
Otoliths within the vestibular system's utricle and saccule provide inertia required for sensing linear acceleration due to head tilts, while the semicircular canals' cupula, lacking otoliths, is adapted to detect angular motion through changes in fluid dynamics caused by rotational head movements.
Step-by-step explanation:
The vestibular system comprises five vestibular receptor organs: the utricle, saccule, and three semicircular canals. The utricle and saccule contain otoliths within the otolithic membrane, which respond to linear acceleration such as gravity. These otoliths add mass to the otolithic membrane, making it sensitive to head tilts and shifts in position due to gravity, causing hair cells to send signals to the brain about head orientation.
Conversely, the semicircular canals detect angular acceleration. Their hair cells are embedded in the cupula, a gelatinous cap without otoliths. This structure responds to changes in fluid dynamics within the canals caused by head rotations. Without otoliths, the cupula can move freely with the fluid, making it sensitive to angular motions. When the head rotates, the fluid displacement within the semicircular canals bends the stereocilia of hair cells appropriately, sending signals to the brain about rotational movement.
Hence, otoliths are crucial in the saccule and utricle for added inertia to sense linear movements but are not necessary in the cupula of semicircular canals because the detection of angular motion relies on fluid dynamics rather than resistance to linear acceleration.