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I know both of the 2 systems work quite similar the first (saccule and utricle) detect linear acceleration and semicircular canals play the role of angular acceleration detectors. I wonder if there is a functional reason for which there are no otoliths in the cupula. Why are they important in the otolithic membrane but not necessary for the cupula ?

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In the ampulla of the semicircular canals, the flow of the endolymph is blocked by the gelatinous cupula into which the hair cells protrude. Thus, during circular motions when the endolymph is accelerated in the semicircular canals, the pressure on each side of the cupula will be different and hence results in some displacement of the cupula, which the hair cells may then detect. When the rotation stops (or the rotation speed is constant), the axonal firing rate of the hair cells will return to the firing rate at rest.

In the maculae, the gelatinous layer is not blocking any canals and hence it isn't displaced effectively by fluid motions as the cupula is in the semicircular canals. The role of the otoliths (which are heavier than the surrounding fluid/tissues) is to cause a temporal displacement of the gelatinous layer during linear accelerations, which happens due to the temporal motional lag between the otoliths and the sensory epithelium with the embedded hair cells. During head tilts, there will also be a displacement of the gelatinous layer, because of the gravitational pull on the otoliths, and the hair cells will then continue to fire their axons at increased/decreased rates (depending on the tilt direction) for the duration of the head tilt.

Thus, the method for displacing the cupula/gelatinous layer is slightly different between the maculae and semicircular canals, and in the semicircular canals, there isn't a need for otoliths to make this happen.

The book Neuroscience edited by Purves et al is a good resource on this.

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  • $\begingroup$ Great first answer Michael, especially liked the description of the physics aspect. Welcome to biology. (From review) $\endgroup$ Dec 18, 2021 at 14:36

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