The crista ampullaris is covered with glyocoprotein material, the cupula (C) that surrounds and protects the "hair cells" in its neuroepithelium (NE). As is the case with the hearing sense, the actual transducers are "hair cells", which are well supplied with afferent nerve fibers from pseudounipolar cells of the spiral ganglion.
When the head is moved, the endolymph in the duct lags behind a bit. You see the same phenomenon when you turn a cup of coffee on a table: the cup moves, but the coffee inside takes a couple of seconds to catch up with it. This relative movement of fluid over the cupula and the "hair cells" causes depolarization of the membranesofthelatter. This in turn is detected by the nerve fibers. The output from this part of the system are axons that constitute the vestibular branch of cranial nerve VIII.
Inner ear; H&E stain; paraffin section, 100x
*Note carefully that the statement you will read in texts that the "semicircular canals" are the site of part of the balance sensation is wrong. The canals are merely channels through the bone in which the semicircular ducts run. The canals are bony labyrinth; and sense perception, whether hearing or balance, is located in the membranous labyrinth.
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