Theampullary cupula, orcupula, is a structure in thevestibular system, providing the sense ofspatial orientation.
The cupula is located within theampullae of each of the threesemicircular canals. Part of thecrista ampullaris, the cupula has embedded within it hair cells that have several stereocilia associated with each kinocilium. The cupula itself is the gelatinous component of the crista ampullaris that extends from the crista to the roof of the ampullae. When the head rotates, theendolymph filling the semicircular ducts initially lags behind due to inertia. As a result, the cupula is deflected opposite the direction of head movement. As the endolymph pushes the cupula, the stereocilia is bent as well, stimulating the hair cells within the crista ampullaris. After a short time of continual rotation however, the endolymph's acceleration normalizes with the rate of rotation of the semicircular ducts. As a result, the cupula returns to its resting position and the hair cells cease to be stimulated. This continues until the head stops rotating which simultaneously halts semicircular duct rotation. Due to inertia, however, the endolymph continues on. As the endolymph continues to move, the cupula is once again deflected resulting in the compensatory movements of the body when spun.
With each rotation, thehair cells undergo eitherdepolarization orhyperpolarization, depending on whether the endolymph moves them toward or away from their adjacentkinocilia, respectively. When the endolymph moves in the opposite direction, the direction of stimulation is reversed accordingly, from depolarization to hyperpolarization or vice versa.[1] The corresponding signal is transmitted to the brain by thevestibulocochlear nerve (CN VIII).
In their natural orientation within the head, the cupulae are located on the medial aspect of the semicircular canals. In this orientation, thekinocilia rest on the posterior aspect of the cupula.
TheBuoyancy Hypothesis posits that alcohol causesvertigo by affecting the neutral buoyancy of the cupula within the surrounding fluid called theendolymph. Linear accelerations (such as that of gravity) should not in theory effect a movement of the cupula when it is neutrally buoyant. The Buoyancy Hypothesis assumes that alcohol, with a different specific gravity from that of the cupula/endolymph, diffuses at different rates into the cupula and the surrounding endolymph. The result is a temporary density gradient between the cupula and endolymph, and a consequent (erroneous) sensitivity to linear accelerations such as that of gravity by a system normally signalling rotational accelerations.[2] This sensation is commonly referred to as "the spins"[citation needed].
This article incorporates text in thepublic domain frompage 1051 of the 20th edition ofGray's Anatomy(1918)
