Anatomy Of The Vestibular System

Anatomy of the vestibular system    Innervated by the vestibulocochlear nerve. Located in close proximity to the facial nerve. A tumour near this region would show signs of both vestibulocochlear nerve and facial nerve dysfunction; facial and balance problems.  3 semicircular canals filled with fluid. o At the base of each canal are the ampulae. o Each ampula contains a door-like membrane called the crista. o Each canal has a specific orientation, each being orientated 90o from the others. o The horizontal canal is not actually horizontal to Earth; it’s inclined at 30o at the anterior portion. The utricle and saccule are important features that provide different signals to the brain.  Histology   Stereocilia are individual cilia of the hair cell, with kinocilium being the largest one. The body of the hair cell is also synapsed with nerve endings that relay information to the vestibule nuclei via the vestibulocochlear nerve. Activating hair cells     There are mechanical-gated K+ channels in the cilium. These channels open with a preferred direction of displacement. If the stereocilia displace towards the direction of the kinocilium, the mechanical-gated K+ channels open to allow K+ influx, leading to depolarisation of the cell. Depolarisation leads to opening of voltage-gated Ca2+ channels, assisting with exocytosis of vesicles that act on the afferent nerve. Otolith organs  Include utricle and saccule.  Concerned with displacements (e.g. head tilt) and linear accelerations. o Unable to determine rate of movement, just that movement has occurred.  The cells of the otolith organs are unique in that they are embedded in a membrane that is below a gelatinous membrane which contains otoconia (calcium carbonate crystals) on top. Motions and the vestibular system  Linear acceleration: o Each individual hair cell that sits within the utricle or saccule has a preferred direction of orientation. o Utricle does not have a flattened shape, with anterior surface projecting upwards whilst posterior surface projecting horizontal (majority); responsible for detection of horizontal acceleration and slight detection of vertical acceleration. o Saccule sits solely in a vertical plane; responsible for detection of vertical acceleration (e.g. in an elevator). o Acceleration in a certain direction will cause otoconia to move in the opposite direction, bringing along with it the hair cells. o Once a constant speed is reached, otoconia return to their original position, bringing along with it the displaced hair cells into neutral position. o Deceleration will cause the same hair cells to bend in the opposite direction.  Head tilts: o The otoconia are subjected to gravity, and so movement of the head causes the hair cells to bend a certain way. o Since not all hair cells in the otolith organs move a certain direction, they will each produce a signal that may be slightly different. o Tilts of the head off the horizontal plane and translational movements of the head in any direction stimulate a distinct subset of hair cells in the saccular and utricular membrane, while simultaneously supressing the response of other hair cells in these organs. o Ultimately, variations in hair cell polarity within the otolith organs produce patterns of vestibular nerve fibre activity that, at a population level, can encode head position and the forces that influence it. o The discharge rate during head tilt can be measured using electrodes. o There is a rapid change in signal frequency when the tilt occurs. o Signal frequency is maintained when head tilt remains stable. Semi-circular canals  Responsible for detection of head rotations.  In contrast to the hair cells of the otolith organs (which project into a gelationous membrane with otoconia on top), hair cells in the semicircular canal project into a cupula.   The cupula serves as a “door” that can flex when pushed by fluid in the area around it. o When the head is moved, the endolymph “flows” in the opposite direction due to inertia. o This pushes onto the cupula, causing it to bend on the contralateral side and activate/inhibit hair cells. Discharge portfolio produced is somewhat different to those produced by the otolith organs.  Information from both vestibule organs are used to determine rotation of the head, much like they are used to determine position.