The inner ear is composed of both the cochlea (the sensory organ required for hearing) and the vestibular apparatus (which provides sensation for orientation and balance). Both of these systems, are located in the fluid filled labyrinth of the inner ear, which is itself encased in the temporal bone of the skull.
The vestibular apparatus is the structure in the inner ear that contributes to the sensation of equilibrium and coordination of head positioning and vestibulo-ocular reflexes. The system consists of the vestibule and the semicircular canals. The patient will normally have two equally functioning vestibular apparati, one for each ear, that are bathed in endolymphatic fluid. The fluid generally has the same consistency and components as cerebrospinal fluid, with the exception of slightly different potassium levels found in endolymph. This system is innervated by the eighth cranial nerve (vestibulocochlear), particularly the vestibular (or balance) division of that nerve.
The vestibule houses the two otolith organs called the utricle and the saccule. These organs respond to linear accelerations. Each of these organs have a structure called the macula, which is a single patch of sensory hair cells, on to which the otoliths attach by a “jelly” like matrix. The otoliths (also known as “crystals”) are calcium oxalate structures that are heavier than their fluid surroundings and deflect the kinocilium hair cells of the maculae toward the vector of linear acceleration (or gravity). Deflecting the kinocilium excites or inhibits the hair cells and results in the transmission of sensory information to the brain stem where balance and eye movement reflexes are generated. In the utricle, the macula lies in a horizontal plane, whereas in the the saccule, the macula lies more in a vertical plane. Thus, the utricle senses gravity predominantly in a linear horizontal direction, while the saccule distinguishes predominantly vertical linear movements.
The semicircular canals are the three looping canals found in each ear. The canals in each ear sit at approximate orthogonal angles to each other and detect rotational head movements (angular accelerations). The canals are named the: bilateral horizontal (lateral), superior, and posterior semicircular canals. Of the three canals in each ear, there will always be certain canals that are optimally stimulated or inhibited when the head is moved in a pitch, roll, or yaw fashion. When the head moves, there is a corresponding movement of fluid (endolymph) within the canals that push on a structure called the cupula, found in each canal. The cupula is an accessory structure that rides on top of the crista ampullaris and deflects the kinocilium of the hair cells on the crista when the endolymph fluid passes over it. Similarly, a deflected kinocilium excites or inhibits the hair cells in the crista and sends sensory information to the brainstem to generate compensatory balance and eye movements. The lateral canals respond to yaw like rotation of the head. The anterior and posterior canals respond to head movements in the pitch and roll axes, similar to those described with orientation of airplane movements. The canals in both ears work in tandem to provide a complementary signal for the brain. For example, with left horizontal canal excitation with head rotation to the left, there is also a right horizontal canal inhibition. The anterior and posterior canals are set up in a cross fashion. This means that when an anterior canal is excited, the contralateral (on the opposite side) posterior canal is inhibited.
Injury of the balance portion of the inner ear often presents in a variety of forms and can include vertigo, disequilibrium, and imbalance.