Developmentally, the inner ear begins as a simple fluid-filled chamber, which gradually changes into the labyrinthine shape shown to the right. The previous page discusses the coiled cochlea. The remaining parts comprise the vestibular system, which sense acceleration. Acceleration refer to a change in velocity, and there are two types. When an object is rotating, it is constantly changing its direction and thus has rotational acceleration. On the other hand, an object with increasing speed in one direction has linear acceleration
Rotational acceleration is sensed by the three semicircular canals, which each lie in a plane perpendicular to the other two. As shown to the right, each semicircular canal has an expanded region with a paddle-like structure, called the cupula (pink in figure), which extends across the canal. When the head is rotating in the plane of a canal, the inertia of the fluid causes the fluid in the canal to lag behind the wall of the canal. This puts pressure on the cupula and causes it to bend.
The bending of the cupula is sensed by a set of hair cells (red in figure) located on a structure called the crista. This is because the stereocilia of the hair cells extend into the cupula.
The utricle and saccule have a similar structure, which is shown to the right. They differ in their exact orientation. A mass of small calcium carbonate stones, called otoconia, lies on top of the otolithic membrane (shown in pink). A "carpet" of hair cells lies below this structure, with the stereocilia of the hair cells extending into the otolithic membrane.
During linear acceleration, the inertia of the mass of otoconia causes it to lag behind the movement of the head, bending the stereocilia of the hair cells.
Meniere's disease is a disorder of the inner ear characterized by incapacitating episodes of vertigo leading to nausea and vomiting. Tinnitus (ringing sound) and hearing loss may also be present. The patients are often in their 30s or 40s. A typical attack lasts roughly an hour. The cause is unknown but involves an excessive accumulation of the endolymph fluid, which is the fluid in the vestibular system and cochlea.
Benign Paroxysmal Positional Vertigo occurs when one of the otoconia breaks free and moves into usually the posterior semicircular canal. When the head position changes, the small stone moves through the canal, creating fluid movement. This deflects the cupula and causes vertigo. See your lab notes for further details about this disorder and its treatment.
In otosclerosis, there is excess formation of bone about the oval window, leading to reduced mobility of the stapes. The progressive hearing loss tends to become apparent either in teenage years or early adulthood. There is a strong genetic component. The treatment is either a hearing aid or surgery, which is usually successful.
See your lab notes for discussion of otitis media. Inflammation of the middle ear is usually the result of an upper respiratory tract infection, which moves up the Eustachian tubes. This is more likely in children with their short Eustachian tubes.
See your lab notes for distinguishing conduction deafness and sensory deafness.
See your lab notes on this topic.