A cilium is a hair-like cellular protrusion found in eukaryotic cells. Long segments of microtubules pair to form a framework that gives structure to the cilium. Cilia can be motile or primary (nonmotile) depending on the arrangement of microtubules and accompanying proteins; cells with primary cilia can be found in many places in the human body and serve as sensory cells, such as the highly specialized stereocilia found in the inner ear’s hair cells. Stereocilia are the apical end of hair cells. Hair cells are mechanosensory cells, meaning they respond to mechanical stimuli, which are physical forces such as sound waves. Stereocilia in the inner ear are responsible for two different detecting senses: hearing and balance.
The inner ear contains an organ called the cochlea. The cochlea is a small bone with a cone-shaped body and a spiral tip, resembling a snail shell; the entire structure is hollow. The inner surface of the cochlea is lined with a delicate layer of epithelial (skin) tissue and the entire structure is filled with fluid. This fluid, or perilymph, aids in the transmission of sound, which travels into the ear in waves. Sound waves make their way into the cochlea where the waves’ vibrations disturb stereocilia on hair cells. The hair cells turn the mechanical stimulus into a neural impulse that is sent to the auditory cortex in the temporal lobe of the brain. It has been found that healthy hair cells are capable of amplifying faint sounds by disturbing the perilymph and mechanically activating surrounding stereocilia.
Connected to the cochlea are three fluid-filled tubes called the semicircular canals. The fluid contained in these tubes is referred to as endolymph. The semicircular canals are oriented in such a way that they can detect head movement in any direction. One of the canals detects side-to-side motion, another detects front-to-back motion, and the third detects angular acceleration (tilt or spin). Stereocilia in the semicircular canals detect mechanical forces like those in the cochlea but, rather than sound wave detection, these stereocilia sense changes in endolymph flow. Mechanical stimulus results in transmission of a neural impulse from the hair cell to the cerebellum, an area at the base of the brain responsible for balance, movement, and coordination. Neural input from a change in head orientation lasts approximately 10 seconds into the movement. After this, if no additional change in position occurs, the input is terminated. This is why you do not feel dizzy when turning your head or feel as if you are falling when you lie on a flat surface. In contrast, vertigo is a feeling of constant dizziness that occurs due to problems that impair the function of the stereocilia, endolymph, neural impulse, or a combination of these.
Stereocilia are incredibly fragile portion of the hair cells and can be easily damaged by exceptionally loud noise, illness, toxins, aging, or head trauma. Damaged or destroyed stereocilia are incapable of regeneration, so any hearing loss associated with stereocilia injury is permanent. This sensorineural hearing loss begins with the inability to hear faint noises and progresses as the level of damage increases. It is estimated that the number of Americans with any type of hearing loss has doubled since 1970. In fact, hearing loss due to environmental noise is such a prevalent occurrence that the U.S. Environmental Protection Agency passed the Noise Control Act of 1972 as a means of protecting U.S. citizens from hazardous levels of urban noise.
See also: Age-Related Hearing Loss; Auditory System; Cochlea; Semicircular Canals; Vestibular System; Vestibulocochlear Nerve
American Speech-Language-Hearing Society. (2016). How we hear. Retrieved from http://www.asha.org/public/hearing/How-We-Hear/
Ciliopathy Alliance. (2014). Structure and function of cilia: What are cilia? Retrieved from http://www.ciliopathyalliance.org/cilia/structure-and-function-of-cilia.html
Martin, Laura J. (2014). Hearing and the cochlea. Medline Plus. Retrieved from https://www.nlm.nih.gov/medlineplus/ency/anatomyvideos/000063.htm
Noise Control Act of 1972, 42 U.S.C. 4901 to 4918. (1972). Retrieved from http://www.gsa.gov/graphics/pbs/Noise_Control_Act_of_1972.pdf