24.1. Hair cells and their sensory organs

The ears provide the senses of hearing and balance. Their middle portion is an air-filled cavity that connects to the mouth or nasopharynx. This connection allows for adjustment to changes in atmospheric pressure, it influences hearing and voice and it can allow for infections to spread between the two cavities. The inner portions of the ears are fluid-filled chambers that contain hair cells. These sensory detectors do not regenerate in mammals and exposure to loud noise produces permanent hearing loss.

Hair cells

Some epithelial cells have apical specializations called stereocilia. When the stereocilia are deflected, ion channels open and the cell is stimulated. This arrangement forms an ancient type of mechanoreceptor called hair cell. Various types of stimuli are detected by animals through hair cells. The stereocilia can be deflected by fluid flow or by contact with a sensory structure. The former case is found in the lateral lines of fishes and in the semicircular canals of all vertebrates, whereas the later case is found in the statocysts of invertebrates, the cochlea of mammals and the otolithic organs of all vertebrates.

The lateral lines of fishes and the statocysts of invertebrates can be found in contact with the medium.

Lateral line

The lateral line system allows the detection of movement, vibration, and pressure gradients in the water surrounding a fish, providing spatial awareness and the ability to navigate in the environment. This plays an essential role in orientation, predatory behavior, defense, and social schooling.

The functional unit of the lateral line is the neuromast. This is a mechanoreceptive organ that detects mechanical changes in water. Superficial neuromasts are located externally on the surface of the body, while canal neuromasts are located along the lateral lines in subdermal, fluid filled canals. Each neuromast consists of a group of hair cells whose hair bundles are covered by a flexible gelatinous cupula.

Hair cells utilize a system of transduction that uses rate coding in order to transmit the directionality of a stimulus. Hair cells of the lateral line system produce a constant, tonic rate of firing. As mechanical motion is transmitted through water to the neuromast, the cupula is deflected, causing deflection of the embedded hair bundles. This results in opening of ion channels in the hair cells. Deflection towards the longest stereocilium results in depolarization of the hair cell, increased neurotransmitter release by the hair cell. Deflection towards the shorter stereocilium has the opposite effect. These electrical impulses are then transmitted by sensory neurons to the brain.

Statocysts

The statocyst is a balance sensory receptor organ present in aquatic invertebrates, including bivalves, cnidarians, ctenophorans, echinoderms, cephalopods, and crustaceans. The statocyst consists of a sac-like structure containing a mineralized mass (statolith) and numerous innervated sensory cells with apical projections (setae). The statolith's inertia causes it to push against the setae when the animal accelerates. Deflection of setae by the statolith in response to gravity activates neurons, providing feedback to the animal on change in orientation and allowing balance to be maintained.

The statolith shifts as the animal moves. Any movement large enough to throw the organism off balance causes the statolith to brush against tiny bristles which in turn send a message to the brain to correct its balance. A key feature of this organ is having labeled lines. This means that the signal from each receptor cell is not merged into a single or a few axons for the entire organ. Such merging would preserve the signal but it would discard the location information. The signal from each region of the organ is transmitted to the brain through separate axons. The brain determines the direction of the movement (or gravity) by identifying the axons that are bringing in the signal. It can also use the rate of action potentials to determine the strength of the signal (acceleration on the statocyst).

Inner ear

In contrast with the lateral line and statocysts which tend to be found near the surface of the body, the cochlea, otolith organs and semicircular canals are contained in a bony enclosure immediately lateral to the braincase, inside the petrous part of the temporal bone. The soft tissues are called the membranous labyrinth, and it is surrounded by dense bone (the bony labyrinth). Together, the membranous and bony labyrinths form the inner ear.

In humans, the inner ear has six sensory regions containing hair cells: three semicircular canals, two otolith organs (saccule and utricle) and the cochlea. Because these are spots where the epithelium is differentiated from the surrounding epithelium, they are called maculae (latin for spot).

Otolith organs

Vertebrates have sensory structures in the inner ears that are analogous (similar function) to the statocysts of invertebrates. They are the otolithic organs: saccule, utricle and lagena (missing in mammals). These otolithic organs share the property of containing mineralized bodies called otoliths (oto = ear, lithos = stone). When the head tissues move, the otholiths lag, producing deflection of the hair bundles of neighboring hair cells. This allows vertebrates to sense head position (gravity), linear movement, or vibration. The neural signals are transmitted through the vestibulocochlear nerve to the brain stem and cerebellum.

In fishes, otoliths receive specific names for each macula. The concretion is called sagitta in the saccule, lapillus in the utricle and asteriscus in the lagena (absent in most mammals). They can reach large sizes (31.4 mm, 11% of the body length), although they remain smaller in most species. The otoliths grow throughout the life of the fish by peripheral deposition of calcium carbonate (mostly aragonite). The growth rate tends to accompany that of the body and vary along the year, resulting in the appearance of rings that resemble tree rings. By counting the rings, it is possible to determine the age of the fishes from habitats with well-defined growth seasons. After the death and decomposition of a fish, otoliths may be preserved within the body of an organism or be dispersed before burial and fossilization. Dispersed otoliths are one of the many microfossils which can be found through a micropalaeontological analysis of a fine sediment.

In addition to detection of linear acceleration, otolith organs are the primary hearing organs in fishes. In amphibians, sound detection is mostly performed by two other vestibular maculae, the amphibian papilla and the basilar papilla. In birds and mammals, the cochlea is the main hearing organ. In all vertebrates however, some auditory sensitivity can be detected at the otolith organs, especially in response to low-frequency sound.

In mammals, the utricle and saccule do not contain a single large otolith, but a large number of tiny otoliths (1-50 um diameter; otoconia) embedded in a gelatinous mass that forms the otolithic membrane. The stereocilia of the hair cells extend into the otolithic membrane. The otoliths essentially increase the inertia of the otolithic membrane, which moves separately from the macula in response to head movements. Tilting the head causes the otolithic membrane to slide over the macula in the direction of gravity. The movement of the otolithic membrane, in turn, deflects the sterocilia, causing some hair cells to depolarize as others hyperpolarize. The exact position of the head is interpreted by the brain based on the pattern of hair-cell depolarization.

The otolithic maculae of mammals are specialized for sensing linear acceleration, such as when gravity acts on the tilting head, or if the head starts moving in a straight line. The difference in inertia between the hair cell stereocilia and the otolithic membrane in which they are embedded leads to a shearing force that causes the stereocilia to bend in the direction of that linear acceleration.

Semicircular canals

A semicircular canal is a ring-like extension of the inner ear. Its enlarged base is called the ampulla. A group of hair cells is found inside the ampulla. They respond to rotational movement, such as turning the head while saying “no.” The stereocilia of these hair cells extend into the cupula, a gelatinous structure that partially obliterates the ampulla. As the head rotates in a plane parallel to the semicircular canal, the fluid lags, deflecting the cupula in the direction opposite to the head movement. The role of the semicircular canal is therefore detecting rotational movement of the head.

Since the hair cells are stimulated by the flow of fluid within the semicircular canal, they are not stimulated by rotational movement within in a plane that is perpendicular with the plane of the canal. This limitation was circumvented by the evolution of three semicircular canals positioned in perpendicular planes in each ear of vertebrates. Hagfishes (jawless fish) have a single semicircular canal, lampreys have two or three, and all jawed vertebrates have three semicircular canals. By comparing the relative amounts of movement detected by each ampulla, the ear can detect the direction of most head rotations within the three-dimensional (3-D) space.

Vertigo is the sensation of spinning or having one's surroundings spin about them when in reality everything is static. This may be associated with nausea, vomiting, sweating, or difficulties walking. It is typically worsened when the head is moved. Vertigo is the most common (25%) type of dizziness. The most common diseases that result in vertigo are benign paroxysmal positional vertigo (BPPV), Ménière's disease, and labyrinthitis. Less common causes include stroke, brain tumors, brain injury, multiple sclerosis, and migraines. In BPPV, some otoconia are dislodged from their usual position within the utricle, and migrate into one of the semicircular canals (the posterior canal is most commonly affected due to its anatomical position). When the head is reoriented relative to gravity, the gravity-dependent movement of the heavier otoconial debris within the affected semicircular canal causes abnormal fluid displacement and a resultant sensation of vertigo. BPPV is often treated with a number of simple movements such as the Epley maneuver or Brandt-Daroff exercises. These maneuvers aim to move the particles from locations in the inner ear where they cause vertigo, to where they do not cause these problems. Additionally, medications may be used to alleviate the nausea.

Hearing in humans

Hearing, or audition, is the transduction of sound waves into a neural signal that is made possible by the structures of the ear. The large, fleshy structure on the lateral aspect of the head is known as the auricle. Some sources will also refer to this structure as the pinna, though that term is more appropriate for a structure that can be moved, such as the external ear of a cat. The C-shaped curves of the auricle direct sound waves toward the auditory canal. The canal enters the skull through the external auditory meatus of the temporal bone. At the end of the auditory canal is the tympanic membrane, or ear drum, which vibrates after it is struck by sound waves. The auricle, ear canal, and tympanic membrane are often referred to as the external ear. The middle ear consists of a space spanned by three small bones called the ossicles. The three ossicles are the malleus, incus, and stapes, which are Latin names that roughly translate to hammer, anvil, and stirrup. The malleus is attached to the tympanic membrane and articulates with the incus. The incus, in turn, articulates with the stapes. The stapes is then attached to the inner ear, where the sound waves will be transduced into a neural signal. The middle ear is connected to the pharynx through the Eustachian tube, which helps equilibrate air pressure across the tympanic membrane. The tube is normally closed but will pop open when the muscles of the pharynx contract during swallowing or yawning.

The external ear contains the auricle and ear canal. The middle ear contains the tympanic membrane and the ossicles, and is connected to the pharynx by the Eustachian tube. The inner ear contains the cochlea and vestibule, which are responsible for audition and equilibrium, respectively.

The inner ear is often described as a bony labyrinth, as it is composed of a series of canals embedded within the temporal bone. It has two separate regions, the cochlea and the vestibule, which are responsible for hearing and balance, respectively. The neural signals from these two regions are relayed to the brain stem through separate fiber bundles. However, these two distinct bundles travel together from the inner ear to the brain stem as the vestibulocochlear nerve. Sound is transduced into neural signals within the cochlear region of the inner ear, which contains the sensory neurons of the spiral ganglia. These ganglia are located within the spiral-shaped cochlea of the inner ear. The cochlea is attached to the stapes through the oval window.

The oval window is located at the beginning of a fluid-filled tube within the cochlea called the scala vestibuli. The scala vestibuli extends from the oval window, travelling above the cochlear duct, which is the central cavity of the cochlea that contains the sound-transducing neurons. At the uppermost tip of the cochlea, the scala vestibuli curves over the top of the cochlear duct. The fluid-filled tube, now called the scala tympani, returns to the base of the cochlea, this time traveling under the cochlear duct. The scala tympani ends at the round window, which is covered by a membrane that contains the fluid within the scala. As vibrations of the ossicles travel through the oval window, the fluid of the scala vestibuli and scala tympani moves in a wave-like motion. The frequency of the fluid waves match the frequencies of the sound waves. The membrane covering the round window will bulge out or pucker in with the movement of the fluid within the scala tympani.

A sound wave causes the tympanic membrane to vibrate. This vibration is amplified as it moves across the malleus, incus, and stapes. The amplified vibration is picked up by the oval window causing pressure waves in the fluid of the scala vestibuli and scala tympani. The complexity of the pressure waves is determined by the changes in amplitude and frequency of the sound waves entering the ear.

A cross-sectional view of the cochlea shows that the scala vestibuli and scala tympani run along both sides of the cochlear duct. The cochlear duct contains several organs of Corti, which tranduce the wave motion of the two scala into neural signals. The organs of Corti lie on top of the basilar membrane, which is the side of the cochlear duct located between the organs of Corti and the scala tympani. As the fluid waves move through the scala vestibuli and scala tympani, the basilar membrane moves at a specific spot, depending on the frequency of the waves. Higher frequency waves move the region of the basilar membrane that is close to the base of the cochlea. Lower frequency waves move the region of the basilar membrane that is near the tip of the cochlea.

The three major spaces within the cochlea are highlighted. The scala tympani and scala vestibuli lie on either side of the cochlear duct. The organ of Corti, containing the mechanoreceptor hair cells, is adjacent to the scala tympani, where it sits atop the basilar membrane.

The organs of Corti contain hair cells, which are named for the hair-like stereocilia extending from the cell’s apical surfaces. The stereocilia are an array of microvilli-like structures arranged from tallest to shortest. Protein fibers tether adjacent hairs together within each array, such that the array will bend in response to movements of the basilar membrane. The stereocilia extend up from the hair cells to the overlying tectorial membrane, which is attached medially to the organ of Corti. When the pressure waves from the scala move the basilar membrane, the tectorial membrane slides across the stereocilia. This bends the stereocilia either toward or away from the tallest member of each array. When the stereocilia bend toward the tallest member of their array, tension in the protein tethers opens ion channels in the hair cell membrane. This will depolarize the hair cell membrane, triggering nerve impulses that travel down the afferent nerve fibers attached to the hair cells. When the stereocilia bend toward the shortest member of their array, the tension on the tethers slackens and the ion channels close. When no sound is present, and the stereocilia are standing straight, a small amount of tension still exists on the tethers, keeping the membrane potential of the hair cell slightly depolarized.

The hair cell is a mechanoreceptor with an array of stereocilia emerging from its apical surface. The stereocilia are tethered together by proteins that open ion channels when the array is bent toward the tallest member of their array, and closed when the array is bent toward the shortest member of their array.

As stated above, a given region of the basilar membrane will only move if the incoming sound is at a specific frequency. Because the tectorial membrane only moves where the basilar membrane moves, the hair cells in this region will also only respond to sounds of this specific frequency. Therefore, as the frequency of a sound changes, different hair cells are activated all along the basilar membrane. The cochlea encodes auditory stimuli for frequencies between 20 and 20,000 Hz, which is the range of sound that human ears can detect. The unit of Hertz measures the frequency of sound waves in terms of cycles produced per second. Frequencies as low as 20 Hz are detected by hair cells at the apex, or tip, of the cochlea. Frequencies in the higher ranges of 20 KHz are encoded by hair cells at the base of the cochlea, close to the round and oval windows. Most auditory stimuli contain a mixture of sounds at a variety of frequencies and intensities (represented by the amplitude of the sound wave). The hair cells along the length of the cochlear duct, which are each sensitive to a particular frequency, allow the cochlea to separate auditory stimuli by frequency, just as a prism separates visible light into its component colors.

The standing sound wave generated in the cochlea by the movement of the oval window deflects the basilar membrane on the basis of the frequency of sound. Therefore, hair cells at the base of the cochlea are activated only by high frequencies, whereas those at the apex of the cochlea are activated only by low frequencies.

Summary

Mechanoreceptors like hair cells are found in most animals from cnidarians to vertebrates. In mammals, they are found only in the inner ears which are composed of vestibule and cochlea. The vestibule contains three semicircular canals, the saccule and the utricle. These organs detect rotational and linear accelerations of the head. The cochlea contains the organ of Corti which is specialized in detecting sound. Acoustic waves enter the ear by reflecting on the auricle, crossing the ear canal and vibrating the eardrum. The vibration is then transferred from the eardrum to oval window by the malleus, incus and stapes, the smallest bones of the body. The oval window vibrates the fluid inside the inner ear, bringing the signal to the hair cells in the organ of Corti. Ion channels open when the hair bundle of hair cell is deflected by a mechanical stimulus and this leads the cell to secrete neurotransmitters and stimulate the associated sensory neuron. The mechanical stimulus is therefore transduced into a neural stimulus. This forms the base of the senses of balance and hearing.

Key terms

Stereocilia, hair cell, hair bundle, statocyst, statolith, lateral line, neuromast, cupula, otolith, cochlea, vestibule, bony labyrinth, saccule, utricle, semicircular canal, lagena, basilar papilla, amphibian papilla, macula, sensory epithelium, vestibulocochlear nerve, sagitta, lapillus, asteriscus, linear acceleration, head rotation, gravity, ampulla, vertigo, auricle, pinna, ear canal, external acoustic meatus, eardrum, tympanic membrane, auditory ossicle, malleus, incus, stapes, auditory canal, Eustachian tube, scala vestibuli, scala tympani, oval window, round window, basilar membrane, tectorial membrane, organ of Corti, inner hair cell, outer hair cell, mechanoreceptor, tonotopy.

Figure credits

Figure 1 by Bechara Kachar - http://irp.nih.gov/our-research/research-in-action/high-fidelity-stereocilia/slideshow, Public Domain, https://commons.wikimedia.org/w/index.php?curid=24468731

Figure 2 by Thomas.haslwanter - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=21451808

Figure 3 by Art of Pogrebnoj-Alexandroff - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=25798632

Figure 4 by Davis, W. J. (1968) - http://caspar.bgsu.edu/~courses/Neuroethology/Labs/Images/Statocyst.jpg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=22495818

Figure 5 by Mouagip. This vector graphics image was created with Adobe Illustrator. - File:Bony labyrinth.png, Public Domain, https://commons.wikimedia.org/w/index.php?curid=15023796

Figure 6 by Uwe Kils - Transferred from de.wikipedia to Commons by JohnnyMrNinja using CommonsHelper., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=6031144

Figure 7 by OpenStax College - Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013., CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=30147997

Figure 8 by OpenStax College - Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013., CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=30147996

Figure 9 by OpenStax - https://cnx.org/contents/FPtK1zmh@8.25:fEI3C8Ot@10/Preface, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=30147991

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Figure 11 by OpenStax - https://cnx.org/contents/FPtK1zmh@8.25:fEI3C8Ot@10/Preface, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=30147994

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Figure 13 by OpenStax College - Anatomy & Physiology, Connexions Web site, with micrograph provided by the Regents of University of Michigan Medical School © 2012. http://cnx.org/content/col11496/1.6/, Jun 19, 2013., CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=30148016

Figure 14 by OpenStax - https://cnx.org/contents/FPtK1zmh@8.25:fEI3C8Ot@10/Preface, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=30147995