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Structure of Ear Lesson Name : Structure of Ear Lesson Developer : Dr Mahtab Zarin and Dr. Zubeda College/Department : Department of Zoology Institute of Life Long Learning, University of Delhi Structure of Ear Table of Contents • Introduction • Structures of Ear 1. 2. 3. External (outer) ear • Auricle • Ear canal • Tympanic membrane Middle Ear • Auditory ossicles • Skeletal muscles • Eustachian tube Inner (Internal) Ear • Bony labyrinth • Membranous labyrinth • Cochlea • Organ of Corti • Semicircular Canals • Utricle and Saccule • Structures of Neural Pathways 1. Structure of auditory nerve pathways 2. Structure of equilibrium (vestibular) nerve pathways • Summary • Exercises • Glossary • References Institute of Life Long Learning, University of Delhi Structure of Ear Learning objectives • Explain the structures of the ear; • Understanding the functions of each part of the ear; • Describe structures and function of receptor organs for equilibrium • To discuss neural pathways/parts of ear. Introduction Hearing is among the five senses in our body that is vision, taste, smell, and touch. The ears collect sound and transmit it to brain, where it is construed, so that speech, music, and other signals can be listened. So, the auditory system needs a source of sound, mechanism for detecting this sound, mechanisms for transmitted sounds to the central nervous system, and pathways of central nervous systems in order to send this sensory information to the brain where it can be processed, interpreted and accumulated. Structure of Ear The ear is the part of auditory system, which perceives sound and helps in maintenance of balance and body position. Ear is partitioned into three major parts (Fig. 1): 1. External ear or outer ear • It collects sound waves and channels them inside of the ear. • It is the most outer part of the ear. • It is subdivided into the auricle, ear canal and tympanic membrane. 2. Middle ear • It is cavity filled with airin the temporal bone, which opens through the eustachian tube into nasopharynx and from nasopharynx to the external. • Three auditory ossicles: the mallaeus, incus and stapes are placed in the middle ear. • Manubrium or handle of the malleus is connected to backside of the tympanic membrane. • The head of the malleus is connected to wall of middle ear and is attached to the incus. • Incus articulates with the head of the stapes. • The foot plate of stapes is connected through an annular ligament to the walls of the oval window. Institute of Life Long Learning, University of Delhi Structure of Ear • The middle ear also contains two small skeletal muscles named as tensor tympani and the stapedius. These muscles control the vibrations of the tympanic membrane. • It pass on the sound waves to the oval window. 3. Internal ear or inner ear • It is also known as labyrinth and consists of two parts i.e. bony and membranous labyrinth, placed one within the other. • Bony labyrinth is a sequence of channels in petrous portion of the temporal bone of the skull. Inside these channels, enclosed by a fluid known as perilymph, is the membranous labyrinth. • Membranous labyrinth encloses a fluid called endolymph. • Labyrinth is provided with a system of passages includes two chief functional divisions that is the cochlea and vestibular system. • Cochlea is a coiled tube, which is 35mm long and makes 2.5 in humans and devoted to hearing. • Vestibular system comprises of three semicircular canals and saccule and utricle. These vestibular apparatus are dedicated to maintain the balance or equilibrium. • The sensory receptors involved are hair cells. There are six groups of hair cells in each inner ear i.e. in the cochlea, each of 3 semicircular canals, saccule and utricle. Institute of Life Long Learning, University of Delhi Structure of Ear Fig. 1. Cross-section of ear consists of outer, middle and inner ear. Source: http://cnx.org/contents/b375ea7d-22d5-4f47-b10a-41dd93637896@4 CC External ear The external ear is the outer part of the ear. It contains the fleshy noticeable outer ear termed as auricle, ear canal, and the tympanic membrane. a) Auricle (Fig. 2) The visible and flap like part is called the auricle or pinna. It is composed of a fine sheet of yellow elastic cartilage, enclosed by skin. It isconnected to the neighboring parts through ligaments and muscles and leading to external acoustic meatus through fibrous tissue. Auricle consists of the curved rim, the helix and it is extended up to the ear canal known as the external acoustic meatus. The inferior portion of the auricle is lobule. Ligaments and muscles connect the auricle to the head. The center of the pinna named as the concha that leads to the external auditory canal. Institute of Life Long Learning, University of Delhi Structure of Ear Fig. 2. The Auricle. Source:http://en.wikipedia.org/wiki/Outer_ear#mediaviewer/File:Gray 904.png Image Credit: This image is in the public domain because its copyright has expired. This applies worldwide. Function of auricle: Outer ear and head have a submissive but significant role in hearing due to their acoustic qualities. The concha, or bowl of the auricle, has a resonance of about 5 kHz, and the irregular surface of the pinna introduces other resonances and antiresonances. These acoustic features are useful to help differentiate whether sound sources are in front of the listener or behind. b) Ear canal The ear canal or external auditory canal is 2.5 cm elongated curved tube, which is placed in temporal bone. It comprised of inner segment enclosed by bone, and an outer segment enclosed by cartilage. The skin adjacent to the ear canal (external acoustic meatus) includes few hairs and specific sweat glands known as ceruminous glands, which release ear wax (cerumen). The combination of hairs and cerumen helps to prevent dust and foreign objects from entering the ear. The ear canal extended upto the external surface of the ear drum (tympanic membrane). Function of ear canal: The external auditory canal (EAC) is basically a tube that is open at one side and closed at the other. Thus the EAC behaves like a quarter-wave resonator. The resonant frequency (f0) is determined by the length of the tube whereas the curvature of the tube is irrelevant. For a tube of 2.5 cm, the resonant frequency is approximately 3.5 kHz. c) Tympanic membrane The eardrum or tympanic membrane is a fine, semi-translucent division between the outer audiory canal and middle ear. The tympanic membrane is surrounded by epidermis and covered through simple cuboidal epithelium tissue. Institute of Life Long Learning, University of Delhi Structure of Ear Value addition: Did you Know Heading text: Perforated eardrum and Otoscope Body text: Tearing of tympanic membrane is known as perforated eardrum and caused by changes in air pressure, generally heals within period of month. It can occur as a consequence of (i) ear infection e.g. otitis media, (ii) trauma e.g. cleaning the ear with pointed instrument, (iii) loud noise e.g. explosion or (iv) surgery e.g. accidental creation of a rupture. (v) Flying or landing when suffering from severe cold can also leads to perforation in ear drum as changes occurs in air pressure and blocked eustachian tubes resulting from the cold. It results in conductive hearing loss that is generally temporary. The tympanic membrane may be examined by an otoscope, a viewinginstrument that light ups and magnifies the external auditory canal and tympanic membrane. Fig. 3a. A view through the Otoscope Source: ILLL in house Institute of Life Long Learning, University of Delhi Structure of Ear Fig.3b. Large perforated eardrum Source: http://commons.wikimedia.org/wiki/File: PerforationTympan.jpg#mediaviewer/File: PerforationTympan.jpg CC Middle Ear(Fig 4.) Middle ear is made up of an air-filled cavity just at the back of the tympanic membrane, consists of three auditory ossicles: the malleus, incus, and stapes. The middle ear also attached to the upper throat through the Eustachian tube (Fig 2). a. Auditory ossicles It is separated from inner ear through a fine bony dividing wall having two small and membrane-enclosed openings termed as oval window and round window. It is attached through ligaments to the three-minute bones called as auditory ossicles. These bones are called as malleus, incus, and stapes and also usually named as the hammer, anvil, and stirrup correspondingly as named attributed to their shapes. The handle of the malleus connected to the inner layer of the tympanic membrane. The head of the malleus articulates along with body of incus. The incus, the central bone in the sequence, articulates along with the head of the stapes. The base or footplate of the stapes fixed into the oval window. Another opening beneath the oval window termed as round window is covered through a membrane known as secondary tympanic membrane. Institute of Life Long Learning, University of Delhi Structure of Ear b. Eustachian tube Front wall of the middle ear contains an aperture that directs into the auditory (phagyngotympanic)tube; commonly we call it as the eustachian tube. It is composed of both bone and elastic cartilage. The Eustachian tubeattaches middle ear with the nasopharynx. When we swallow and yawn, it opens in order to allow air to move in and outside the middle ear until the pressure in middle ear equilibrates with the new atmospheric pressure. Fig. 4.Middle Ear Anatomy Source: http://en.wikipedia.org/wiki/Middle_ear#mediaviewer/File: Blausen_0330_EarAnatomy_MiddleEar.png CC "Blausen gallery 2014". Wikiversity Journal of Medicine. DOI: 10.15347/wjm/2014.010. ISSN 20018762. - Own work Image Credit: CC BY-SA 3.0 c. Skeletal muscles Institute of Life Long Learning, University of Delhi Structure of Ear There are also two tiny skeletal muscles, which are connected to the ossicles. (1) Tensor tympani muscle is among the tinyskeletal muscles that are supplied with the mandibular branch of the trigeminal (V) nerve. It is enclosed in the bony canal above the bone part of the eustachian tube. It dampens sounds such as those produced from chewing. This restricts movement and enhances tension on the eardum or tympanic membrane to avoid damage to the internal ear because of loud sounds. (2) Second skeletal muscle is stapedius muscle that is supplied with the facial (VII ) nerve. It is known to be the smallest skeletal muscle in the body of human. It controls the intensity of vibrations in the stapes arises from loud sounds; in turn it protects the oval window, although it minimizes the receptivity of hearing. Value addition: Did you Know Heading text: Infection of Eustachian tube Body text:The purpose of the Eustachian tube within the ear is to drain the middle ear so that air pass through to clean it and also to protect it. The tube attaches itself from the middle ear to the pharynx. If this will not occur the ear can cause to middle ear inflammation. Infection in the middle ear more common to children as they possess shorter Eustachian tube, which results weak protection. As the humans grow they get the longer tube. With the closure of the Eustachian tube by infection such as sore throat or common cold, the swallowing mechanism becomes ineffective. The mucous membrane absorbs the air within the middle ear and its pressure decreases; as a result the tympanic membrane bulges inwards and its vibrations get decreased or abolished, causing discomfort and loss of hearing. Institute of Life Long Learning, University of Delhi Structure of Ear Fig.5.Blockage ofEustachian tube 3. Source:http://www.webmd.com/a-to-z-guides/eustachian-tubes Image Credit: WebMD for non-commercial purpose only Inner (Internal) Ear(Fig.6.) Inner (internal) ear is also termed as the labyrinth because of its complex series of canals (Fig 3). Anatomically, it has two major divisions: (i) external bony labyrinth and (ii) internal membranous labyrinth. It resembles to long balloons put inside a rigid tube. a. Bony labyrinth The bony labyrinth has the sensory organs for sense of balance and movement, viz. (i) the vestibules and (ii) the semicircular canals and (iii) sensory organ for hearing, the cochlea. The bony labyrinth encloses perilymph, a fluid that is chemically equivalent to cerebrospinal fluid that lies around the membranous labyrinth. The membranous labyrinth is a sequence of epithelial sacs and tubes contains endolymph, situated within the bony labyrinth. Membranous labyrinth contains the receptors for hearing and equilibrium. b. Membranous labyrinth The membranous labyrinth in the vestibule composed of two sacs known as utricle and the saccule that are filled with endolymph. There are three bony semicircular canals protruding superiorly and posteriorlyfrom the vestibule. Each one of bony semicircular canals is placed at right angles to the other two. Based on their positions, they are called as anterior, posterior, and lateral Institute of Life Long Learning, University of Delhi Structure of Ear semicircular canals. There is a swollen enlargement at one end part of each canal known as the ampulla. The regions of the membranous labyrinth that are placed within the bony semicircular canals are known as the semicircular ducts. Fig.6.Internal Ear Anatomy Source: n.wikipedia.org/wiki/Inner_ear#mediaviewer/File:Blausen_0329_EarAnatomy_Int ernalEar.png CC "Blausen gallery 2014". Wikiversity Journal of Medicine. DOI: 10.15347/wjm/2014.010. ISSN 20018762. - Own work Image Credit: CC BY-SA 3.0 c. Cochlea(Fig.7) Cochlea is situated at anterior of the vestibule. It is a bony spiral canal, which resembles to a snail’s shell and makes around 2.5 turns in the region of a central and conical bony core called modiolus. In cochlea, waves propagate from the base (close to the middle ear and the oval window) to the apex (the top or center Institute of Life Long Learning, University of Delhi Structure of Ear of the spiral). Sections through the cochlea reveal that it is partitioned into three chambers (Scalae): (vi) Scala vestibuli: It contains perilymph and lies superior to the cochlear duct and adjacent to the oval window. (vii) Scala tympani: Contain perilymph and lies inferior to the cochlear duct and terminates at the round window. (viii) Cochlear duct or scala media: It has endolymph and high potassium ion concentration that the stereocilia of the hair cells protrude into helicotrema. Fig 7.The cochlea and organ of Corti. a | The cochlea is a part of the internal ear, which also includes the vestibular apparatus and lies in the temporal bone of the skull. Sounds impinge upon the eardrum, whose vibrations are communicated through the three miniscule bones of the middle ear to initiate oscillatory changes in the pressure within the coiled Institute of Life Long Learning, University of Delhi Structure of Ear cochlea. b | As shown in a cross-section, the cochlea comprises three liquid-filled compartments, the scalae, which are separated by two elastic partitions: Reissner's membrane and the basilar membrane. The scala vestibuli and scala tympani contain perilymph, an ordinary extracellular fluid similar to that surrounding most neurons. The scala media contains K+-rich endolymph, which is secreted by cells of the stria vascularis and which also endows the scala media with a potential of about +80 mV with respect to other extracellular compartments. The somata of the afferent neurons that innervate hair cells lie in the spiral ganglion. c | Upon the collagenous basilar membrane rests the organ of Corti, a strip of epithelial cells that have highly varied structures. The human cochlea includes a single row of inner hair cells, which detect and transmit most of the afferent information to the brain. The three rows of outer hair cells have a motor function, implementing the active process that enhances hearing. The tectorial membrane is a gelatinous strip that is attached at its bottom surface to the tips of the longest stereocilia in the hair bundles of outer hair cells. Movement of the membrane relative to the bundles accordingly deflects the outer hair cell bundles. The hair bundles of inner hair cells are instead deflected by motion of the liquid beneath the tectorial membrane. d | When the cochlea is excited by sound, the back-and-forth motion of the stapes produces alternate increases and decreases in the pressure of the liquid at the base of the scala vestibuli. The pressure difference across the basilar membrane elicits a series of travelling waves that progress along the membrane at a speed of some metres per second, far below the velocity of sound in water. Source: A. J. Hudspeth (2014). From integrating the active process of hair cells with cochlear function. Nature Reviews Neuroscience15,600–614doi: 10.1038/nrn3786. http://www.nature.com/nrn/journal/v15/n9/fig_tab/nrn3786_F 1.html Displayed with permission. The cross section of the cochlear duct shows: a. Helicotrema : a small opening through which the scala tympani and the scala vestibuli join together at the apex of the cochlea. b. The cochlear duct is bounded on three sides through the basilar membrane, the stria vascularis, and Reissner's membrane. (i) Stria vascularis is a rich bed of capillaries and secretory cells. (ii) The vestibular membrane also called Reissner’s membrane divides cochlear duct from the scala vestibuli. (iii) The basilar membrane divides the cochlear duct from the scala tympani. Spiral lamina isa thin spiral and bony protuberance and an extension of the modiolus. It serves as the medial support for the basilar membrane. Institute of Life Long Learning, University of Delhi Structure of Ear Fig.8. Cross section of the cochlea. Source :http://www.med.umich.edu/histology/cns/ear.html#cortiCC d. Organ of Corti • The spiral organ or organ of Cortiis positioned on the basilar membrane. The spiral organ is a coiled area of epithelial cells extends from apex to the base of the cochlea. • The receptor cells in organ of Corti are known as hair cells. Receptors of hearing are located on the hair cells. Sensory hair cells are motorized through the potential variation among perilymph and endolymph. • Hair cellsare supported by the phalangeal cells (also called Deiter's Cells). Institute of Life Long Learning, University of Delhi Structure of Ear • These hair cells are mechanoreceptors which contains hair like projections from one end called stereocilia. • Stereocilia of the hair cells pierce the tough, membrane like reticular lamina (also knownas apical cuticular plate orreticular membrane) which is formed by inner and outer rods of corti or pillar cells. • Reticular lamina or reticular membrane divides endolymph in the scala media from underlying corticolymph and perilymph of scala tympani. • The hair cells are arranged in four rows: 3 rows of outer hair cells lateral to the tunnel shaped by the rods of corti, and one row of inner hair cells which is medially situated to the tunnel. There are around 20,000 outer hair cells and 3500 inner hair cells found in each of both (right and left) human cochlea. • Outer and inner pillar cells form a triangular shaped tunnel, named as inner tunnel, thatcontainsperilymph-type fluid known as corticolymph. • The organ of Corti is cover through atectorial membranewhich is thin, gelatinous but elastic in nature. Tectorial membrane is formed and maintainedthrough the columnar cells positioned on apex of the spiral limbus just medial to the organ of Corti. • Nerve fibers enter the organ of Corti through openings in a bone-shelf which extends from modiolus. Nerve fibers enter through the supporting cells so that synapse with hair cells. Function of outer hair cells: The main role of outer hair cells is essentially to contract upon stimulation, so that "draging" on tectoral membrane in turn stimulate inner hair cells (see weblink an animated drawing in the following http://www.neuroreille.com/promenade/english/corti/cofunct/cofunct.ht m). This mechanically alters movement of hair cells in a complex mode which sharpens frequency turning at each point beside the basilar membrane. Function of inner hair cells: Stereocilia of inner hair cells project into endolymph fluid and convert sound waves originated by fluid movement in cochlear duct into receptor potentials. Institute of Life Long Learning, University of Delhi Structure of Ear Fig.9. a. Cross section of Organ of Corti Institute of Life Long Learning, University of Delhi Structure of Ear b. . Pictoral representation of Cross section of organ of Corti. Source :a.http://www.med.umich.edu/histology/cns/ear.html#corti CC b.http://en.wikipedia.org/wiki/Organ_of_Corti#mediaviewer/File:Gray931.png CC Value addition: Did you Know Heading text: Mechanoelectrical transduction Body text:“Hair cells, the sensory receptors of the auditory and vestibular systems, are the best-understood mechanoreceptors in eukaryotes. These cells are specialized to detect stimuli of amplitudes well below a nanometre and frequencies far above a kilohertz. When a stimulus deflects the hair cell's mechanoreceptive organelle, the hair bundle (which can be due to sound in the auditory system, acceleration in the vestibular or water motion in the lateral-line system), transduction commences through an incompletely characterized group of proteins to producea stimulus force into an electrical response Fundamentally the hair bundle uses an, the receptor potential, which is a change in the voltage across the cell's membrane. The receptor potential in turn modulates the release of the neurotransmitter glutamate from synapses at the hair cell's base and thus transmits the response to the CNS” (Fig. 9). Fig. 9: The hair cell and hair bundle. a | A schematic drawing depicts a cylindrical hair cell surrounded by supporting cells. A ribbon synapse at the base of the cell releases glutamate, which excites the afferent nerve terminal and sends information into the brain. Neural centres in the brainstem can reduce the hair cell's sensitivity by activating an efferent nerve terminal that releases acetylcholine. The mechanoreceptive hair bundle consists of 10–300 cylindrical processes, the stereocilia, that project as a compact cluster from the cell's apical surface. As hypertrophic derivatives of microvilli, the stereocilia are filled with parallel actin filaments that are held in register by crosslinks of plasmin, fimbrin, espin and other proteins. The stereocilia vary systematically in length across each hair bundle so that the structure is bevelled like the tip of a hypodermic needle. Tip links interconnect successive stereocilia along the bundle's axis of mirror symmetry. A single true cilium, the Institute of Life Long Learning, University of Delhi Structure of Ear kinocilium, stands at the bundle's tall edge. During development, this structure has a role in organizing and orienting the bundle. b | In a lateral view, a scanning electron micrograph shows an individual hair bundle from the frog's sacculus, an organ that detects gravity and ground-borne vibration. About 8 μm in height, the bundle comprises a single kinocilium with a bulbous swelling at its tip and about 60 cylindrical stereocilia with tapered basal insertions. c | A top view shows the specialized hair bundle of an outer hair cell from the bat's cochlea, which is Vshaped and has only three ranks of stereocilia, the shortest of which is 0.5 μm in height. Source: A. J. Hudspeth (2014). From integrating the active process of hair cells with cochlear function. Nature Reviews Neuroscience15,600–614doi: 10.1038/nrn3786http://www.nature.com/nrn/journal/v15/n9/fig_tab/nrn3786_F2 .html Displayed with permission. Tether in stereocilia:(Fig.10) The stereocilia are collection of microvilli-type structures arranged in an order of tallest to shortest. Protein fibers tether neighboring hairs collectively within each group, in a way that this group will bend in reaction to movements of basilar membrane. Fig.10. Tether in stereocilia of hair cells. Source: http://cnx.org/resources/0dfe4b4792b7cde18817f853a971455 6/1407_The_Hair_Cell.jpg CC Institute of Life Long Learning, University of Delhi Structure of Ear Function of cochlea The perilymph in cochlea moves in response to vibrations, which processed and comes from middle ear through the oval window. When the perilymph moves, cochlear partbetween basilar membrane and organ of Cortimoves. Evantually, thousands of hair cells detect this motion through their stereocilia, and transduce that motion into electrical signals. These signals aretransformed into electrochemical impulses action potentials that travel through the auditory nerve to the brainstem for further processing. The perilymph Perilymph is an extracellular fluid, which fills the bony labyrinth of internal ear. It is placedwithin two of three chambers in cochlea of internal ear: scala tympani and scala vestibuli.The ionic composition of perilymph can be compared with that of plasma and cerebrospinal fluid. The major cation in perilymph is sodium ion. The endolymph Endolymph is the fluid which fills the membranous labyrinth of internal ear. It is also known asScarpa's fluid. Main constituent of this unique extra-cellular fluid is potassium, which is released from stria vascularis. Due to the high content of potassium in the endolymph, potassium is carried as the de-polarizing electrical current in hair cells. This is called the mechano-electric transduction (MET) current. Table 1: Difference between Perilymph and endolymph Features Perilymph Endolymph Location in labyrinth Within the osseous or bony In the membranous labyrinth labyrinth of the inner ear of the inner ear the scala tympani and scala the cochlear duct Location in cochlea chamber Composition vestibuli of cochlea K+ 4.8 mmol/L 144.8mmol/L Na+ 150.3 mmol/L 15.8 mmol/L Cl- 121.5 mmol/L 107.1 mmol/L Protein 50mg/dL 15mg/dL Institute of Life Long Learning, University of Delhi Structure of Ear Electric potential Less than that of endolymph 80–120 mV in the cochlea Function of perilymph • The perilymph enclosing the saccule, utricle, and semicircular canals, provides a padding support for the membranous labyrinth. • Perilymph of scala vestibuli and scala tympani in the cochlea conveys sound waves across the scala media. Sound waves bend the basilar membrane so that stimulate hair cells of the organ of Corti. Function of endolymph Movement occurs in the endolymph in different sections of the membranous labyrinth in reaction to the movement in the perilymph due to sound vibrations from middle ear. For example in Cochlear duct, Sound waves-induced movement in the endolymph excite the receptor cells, so that itconvert their movement into nerve impulses whichis perceived as sound by brain. Semicircular canals:Angular movement of endolymph in semicircular ducts excites the vestibular nerve receptors. The semicircular canals of both inner ears act in order to coordinate balance. Vestibular system Collectively, the receptor organs to maintain the equilibrium/balance called asvestibular apparatuswhich includessaccule, utricle, and semicircular ducts Hair cells are also present in the vestibular apparatus of the inner ear. The hair cells perceive alterations in the movement and head position through mechanism of stereocilia transduction like that discussed for hair cells in cochlea. (i) Semicircular Canals The three semicircular canalsare a part of the bony labyrinth in inner ear on each side of head. • Inside the bony canals, the membranous canals are suspended in perilymph. • The three canals are the horizontal, superior and posterior semicircular canals. • The three canals/ducts are oriented in different planesso that different ducts stimulated through the head rotation in different planes (Fig. 11). • Among various species of mammals, the dimension of the semicircular ducts is interrelated to their different kinds of locomotion. Particularly, species that are responsive and have speedy, jerky movementspossess larger Institute of Life Long Learning, University of Delhi Structure of Ear semicircular canals in relation to their body size as compared tothose, which move more cautiously. • Dilated portion of each membranouscanalcalledampullain which there is a small elevation termed as crista. Receptor cells i.e. hair cells and stereocilia are contained in the crista. Each crista is encapsulated within a gelatinous mass, the cupula.Endolymph contained in the ampulla plays a role in detecting rotational movement.The streocillia of the hair cells lies in the cupula and the bases of the hair cells are innervate by the afferent neurons of the vestibular division of the vestibulocochlear nerve(Fig. 11). Fig 11. Cross section of a semicircular canal Source: http://en.wikipedia.org/wiki/Endolymph#mediaviewer/File:Vestibular_sy stem%27s_semicircular_canal-_a_cross-section.jpgCC Table 2: Features ofSemicircular canals Features Semicircular canals Institute of Life Long Learning, University of Delhi Structure of Ear Different Lateral or horizontal or external semicircular canal Superior or anterior semicircular canal Posterior semicircular canal Position At an angle of 30degree from the horizontal plane. Positioned vertically at a right angle to posterior semicircular canal Positioned vertically at a right angle to superior semicircular canal Length 12 to 15 mm 15 to 20 mm 18 to 22 mm Shortest of the three canals Medium of the three canals Longest of the three Movement of fluid inside canal in response to Rotation of head around a vertical axis Rotation of head around a anteriorposterior axis Rotations of head in around a lateral axis Detects Rotation in transverse plane Rotation in the coronal plane plane Rotation names of canals the body canals in the sagittal body plane Click the hyperlink to view the different body planes http://en.wikipedia.org/wiki/Transverse_plane#mediaview er/File:BodyPlanes.jpg Example When you turn your head to the left and right hand sides before crossing a road or when you spin in a rotating chair When you move your head to touch your shoulders, or when doing a cartwheel. when nodding your head. or when you bend forward to pick something up from the floor Institute of Life Long Learning, University of Delhi Structure of Ear Pictoral depictionof function of semicircular canals in different directions of head movement Fig. 12. Orientation and function of three semicircular canals Source: ILLL in house Function of equilibrium. semicircular The canals:Semicircular canalsfunction in dynamic semicircular canals perceive information about angular accelerations in rotational movements and position of head in three perpendicular axes. (ii) Utricle and Saccule The utricle and saccule are slightly more complex than ampullae in semicircular canals in inner ear. • Within each membranous labyrinth, on the surface of the utricle, there is an otolithic organ called macula. • Another macula placed in on the surface of the saccule in a semi-vertical position. • Maculas constitute supporting cells and hair cells. • Hair cells in maculaswork, as mechano-receptorscomprise of 40-70 stereocilia and one true cilium known askinocilium. The kinocilium is only sensory part of hair cell that causespolarization of hair cell. • The tip of stereocilia and kinocilium protruding outward from the hair cells are enclosed by gelatinous otolithc membrane. Institute of Life Long Learning, University of Delhi Structure of Ear • There are minute stones termed as otoliths, embedded in gelatinous otolithic membrane. • The otoliths are composed of calcium carbonate crystals that make the gelatinous membrane heavier than the surrounding fluid. The otoliths also called otoconiaor statoconia or ear dustrange from 3 to 19 µm in length in humans and more dense than the endolymph. • The nerve fibers from the hair cells join those from the cristae in the vestibulocochlear nerve. Fig. 13. The membranous labyrinth of the vestibular system, which contains the organs of balance (Lower left) the cristae of the semicircular ducts and (lower right) the maculae of the utricle and saccule. Source: "macula". Art. Encyclopædia Britannica Online. Web. 04 Mar. 2015. <http://www.britannica.com/EBchecked/media/68878/The-membranouslabyrinth-of-the-vestibular-system-which-contains-the> Institute of Life Long Learning, University of Delhi Structure of Ear Fig 14. Macula of utricle and saccule and crista of semicircular canal form vestibular system. Source: http://www.med.umich.edu/histology/cns/ear.html#vestibule CC Table 3: Features of utricle and saccule Features Utricle Saccule Meaning Leather bag Money bag Function It senses linear accelerations in the and head tilts horizontal plane. It senses linear accelerations and head tilts in the vertical plane. Size comparison The utricle is larger than the saccule. It is smaller than the utricle. Shape Oblong shaped, compressed transversely. Globular Location Located in upper and back part of the vestibule. Located near the opening of scala vestibuli of cochlea. Institute of Life Long Learning, University of Delhi Structure of Ear Position macula of Hair cells in macula Macula of horizontally utricle. utricle placed on floor of Macula of saccule placed in a nearly vertical position. Hair cells comprise of 40-70 stereocilia and one kinocilium. Hair cellscomposed of many (at least 70) stereocilia and single kinocilium. Importance of otoliths: Otoliths composed gelatiouns otolithic ofcalcium carbonate-protein membrane. Eventually, granules gelatinous adds otolithic weight to membrane increasethe weight of the ends of hair cells and enhances their inertia. The addition in weight and inertia is essential to the capability of utricle and saccule to sense linear acceleration and to detect theorientation of head. Function ofUtricle and Saccule: The utricle and saccule are part of the equilibrium apparatuswithin the vestibule. The utricle and saccule convey sensory massage related to speed of positioning up and down, back and forth and alterations in head position in response to the forces of gravity. This is important to detects linear accelerationsand maintain the static equilibrium. Structure of Neural Pathways It consists of vestibular nerve and cochlear nerve pathways. (I) Structure of auditory nerve pathways Cochlear (auditory) nerve pathways represent spiral ganglia, cochlear nerve and central auditory pathway. a. Spiralganglion The spiral (cochlear) ganglion is an assembly of nerve cells which functions in the sense of hearing through sending a depiction of sound from the cochlea to the brain. • Hair cells synapse with first order sensory neurons as well as with motor neurons arise from the cochlear branch of vestibulocochlear (VIII) nerve. • The cell bodies of the afferent (sensory) neurons that branched around the bases of the hair cells are located in the spiral ganglion within the modiolus. 90-95% of these afferent neurons innerwate the inner hair cells whereas only 5-10% innervate the more and numarous outer hair cells. Institute of Life Long Learning, University of Delhi Structure of Ear There are between 35,000 and 50,000 neurons which exist in the spiral ganglion. • Most of efferent (motor) neurons in the auditory nerve ends at the outer hair cells rather on the inner hair cells. b. Cochlear (Auditory) Nerve • The fiber that connectsthe spiral ganglion with the central nervous system constitutes the cochlear nerve. • The axons of the neurons that innervate hair cells(which form cochlear (auditory) division of the vestibulocochlear acoustic nerve)terminate in the dorsal and ventral cochlear and ventral cochlear nuclei of the medulla oblongata. • The total number of afferent and efferent fibers in each auditory nerve is around 28,000. Fig. 15. Spiralganglion and Cochlear (Auditory) Nerve Source: http://www.med.umich.edu/histology/cns/ear.html#spiralGanglionOrient CC Institute of Life Long Learning, University of Delhi Structure of Ear c. Central auditory pathway (Fig. 16) • After moving through cochlear nuclei, axons carrying auditory impulses move 1. Throughvarious pathways to the inferior colliculi which are the centers for auditory reflexes 2. Through the medial geniculate body in the thalamus to the auditory cortex. Fig. 16. Auditory pathway from cochlea to brain Source: http://cnx.org/contents/c672e6c1-83aa-4c17-82f5da5be3643a5f@3#eip-id1170527734470 CC • The auditory cortex is partitioned into three divisions: Primary Auditory Cortex, Secondary Auditory Cortex and Tertiary Auditory Cortex. These structures are located concentrically to one another with the primary auditory cortex placed in middle and tertiary auditory cortex placed on the outside. The primary auditory cortex is located in Broadmann areas 41 and 42in the superior portion of the temporal lobe (Fig 17). Institute of Life Long Learning, University of Delhi Structure of Ear Fig 16. Primary auditory cortex Source: http://cnx.org/contents/c672e6c1-83aa-4c17-82f5da5be3643a5f@3#eip-id1170527734470 CC Image Credit: CC BY-SA 4.0 • The auditory association areas adjacent to the primary auditory receiving area are widespread, extending onto the insula. • The olivocochlear bundle is a prominent bundle of efferent fibers in each auditory nerve that arises from ipsilateral as well as contralateral superior olivary complex and ends primarily around the bases of outer hair cells of the organ of Corti. (II) Structure of equilibrium (vestibular) nerve pathways It represents vestibular ganglia and neural pathways for equilibrium. a. Vestibular ganglia (Fig. 17) The vestibular branch of the vestibulocochlear (VIII) nerve is composed of ampullary, utricular, and saccular nerves. These nerves contain first- ordersensory neurons as well as motor neurons that synapse with receptors of equilibrium. The first order sensory neurons transmit sensory information from the receptors, and the motor neurons transmit responsive signals to the receptors, so to change their sensitivity. Cell bodies of the 19,000 sensory neurons innervating the cristae and maculas on each side are situated in the vestibular ganglia (also called Scarpa'sganglion). Institute of Life Long Learning, University of Delhi Structure of Ear Fig. 17. Cross section showing cochlear and vestibular nerve of vestibulocochlear (VIII) nerve Source: http://www.med.umich.edu/histology/cns/ear.html#vestibCochlearNerve Orient CC b. Neural pathways for equilibrium (Fig. 17) Each vestibular nerve terminates in the ipsilateral 4-part vestibular nucleus and in the flocculonodular lobe of the cerebellum. Second-order neurons pass down the spinal cord from the vestibular nuclei in vestibulospinal tracts and ascend through the medial longitudinal fasciculi toward the motor nuclei of cranial nerves concerned with the control of eye movement. Impulses from the vestibular receptors are relayed through the thalamus to the cerebral cortex. Institute of Life Long Learning, University of Delhi Structure of Ear Fig. 17. Neural pathways for equilibrium Source: ILLL in house Institute of Life Long Learning, University of Delhi Structure of Ear Summary • Each ear (left or right)has three main regions, external ear, middle ear, internal ear. • External ear contains auricle, ear canal and tympanic membrane. Itreceives sound waves and directs them inward. • Middle ear consists of auditory ossicles (the malleus, incus, and stapes), skeletal muscles and eustachian tube. It conveys sound waves to oval window. • Internal ear constitutes bony labyrinth and membranous labyrinth. It contains the receptors for hearing and equilibrium. • Membranous labyrinth encloses a fluid known as endolymph. Perilymph contained in bony labyrinth • Labyrinth is provided with two chief functional components i.e. the cochleaand vesibular system. Cochlea is partitioned into three chambers (Scalae): Scala vestibuli, Scala tympani and Scala media or Cochlear duct. • Helicotrema is a small opening through which the scala tympani and the scala vestibuli join together at top of the cochlea. Cochlear duct is bordered on three sides through basilar membrane, stria vascularis and Reissner's membrane. • Cochlear duct contains organ of Corti which is positioned on the basilar membrane. The organ of Corti is overlaid by a tectorial membrane. • The receptor cells for hearing in organ of Corti are known as hair cells. hair cells areorganized in 3 rows of outer hair cells lateral to tunnel shaped by the rods of corti, and one row of inner hair cells which is medially situated to the tunnel. • Vestibular system contains saccule, utricle, and semicircular canals or ducts the receptor organs to maintain the equilibrium. Receptor cells includes hair cells and stereocilia are contained in the crista of semicircular canals to detects rotational movement and angular accelaration. Similarly, macula in the saccule and utricle contains hair cells to detects linear accelaration. • The cochlear branch of the vestibulocochlear (VIII) nerve is composed of spiral ganglion and cochlear nerves. Vestibular branch of the vestibulocochlear (VIII) nerve is composed of nerves from ampulla, utricule, and saccule. Institute of Life Long Learning, University of Delhi Structure of Ear Exercises A. Multiple choice question: 1. Chamber in the cochleawhich encloses the hearing receptors is A. Vestibular apparatus B. Scala vestibuli C. Cochlear duct D. Semicircular duct Answers: C 2. Organs for static equilibrium are placed in the ______________ and utilize bending of ___________ so togenerate nerve impulse. A. semicircular canals; gelatinous substance B. cochlea; perilymph C. vestibule; ampulla D. vestibule; otoliths Answers: D 3. The auditory canal is part of the ____ A. Internal ear B. Middle ear C. External ear D. Auricle Answers: C 4. The auditory ossicle that is attached to oval window of vestibule is A. Malleus B. Stapes C. Incus D. Stapedius Answers: B 5. Hair cells are the hearing receptors which are placed in A. Cochlea B. Semicircular canals Institute of Life Long Learning, University of Delhi Structure of Ear C. Vestibule D. Tympanic membrane Answers: A 6. A small aperture which allowscommunication between scala vestibuli to scala tympani known as A. Modiolus B. Spiral lamina C. Stria vascularis D. Helicotrema Answers: D 7. Organ situated just medial to the oval window is A. vestibule B. semi-circular canals C. cochlea D. concha Answers: A 8. Number of turns in the cochlea is A. 2 B. 2.5 C. 3 D. 3.5 Answers: B 9. The last organ for hearing in the inner ear is A. Crista B. Helicotrema C. Macula D. Organ of Corti Answers: D 10. Part of internal ear detects linear acceleration is A. semi-circular duct B. utricle and saccule Institute of Life Long Learning, University of Delhi Structure of Ear C. organ of Corti D. crista Answers: B 11. Part of internal ear detects angular acceleration is A. macula B. organ of Corti C. utricle D. semicircular canals Answers: D 12. Organ of corti placed over A. Tectorial membrane B. Reticular lamina C. Basillar membrane D. Reissner membrane Answers: C 13. Perilymph in internal ear: A. is extra-cellular fluid whichplaced inosseous labyrinth. B. has high concentration of sodium ions. C. has low concentration of potassium ions. D. is directlyconnected with oval window. E. ALL of the above are correct. Answer: E 14. Crista is the organ placed inside A. utricle and saccule B.auditory canal C. cochlea D. semi-circular canals Answer: D 15. Fluid filled in the membranous labyrinth is A.Endolymph B. Perilymph C. Corticolymph D.Blood Answer: A 16. Macula is component for equilibrium placed within A.Organ of corti B. Semi-circular canals Institute of Life Long Learning, University of Delhi Structure of Ear C.Utricle and saccule D. Ear drum Answer: C 17. ___________ does not formboundary of scala media A.Stria vascularis B.Tectorial membrane C. Basilar membrane D. Reissner’s membrane Answer: B 18. Endolymph is the fluid found in A. Stria vascularis B. Cochlear duct C. Scala tympani D. Scala vestibuli Answer: B 19. A central and conical bony core around which cochlea coils is called as A. Reissner’s membrane B. Helicotrema C. Spiral lamina D. Modiolus Answer: D 21. Stereocilia of outer hair cells are embedded in A. Reissner’s membrane B. Reticular lamina C.Tectorial membrane D.Basilar membrane Answer: C 22. Identify the area in following section marked with arrow A. Reissner’s membrane B.Tectorial membrane C.Helicotrema D. Basilar membrane Answer: B Institute of Life Long Learning, University of Delhi Structure of Ear B. Short answer type question: 1.Define following: I. II. Otoliths Tympanic membrane III. Modiolus IV. Reissner’s membrane V. tectorial membrane VI. Hair cells VII. Streocillia 2. Differentiate between followings: I. Endolymph and perilymph II. Among Three semicircular canals III. Uricle and saccule IV. Outer and inner hair of organ of corti 3. Write short notes on: I. Eustachian tube II. Organ of the corti III. Cochlear (auditory) nerve IV. Spiral ganglion V. VI. Vestibulocochlear nerve Hair cells C. Long answer type questions: 1. Explain with the labelled diagram the structures of different parts of middle ear. 2. Discuss various parts through the section of cochlea and their importance in the hearing. 3. Discuss about the cochlear nerve pathways 4. What are the vestibular system and their vestibular nerve pathway. Expalain with the labelled diagrams. Institute of Life Long Learning, University of Delhi Structure of Ear 5. Label the parts of the inner ear in the following figure with the help of names provided in list. Institute of Life Long Learning, University of Delhi Structure of Ear Glossary • Auditory ossicles: These are part of the middle ear and are the three smallest bones namely, malleus, incus, and stapes and plays important role in perceiving sound waves. • Auricle: It is a part of outer ear also known as pinnae and is flap like visible structure of elastic cartilage projecting outward and covered with skin. • Dynamic equilibrium: It is meant to the maintain body position (primarily head) in relation to rotational movement and speed of the movement. • ear canal: Part of external ear which leads inside to the eardrum and middle ear. The ear canal is covered with ear wax and hairs which prevent from infection and clean the ear. • Earwax or cerumen: It is a yellowish wax like substance released from the sebaceous glands located in the ear canal of outer ear and it helps in cleaning and lubricating the ear. • Organ of Corti: It is a receptive organ of hearing placed within the cochlear duct. The organ of Corti has specialized cells called hair cells which transduce sound vibrations into electrical impulses. • Otoliths: Stereocilia in saccule and utricle are covered through gelatinous substances which enclose minute stones termed as otoliths and it is responsible for maintaining static equilibrium. • Scala media: Also known as cochlear duct. This is an area between upper (Scala vestibuli) and lower (Scala tympani) chambers of the cochlea and contains hearing receptors in the organ of Corti. • Static equilibrium: It is meant to maintain the position of body (primarily the head) in response to gravitational force. • Stereocilium:It is a finger-like projection on the apical surface of sensory hair cells in the organ of Corti and vestibular apparatus of internal ear and have an important role in hearing and equilibrium. • Tectorial membrane: Situated in the organ of Corti of the cochlea. This is a sheet of cells lies above the stereocilia of the hair cells. Movement of Institute of Life Long Learning, University of Delhi Structure of Ear the hair cells attached to basilar membrane leads the stereocilia to move against the tectorial membrane, thereby initiate a nerve impulse which pass from hair cell to brain. • Tympanic membrane: It is a fine, semi-translucent division between the ear canal and middle ear. • Vestibular apparatus: It is group of the receptor organs to maintain the equilibrium in response to head position and various body movements and it includes saccule, utricle, and semicircular canals. References 1. Tortora, G.J. & Grabowski, S. Principles of Anatomy & Physiology. 13th Edition, p.642. 2. Moyes, C. D. and Schulte, P. M. (2006). Principles of Animal Physiology, p. 248. 3. Hill, R. W., Wyse, G. A. and Anderson, M. (2006). Animal Physiology. p.355. 4. Randall, D., Burggren W. and French, Kathleen (2001). Eckert Animal Physiology. 5. Widmaier, E.P., Raff, H. and Strang, K.T. (2008). Vander’s Human Physiology, XI Edition, McGraw Hill. 6. Guyton, A.C. and Hall, J.E. (2011). Textbook of Medical Physiology, XII Edition, Harcourt Asia Pvt. Ltd./W.B. Saunders Company. 7. Ganong, William F. Review of Medical Physiology. XXI Edition. Mc Graw Hill 8. Textbook of Physiology by Prof. A.K. Jain. 9. Anatomy And Physiology: In health and illness. Ross and Wilson (Tenth Edition) Weblinks • http://www.med.umich.edu/histology/cns/ear.html#EAR4 • http://apbrwww5.apsu.edu/thompsonj/AnatomyPhysiology/2010/2010Exa mReviews/Exam204Review/CH2015SpiralOrganofCorti.htm • http://www.austincc.edu/apreview/PhysText/PNSafferentpt2.html • http://www.d.umn.edu/~floven/Courses/CSD3103/homework%202.htm Institute of Life Long Learning, University of Delhi