INNER EAR

Inner Ear Anatomy

Introduction zAuditory transduction takes place in the inner ear zTransduction refers to the transformation of energy from one form to another. yIn the case of the auditory system, acoustic (mechanical) energy is transformed to electrochemical energy

Inner Ear - Anatomy zAlso known as the Labyrinth b’cos it is similar to a complex, winding cave. zConsists of two parts: ythe osseous or bony labyrinth, a series of cavities within the petrous part of the temporal bone, yand the membranous labyrinth, a series of communicating membranous sacs and ducts, contained within the bony cavities.

Bony Labyrinth

zConsists of three parts: the vestibule, semicircular canals, and cochlea. zcontain a clear fluid, the perilymph, in which the membranous labyrinth is situated.

Bony Labyrinth zThe vestibule (similar to that in a house) is a oval cavity which is located between the cochlea and the semi-circular canals. zThe vestibule has two windows - oval and round, which open into the middle ear.

Bony Labyrinth zThe Bony Semicircular Canals – Are three in number, superior, posterior, and lateral, and are situated above and behind the vestibule. zThey enclose the vestibular apparatus. zEach semicircular canal has a swelling at one end - Ampulla.

Bony Labyrinth zThe cochlea bears some resemblance to a common snail-shell. yEarlier it was believed that the primary purpose of the coiling was to save space. yRecent research has however suggested a more important function. z It forms the anterior part of the labyrinth, is conical in form, and placed almost horizontally in front of the vestibule. zIt is perforated in the base through which the VIII CN enters the cochlea.

Bony Labyrinth zIt consists of a conical shaped central axis, the modiolus. zThe bony canal of the cochlea takes two turns and three-quarters around the modiolus. z1 cm wide; 5 mm from base to apex zIf straightened out, it is about 30 mm. in length and decreases in width from base to apex.

Membranous Labyrinth zIt lies within the bony labyrinth and has the same shape. zIt bathes in the perilymph (fluid of the bony labyrinth). zContains the fluid- the endolymph.

Membranous Labyrinth A human cochlea at 5 months gestation Oval Window Round Window

Membranous Labyrinth

Bony Labyrinth - Review Oval Window Cochlea Superior (anterior) semicircular canal Horizontal (lateral) semicircular canal Ampullae Round window Inferior (posterior) Semicircular canal 6

Membranous Labyrinth zThe membranous vestibule and semicircular canals contain the vestibular organs. zThe membranous vestibule (enclosed within the bony vestibule) consists of the Utricle and Saccule.

Membranous Labyrinth

Semicircular Canals zThe semicircular canals help to maintain a sense of balance by responding to gravity and changes in angular acceleration of the head (up/down, forward/back, and side to side). zThe semicircular canals are arranged roughly at right angles with each other so that they represent all three planes in three-dimensional space.

Membranous Labyrinth zThe utricle the larger of the two, is in contact with the membranous semi-circular canals. zThe Saccule is in contact with the membranous cochlea by the ductus reuniens. zBoth the Utricle and Saccule are responsible for sensing linear acceleration. zThe sensory region of the utricle and saccule is known as macula. zThe sensory regions of the semi-circular canals are located at the ampulla’s and are known as cupula.

Membranous Cochlea zThe membranous cochlea lies within the snail-shaped bony cochlea. zThe cochlea coils around the bony modiolus. zIt consists of three ducts called Scalae yScala vestibuli, scala tympani & scala media.

Membranous Cochlea zThe scala media (also known as cochlear duct) lies between the scala vestibuli and scala tympani. zThe scala vestibuli lies above the scala media and is separated by the Reissner’s membrane. zThe scala tympani lies below the scala media & is separated by the basilar membrane.

Cross section of the whole cochlea zScala media (1), scala vestibuli (2), and scala tympani (3). zThe red arrow is from the oval window, the blue arrow points to the round window. zWithin the modiolus (6), the spiral ganglion (4) and auditory nerve fibers (5) are seen. 6

Membranous Cochlea zWhile both scala vestibuli and scala tympani are filled with perilymph, the scala media is filled with endolymph. zAt the apex of the cochlea, the scala vestibuli and scala media are connected by an opening called the helicotrema. zThe endolymph in the scala media is sealed from both the other scala’s.

Membranous Cochlea

Cochlear Scala

Perilymph and Endolymph

Membranous Cochlea zThe scala vestibuli communicates with the middle ear by means of the oval window. zThe scala tympani communicates with the middle ear by means of the round window.

Basilar Membrane zIs about 35mm long and varies in width from less than 0.1 mm at the basal turn to about 0.5mm at the apical turn. zThis in reverse to that of the cochlear duct which decreases in width from base to apex.

Basilar Membrane zThis varying width of the basilar membrane is important since it determines where different frequencies are represented. yThe stiffness of the basilar membrane decreases by at least a factor of 100 from base to apex, in an approximately exponential fashion zThe Basilar membrane separates the scala media from the scala tympani.

Basilar Membrane zThe Basilar membrane is held by bony projections from the bony labyrinth. zIt is held by the spiral lamina at its medial margin (ie, towards the modiolus) and the spiral ligament at its outer edge (ie towards the outer wall of the bony labyrinth). zSituated on the superior surface of the basilar membrane (ie, inside the scala media) is the organ of Corti.

Basilar Membrane

Cochlear partition Base: Narrowest Stiffest  High Freq. Apex: Widest Least stiff  Low Freq. Denes&Pinson 1993 Animation

Inner Ear - Anatomy

Organ of corti (the real thing!) Organ of Corti

Organ of corti

Fluids of the Inner Ear zEndolymph & Perilymph - Differ in chemical composition and in their DC (direct current) voltage. zThe endolymph has a high potassium and a low sodium concentration. zPerilymph - low potassium but a high sodium content.

Inner Ear Fluids

zThe Endolymph is similar to the fluid in the central nervous system (ie, cerebrospinal fluid). zThe Perilymph is similar to extracellular fluids. zThe Endolymph has a strong positive potential because of its high pottasium concentration (~ 80 millivolts) when compared to the perilymph of the scala tympani

Inner Ear Fluids zThe Perilymph in the scala vestibuli demonstrates a positive, but much smaller potential of about 3 mV when compared to the perilymph of the scala tympani. zThe remaining cochlear structures exhibit a negative DC potential.

Inner Ear Fluids zThus, yEndolymph of scala media - +80mV yPerilymph of scala vestibuli - +3mV yPerilymph of scala tympani - 0 mV yOther cochlear structures - Negative potential.

Innervation zThe cochlea is innervated by the VIII CN. zThe human cochlea contains about 30,000 afferent (sensory) neurons and about 1,800 efferent (motor) neurons zBoth inner and outer hair cells receive afferent and efferent innervations. zThe inner hair cells receive the majority of the afferent innervations (about 95%).

Innervation of hair cells

Innervation Pattern of IHC & OHC

Innervation of hair cells zEach inner hair cell is innervated by about 20 afferent nerves (ie, cell:neuron ratio is about 1:20). zWhile a single afferent nerve innervates about 10 outer hair cells (ie, cell:neuron -10:1 ratio). zThe outer hair cells, however, receive the majority of the efferent neurons.

Organ of Corti zOrgan of Corti includes the inner and outer hair cells, and their supporting cells. zMore central of the organ of Corti are two rows of rod-like bodies, the inner and outer rods (or pillars of Corti). zThe inner & outer rods form the tunnel of Corti.

IHC and OHC

Organ of Corti – Hair cells zSituated on the basilar membrane are 3 to 5 rows of about 12, ,000 outer hair cells and one row of about 3,000 inner hair cells. zThe inner and outer hair cells are separated by the tunnel of corti. zThe inner hair cells lie towards the medial end of the tunnel of corti.

Organ of Corti zThe rows of the outer hair cells lie in the outer or lateral end of the tunnel. zThe hair cells are about 0.01mm long and 0.001mm in diameter. zLocated on the top of each hair cell are hairlike projections called stereocilia. z Outer and Inner hair cells differ in their shape, manner of innervation and organization of stereocilia.

Floor of Scala media & Organ of Corti 1-Inner hair cell 2-Outer hair cells 3-Tunnel of Corti 4-Basilar membrane 5-Habenula perforata 6-Tectorial membrane 7. Deiters Cell 8. Inner Rod Cells 9. Outer Rod Cells 7 89

Organ of Corti- the real thing!

Organ of Corti - Hair cells zThe inner hair cells are more round shaped more like the base of a vase. zThe outer hair cells are longer and thinner and are more tubular (test tube) shaped. zWhile the inner hair cells stand alone next to the inner rod cells, the outer hair cells are supported and embedded in the cells of Deiters.

Hair Cells Inner hair cellOuter hair cell

Organ of Corti - Hair cells zThe stereocilia of the inner hair cells are organized in a single row,while that of the outer hair cells are arranged in a V or W-shaped pattern.

Sterocilia of Outer hair cells (rat)

Sterocilia of Inner Hair cells

Organ of Corti - Hair cells zThe reticular lamina is a delicate membrane perforated by rounded holes which are occupied by the free ends of the outer hair cells. It extends from the heads of the outer rods of Corti to the external row of the outer hair cells.

Organ of Corti & the Reticular Lamina

Organ of Corti - Hair cells zThe Tectorial membrane covers the sterocilia of the hair cells. It is a thin fibrous membrane that extends from the modiolus and barely touches the ends of the sterocilia of the outer and inner hair cells.

Tectorial Membrane