Ear Anatomy PDF

EAR (AURIS, VESTIBULOCOCHLEAR ORGAN) Has three parts External ear Middle ear (tympanic cavity) Internal ear (inner ear) External and middle ear is separated by the tympanic membrane  EXTERNAL EAR Auricle (collect sounds) External acoustic meatus (conducts th...

EAR (AURIS, VESTIBULOCOCHLEAR ORGAN) Has three parts External ear Middle ear (tympanic cavity) Internal ear (inner ear) External and middle ear is separated by the tympanic membrane  EXTERNAL EAR Auricle (collect sounds) External acoustic meatus (conducts the sound to the tympanic membrane) EXTERNAL EAR (continued) Auricle Formed of cartilage and connective tissue Deepest part is called concha Lobule (earlobe) is devoid of cartilage (formed of fat and connective tissue) EXTERNAL EAR (continued) Arterial supply is mainly from the posterior auricular and the superficial temporal arteries Nerve supply is from great auricular, auriculotemporal and lesser occipital Lymph drainage is to the superficial parotid, retroauricular and superficial cervical lymph nodes EXTERNAL EAR (continued) External acoustic meatus Extends between the concha and the tympanic membrane Length is about 2.5-3 cm “S” shaped Outer 1/3 is formed of cartilage Inner 2/3 is bonny (temporal bone) Glands secrete cerumen (earwax) EXTERNAL EAR (continued) Innervation of the external acoustic meatus is through Auriculotemporal nerve Auricular branch of the vagus nerve Posterior auricular Branch from glossopharyngeal nerve  Tympanic membrane Thin oval shaped membrane Lateral surface of the tympanic membrane faces anteriorly and inferiorly Outer surface is formed of epidermis, inner surface is covered by the mucosa of the middle ear, middle portion is formed of fibroelastic connective tissue External surface is innervated by the – Auriculotemporal nerve (CNV3) – Auricular branch of the vagus nerve Inner surface is by tympanic branch of glossopharyngeal (CN9) Tympanic membrane (continued) Tympanic membrane has two parts Pars tensa (bigger and tense lover part) Pars flaccida (small and flaccid upper part) ❖ Sound waves cause vibration ❖ Postero-inferior part is safest for incisions ❖ Perforation may lead to deafness  MIDDLE EAR Lies inside the petrous part of the temporal bone Has two parts Tympanic cavity (proper) Epitympanic recess o Extends superiorly from the upper border of tympanic membrane MIDDLE EAR (continued) Postero-superiorly connects with the mastoid cells through the mastoid antrum, which leads to the mastoid air cells Contents of the middle ear are: Auditory ossicles (malleus, incus, stapes) Stapedius muscle Tensor tympani muscle Chorda tympani (branch of facial nerve) Tympanic nerve plexus MIDDLE EAR (continued) Tympanic nerve plexus is formed of Tympanic branch of the glossopharyngeal nerve (Jacobson's nerve) – Contains sensory fibers (to middle ear mucosa) and preganglionic parasympathetic fibers – Lesser petrosal nerve arises from the plexus to synapse in the otic ganglion and innervate the parotid gland Caroticotympanic nerves (post-ganglionic sympathetic branches of the internal carotid plexus) MIDDLE EAR (continued) Walls of the tympanic cavity Lateral wall (membranous) Tympanic membrane (inf) Bonny wall of the epitympanic recess (sup) Medial wall (labyrinthine) Promontorium (basal turn of cochlea) – Oval window (base of stapes) – Round window (closed by secondary tympanic membrane) Roof (tegmental) Tegmen tympani (thin plate of bone separating the tympanic cavity from the middle cranial fossa) Floor (jugular) Layer of bone separating from the superior bulb of internal jugular vein Aditus to mastoid antrum Oval Lesser petrosal nerve from glossopharyngeal en route to otic ganglion Round Sympathetics Chorda tympani Anterior view MIDDLE EAR (continued) Anterior wall (carotid) Tensor tympani muscle (sup) Auditory tube (mid) Plate of bone separating from carotid canal (inf) Posterior wall (mastoid) Aditus to mastoid antrum (entrance to mastoid antrum) Pyramidal eminence (origin for stapedius muscle) Stapedius muscle MIDDLE EAR (continued) Auditory tube (pharyngotympanic tube, eustachian tube) Connects the tympanic cavity with nasopharynx About 3.5-4 cm long Equalize the pressure inside the tympanic cavity with the outside pressure Contraction of tensor veli palatini muscle opens the pharyngeal opening tube which is normally closed Opens during swallowing and yawning, also with valsalva manevour MIDDLE EAR (continued) Auditory ossicles (malleus, incus, stapes) These bones conduct the movements of the tympanic membrane to the inner ear. Malleus attaches to the tympanic membrane Base of the stapes fits into the oval window Incus lies between these two bones MIDDLE EAR (continued) Muscles controlling the movements of the auditory ossicles Tensor tympani muscle Inserts to the handle of malleus Tenses the tympanic membrane thus reducing the oscillations of the tympanic membrane Prevents damage to the inner ear due to the excessively loud sounds Innervated by medial pterygoid nerve (br of V3) Stapedius muscle Extends between the pyramidal eminence and stapes Reduces the oscillatory range of the base of the stapes Prevents damage to the inner ear due to the excessively loud sounds Innervated by the nerve to stapedius muscle (branch of facial nerve) Lesions of the facial nerve may lead to to hyperacusis MIDDLE EAR (continued) Facial nerve (CN VII) Leaves cranium through the internal acoustic meatus Traverses the petrous part of the temporal bone within the facial canal In the facial canal it is related to the medial and posterior walls of the tympanic cavity Exits the skull through the stylomastoid foramen MIDDLE EAR (continued) Facial nerve (CN VII) (continued) Within the facial canal, the facial nerve gives of three branches – Greater petrosal nerve – Nerve to the stapedius muscle – Chorda tympani Leaves the skull through the stylomastoid foramen and gives rise to the – Posterior auricular nerve Then it enters into the tissue of the parotid gland and here it gives of its five terminal (muscular) branches: – Temporal – Zygomatic – Buccal – Marginal mandibular – Cervical  INNER EAR Lies inside the petrous part of the temporal bone Contains the vestibulocochlear organ Vestibulocochlear organ is composed of three parts – Cochlea (contains receptor for hearing) – Vestibule (contains receptors for detecting linear acceleration) – Labyrinth (contains receptors detecting angular head movements) MIDDLE EAR (continued) Inner Ear Bony labyrinth Membranous labyrinth (lies inside the bony labyrinth) INNER EAR (continued) Bony labyring parts Membranous labyring parts Cochlea Cochlear duct Contains receptor for hearing (organ of corti) Vestibule Utriculus and sacculus Contains receptors for detecting acceleration Labyrinth Semicicrcular ducts Contains receptors detecting angular head movements INNER EAR (continued) Between the bony labyrinth and the membranous labyrinth there is a fluid called the perilymph Secreted by the epithelium lining the inner surface of the fibrous membrane that lines the inner surface of the bony labyrinth Drains into the CSF through cochlear aquaduct Inside the membranous labyrinth there is a fluid called the endolymph Secreted from the stria vascularis of cochlear duct Drains into the CSF through endolymhpatic duct and sac INNER EAR (continued) Bony labyrinth Cochlea (ant) Vestibule (mid) Semicircular canals (post) Inner surface of the bony labyrinth is lined with a thin fibrous membrane – Inner surface of this fibrous membrane is lined by a thin layer of epithelium which secretes the perilymph (perilymph drains into the CSF) INNER EAR (continued) Cochlea Begins at the vestibule and makes 2.5 turns around a bony core called the modiolus Upper part is called the cupula INNER EAR (continued) Osseous spiral lamina partially divides the cochlea into two parts – Scala vestibuli – Scala tympani Both scala are filled with perilymph The two scalae are connected with each other through the helicotrema at the apex Between the two scala, lies the cochlear duct of the membranous labyrinth INNER EAR (continued) Vestibule Lies between the cochlea and the semicircular canals Contains the utricle and saccule (parts of the membranous labyrinth) Medial wall bears – Oval window (base of the stapes fits) – Round window (closed by secondary tympanic membrane) INNER EAR (continued) Semicircular canals There are three semicircular canals – Anterior (superior), posterior, lateral (horizontal) Each semicircular canal extends from the vestibule and ends with a swelling called ampulla Contains the semicircular ducts of the membranous labyrinth INNER EAR (continued) Membranous Labyrinth Lies inside the bony labyrinth Contains endolymph Composed of ducts and sacs that communicate with each other INNER EAR (continued) Membranous labyrinth has two parts Cochlear duct Vestibular labyrinth - Utricle and saccule - Semicircular ducts INNER EAR (continued) Cochlear duct Lies in between the scala vestibuli and scala tympani Contains endolymph INNER EAR (continued) Base of the stapes starts waves of hydaulic pressure (base of the stapes fits onto the oval window) Wave is transmitted through the scala vestibuli Passing through helicotrema waves reach the scala tympani The pressure is finally absorbed by the secondary tympanic membrane of the round window. INNER EAR (continued) Cochlear duct lies between two membranes: Vestibular membrane (forms the roof) Basilar membrane (forms the floor) INNER EAR (continued) Structure of the organ of Corti and the formation of the cochlear nerve Spiral organ of Corti is situated on the basillar membrane and it is overlaid by the tectorial membrane There are hair cells in the organ of Corti Tips of these hair cells are embedded in the tectorial membrane The sensory cells for hearing lie in the spiral ganglion (situated in the modiolus) The peripheral processes of these neurons terminate at the base of the hair cells in the organ of corti The central processes of these neurons form the cochlear nerve INNER EAR (continued) Basic mechanism of hearing The wave motion of perilymph is transmitted to the endolymph inside the cochlear duct As a result, the basilar membrane vibrates, which causes the organ of Corti to move against the tectorial membrane, stimulating generation of nerve impulses Movements of the tectorial membrane stimulate the hair cells, which is transmitted to the neurons of the spiral ganglion Central axons of the neurons of spiral ganglion form the cochlear nerve which is a part of the CN8 INNER EAR (continued) Semicircular ducts, utricle and saccule Semicircular ducts open into the utricle through five openings (ant. and post. canals shares a single canal at one of the limbs) Utricle and saccule communicates through the utriculosaccular canal (duct) The saccule is continuous with the cochlear duct through ductus reuniens INNER EAR (continued) The semicircular canals detect angular acceleration The utricle and saccule detect linear acceleration – Utricule detect horizontal, antero-posterior and right to left, acceleration – Saccule detect vertical acceleration (up and down) INNER EAR (continued) Utricle and saccule have regions called macula where the hair cells are located (receptors for accelaration) Ampulla of the semicircular canals also bear hair cells at crista ampularis (receptors for head rotation) Peripheral processes of the neurons in the vestibular ganglion (in the internal acoustic meatus) has nerve ending that terminate at the base of the hair cells Central processes of the neurons of the vestibular ganglion constitute the vestibular nerve INNER EAR (continued) Cochlear and vestibular nerves form the vestibulocochlear nerve (CN VIII), which passes through the internal acoustic meatus (together with the facial nerve) enters the brainstem. As the axons enter the brainstem, they synapse with the second order neurons of the ventral and dorsal cochlear nuclei Auditory cortex Medial geniculate body (thalamic nucleus of hearing) Inferior collicuclus Nucleus of lateral lemniscus Superior olivary complex Ventral and dorsal cochlear nuclei Clinical notes Otitis externa Otoscopic examination Otitis media Perforation of the tympanic membrane Otosclerosis Blockage of the pharygotympanic tube Paralysis of the stapedius muscle (lesion to facial nerve) Hearing loss (conductive, sensorineural) Meniere’s disease High tone deafness