Sense of Hearing Anatomy PDF

Sense of hearing 1. External ear 2. Middle ear 3. Auditory tube 4. Inner ear 5. Development  The organ of hearing External ear Middle ear Inner ear Auricle and external Tympanic cavity labyrinth auditory meatus  External ear The...

Sense of hearing 1. External ear 2. Middle ear 3. Auditory tube 4. Inner ear 5. Development  The organ of hearing External ear Middle ear Inner ear Auricle and external Tympanic cavity labyrinth auditory meatus  External ear The external ear consists of two parts: 1. The auricle or pinna is a funnel-like structure 2. The external acoustic meatus is an S-shaped tube. The auricle is made up of elastic cartilage which is lined on both sides by skin. The lowest part of the auricle is soft and consists only of fibrofatty tissue covered by skin. This part is known as lobule. The external auditory meatus The external acoustic meatus conducts sound waves from the concha to the tympanic membrane. The canal is S- shaped. Its outer part is directed medially, forwards and upwards. The middle part is directed medially, backwards and upwards. The inner part is directed medially, forwards and downwards. The meatus can be straightened for examination by pulling the auricle upwards, backwards and slightly laterally. The meatus is about 24 mm long, of which the medial 2/3 (16mm) is bony, and the lateral 1/3 (8mm) is cartilaginous. Due to the obliquity of the tympanic membrane, the anterior wall and floor are longer than the posterior wall and roof. The external acoustic meatus is lined by skin and contains hairs, sebaceous glands, and ceruminous or wax glands. Ceruminous glands are modified sweat glands.  Tympanic membrane This is a thin, translucent partition between the external acoustic meatus and the middle ear. It is placed obliquely at an angle of 55 degrees with the floor of the meatus. It faced downwards, forwards and laterally. The membrane has outer and inner surfaces. The outer surface of the membrane is lined by thin skin. It is concave. The inner surface provides attachment to the handle of the malleus which extends up to its centre. The inner surface is convex. And lined by a low ciliated columnar epithelium. The point of maximum convexity lies at the tip of the handle of the malleus and is called the umbo. The membrane is thickened at its circumference which is fixed to the tympanic sulcus of the temporal bone on the tympanic plate. Superiorly, the sulcus is deficient. Here the membrane is attached to the tympanic notch. From the ends of the notch, two bands, the anterior and posterior malleolar folds, are prolonged to the lateral process of the malleus. While the greater part of the tympanic membrane is tightly stretched, and is called the pars tensa, the part between the two malleolar folds is loose and is called the pars flaccida. The middle fibrous layer made up of Tympanic membrane superficial radiating fibres and deep circular fibres. The circular fibres are minimal at the centre and maximal at the periphery. The fibrous layer is replaced by loose areolar tissue in the pars flaccida. Tympanic membrane comprises all the three embryonic layers – outer layer is ectodermal, inner layer is endodermal while middle one is mesodermal in origin.  Middle ear The middle ear or tympanic cavity is a narrow air filled space situated in the petrous part of the temporal bone between the external ear and the internal ear. The middle ear is shaped like a cube, that is compressed from side to side. The cavity of the middle ear can be subdivided into the tympanic cavity proper which is opposite the tympanic membrane; and the epitympanic recess which lies above the level of the tympanic membrane. Features Shape and size Parts Boundaries Medial or labyrinthine wall Boundaries Thee medial wall separates the middle ear from the internal ear. The promontory is a rounded bulging produced by the first turn of the cochlea. The fenestra vestibuli is an oval opening posterosuperior to the promontory. It leads into the vestibule of the internal ear and is closed by the foot-plate of the stapes. The fenestra cochleae is a round opening at the bottom of a depression posteroinferior to the promontory. It opens into the scala tympani of the cochlea, and is closed by the secondary tympanic membrane. The prominence of the facial canal runs backwards just above the fenestra vestibuli, to reach the lower margin of the aditus. The canal then descends behind the posterior wall to end at the stylomastoid foramen. Medial or labyrinthine wall  Roof or tegmental wall The roof separates the middle ear from the middle cranial fossa. It is formed by thin plate of bone called the tegmen tympani.  Floor or jugular wall The floor is formed by a thin plate of bone which separates the middle ear from the superior bulb of the internal jugular vein.  Boundaries Anterior or carotid wall The uppermost part of the anterior wall bears the opening of the canal for the tensor tympani. The middle part has the opening of the auditory tube. The inferior part of the wall is formed by a thin plate of bone which forms the posterior wall of the carotid canal. The plate separates the middle ear from the internal carotid artery Anterior wall or carotid wall  Boundaries Posterior or mastoid wall Superiorly, there is an opening or aditus through which the epitympanic recess communicates with the mastoid or tympanic antrum. Fossa incudis is a depression which lodges he short process of the incus. Pyramid is a conical projection. It has an opening at its apex for passage of the tendon of the stapedius muscle. Posterior canaliculus for the chorda tympani through which the nerve enters the tympanic cavity. Posterior or mastoid wall  Lateral or membranous wall The lateral wall separates the middle ear from the external acoustic meatus. It is formed mainly by the tympanic membrane and partly by the squamous temporal bone  Scutum The term scutum derives from the Latin word for shield: the scutum in the ear is a sharp bony spur formed by the superior wall of the external auditory canal and the lateral wall of the tympanic cavity.  Prussak’s space Prussak's space is the small middle ear recess, bordered laterally by the flaccid part of Shrapnell's membrane, superiorly by the scutum (a sharp bony spur that is formed by the superior wall of the external auditory canal) and lateral malleal ligament, inferiorly by the lateral process of the malleus, and medially by the neck of the malleus. Boundaries  Contents 1. Three small bones or ossicles (malleus, incus, stapes) 2. Ligaments of the ear ossicles 3. The tensor tympani muscle and the stapedius muscle. 4. Chorda tympani and tympanic plexus. 5. Vessels supplying and draining the middle ear 6. Air  Ear ossicles The ossicles of the ear are the only bones fully formed at birth. Ear ossicles Malleus Incus Stapes  Muscles of the middle ear Tensor tympani muscle Stapedius muscle Both act simultaneously to damp down the intensity of high- pitched sound waves and thus protect the internal ear. The tensor tympani lies in a bony canal that opens at its lateral end on the anterior wall of the middle ear, and at the medial end on the base of the skull. The auditory tube lies just below this canal. The muscle arises from the walls of the canal in which it lies. The muscle ends in a tendon which reaches the medial wall of the middle ear and bends sharply around the processus cochleariformis. It then passes laterally across the tympanic cavity to be inserted into the handle of the malleus.  Muscles of the middle ear The stapedius lies in a bony canal that is related to the posterior wall of the middle ear. Posteriorly, and below, this canal is continuous with the vertical part of the canal for the facial nerve. Anteriorly, the canal opens on the summit of the pyramid. The muscle arises from the walls of this canal. Its tendon emerges through the pyramid and passes forwards to be inserted into the posterior surface of the neck of the stapes.  Auditory tube Auditory tube is also known as the pharyngotympanic tube or the eustachian tube. The auditory tube is a trumpet-shaped channel which connect the middle ear cavity with the nasopharynx. It is about 4 cm long, and is directed downwards, forwards and medially. The tube is divided into bony and cartilaginous parts. The bony part forms the posterior and lateral 1/3 of the tube. It is 12 mm long, and lies in the petrous temporal bone near the tympanic plate. The cartilaginous part forms the anterior and medial 2/3 of the tube. It is 25 mm long. The tube provides a communication of the middle ear cavity with the exterior, thus ensuring equal air pressure on both sides of the tympanic membrane. The tube is usually closed. It opens during swallowing, yawning and sneezing, by the actions of the tensor and levator veli palatini muscles. Auditory tube Arterial supply Nerve supply  Internal ear The internal ear, or labyrinth, lies in the petrous part of the temporal bone. It consists of the bony labyrinth within which there is a membranous labyrinth. The membranous labyrinth is filled with a fluid called endolymph. It is separated from the bony labyrinth by another fluid called the perilymph.  Bony labyrinth The bony labyrinth consists of three parts: 1. Cochlea, anteriorly. 2. Vestibule, in the middle. 3. Semicircular canals, posteriorly.  Vestibule This is the central part of the bony labyrinth. Its lateral wall opens into the middle ear at the fenestra vestibuli which is closed by the footplate of the stapes. Three semicircular canals open into its posterior wall. The medial wall is related to the internal acoustic meatus, and presents the spherical recess in front, and the elliptical recess behind. The two recesses are separated by a vestibular crest which splits inferiorly to enclose the cochlear recess. Just below the elliptical recess, there is the opening of a diverticulum, the aqueduct of the vestibule which opens at a narrow fissure on the posterior aspect of the petrous temporal bone, posterolateral to the internal acoustic meatus. It is plugged in life by the ductus endolymphaticus and a vein; no perilymph escapes through it.  Semicircular canals There are three bony semicircular canals: 1. Anterior 2. Posterior 3. Lateral Each canal has two ends. They lie posterosuperior to the vestibule, and are set at right angles to each other. Each canal describes 2/3 of a circle, and is dilated at one end to form the ampula. These three canals open into the vestibule by five openings. Because, posterior end of the anterior semicircular canal unites with the upper end of the posterior canal to form the crus commune which opens into the medial wall of the vestibule. The anterior semicircular canal lies in a vertical plane. It is convex upwards. The posterior semicircular canal also lies in a vertical plane. It is convex backwards. The lateral semicircular canal lies in the horizontal plane with its convexity directed posterolateral. Semicircular canals  Cochlea It forms the anterior part of the labyrinth. It has a conical central axis known as the modiolus around which the cochlear canal makes two and three quarter turns. The modiolus is directed forwards and laterally. A spiral ridge of the bone, the spiral lamina, projects from the modiolus and partially divides the cochlear canal into the scala vestibuli above, and the scala tympani below. The division between the two passages is completed by the basilar membrane. The scala vestibuli communicates with the scala tympani at the apex of the cochlea by a small opening, called the helicotrema.  Membranous labyrinth Like the bony labyrinth, the membranous labyrinth also consists of three main parts: 1. The spiral duct of the cochlea or organ of Corti, anteriorly. 2. The utricle and saccule with maculae, the organs of static balance, within the vestibule. 3. The semicircular ducts with cristae, the organs of kinetic balance, posteriroly.  Duct of the cochlea or the scala media The spiral duct occupies the middle part of the cochlear canal between the scala vestibuli and the scala tympani. It is triangular in cross-section. The floor is formed by the basilar membrane; the roof by the vestibular or Reissner’s membrane; and the outer wall by the bony wall of the cochlea. The basilar membrane supports the spiral organ of Corti which is the end organ for hearing.  Duct of the cochlea or the scala media It comprises rods of Corti and hair cells. Hair is embedded in a gelatinous membrane called the membrana tectoria. The organ of Corti is innervated by peripheral processes of bipolar cells located in the spiral ganglion. This ganglion is located in the spiral canal present within the modiolus at the base of spiral lamina. The central processes of the ganglion cells form the cochlear nerve. Posteriorly, the duct of the cochlea is connected to the saccule by a narrow ductus reuniens. Duct of the cochlea or the scala media  Saccule and utricle The saccule lies in the anteroinferior part of the vestibule, and is connected to the basal turn of the cochlear duct by the ductus reuniens. The utricle is larger than the saccule and lies in the posterosuperior part of the vestibule. It receives the end of three semicircular ducts through five openings. The duct of the saccule unites with the duct of the utricle to form the ductus endolymphaticus. The ductus endolymphaticus ends in a dilatation, the saccus endolymphaticus. The ductus and saccus occupy the aqueduct of the vestibule. The medial walls of the saccule and utricle are thickened to form a macula in each chamber. The maculae are end organs that give information about the position of the head. They are static balance receptors. They are supplied by peripheral processes of neurons in the vestibular ganglion. Saccule gets stimulated by linear motion (going in “lift”). Utricle gets stimulated by horizontal linear motion (going in car).  Semicircular ducts The three semicircular ducts lie within the corresponding bony canals. Each duct has an ampulla corresponding to that of the bony canal. In each ampulla, there is an end organ called the ampullary crest or crista or cupola. Cristae respond to pressure changes in the endolymph caused by movements of the head.  Orientation of Labyrinths in Skull  Development Internal ear develops from the otic vesicle, which is a derivative of the otic placode, in the fourth week of development. This vesicle divides into a ventral component, which gives rise to the saccule and cochlear duct and a dorsal component, which gives rise to the utricle, semicircular canals, and endolymphatic duct. Otic capsule develops from the mesenchyme around the otocyst and forms the perilymphatic space, which develops into the scala tympani and scala vestibuli. The cartilaginous otic capsule ossifies to form the bony labyrinth. Auricle develops from six tissue elevations (auricle hillocks), which form aroud the margins of the dorsal portion of the first pharyngeal cleft (external auditory meatus). First three develop on the caudal edge of the first pharyngeal (mandibular) arch and next three on the cranial edge of the second pharyngeal arch. The tragus, helical root, and helical crus develop from the first arch. Second brachial arch forms antihelix, antitragus, lower helix and ear lobule. Development  Question 1. All of the following are true about the middle ear cavity EXCEPT: a) Roof is formed by tegmen tympani b) Anterior wall has opening of two canals c) Medial wall is formed by tympanic membraine d) Floor has bulb of internal jugular vein  Question 2. True about anatomy of Eustachian tube: a) Open in oropharynx b) Larger and wider in adult than children c) More horizontal in infant and children d) 16 mm in lenght  Question 3. Not a part of bony labyrinth: a) Cochlea b) Vestibule c) Utricle d) Semicircular canal  Question 4. TRUE about internal ear anatomy a) Three semicircular canals here 6 opening into the vestibule b) The angle between anterior and posterior SCC is 180 degree c) Vestibule is the central chamber d) Spiral canal makes 2 turns  Question 5. By how many openings do the semicircular canals open in the vestibule? a) 3 b) 4 c) 5 d) 6