1 Lecture: 5 The Sense of Hearing Dr. Eyad M. Hussein Ph.D of Neurology Consultant in Neurology Department, Nasser Hospital, Assistant Professor, Faculty of Medicine, Islamic University Faculty of Dentistry, University of Palestine

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3 1.Communication: hearing is essential to Language. 2.Localization: determining the location of unseen sound sources. Is Hearing Important?

The Sense of Hearing  It is the ability to detect sounds.  The most important function of hearing is involved in language and social communications.  Sound is a stimulus (energy) for hearing.  In air sound travels at about 330 m/sec. While in water sound travels at about 1500 m/sec.  The ear with all its parts is located in the temporal bone. 4

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6 1.Adequate stimulus (Sound). 2.Conduction of stimulus to sensory organs of hearing. 3.Sensory transduction of stimulus. 4.Neural transmission of the signal. 5.Central auditory processing of the signal. What needed for normal hearing?

The Peripheral Organ of Hearing The peripheral organ of hearing is the ear, which has three divisions: 1.The external ear. 2.The middle ear. 3.The inner ear. 7

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The External Ear 1.Auricle = Pinna. 2.External auditory canal (meatus). The auricle: elastic cartilage except the lobule (fat). The auricle consist of: Lobule. Concha. Helix. Antihelix. Tragus. 9

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Relations of the External Auditory Canal  Anterior: the temporo-mandibular joint.  Posterior: the mastoid.  Superior: the temporal lobe of the brain.  Inferior: the parotid gland.

Functions of the External Ear 1.Collection of sound stimuli. 2.Increase the sound pressure at the tympanic membrane. 3.Sound localization. 4.Protective function: skin of the auditory canal secretes wax (a bactericidal effect). 12

It is a thin membrane about 1 cm in diameter located between the external and middle ear. Function: 1.Separates the external auditory canal from the tympanic cavity (middle ear). 2.Transmits vibrations from outside to middle ear. 13 The Tympanic Membrane Eardrum

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15 The Tympanic Membrane

Nerves Supply of the External Ear and Tympanic Membrane 1. Greater auricular nerve: from C2, C3 roots. 2. Lesser occipital nerve: from C2 root. 3. Auriculotemporal branch of the mandibular nerve. 4. Posterior auricular branch of the facial nerve. 5. Auricular (Arnold) branch of vagus nerve. 16

The Middle Ear  It is an air filled cavity within the temporal bone.  The middle ear opens through Eustachian tube into the nasopharynx. The middle ear consists of: 1.Tympanic cavity. 2.Mastoid air cells. 3.Eustachian tube. 17

Structures in the Middle Ear  Ossicles.  Stapedius muscle.  Tensor tympani muscle.  Facial nerve and branches (chorda tympani and nerve to stapedius muscle).  Jacobson nerve (tympanic plexus).  Eustachian tube.  Mastoid air cells.

 These are the chain of bones in the middle ear that connect the eardrum to the inner ear. 1.Malleus: the malleus is the bone embedded in the eardrum. 2.Incus: the incus is a small bone which connects the malleus to the stapes. 3.Stapes: the stapes is the last bone in the chain. It connects to the fluids of the inner ear. 19 The Auditory Ossicles

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Function of Middle Ear Conduct sound from the tympanic membrane through the middle ear to the cochlea. 2. Physical protection of the cochlea. 3.The malleus with tensor tympani muscle keeps the tympanic membrane tensed. 4.The stapedius muscle reduce excessive vibration and sound transmission to the inner ear.

There are two middle ear muscles: 1. The tensor tympani muscle: inserts on the malleus. Action: dampens down vibrations of tympanic membrane. 2. The stapedius muscle: inserts on the neck of stapes. Action: dampens down vibrations of stapes. Function: 1.They regulate the range of movement in the tympanic membrane and ossicles when sound vibrations strike the membrane. 2.Filter function: Reduce excessive vibration and sound transmission to the inner ear. 23 Muscles of the Middle Ear

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The Eustachian Tube (Auditory Tube or Pharyngotympanic Tube) It begins in the anterior wall of the tympanic cavity and ends in the nasopharynx. It is 18mm at birth and 35 mm in adults. The Eustachian tube is normally closed. During swallowing, the action of the tensor veli palatini muscle opens the tube.

Functions of the Eustachian Tube 1. It connects the middle ear with the nasopharynx: the tube is closed at rest, but it opens during swallowing. 2. Equalization of pressure in the tympanic cavity. 3. Drainage and ventilation of the middle ear. 4. Protection of the middle ear. 5. Drainage of middle ear secretion.

Eustachian Tube Open Closed

Mastoid  Mastoid antrum:  Lies behind the middle ear in the petrous part of the temporal bone.  Mastoid air cells:  A series of communicating cavities that are continous with the antrum and the middle ear.

32 The inner ear is divided into two parts: 1. Osseous (Bony) labyrinth. 2. Membranous labyrinth. The Inner Ear (Labyrinth)

33 The hardest bone in the body!!! 1. Vestibule. 2. Semicircular canals. 3. Cochlea. 4. Oval window. 5. Round window. It is filled with a fluid called the perilymph (between the bony and the membranous labyrinth). Bony Labyrinth

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35 The Membranous Labyrinth  It lies within the bony labyrinth.  3 semicircular ducts: superior, posterior, lateral.  Utricle  Saccule  Cochlear duct.  Endolymphatic duct and sac.  It is filled by a fluid called the endolymph.

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◦ The cochlea (end organ for hearing). ◦ The vestibule and semicircular canals (end organ for balance). The Inner Ear

39 The Internal Auditory Canal Contains:  The facial nerve,  The acoustic nerve.  The superior vestibular nerve.  The inferior vestibular nerve.  Labyrinthine artery.

 It is a special sensory (hearing and balance) nerve.  It has two divisions: 1.Cochlear division. 2.Vestibular division. The Vestibulo-Cochlear Nerve (VIII) (Auditory or Stato-Acustic Nerve) 40

 It is a special sensory (hearing and balance) nerve.  It emerges from the anterolateral surface between the pons and medulla oblongato with the facial nerve.  It has two divisions: 1.Cochlear division. 2.Vestibular division. The Vestibulo-Cochlear Nerve (VIII) (Auditory or Stato-Acustic nerve)

The Pathway of Hearing Receptors: the organ of the corti in the cochlea of inner ear. First order neuron: cells of the spiral (cochlear) ganglion. Carry hearing sensation from the corti in the cochlea of inner ear → cochlear nerve enters the brain stem between the pons and medulla oblongata → ventral and dorsal nuclei of the pons and upper medulla. 42

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Cross Section Through Cochlea

Second order neuron: cells of cochlear nuclei: Few axons ascend in the lateral lemniscus of the same side. Most of the axons of these cells form the trapezoid body → decussate to opposite side and joined by few fibers from the cochlear nuclei of that side ascends as lateral lemniscus → inferior colliculus → medial geniculate body. Third order neuron: cells of medial geniculate body → posterior limb of internal capsule → auditory sensory area in superior temporal gyrus (area 41 & 42) where hearing is bilaterally represented. 48

49 The auricle → the external auditory meatus → vibration of the tympanic membrane → vibration of the malleus → incus → stapes → vibration of the perilymph of the inner ear → vibrationo of the endolymph causes → vibration of the basilar membrane → hair cells in the organ of Corti → cochlear nerve → auditory center of the brain the CNS (area 41 & 42 in the superior temporal lobe). Steps in Hearing

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A. Lesion of the Cochlear Division: 1.Tinnitus in irritative lesion. 2.Deafness in destructive lesion. B. Lesion of the Vestibular Division: 1.Vertigo. 2.Spontaneous nystagmus. 3.Ipsilateral incoordination. Lesion of the Vestibulo-Cochlear Nerve 52

Causes of Deafness 1.Conductive Deafness: otitis media, cholesteatoma, tympanic membrane rupture, carcinoma. 2.Sensorineural Deafness: Congenital Infection Trauma Drugs Meniere’s disease Cerebellopontine angle tumor Brain stem disease 53

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