LECTURER IN CHARGE BAMIDELE 1 O The Special

LECTURER IN CHARGE : BAMIDELE 1 O.

The Special Senses • There are five special organs responsible for the following senses: 1. The ear - Hearing - Equilibrium 2. The eye -Vision (sight) 3. The nose -Smell (olfaction) 4. The tongue -Taste (gustation) Smell and taste are Chemical senses. 5. The skin - Touch 2

THE EAR • In addition to its role in hearing, the ear also function as the sense organ of equilibrium and balance. • The ear is much more than a mere appendages on either side of the head. • A large part of it lies hidden from view, deep inside the temporal bone. • Receptors activating hearing and equilibrium are mechanoreceptors.

The Ear Parts of the ear • Outer (external) ear • Middle ear (ossicles) for hearing) • Inner ear (labyrinth) for hearing & equilibrium 4

FUNCTIONS OF PARTS OF THE EAR • EXTERNAL EAR: It has two parts namely: -Auricle/ Pinna: It is the appendages on the side of the head. It picks up the sound wave transmitted to the middle ear. -External auditory canal: It is a curving tube about 2. 5 cm in length. It ends at the tympanic membrane. It is a conduit through which sound wave is transmitted to the middle ear. Outer 1/3 rd contains short hairs and ceruminous glands which produce cerumen.

Sound in external acoustic meatus hits tympanic membrane (eardrum) – it vibrates Pressure is equalized by the pharyngotympanic tube (AKA eustachian or auditory tube) 6

FUNCTIONS OF THE PARTS OF EAR • MIDDLE EAR: It is cavity between the tympanic membrane on the outer side and cochlea on the inner side. The cavity contains three ossicles: • Malleus (hammer) • Incus (anvil) • Stapes (stirrup) • The ossicular bones amplifier the sound waves.

TM causes ossicles in air filled middle ear to move: – Malleus (hammer) – Incus (anvil) – Stapes (stirrup) These are 3 of the smallest bones of the body Ossicles articulate to form a lever system that amplifies and transmits the vibratory motion of the TM to fluids of inner ear, cochlea via oval window 8

Skeletal muscles of middle ear When loud sound enters the ear, muscles contract, limiting vibration and dampening the noise 9

FUNCTIONS OF THE PARTS OF EAR • INNER EAR: Comprises cochlea, vestibular apparatus and Semicircular canal. • Cochlea is also called labyrinth. It has two openings at its base which are oval and round windows. The lumen of the cochlea is divided by two membranes namely: • basilar membrane • reissner's (Vestibular)membrane • The two membranes divide the cochlea into three compartments which are: • Scala vestibuli • Scala media • Scala tympani

Inner ear = bony “labyrinth” made up of 3 parts 1. Cochlea - hearing 2. Vestibule - equilibrium 3. Semicircular canals equilibrium In petrous part of the temporal bone Semicircular canals____ Filled with perilymph and endolymph fluids Vestibule______ Cochlea____________ 11

Spiral organ of Corti in cochlea contains receptors for hearing (amplifies sound 100 times) • Vibration of stapes at oval window starts traveling waves displacing basilar membrane • Sensory hair cells stimulated • Nerve ending of cochlear nerve division of VIII (Vestibulocochlear n. ) 12

THE ORGAN OF CORTI • The association of the basilar membrane, hair cells with sensory fibers and tectorial membrane forms a functional unit called the spiral organ or organ of corti. • The junction between the basilar membrane and the bony spiral lamina projects into the scala media and supported by two rods called rods of corti. • It has two hair cells which are: Internal and external hair cells. • A single roll of internal hair cells (about 3, 500) is found internally to inner rod of corti. • 3 -4 rows of external hair cells (about 20, 000) are located externally to the outer rod of corti. • The hair cells are innervated by nerve fibres of cochlear division of the 8 th cranial nerve.

HAIR CELLS • Each hair cells has diameter of 8 -12 microns, length of about 4 microns, thickness of 0. 1 micron. • They have tiny projections/ processes called hairs or stereocilia on them. • The processes are floating in the organ of corti. • They are embedded in a dense granular, reticular lamina and covered by a thin viscous, elastic and ribbon-like glycoprotein called tectorial membrane. • Inner edge of tectorial membrane is attached to limbus while the outer edge to the Henser's cells. • Between 90 -95% and 5 -10% afferent neurons innervates the inner and outer hair cells repectively.

Auditory pathway 15

AUDITORY PATHWAYS • Afferent neurons innervates the hairs cells. • The axon of the neurons of the cochlear nerve form the auditory division of the vestibulocochlear acoustic nerve. • These nerves send fibres centrally to terminate in the dorsal and ventral cochlear nuclei of medulla oblongata. • From here, axon carries auditory impulse via the variety of pathways to the inferior colliculi of the midbrain. • From the inferior colliculi, fibres are sent to the medial geniculate bodies in the thalamus. • Finally, the fibres terminate in the auditory cortex in temporal lobe. • Collateral fibres enter cerebellum and the reticular formation areas.

HAIRING IMPAIRMENTS • There two main categories of deafness/ impairment: 1. Conduction deafness 2. Sensorineural or Perceptive deafness • Conduction deafness: It occurs when the transmission of sound through the external auditory canal to the oval window is impaired. • Sensorineural deafness: It occurs when the transmission of nerve impulse anywhere frm the cochlea to the auditory cortex is impaired. • Causes of conduction deafness: Ø Otitis media Ø Allergic reactions Ø Respiratory diseases or inflammation Ø Otosclerosis

HAIRING IMPAIRMENTS • Causes of sensorineural deafness: Ø Results from wide variety of pathological processes Ø Exposure to extremely loud sounds. Ø Age related (presbycusis) - begins after age 20 when ability to hear high frequencies diminishes. - affect men to a greater degree than women These impairments can be detected by audiometry • Treatment of conduction deafness • People with conduction deafness can be helped by hearing aid to amplify sound and conduct its waves via the bone to the inner ear.

HAIRING IMPAIRMENTS • Treatment of sensorineural deafness • People with this deafness sometimes choose to have cochlear implants which electrically stimulate the fibres of the vestibulocochlear nerve in response to sounds. • Tests for deafness • Conductive deafness can be distinguished clinically from sensorineural deafness by performing: Ø Rinne's test Ø Weber's test

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