Vestibulocochlear nerve VIII D.Nimer D.Rania Gabr D.Safaa D.Elsherbiny

Objectives Describe the structures making the external and middle ear. Discuss the features of the tympanic membrane. Describe the ossicles and related muscles. Describe the auditory tube, its openings and structure. Understand the mastoid air cells and their connection to the middle ear. Describe the structure of cochlea, semicircular canals, utricle and saccule Describe the course of vestibulocochlear nerve and its central connections

Anatomy of the Ear Three Main Sections The ear consists of: -External ear -Middle ear(tympanic cavity) -Internal ear( labyrinth)

The External Ear Consists of: Auricle (pinna) Made of elastic cartilage Helix (rim) Lobule (ear lobe) External auditory canal Lies within temporal bone & connects to ear drum (tympanic memb) Its outer 1/3 is lined by skin with hair, sebaceous and wax secreting glands. Its internal 2/3 are lined with sensitive hairless skin.

Nerve supply of external ear : Auriculotemporal & vagus nerves.

Middle Ear •Middle Ear: Is an air-containing cavity in the petrous part of the temporal bone – 3 ossicles – Mastoid – Eustachian Tube

Auditory Ossicles (smallest bones in body) Together, they are only one inch long. Malleus Attaches to ear drum Articulates with incus Incus Articulates with stapes Stapes (stirrup) Footplate of stapes fits into oval window Their function is to amplify sound vibrations. The malleus vibrates the incus, which vibrates the stapes. The stapes then transmits vibrations to the fluid-filled inner ear at the oval window.

limits movements of malleus & stiffens eardrum to prevent its damage. Muscles of middle ear Stapedius Smallest skeletal muscle attaches to stapes prevents very strong vibrations of stapes from loud noises. N.S.: branch of facial nerve. Tensor Tympani attaches to handle of malleus. limits movements of malleus & stiffens eardrum to prevent its damage. N.S.: mandibular division of Trigeminal nerve.

Middle ear cavity Walls of the middle ear: Anterior Posterior Medial Lateral Roof Floor

Outer ear covered by tympanic membrane (not seen) Communications of middle ear: Outer ear covered by tympanic membrane (not seen) Inner ear = oval and round windows Eustachian tube connects middle ear to the nasopharnyx for air passage (normally closed, opens with swallowing/yawning)

Eustachian Tube The eustachian tube (auditory tube) extends from the middle ear to the nasopharynx and permits air to enter or leave the middle ear cavity. The air pressure in the middle ear must be the same as the external atmospheric pressure in order for the eardrum to vibrate properly.

Eustachian Tube 35-38 mm long Oriented downward, forward, medially Osseous portion (post. 1/3) Cartilaginous portion (ant. 2/3) Isthmus The eustachian tubes of children are short and nearly horizontal and may permit bacteria to spread from the pharynx to the middle ear.

Auditory Tube (Eustachian tube) Is a route for pathogens to travel from nose and throat to ear causing Otitis Media During swallowing and yawning it opens to equal pressure in middle ear. Normal Ear Drum Inflamed Ear Drum

Mastoid Air Cells: They are a series of communicating cavities within mastoid process that are continuous above with mastoid antrum and middle ear. They are lined with mucous membrane.

Inner Ear 2 Parts: 1- Auditory system: Cochlear part: The scala tympani, scala media and scala vestibuli make up the cochlea which is involved in hearing. 2- Vestibular system: The semi-circular canals, utricle and saccule

Inner Ear Bony labyrinth Membranous labyrinth Contains perilymph Semicircular canals Anterior, posterior, and lateral Lie right angles to each other Vestibule Oval portion Cochlea Looks like a snail Converts mechanical energy into electrical energy Membranous labyrinth Contains endolymph, high in K+ ions

The Cochlea Divided into 3 channels Cochlear duct (scala media) Contains the Organ of Corti Scala vestibuli Ends at the oval window Scala tympani Ends at the round window

Cochlear Anatomy 3 fluid filled channels found within the cochlea, scala vestibuli, scala tympani and cochlear duct (scala media). Vibration of the stapes upon the oval window sends vibrations into the fluid of the scala vestibuli.

Tubular Structures of the Cochlea Stapes pushes on fluid of scala vestibuli at oval window. At helicotrema, vibration moves into scala tympani. Fluid vibration dissipated at round window which bulges. The central structure is vibrated (cochlear duct).

The Organ of Corti : is the sensitive element in the Inner ear and can be thought of as the body's microphone. It is found on the scala media side of the basilar membrane There are two types of receptor cells, which have different functions: 1.Inner Hair Cells 2.Outer Hair Cells It contains four rows of Hair cells which protrude from its surface. Above them is the tectorial membrane which can move in response to pressure variations in the fluid- filled tympanic and vestibular canals.

CN VIII = Vestibulocochlear nerve Special sensory : Hearing and balance 21

CN VIII = Vestibular nerve It is a pure sensory nerve concerned with equilibrium. Origin: It is formed by the central processes (axons) of the bipolar cells of the Scarpas ganglion (vestibular ganglion). The peripheral processes (dendrites) terminate in the vestibular receptors in the semicircular canals Termination: Cerebellum: in the archicerebellum (flocculonodular lobe). The 4 vestibular nuclei: lateral & superior in the pons. Medial & inferior in medulla.

Connections of the vestibular nuclei: Input: from the: Vestibular nerve. Cerebellum: cerebellovestibular fibers. Output: they send fibers to: Cerebellum (archicerebellum): through the inferior cerebellar peduncle. Spinal cord: via the lateral vestibulospinal tract (from the lateral vestibular nucleus) and the medial vestibulospinal tract (from the medial vestibular nucleus). Nuclei that move the extraocular muscles: of the two sides (III, IV, VI) via the MLF to coordinate movements of the two eyes. Thalamus (VPMN) and cerebral cortex: (vestibular area) for conscious awareness of the position of the head in space.

CN VIII = Cochlear nerve It is a pure sensory nerve concerned with hearing. Origin: It is formed by the central processes (axons) of the bipolar cells of the spiral ganglion (cochlear ganglion). The peripheral processes terminate in organ of Corti in the cochlea. Termination: in the dorsal and ventral cochlear nuclei at the pontomedullary junction. Auditory Pathway: Organ of Corti  spiral ganglion  cochlear nerve  dorsal & ventral cochlear nuclei  trapezoid body  lateral lemniscus  inferior colloculus  medial geniculate body  auditory radiation  primary auditory area (41 & 42)  auditory association area (22)

Lesions of the auditory pathway: Lesion in the cochlea, cochlear nerve or cochlear nuclei: complete ipsilateral deafness. Lesion in the lateral lemniscus, MGB or auditory area: partial deafness on the two sides mainly on the opposite side (bilateral diminution of hearing).

Medial Longitudinal Fasciculus Extends through out the brain stem and formed of both descending & ascending fibers Projects bilaterally Has two components: Ascending fibers: to the motor nuclei that move the eyes (III, IV & VI) Descending: extends into the spinal cord as the medial vestibulospinal tract. These fibers are derived from: Vestibular fibers: from the vestibular nuclei of both sides. Auditory fibers: from the nucleus of lateral lemniscus and trapezoid body. Gaze movement fibers: are derived from the: Interstitial nucleus of Cajal for the downward gaze movement. Paraabducent nucleus for the lateral gaze movement.

Medial Longitudinal Fasciculus

Clinical Notes Acoustic neuroma Lesion of vestibulocochlear nerve produces deafness (disturbance of cochlear nerve functions), tinnitus, vertigo, dizziness, nausea, nystagmus, loss of balance and ataxia (disturbance of vestibular nerve functions) A benign tumour of 8th nerve leads to compression of the nerve leading to attacks of dizziness, and profound deafness and ataxia The representation of cochlea is essentially bilateral at all levels ABOVE the cochlear nuclei Lesions anywhere along the pathway usually have NO obvious effect on hearing. Deafness is essentially only caused by damage to the middle ear, cochlea, or auditory nerve. Acoustic neuroma

Deafness Nerve deafness (sensorineural) Conduction deafness damage to hair cells from antibiotics, high pitched sounds, anticancer drugs. Conduction deafness perforated ear drum Otosclerosis: an abnormal growth of bone near the middle ear