Objectives 1-Describe anatomy of middle ear 2-Describe anatomy of inner ea 3-Describe physiology of hearing 4-Describe physiology of balance

2- Eustachian Tube: The Eustachian tube connects the tympanic cavity with the nasophrynx, in the adult it is about 36 mm in length. From its pharyngeal end it runs upwards, laterally and backwards. In infants the tube is shorter, wider and more horizontal than in adults. The tube has 2 parts, a pharyngeal cartilaginous part which forms 2/3 of its length, and a tympanic bony 1/3 portion. The upper and medial walls of the pharyngeal portion of the tube are formed by a plate of cartilage. The lateral wall is membranous.

There are 3 muscles attached to Eustachian tube these are: Tensor palate. Levator palate. Salpingopharyngeus.

In resting state the tube is closed, and opened only during swallowing and yawing (by the action of tensor palate and to lesser extent by the action of levator palate and salpingopharyngeus). Opening of the tube will result in equality of pressure on both sides of tympanic membrane to achieve best hearing. The tube is lined by columnar ciliated epithelium.

3- Mastoid antrum and mastoid air cell system: Mastoid antrum is an air-filled sinus within the petrous temporal bone. It lies posterior to tympanic cavity and external auditory canal, inferior to middle cranial fossa and anterior to sigmoid sinus. The antrum communicates with the tympanic cavity by the aditus. Mastoid air cell system a rising from the wall of mastoid antrum, these are interconnecting air- filled cavities a rises from the walls of mastoid antrum.

Surgical Anatomy of Inner Ear (Cochlea) Inner ear lies within the petrous temporal bone. It consists of: 1- Membranous labyrinth enclosed in a 2- Bony labyrinth. The membranous labyrinth contains a fluid known as endolymph, and the space between the walls of bony labyrinth and membranous labyrinth contain a fluid known as perilymph.

1- Bony Labyrinth: It consists of: A-Vestibule: The vestibule is the central portion of bony labyrinth, ovoid chamber lying between the middle ear and fundus of internal auditory meatus. On the lateral wall there is the opening of oval window closed by stapes footplate and annular ligament. On the medial wall anteriorly there is spherical recess houses the saccule and perforated by fibers of inferior vestibular nerve. Behind the spherical recess is a ridge, vestibular crest (crista vestibuli).

Above and behind the crest is an elliptical recess which houses the utricle. The posterior wall of the vestibule contains 5 openings of the semicircular canals. The anterior wall of the vestibule contains opening into scala vestibuli, of cochlea.

B-Semicircular canals: There are 3 semicircular canals (lateral, superior and posterior) situated above and behind the vestibule, each about 2/3 of a circle. At one end of each canal there is a dilation called the ampulla which contains the sensory vestibular epithelium, these ampullated ends open separately into the vestibule, the non-ampullated ends of superior and posterior semicircular canals meet and join to form the crus commune which opens in the posterior wall of vestibule.

The non-ampullated end of lateral semicircular canal opens separately into the posterior wall of the vestibule, thus there are 5 opening of semicircular canals into the posterior wall of the vestibule.

C- Cochlea: It lies in front of vestibule, its appearance is like the shell of a snail. The cochlea is about 2 and one half turns. It is a spiral tube wound two and a half times round central axis called modiolus. Arising from the modiolus which contains the cochlear nerve a thin shelf of bone (bony spiral lamina), the membranous spiral lamina (basilar membrane) extends from the edge of bony spiral lamina to the outer wall of cochlea, thereby dividing the cochlea into the scala vestibuli and scala tympani.

At the apex of cochlea, the bony spiral lamina is not attached to modiolus and thus there is communication between perilymph of scala vestibuli and scala tympani; this channel is called helicotrema. At the base of cochlea the scala vestibuli opens into the vestibule and has the oval window closed by stapes footplate and annular ligament on the lateral wall. The scala tympani is blind-ended tube and has round window which is closed by round window membrane in its floor. in the

The modiolus contains small: canals that enter the bony spiral lamina, these canals near the origin of bony spiral lamina from the modiolus dilates to accommodate the bipolar ganglion cells of cochlear (spiral) ganglion.

2- Membranous labyrinth: Vestibular component (3 semicircular ducts, utricle and saccule) supplied by vestibular component of VIII nerve. Auditory component (cochlear duct or called Scala media) supplied by cochlear component of VIII cranial nerve.

A- Vestibular membranous labyrinth: consists of 1-Semicircular ducts: there are semicircular ducts lie in the bony semicircular canals. Each duct has ampulla at one end which contains the sensory vestibular cells (the crista), the hair cells of the crista have long filaments which project into a mass of gelatinous material called the cupula. During angular acceleration, the movement of endolymph through the space between the crista and cupula leads to deflection of the cupula and cilia of the hair cells resulting in ↑ or ↓ neural output of the sensory hair cells according to the direction of deflection of the cilia.

2- Saccule: the saccule lies in the spherical recess of the bony vestibule, it is globular in shape. The sensory vestibular cells are present in the anterior wall of the saccule called the macula.

3- Utricle: the utricle lies in the elliptical recess of the bony vestibule, the sensory vestibular cells are present in the lateral wall of the utricle called the macula. The semicircular ducts communicate with utricle by 5 openings.

The sensory vestibular cells (vestibular sensory epithelium The sensory vestibular cells (vestibular sensory epithelium.) of the crista and macula are the same. The surface of the cells has a single kinocilium and between 20−100 stereocilia. In the ampulla of semicircular ducts the kinocilium and stereocilia project into a gelatinous material called the cupula; there is a space between the hair cells and the cupula. In angular acceleration there will be movement of endolymph through the space between the hair cells and the cupula causing deflection of stereocilia, when the deflection of stereocilia is in the direction of kinocilium resulting in ↑ neural output, while deflection away from kinocilium results in ↓ neural output.

In the macula, a gelatinous material overlies the sensory hair cells, the stereocilia and kinocilium also project into the under surface of this gelatinous membrane which is called otoconial membrane.

B- Auditory membranous labyrinth (cochlear duct) (scala media): Cochlear duct (scala media) is triangular in cross section has a floor, lateral wall and roof.

1-Floor of cochlear duct: The floor is formed by 1 Bony spiral lamina which has upper and lower ridges and, 2 Membranous spiral lamina which extends from the lower ridge of bony spiral lamina to the lateral wall of cochlear duct. The upper ridge gives rise to tectorial membrane. The cochlear nerve fibers pass from the modiolus in canals in the bony spiral lamina and then through membranous spiral lamina to organ of Corti (the sensory auditory organ) which lies on the upper surface of membranous spiral lamina.

The organ of Corti is a ridge-like structure containing the auditory sensory cells and supporting cells. The sensory cells are arranged as single row of inner hair cells and 3, 4 or 5 rows of outer hair cells. The inner hair cells are separated from outer hair cells by tunnel of Corti. The hair cells have stereocilia or "hairs" arising from its upper surface.

The tectorial membrane (an acellular jell- like membrane) arise from the upper ridge of bony spiral lamina and extends over the organ of Corti. The tips of stereocilia of outer hair cells are embedded in the under surface of tectorial membrane.

2- Lateral wall of cochlear duct: It is formed by: Stria vascularis above (stria vascularis plays a role in the maintenance of ionic composition of endolymph). Spiral prominence below and, Transitional zone between (1) & (2).

3- Roof of cochlear duct (Reissner's membrane): Reissner's membrane is stretched between the bony spiral lamina to the upper part of the lateral wall of cochlear duct.

Note: Scala media is separated by Reissner's membrane from scala vestibuli which lies above scala media, and the scala media is separated by its floor (basilar membrane) from scala tympani below. Scala media contains endolymph, while scala vestibuli and scala tympani contain perilymph.

The Endolymphatic System: The endolymphatic system consists of: A duct formed from the endolymphatic duct of the saccule and the utrculosaccular duct of the utricle which joins the endolymphatic duct and Endolymphatic sac.

Fluids of Inner Ear: Endolymph: It is contained in the membranous labyrinth. It has high potassium and low sodium concentration. It is produced by stria vascularis and absorbed by endolymphatic sac. Perilymph: It is contained in the perilymphatic space between the bony labyrinth and membranous labyrinth. It is similar to cerebrospinal fluid. It has a high sodium and low potassium concentration. Perilymph is derived from blood vessels (mainly) and some is derived from cerebrospinal fluid .

Arterial blood supply of inner ear: from labyrinthine artery which arises from the basilar or anterior inferior cerebellar artery.

Nerve supply of inner ear: By vestibulocochlear nerve (VIII cranial nerve) which is formed by cochlear and vestibular nerves. Vestibulocochlear nerve enters brain stem between the pons and the medulla.

Physiology of Hearing: The purpose of the auditory apparatus is to convert vibrations in the air to vibrations in the inner-ear fluids, and then to nerve impulses to be transmitted by the auditory nerve to higher centres of hearing. Airborne sound consists of vibrations of the atmosphere (alternate phases of condensation and rarefaction).

1- External and middle ears: The auricle collects the sound waves, and they pass along the external auditory meatus to the tympanic membrane which is set in motion. The vibrations of tympanic membrane are transmitted to the malleus, incus and stapes. Vibration of stapes (which lies in the oval window) causing vibrations in the endolymphatic compartments of inner ear. The ratio of functioning area of the tympanic membrane to the area of the footplate is 14:1, the ossicular lever ratio of 1.3:1. The product of these area and lever ratios (14 and 1.3) is about 18:1 which represents the transformer ratio.

This will increase the sound pressure at the footplate to a degree which causes the fluids of inner ear to vibrate. The stapes moves in rocking motion and, as fluids cannot be compressed, these vibrations are transmitted to round window membrane. This reciprocal action of the oval and round windows is essential. The tympanic membrane is most efficient when the air pressure in the external auditory canal and middle ear is equal, and this is achieved by Eustachian tube which normally opens during swallowing and yawning.

2- Inner ear: The vibrations, transmitted by the stapes, produce displacement of the floor of cochlear duct (the basilar membrane) and shearing movements between the hair cells of organ of Corti and tectorial membrane which initiates nerve impulses in the fibers of cochlear n. These fibers then transmit impulses to the auditory nuclei in the brain stem, and from there the fibers pass through the midbrain to the auditory cortex where the impulses are perceived as sound.

Physiology of Vestibular Apparatus: The balance of the body is maintained by coordinated information from 3 systems: Proprioception i.e. joints, muscles, tendons... The eyes. The vestibular system.

The utricle and saccule of vestibular system respond to linear acceleration (the gravity). Change in the position of the head in relation to gravity direction will stimulate the utricle and saccule and lead to impulses which give information about the position of the head in the space and initiate reflexes which tend to keep the head in upright position.

The semicircular canals respond to angular (rotatory) acceleration, and stimulation of the canal gives rise to sensation of rotation and nystagmus. Normally there is resting impulse rate (10−20 impulse/second) in the nerve fibers leaving the crista. Movement of endolymph and cupula towards the ampulla causes ↑ in impulse rate, and movement away from ampulla causes ↓ in the impulse rate.

This mechanism can be explained by considering rotation in a rotating chair about vertical axis, when the rotation (for example) to the right, this result in movement of endolymph and cupula to the left, which means that the cupula and endolymph in the right side move towards the ampulla which result in ↑ impulse rate from the right crista, and the cupula and endolymph move away from the ampulla of the left side which results in ↓ impulse rate from the left crista. This ↑ impulse rate from the right crista and ↓ impulse rate from left crista will lead to sense of rotation to the right side. Also there will be deviation of the eye to the left side (slow component of nystagmus) and then rapid deviation of the eye to the right side (fast component of nystagmus), so there will be nystagmus directed to the right side (direction of nystagmus is determined by the direction of fast component).