Anatomy and Physiology of Ear1
Look at the anvil of a blacksmith—how it is hammered and beaten; yet it moves not from its place. Let men learn patience and endurance from it.
—Sri Ramakrishna Dev
TEMPORAL BONE
The temporal bone has an interesting multifaceted anatomy. The important structures present and their complicated anatomic interrelations make the temporal bone surgery a challenge.
- Relations: It articulates with five cranial bones: parietal, sphenoid, occipital, zygomatic and mandible. This pyramidal-shaped bone forms part of the base and lateral side of skull (Fig. 1). The petrous part separates middle cranial fossa from the posterior cranial fossa.
- Contents: It houses the hearing and vestibular organs. The important structures which pass through it include internal carotid artery, internal jugular vein, and facial nerve. So the temporal bone houses following structures:
- ▸ Bony portion of external auditory canal (EAC)
- ▸ Middle ear containing malleus, incus and stapes
- ▸ Internal ear containing peripheral portions of auditory and vestibular system
- ▸ Fallopian canal containing facial nerve
- ▸ Osseous canal for the internal carotid artery
- ▸ Bony covering for the sigmoid sinus and the jugular bulb.
- Parts: The five portions of temporal bones are referred to as separate bones and include:
- ▸ Squamous
- ▸ Petrous
- ▸ Tympanic
- ▸ Mastoid
- ▸ Styloid process.
SQUAMOUS
It forms lateral wall of middle cranial fossa and roof of bony EAC. Its zygomatic process forms roof of the mandibular fossa (Fig. 2).
PETROUS
- Anterior surface (Fig. 3): Posterior boundary of middle cranial fossa
- ▸ Medially arcuate eminence of superior semicircular canal (SCC)
- ▸ Laterally tegmen tympani and tegmen antri
- ▸ Anteriorly canal for tensor tympani and eustachian tube
- ▸ At apex depression for trigeminal ganglion
- ▸ Foramina and sulci of greater and lesser superficial petrosal nerves.
- Posterior surface (Fig. 4): Anterior boundary of posterior cranial fossa
- ▸ Sulci for sigmoid sinus and superior petrosal and inferior petrosal sinuses
- ▸ Internal auditory meatus (IAM): Facial and auditory nerves and labyrinthine vessels
- ▸ Subarcuate artery depression lies superior and lateral to IAM
- ▸ Operculum opening for vestibular aqueduct lies inferolateral to IAM
- ▸ Jugular foramen is formed at the union of petrous and occipital bone.
- Inferior surface (Fig. 5):
- ▸ Opening for cochlear aqueduct at the superior-most limit of jugular foramen
- ▸ Inferior foramen of carotid canal for internal carotid artery is situated anterior to jugular fossa and separated from it by a wedge of bone called keel
- ▸ Tympanic canaliculus for tympanic branch (Jacobson's nerve) of glossopharyngeal nerve penetrates keel.
TYMPANIC
It forms anterior wall, floor and posterior wall of bony EAC, anterior wall and floor of middle ear and posterior part of mandibular fossa.
- Two sutures in bony EAC: Anteriorly tympanosquamous and posteriorly tympanomastoid
- Petrotympanic suture in middle ear through which chorda tympani nerve exits.
MASTOID
Sternocleidomastoid (SCM) muscle is inserted on mastoid tip. The posterior belly of digastric muscle is attached to the digastric sulcus that terminates anteriorly at the stylomastoid foramen.4
Fig. 5: Inferior view of left temporal bone. Note that some adjacent part of greater wing of sphenoid is also seen
Emissary vein draining into the sigmoid sinus travels through mastoid foramen. Mastoid antrum is situated deeper to Macewen's triangle.
STYLOID PROCESS
It is situated anterior to stylomastoid foramen and gives origin to three muscles (stylohyoid, styloglossus and stylopharyngeus) and attachment to two ligaments (stylohyoid and stylomandibular).
ANATOMY OF EAR
The ear is a complex organ. Its functional components, the hearing apparatus and balancing organ are situated in the temporal bone of skull. Ear has intimate relationship to the brain occupying middle and posterior cranial fossa, jugular bulb, sigmoid sinus, internal carotid artery and cranial nerve (CN) V, VI, VII, VIII, IX, X, XI and XII. For the sake of description, the ear is divided into three parts (Fig. 6):
- External ear
- Middle ear
- Internal ear.
EXTERNAL EAR
The external ear is divided into auricle (pinna) and external acoustic or auditory canal (EAC). The TM separates external ear from the middle ear.5
Auricle
The auricle except its lobule is made up of (Figs 7 and 8) a framework of a single piece of yellow elastic cartilage (Fig. 8), which is covered with skin. The skin is adherent to the perichondrium on its lateral surface while it is comparatively loose on the medial surface. Epithelium is squamous keratinizing. Sebaceous glands and hair follicles are found in the subcutaneous tissue. Adipose tissue is present only in the lobule. There are various elevations and depressions, which can be seen on the lateral surface of pinna (Fig. 8).
- Incisura terminalis: This area is devoid of cartilage and lies between the tragus and crus of the helix.
- Nerve supply (Fig. 9):
- ▸ Auriculotemporal nerve (CN V3): It is a branch of mandibular division of trigeminal nerve and supplies anterosuperior part of lateral surface of pinna, including tragus and crus of helix.
- ▸ Cranial nerve VII (facial nerve): It innervates the skin of concha and antihelix, lobule and mastoid.
- ▸ Cranial nerve X (vagus nerve): Its auricular branch (Arnold's nerve) supplies to concha and postauricular skin.
- ▸ Lesser occipital nerve (C2): This nerve of cervical plexus supplies upper part of medial surface of auricle and postauricular region.
External Auditory Canal
- Dimensions: EAC is 24 mm long and extends from the concha to the TM. Its anterior wall is 6 mm longer than the posterior wall. EAC is usually divided into two parts: (1) cartilaginous and (2) bony. Its outer one-third (8 mm) is cartilaginous and its inner two-thirds (16 mm) is bony.
- Direction: EAC is “S” shaped and not straight. Its outer one-third cartilaginous part is directed upward, backward and medially while its inner two-thirds bony part is directed downward, forward and medially.
- Cartilaginous EAC: It is a continuation of the cartilage that forms the framework of the pinna.
- ▸ Skin glands: The skin of the cartilaginous canal (Fig. 10) is thick and contains ceruminous and pilosebaceous glands that secrete wax. The hydrophobic, slightly acidic (pH 6.0–6.5) cerumen is formed in this part of EAC.
- Bony EAC: It is mainly formed by the tympanic portion of temporal bone but roof is formed by the squamous part of the temporal bone (Figs 2 and 11). In the anterosuperior region, squamous part articulates with tympanic bone (tympanosquamous suture). Inferiorly and medially squamous part joins with the lateral superior portion of the petrous bone (petrosquamous suture). Skin of the bony EAC is thin and continuous over the TM. Here the skin is devoid of subcutaneous layer, hair follicles, and ceruminous glands.
- ▸ Recess: Anteroinferior part of the deep bony meatus, medial to the isthmus has a recess, which is called the anterior recess.
- ▸ Foramen of Huschke: In children and occasionally in adults, anteroinferior bony EAC may have a deficiency that is called as foramen of Huschke.
- Relations of Bony EAC
- ▸ Superior: Middle cranial fossa
- ▸ Inferior: Parotid gland
- ▸ Posterior: Mastoid antrum and air cells and the facial nerve
- ▸ Anterior: Temporomandibular joint (TMJ)
- ▸ Medial: Tympanic membrane
- ▸ Lateral: Cartilaginous EAC.
- Epithelial migration: The skin of EAC has a unique self-cleansing mechanism. This migratory process continues from the medial to lateral side. The sloughed epithelium is extruded out as a component of cerumen.
- Nerve supply (Fig. 12):
- ▸ Auriculotemporal nerve (CN V3): It is a branch of mandibular division of trigeminal nerve and supplies anterosuperior wall of EAC.
- ▸ CN X (vagus nerve): Its auricular branch (Arnold's nerve) supplies to inferoposterior EAC.
- ▸ CN VII (facial nerve): It innervates the skin of the mastoid and posterior EAC.
Tympanic Membrane (Fig. 13)
- Dimensions: Its dimensions are: 9–10 mm height and 8–9 mm width. It is 0.1 mm thick.
- Position: Tympanic membrane is a partition wall between the EAC and the middle ear. It is positioned obliquely. It forms angle of 55° with deep EAC. Its posterosuperior part is more lateral than its anteroinferior part.
- Parts: Tympanic membrane consists of two parts: (1) pars tensa and (2) pars flaccida.
- ▸ Pars tensa: It forms most of TM.
- Annulus tympanicus: TM is thickened in the periphery and forms a fibrocartilaginous ring called the annulus tympanicus that fits in the tympanic sulcus.
- Umbo: The central part of TM near the tip of malleus is tented inwards and is called the umbo.
- Cone of light: A bright cone of light radiating from the tip of malleus to the periphery in the anteroinferior quadrant is usually seen during otoscopy.
- ▸ Pars flaccida (Shrapnell's membrane): It is situated above the lateral process of malleus between the notch of Rivinus and the anterior and posterior malleal folds. It is not as tense as pars tensa and may appear little pinkish.
- Structure: Tympanic membrane consists of the following three layers (Fig. 14):
- Middle fibrous layer: It encloses the handle of malleus and consists of three types of fibers: radial, circular and parabolic. In comparison to pars tensa, this layer is very thin in pars flaccida and not organized into various fibers.
- Inner mucosal layer: It is continuous with the middle ear mucosa.
- Otoscopy (Fig. 13): Normal TM is shiny and pearly gray in color. Its lateral surface is concave, which is more marked at the tip of malleus (umbo). Attic area lies above the lateral process of malleus and is slightly pinkish. Its transparency varies from person to person. Some middle ear structures can usually be seen through TM such as incudostapedial joint, round window (RW) and Eustachian tube.
- Mobility (Seigalization): A normal TM is mobile. It can be tested with pneumatic otoscope or Siegel's speculum.
- Nerve supply:
- ▸ Auriculotemporal nerve (CN V3): It is a branch of mandibular division of trigeminal nerve and supplies anterior half of lateral surface of TM.
- ▸ CN X (vagus nerve): Its auricular branch (Arnold's nerve) supplies to posterior half of lateral surface of TM.
- ▸ CN IX (glossopharyngeal nerve): Its tympanic branch (Jacobson's nerve) supplies to medial surface of TM.
MIDDLE EAR
The middle ear cleft (Fig. 15), which is lined by mucous membrane and filled with air, consists of the middle ear, Eustachian tube, aditus ad antrum, mastoid antrum and mastoid air cells. Middle ear is a 1–2 cm3 air-filled cavity that houses ossicles (malleus, incus, stapes), muscles (tensor tympani, stapedius) and nerves (chorda tympani, tympanic plexus).
Relations of Middle Ear Cleft
- Roof: Tegmen plate separates it from middle cranial fossa and its contents like meninges and temporal lobe of cerebrum.
- Floor: Jugular bulb
- Medial: Labyrinth. Lateral SCC lies posterosuperior to facial nerve.
- Posterior: Sigmoid intracranial venous sinus
- Anterior: Petrous part of internal carotid artery lying in carotid canal
- Posteromedial: Posteromedial to mastoid air cells is situated cerebellum in the posterior cranial fossa.
- Cranial nerves:
- ▸ CN V and CN VI: They lie close to the apex of the petrous pyramid.
- ▸ CN VII: The horizontal tympanic part is situated in the medial wall of middle ear, while vertical mastoid part runs between the middle ear and mastoid air cells system.
Parts of Middle Ear (Tympanum)
The dimensions of middle ear are shown in Figure 16. The tympanum (Fig. 17) is traditionally divided into three parts—mesotympanum, epitympanum and hypotympanum.
- Mesotympanum: This is the portion of middle ear that lies at the level of pars tensa.
- Epitympanum (attic): This is the portion of middle ear that lies above the level of pars tensa and medial to Shrapnell's membrane and the bony lateral attic wall.
- Hypotympanum: This is the portion of middle ear that lies below the level of pars tensa.
- ▸ Protympanum: The portion of middle ear around the Eustachian tube opening is termed as protympanum.
Boundaries of Middle Ear (Fig. 18)
Middle ear has six boundaries: roof, floor, medial, lateral, anterior and posterior walls.
- Roof (tegmental wall): It is formed by tegmen tympani (a thin plate of bone), which extends posteriorly to form the roof of the aditus and antrum (tegmen antri). Tegmen tympani separates middle ear from the middle cranial fossa.
- Floor (jugular wall): A thin plate of temporal bone separates tympanic cavity from the jugular bulb.
- Anterior (carotid wall): The anterior wall, a thin plate of bone, which separates the middle ear cavity from internal carotid artery, has following features:
- ▸ Eustachian tube: It connects the middle ear with nasopharynx. It aerates and drains the middle ear. (Eustachian tube is discussed in detail in Chapter “Disorders of Eustachian Tube”).
- ▸ Canal of tensor tympani muscle: It is situated in the roof of Eustachian tube.
- ▸ Canal for chorda tympani nerve.
- ▸ Attachment of anterior malleolar ligament.
- Posterior (mastoid wall): It lies close to the mastoid air cells and presents following structures:
- ▸ Pyramid: It is a bony projection through the summit of which appears the tendon of the stapedius muscle that is inserted to the neck of stapes.
- ▸ Aditus ad antrum: It is an opening through which mastoid antrum opens into the attic. It lies above the pyramid. Its relations are following:
- Medial: Bony prominence of the horizontal SCC
- Lateral: Fossa incudis (short process of incus lies and attached here).
- Inferior: Fallopian canal for facial nerve
- Superior: Tegmen antri.
- ▸ Facial nerve: The vertical mastoid part of the fallopian canal for facial nerve runs in the posterior wall just behind and below the pyramid.
- ▸ Facial recess or suprapyramidal recess (Fig. 19): This recess is a depression in the posterior wall lateral to the pyramid. Its boundaries are following:
- Medial: Vertical part of CN VII
- Lateral: Chorda tympani (branch of 7th CN) and tympanic annulus
- Superior: Fossa incudis (short process of incus lies and attached here).
- ▸ Sinus tympani or infrapyramidal recess: This deep recess lies medial to the pyramid. It is bounded by the bony ridges (subiculum below and the ponticulus above).
- Medial (labyrinthine wall) (Figs 21 and 22): It is formed by the lateral wall of labyrinth. It presents following structures:
- ▸ Promontory: It is a bony bulge which is due to the basal coil of cochlea.
- ▸ Oval window (fenestra vestibuli): The footplate of stapes is placed in this window.
- ▸ Round window (fenestra cochleae): It is covered by the secondary TM.
- ▸ Horizontal tympanic part of fallopian canal for facial nerve: It lies above the oval window.
- ▸ Lateral SCC: It lies above the fallopian canal of facial nerve.
- ▸ Processus cochleariformis: It is a hook-like projection, which lies anterior to the oval window. The tendon of tensor tympani takes a turn on this process and then is inserted on the neck of malleus.
- Lateral (membranous wall) (Fig. 23):
- ▸ Tympanic membrane: Lateral wall is formed mainly by the TM. Some structures of the middle ear (such as long process of incus, incudostapedial joint, RW and Eustachian tube) can be seen through the normal semitransparent TM.
- ▸ Scutum: An upper part of epitympanum is formed by outer bony attic wall called scutum.
Ossicles
The ossicles (Fig. 24) conduct sound energy from the TM to the oval window. There are three middle ear ossicles—malleus, incus and stapes.
- Malleus (hammer): It consists of a head, neck, handle (manubrium), a lateral and an anterior process. It is the largest ossicle and measures 8 mm in length.
- ▸ Head and neck: They lie in the attic.
- ▸ Manubrium: It is embedded in the fibrous layer of the TM.
- ▸ Lateral process: It appears as a knob-like projection on the outer surface of the TM and provides attachments to the anterior and posterior malleal folds.
- Incus (anvil): It consists of following parts:
- ▸ Body and short process: They lie in the attic.
- ▸ Long process: It hangs vertically and forms incudostapedial joint with the head of stapes.
- Stapes (stirrup): This smallest bone of body measures about 3.5 mm. It consists of head, neck, anterior and posterior crura and footplate. The footplate is positioned in the oval window by annular ligament.
Intratympanic Muscles
There are two middle ear muscles: tensor tympani and the stapedius.
- Tensor tympani: It runs above the Eustachian tube. Its tendon turns around the processus cochleariformis and passes laterally. It tenses the TM.
- ▸ Origin: Bony tunnel above the osseous part of Eustachian tube.
- ▸ Insertion: Just below the neck of malleus.
- ▸ Nerve supply: It develops from the 1st branchial arch and is supplied by a branch of mandibular division of trigeminal nerve (CN V3).
- Stapedius: On contraction, it dampens the loud sounds and prevents noise trauma to the inner ear.
- ▸ Origin: Conical cavity and canal within pyramid.
- ▸ Insertion: It inserts to the neck stapes.
- ▸ Nerve supply: It is developed from the second branchial arch and is supplied by nerve to stapedius of facial nerve.
- Functions: Acoustic reflex protects ear from loud sounds.
- ▸ Dampening of middle ear mechanics: Loud sounds (80 dB and above) cause contraction of stapedius that limits stapes movement.
- ▸ Gain control mechanism: Acoustic reflex keeps cochlear input more constant and expands dynamic range.
- ▸ Reduction in self-generated noise: Stapedius muscle contracts with chewing and vocalization.
Intratympanic Nerves (Fig. 25)
- Tympanic plexus (nerve supply of middle ear): The tympanic nerve plexus, which lies on the promontory, supplies to the medial surface of the TM, tympanic cavity, mastoid air cells, and the bony Eustachian tube. It is formed by following nerves:
- ▸ Tympanic branch (Jacobson) of glossopharyngeal: It carries secretomotor fibers to the parotid gland. The pathway of secretomotor fibers to the parotid gland consists of inferior salivary nucleus > CN IX > Jacobson's tympanic branch > Tympanic plexus > Lesser petrosal nerve > Otic ganglion > Auriculotemporal nerve > Parotid gland.
- ▸ Sympathetic fibers: Caroticotympanic nerves come from the sympathetic plexus, which is present around the internal carotid artery.
- Chorda tympani nerve: This branch of the facial nerve enters the middle ear through posterior canaliculus. It runs on the medial surface of the TM. It lies between the malleus and long process of incus, above the insertion of tensor tympani. It carries gustatory fibers from the anterior two-thirds of tongue and parasympathetic secretomotor fibers to the submaxillary and sublingual salivary glands.
Middle Ear Mucosa
Middle ear mucosa wraps ossicles, muscles, ligaments and nerves like peritoneum wraps various viscera in the abdomen. These mucosal folds divide the middle ear into various compartments. So, all the middle ear structures lie outside the mucous membrane.12
Fig. 25: Nerves in relation with the middle ear. Note secretomotor pathway of salivary, lacrimal and nasal glands
Mucous membrane of the nasopharynx is continuous with that of the middle ear cleft.
Middle ear cavity is lined by ciliated columnar epithelium in its anterior and inferior part and mucosa changes to cuboidal type in the posterior part. Attic and mastoid air cells are lined by flat, nonciliated epithelium. Eustachian tube is lined by ciliated pseudostratified columnar epithelium with several mucous glands in the submucosa.
Ossicles and their mucosal folds separate mesotympanum from epitympanum (attic).
- Compartments of epitympanum:
- ▸ Prussak's space: Its boundaries limit spread of cholesteatoma to other compartments.
- Lateral: Membrana flaccida (Shrapnell's membrane)
- Medial: Neck of malleus
- Floor: Lateral process of malleus
- Roof: Fibers of lateral malleolar ligament arising from neck of malleus and inserting along the rim of notch of Rivinus
- ▸ Attic compartments: Transversely placed superior malleolar fold divides attic into two compartments—smaller anterior and larger posterior. The gap between the lateral malleolar fold and lateral incudal fold provides communication between Prussak's space and posterior attic compartment.
- Anterior attic compartment
- Posterior attic compartment: Superior incudal fold divides this space into following two divisions: medial and lateral spaces.
- Compartments of mesotympanum: In the upper part of mesotympanum, there are following three compartments:
- ▸ Inferior incudal space: Its boundaries are following:
- Superior: Lateral incudal fold
- Medial: Medial incudal fold
- Lateral: Posterior malleolar fold extending from neck of malleus to posterosuperior margin of tympanic sulcus.
- Anterior: Interossicular fold that lies between long process of incus and upper two-thirds of handle of malleus.
- ▸ Anterior pouch of von Troeltsch: It lies between the following boundaries:
- Medial: Anterior malleolar fold extending from neck of malleus to anterosuperior margin of tympanic sulcus
- Lateral: Portion of the TM anterior to handle of malleus
- ▸ Posterior pouch of von Troeltsch: It is situated between the following boundaries:
- Medial: Posterior malleolar fold extending from neck of malleus to posterosuperior margin of tympanic sulcus.
- Lateral: Portion of the TM posterior to handle of malleus.
Mastoid Antrum
This air-containing space (9 mm height, 14 mm width and 7 mm depth) is situated in the upper part of mastoid. Its boundaries are following:
- Roof: It is formed by the tegmen antri, which separates mastoid antrum from the middle cranial fossa.
- Lateral wall: It is formed by a 1.5 cm thick plate of squamous part of temporal bone which is marked on the lateral surface of mastoid by suprameatal (Macewen's) triangle (Figs 2 and 28). It is covered by postaural skin.
- ▸ Boundaries of Macewen's triangle:
- Linea temporalis (temporal line): A ridge of bone extending posteriorly from the zygomatic process (marking the lower margin of temporalis muscle and approximating the floor of middle cranial fossa)
- EAC: Posterosuperior margin of EAC.
- Tangent: A tangent to the posterior margin of EAC.
- Medial wall: It is formed by the petrous bone and related to the:
- ▸ Posterior SCC
- ▸ Endolymphatic sac
- ▸ Dura of posterior cranial fossa
- Anterior: Anteriorly mastoid antrum communicates with the attic through the aditus ad antrum. Medial to lateral, the relations are following:
- ▸ Facial nerve canal
- ▸ Aditus ad antrum and facial recess lie between tympanum and mastoid antrum (see posterior wall of middle ear in the section of boundaries of middle ear)
- ▸ Deep bony EAC
- Posterior wall: It is formed by mastoid bone and communicates with mastoid air cells.
- ▸ Sigmoid sinus curves downwards.
- Floor: It is formed by mastoid bone and communicates with mastoid air cells. Other deeper relations from medial to lateral sides are:
- ▸ Jugular bulb medial to facial canal
- ▸ Digastric ridge gives origin to posterior belly of digastric muscle
- ▸ Origin of SCM muscle.
Types of Mastoid (Fig. 29)
The mastoid consists of “honeycomb” air cells, which lie underneath the bony cortex. Depending on its development, three types of mastoid are described: cellular, diploic and acellular.
- Cellular (well-pneumatized): Mastoid cells are well developed with thin intervening septa. Eighty percent people have pneumatized mastoids.
- Diploic: Mainly there are marrow spaces with few air cells.
- Acellular (sclerotic): There are neither cells nor marrow spaces.
Theories of Deficient Mastoid Pneumatization
- Tumarkin: Failure of aeration of middle ear cleft that is the cause of “frustration of pneumatization”, results from the blockage of the Eustachian tube. It happens in patients with upper respiratory tract infections.
- Diamond and Dahlberg: Different types of mastoids are the normal congenital variations.
- Wittmack: Infantile otitis media interferes with normal absorption of diploic bone and leads to failure of pneumatization and dense mastoid bone. Evidence for this theory is lacking.
Mastoid antrum is present in all types of mastoids. It is the most constant mastoid air cell. In sclerotic mastoid, antrum is usually small and sigmoid sinus may be anteriorly positioned. In cases of mastoiditis, abscesses may form in these air cells and result in various types of intra- and extracranial complications For further details, see Chapter 20 “Complications of Suppurative Otitis Media”.
Fig. 31: X-ray mastoid left showing normal pneumatizationSource: Dr Jayesh Patel, Consultant Radiologist, Anand, Gujarat.
Fig. 32: X-ray mastoid right showing partial loss of pneumatizationSource: Dr Jayesh Patel, Consultant Radiologist, Anand, Gujarat.
The mastoid air cells are traditionally divided into several groups, which include:
- Zygomatic cells: In the root of zygoma.
- Tegmen cells: In the tegmen tympani.
- Perisinus cells: Present over the sinus plate.
- Retrofacial cells: Present behind the fallopian canal of facial nerve.
- Perilabyrinthine cells: They are located above, below and behind the labyrinth. The cells, which are present in the arch of superior SCC, may communicate with the petrous apex.
- Peritubal: They are present around the Eustachian tube. These and the hypotympanic cells communicate with the petrous apex.
- Tip cells: These large cells lie in the tip of mastoid, medial and lateral to the digastric ridge.
- Marginal cells: These cells, which lie behind the sinus plate, may extend into the occipital bone.
- Squamous cells: They lie in the squamous part of temporal bone.
Korner's Septum
Mastoid develops from the squamous and petrous parts of temporal bone. In some cases, petrosquamosal suture persists as a bony plate called Korner's septum, which separates superficial squamosal cells from the deep petrosal cells. During the mastoid surgery, Korner's septum causes difficulty in locating the antrum and the deeper cells.
Blood Supply
Arterial Supply
Following branches of external and internal carotid arteries supply blood to middle ear:
- External carotid artery:
- ▸ Maxillary artery:
- Anterior tympanic artery: Major contributor
- Middle meningeal artery
- Petrosal branch
- Superior tympanic artery: It traverses along the canal for tensor tympanic muscle.
- Artery of pterygoid canal: Branch that runs along Eustachian tube.
- ▸ Posterior auricular artery:
- Stylomastoid artery: Major contributor
- ▸ Ascending pharyngeal artery:
- Tympanic branch
- Internal carotid artery: Petrous part
- ▸ Caroticotympanic branches.
Venous Drainage
Veins from the middle ear cleft drain into pterygoid venous plexus, superior petrosal sinus and sigmoid sinus.15
Lymphatic Drainage of Ear
The lymphatics of middle ear drain into retropharyngeal and parotid nodes. Eustachian tube lymphatics drain into retropharyngeal group of lymph nodes (Table 1). Internal ear does not have any lymphatics.
INTERNAL EAR
The internal ear (labyrinth), which has organs of both hearing and balance, is divided into bony and membranous labyrinth. The membranous labyrinth is filled with endolymph. Perilymph is filled in the space present between membranous and bony labyrinths.
Bony Labyrinth
Bony labyrinth (Fig. 33) consists of three parts: vestibule, semicircular canals (SCCs) and cochlea. The lateral wall of labyrinth is medial wall of middle ear. The medial wall of labyrinth is the lateral limit of IAC.
- Vestibule: This central chamber of the labyrinth (5 mm) has following structures:
- ▸ Lateral wall: It has oval window.
- Oval window (Fenestra vestibuli): It lies in the lateral wall and is closed by footplate of stapes surrounded by annular ligament.
- ▸ Medial wall (Fig. 34): It shows following structures:
- Spherical recess: It is situated anteriorly and lodges the saccule. Perforations of maculae cribrosa media provide passage for fibers of inferior vestibular nerve.
- Elliptical recess: It is situated posteriorly and lodges the utricle. The perforations of maculae cribrosa superior (Mike's dot) provide passage to nerve fibers that supply to utricle and ampulla of superior and lateral SCC.
- Vestibular crest and cochlear recess: The spherical and elliptical recesses are separated from each other by vestibular crest. Inferiorly, vestibular crest splits to enclose cochlear recess for cochlear nerve fibers.
- Opening of aqueduct of vestibule: It is present below the elliptical recess. Through this passes the endolymphatic duct.
- ▸ Posterosuperior region:
- Five openings of SCCs: They are present in the posterosuperior part of vestibule.
- ▸ Anterior: Cochlea opens into the anterior region of vestibule.
- Semicircular canals (Fig. 35): There are three SCCs: lateral (horizontal), posterior and superior (anterior). Each canal occupies two-thirds of a circle and has a diameter of 0.8 mm. They lie in planes at right angles to one another. Each canal has two ends: ampullated and nonampullated. All the three ampullated ends and nonampullated end of lateral SCC open independently and directly into the vestibule.
- ▸ Superior SCC: It is 15–20 mm long and situated transverse to the axis of petrous part of temporal bone. Its anterolateral end is ampullated and opens in the superolateral part of vestibule.
- ▸ Lateral SCC: It is 12–15 mm long and projects as a rounded bulge into the middle ear, aditus and antrum. It makes an angle of 30° with the horizontal plane. Its anterior end is ampullated and opens into the upper part of vestibule. The posterior nonampullated end opens into the lower part of vestibule below the orifice of crus commune.16
- ▸ Posterior SCC: It is 18–22 mm long and situated parallel and close to the posterior surface of petrous part of temporal bone. Its lower end is ampullated and opens into the lower part of vestibule. Its upper limb joins the crus commune.
- Crus commune: The nonampullated ends of posterior and superior canals join and form a crus commune (4 mm length), which then opens into the medial part of vestibule. So, the three SCCs open into the vestibule by five openings.
- ▸ Cochlea (Figs 36 and 37): The bony cochlea, which is a coiled tube, looks like snail. Cochlear canal makes 2.5–2.75 turns round a central pyramid of bone called modiolus. The cochlear tube is 30 mm long. It is 5 mm from base to apex and 9 mm around its base.
- Modiolus: The base of modiolus, which is directed towards internal acoustic meatus, transmits vessels and nerves to the cochlea. The apex lies medial to tensor tympani muscle.
- Osseous spiral lamina: A thin plate of bone called osseous spiral lamina, winds spirally around the modiolus like the thread of a screw. This bony lamina gives attachment to the basilar membrane and divides the bony cochlear tube into three compartments: scala vestibuli, scala tympani and scala media (membranous cochlea).
- Rosenthal's canal: The spiral ganglions are situated in Rosenthal's canal, which runs along the osseous spiral lamina.
- Scala vestibuli: This upper-most channel is continuous with vestibule and closed at oval window by the stapes footplate.
- Scala tympani: This lowermost channel is closed by secondary TM of RW.
- Promontory: The promontory, a bony bulge in the medial wall of middle ear, represents the basal coil of cochlea.
- Helicotrema: This opening is situated at the apex of cochlea which provides communication between the scala vestibuli and scala tympani which are filled with perilymph.
- Round window (RW) (fenestra cochlea): On the lateral wall of internal ear (medial wall of middle ear), scala vestibuli is closed by the stapes footplate, while the scala tympani is closed by secondary TM of RW.
- Aqueduct of cochlea: The scala tympani is connected with the subarachnoid space through the aqueduct of cochlea. It is thought to regulate perilymph and pressure in bony labyrinth.
Membranous Labyrinth
Membranous labyrinth (Fig. 38) consists of cochlear duct, utricle, saccule, three semicircular ducts and endolymphatic duct and sac.
- Cochlear duct (membranous cochlea or scala media) (Fig. 39): This blind coiled tube, which appears triangular on cross-section, is connected to the saccule through ductus reunions. It is bounded by the following three walls:
- ▸ Basilar membrane: It supports the organ of Corti. Its length increases as it proceeds from the basal coil to the apical coil. So, the higher frequencies of sound are heard at the basal coil while lower tones at the apical coil. The inner thin area is called zona arcuata while outer thick area is called zona pectinata.
- ▸ Reissner's membrane: It separates scala media from the scala vestibuli.
- Utricle: The utricle, which is oblong and irregular, has anteriorly upward slope at an approximate angle of 30°. It lies in the posterior part of bony vestibule and receives the five openings of the three semicircular ducts. The utricle (4.33 mm2) is bigger than saccule (2.4 mm2) and lies superior to saccule. The utricle is connected with the saccule through utriculosaccular duct. Its sensory epithelium, which is called macula, is concerned with linear acceleration and deceleration.
- Saccule: The saccule lies anterior to the utricle opposite the stapes footplate in the bony vestibule. Its sensory epithelium, macula responds to linear acceleration and deceleration. The saccule is connected to the cochlea through the thin reunion duct.
- Semicircular ducts: The three semicircular ducts, which open in the utricle, correspond exactly to the three bony canals. The ampullated end contains a thickened ridge of neuroepithelium, which is called crista ampullaris.
- Endolymphatic duct and sac: The ducts from utricle and saccule unite and form utriculosaccular duct, which continues as endolymphatic duct that passes through the vestibular aqueduct. The terminal part of the endolymphatic duct is dilated and forms endolymphatic sac that is situated between the two layers of dura on the posterior surface of the petrous bone. Endolymphatic sac consists of both an intraosseous and an extraosseous portion. The endolymphatic duct and sac are thought to be involved in the reabsorption and regulation of endolymph.
Inner Ear Fluids
Perilymph fills the space between bony and membranous labyrinth while endolymph fills the entire membranous labyrinth (Table 2).
Perilymph
It resembles extracellular fluid and is rich in sodium ions. The aqueduct of cochlea provides communication between scala tympani and subarachnoid space. Perilymph percolates through the arachnoid type connective tissue present in the aqueduct of cochlea.
- Source: There are two theories:
- Filtrate of blood serum from the capillaries of spiral ligament
- CSF reaching labyrinth via aqueduct of cochlea.
Endolymph
It resembles intracellular fluid and is rich in potassium ions. Protein and glucose contents are lesser than that in perilymph.
- Source: They are believed to be following:
- Stria vascularis
- Dark cells of utricle and ampullated ends of semicircular ducts.
- Absorption: There are following two opinions regarding the absorption of endolymph:
- Endolymphatic sac: The longitudinal flow theory believes that from cochlear duct the endolymph reaches saccule, utricle and endolymphatic duct and is then absorbed by endolymphatic sac.
- Stria vascularis: The radial flow theory believes that endolymph is secreted as well as absorbed by the stria vascularis.
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Organ of Corti
This (Fig. 40) sensory organ of hearing is situated on the basilar membrane. It is spread like a ribbon along the entire length of basilar membrane. It consists of following important components:
- Tunnel of Corti: This tunnel, which is situated between the inner and outer rods, contains a fluid called cortilymph. The functions of the rods and cortilymph are not yet clear.
- Hair cells: These important receptor cells of hearing transduce sound energy into electrical energy. There are two types of hair cells—inner and outer. At low magnification stereocilia (evaginations of membrane on the apical surface) appears as hairs. The stereocilia have mechanically activated ion channels which are opened by the sound stimuli. With the advancement of age there is generalized reduction in the number of hair cells. Differences between inner and outer hair cells are given in Table 3.
- ▸ Inner hair cells: Inner hair cells (IHCs) form a single row and are richly supplied by afferent cochlear nerve fibers. These flask-shaped cells are very important in the transmission of auditory impulses.
- ▸ Outer hair cells: Outer hair cells (OHCs) are arranged in three or four rows and mainly receive efferent innervation from the olivary complex. These cylindrical cells modulate the function of inner hair cells.
- Nerve supply: Ninety-five percent of afferent fibers of spiral ganglion of cochlear nerve supply the IHCs. The OHCs get only 5% of the cochlear nerve fibers. Efferent fibers, which are mainly for the OHCs, come from the superior olivary complex through the olivocochlear bundle. Hair cells are innervated by dendrites of bipolar cells of spiral ganglion. Each cochlea sends auditory information to both sides of brain.
- ▸ Supporting cells: Deiter's cells, which are situated between the outer hair cells, provide support to OHC. Cells of Hensen are situated outside the Dieter's cells.
- ▸ Tectorial membrane: The tectorial membrane, which overlies the organ of Corti, consists of gelatinous matrix and delicate fibers. The shearing force between the hair cells and tectorial membrane stimulate the hair cells.
Vestibular Receptors
Peripheral vestibular receptors are of two types: cristae and maculae.
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- Cristae (Fig. 41): They lie in the ampullated ends of the three semicircular ducts and respond to angular acceleration. On a crest-like mound of connective tissue lie the sensory epithelial hair cells, which are covered by cupula.
- ▸ Cupula: The cilia of epithelial hair cells project into cupula that consists of a gelatinous mass (complex carbohydrates or glycoproteins and proteoglycans arranged in filamentous network), which extends from the surface of crista to the ceiling of the ampulla. The cupula, which is thought to be secreted by the supporting cells, forms a water-tight partition. With the movements of endolymph, cupula can be displaced to any one side like a swing door. The gelatinous mass of cupula, which consists of polysaccharide, contains canals into which project the cilia of sensory hair cells.
- The mechanism governing caloric nystagmus under earth gravity and zero gravity in space is not clear. It seems that a direct thermal effect on the SCC afferents play only a small role.
- ▸ Sensory epithelial hair cells (Fig. 42): The sensory hair cells are of two types: type 1 and type 2. From the upper surface of each cell projects a kinocilium and multiple stereocilia. The kinocilium, which is thicker than stereocilia, is located on the edge of the cell. Sensory cells are surrounded by supporting cells which have microvilli on their upper surface. Hair cells of both types may have contact with the same nerve calyx.
- Type 1 cells: These cells are found only in birds and mammals. They are flask-shaped and correspond to the IHC of organ of Corti. Each cell has a single large cup-like nerve terminal that surrounds the base.
- Type 2 cells: They are cylindrical and have multiple nerve terminals at the base. They resemble OHC of organ of Corti.
- Maculae (Fig. 43): They lie in otolith organs (utricle and saccule). Macula of the utricle is situated in its floor in a horizontal plane in the dilated superior portion of the utricle. Macula of saccule is situated in its medial wall in a vertical plane. The macula utriculi (approximately 33,000 hair cells) are larger than saccular macula (approximately 18,000 hair cells). The striola, which is a narrow curved line in center, divides the macula into two areas. They appreciate position of head in response to gravity and linear acceleration. A macula consists mainly of two parts: a sensory neuroepithelium and an otolith membrane.
- ▸ Sensory neuroepithelium: It is made up of type 1 and type 2 cells, which are similar to the hair cells of the crista.Type I cells are in higher concentration in the area of striola and change orientation (mirror-shaped) along the line of striola with opposite polarity. The kinocilia face striola in the utricular macula, whereas in saccule, they face away from the striola. The polarity and curvilinear shape of striola offer central nervous system (CNS) a wide range of neural information of angles in all the three dimensions for optimal perception and compensatory correction. During tilt, translational head movements and positioning, visual stimuli combined with receptors of neck muscles, joint and ligaments also play an important part.
- ▸ Otolithic membrane: The otoconial membrane consists of a gelatinous mass, a subgelatinous space and the crystals of calcium carbonate called otoliths (otoconia or statoconia). The otoconia, which are multitude of small cylindrical and hexagonally shaped bodies with pointed ends, consists of an organic protein matrix together with crystallized calcium carbonate. The otoconia (3–19 µm long) lie on the top of the gelatinous mass. The cilia of hair cells project into the gelatinous layer. The linear, gravitational and head-tilt movements result into the displacement of otolithic membrane, which stimulate the hair cells lying in different planes.
Blood Supply of Labyrinth
- Internal auditory (labyrinthine) artery: Labyrinth is supplied by internal auditory artery which is a branch of anterior inferior cerebellar artery that arises from basilar artery. The labyrinthine artery may directly arise from the basilar artery.
- ▸ Branches: Internal auditory artery divides into two following branches:
- Anterior vestibular artery: It supplies to utricle and lateral and superior SCC.
- Common cochlear artery: It further divides into two following branches:
- Main cochlear artery: It supplies to cochlea (80%).
- Vestibulocochlear artery: It again divides into two following branches:
- Posterior vestibular artery: It supplies to saccule and posterior SCC.
- Cochlear branch: It supplies to cochlea (20%).
- Venous drainage: It is through internal auditory vein, vein of cochlear aqueduct and vein of vestibular aqueduct. These veins drain into the inferior petrosal and sigmoid sinuses.
Internal Auditory Canal
Internal auditory canal is about 1 cm long and passes into petrous part of temporal bone in a lateral direction. It is lined by dura. At its lateral end (fundus), IAC is closed by a vertical cribriform plate of bone that separates it from labyrinth (Fig. 44). A transverse crest divides this plate into smaller upper and larger lower parts. Upper part is further divided into anterior and posterior quadrant by a vertical crest called Bill's bar.
Contents
- Vestibulocochlear nerve
- Facial nerve including nervous intermedius
- Internal auditory artery and vein.
For further details of IAC and cerebellopontine (CP) angle, see Chapter 25 “Tumors of Ear and Cerebellopontine Angle”.
Vestibulocochlear (Auditory) Nerve
In the IAC, the vestibular and cochlear nerves merge and form vestibulocochlear nerve (CN VIII). A small branch of anterior inferior cerebellar artery (AICA), which can be used as a landmark during vestibular schwannoma surgery, runs between the CN VII and CN VIII on the brainstem.
DEVELOPMENT OF EAR
Auricle
In the sixth week of embryonic life, six tubercles (Hillocks of His) appear around the first branchial cleft (Fig. 45). They progressively grow and coalesce and form the auricle. Tragus develops from the tubercle, which arises from the first branchial arch. The remaining pinna develops from the rest of the five tubercles of second arch. By the 20th week, pinna attains adult shape. Initially, pinna is located low on the side of the neck but later on, it moves to a more lateral and cranial position.
External Auditory Canal
External auditory canal develops from the first branchial cleft (Fig. 46). External ear canal gets fully formed by the 28th week. In the 16th embryonic week, cells proliferate from the bottom of ectodermal cleft and form a meatal plug.
Fig. 45: Development of pinna (A) from six hillocks of His (B) around the first branchial cleft (1 from first and 2–6 from second branchial arch)
Fig. 46: Development of external and middle ears. 1st (Meckel's cartilage) and 2nd (Reichert's cartilage) branchial arches
Tympanic Membrane
It develops from all the three germinal layers.
- Ectoderm: Outer epithelial layer is formed by the ectoderm.
- Mesoderm: The middle fibrous layer develops from the mesoderm.
- Endoderm: Inner mucosal layer is formed by the endoderm.
Middle Ear (Fig. 46)
- Endoderm of tubotympanic recess: The Eustachian tube, tympanic cavity, attic, antrum and mastoid air cells are derived from the endoderm of tubotympanic recess which arises from the first and partly from the second pharyngeal pouches.
- First branchial arch: Malleus and incus develops from mesoderm of the first arch.
- Second branchial arch: The stapes superstructures develop from the second arch.
- Otic capsule: The stapes footplate and annular ligament are derived from the otic capsule. The details of branchial apparatus are covered in Chapter “Anatomy and Physiology of Oral Cavity, Pharynx and Esophagus”.
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Inner Ear
Development of the inner ear, which begins in third week of fetal life, is complete by the 16th week (Fig. 47).
- Auditory placode: The auditory placode, which is the thickened ectoderm of hindbrain, gets invaginated and forms auditory vesicle (otocyst).
- Auditory vesicle: The auditory vesicle differentiates into endolymphatic duct and sac, utricle, semicircular ducts, saccule and cochlea.
- ▸ Development of pars superior (SCCs and utricle) takes place earlier than pars inferior (saccule and cochlea). The pars superior is phylogenetically an older part of labyrinth.
CENTRAL CONNECTIONS (NEURAL PATHWAYS)
Auditory Neural Pathways
The auditory pathway (Fig. 48) from peripheral to center consists of eighth nerve, cochlear nuclei, olivary complex (superior), lateral lemniscus, inferior colliculus, medial geniculate body and auditory cortex (ECOLIMA mnemonic) (Table 5).
- Cochlear nerve: Axons of these spiral ganglion bipolar cells form the cochlear nerve, which ends in the dorsal and ventral ipsilateral cochlear nuclei.
- Brainstem:
- ▸ Cochlear nuclei: The cochlear nuclei send neural information to both sides of the brain.
- ▸ Superior olivary nucleus, lateral lemniscus and inferior colliculus: From the cochlear nuclei, some of the axons go directly to inferior colliculus (both ipsilateral and contralateral) while other goes via superior olivary nucleus and lateral lemniscus (both ipsilateral and contralateral).22
TABLE 5 Ascending auditory pathways, from below upwards First-order neuronsBipolar neurons of spiral ganglion in cochlear nerveSecond-order neuronsDorsal and ventral cochlear nucleiThird-order neuronsSuperior olivary complex in pons. From here fibers travel in lateral lemniscus in ponsFourth-order neuronsInferior colliculus in midbrainFifth-order neuronsMedial geniculate body in thalamus. From here fibers go to auditory cortex in temporal lobe of the cerebrum through the auditory radiations in sublentiform part of internal capsuleSo, the auditory fibers travel via both ipsilateral as well as contralateral routes and have multiple decussation points.
- Thalamus: From the inferior colliculus, axons go to the medial geniculate body of metathalamus via inferior brachium.
- Cerebrum: From the medial geniculate body, axons go to the primary auditory cortex of temporal lobe of the cerebrum via the sublentiform part of internal capsule. Each side of ear is represented in both the cerebral hemispheres. The area of hearing is situated in the superior temporal gyrus (Brodmann's area 41).
Central Vestibular Connections (Fig. 49)
Vestibular Nerve
The Scarpa's ganglion, which lies in the lateral part of the internal acoustic meatus, contains bipolar cells. The peripheral processes of these bipolar cells innervate the sensory epithelium of the labyrinth. The central processes aggregate and form the vestibular nerve. A significant feature of vestibular neurons is their high frequency of resting discharge with an average of 90/sec. The majority of vestibular nerve fibers terminate in vestibular nuclei but some go directly to the cerebellum.
- Branches: The vestibular nerve has two branches—superior and inferior.
- Superior vestibular nerve: It supplies the cristae of superior and lateral SCC, macula of utricle and the anterosuperior portion of the macula of the saccule.
- Inferior vestibular nerve: It innervates the crista of posterior SCC and main portion of the macula of the saccule.
Vestibular Nuclei
They are four in number: superior, inferior (descending), medial and lateral. They receive afferents not only through vestibular nerve but also from cerebellum, reticular systems, spinal cord and contralateral vestibular nuclei (Table 6).23
Functions of Efferents from Vestibular Nuclei
The information received from the labyrinths, eyes and proprioceptive systems is integrated in CNS. The efferents from vestibular nuclei perform following functions:
- Vestibulo-ocular reflexes: The medial longitudinal bundle is the pathway for vestibulo-ocular reflexes and explains the genesis of nystagmus. It helps in stabilizing the gaze so that image is fixed on the fovea of retina during the head movement.
- Equilibrium:
- ▸ Vestibulospinal tract: It coordinates the movements of head, neck and body in the maintenance of balance.
- ▸ Vestibulocerebellar tract: It coordinates input information to maintain the body balance.
- Autonomic symptoms: Autonomic nervous system explains nausea, vomiting, palpitation, sweating and pallor seen in vestibular disorders such as Meniere's disease.
- Motion awareness: The temporal lobe is responsible for subjective awareness of motion.
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PHYSIOLOGY OF HEARING
The pinna collects sound signal1 from the environment, which passes through EAC and vibrates the TM (Fig. 50). Vibrations of the TM are transmitted to the stapes footplate through the chain of ossicles.2 Vibrations of stapes footplate result in the pressure changes in the labyrinthine fluids3 that lead to movement of the basilar membrane and thus stimulate the hair cells of the organ of Corti. The IHC of cochlea act as transducers and convert the mechanical energy into electrical impulses which travel along the auditory nerve.
COMPONENTS OF HEARING PHYSIOLOGY
The physiology of hearing is broadly divided into three divisions:
- Conduction of mechanical sound energy (external and middle ear conductive apparatus)
- Transduction of mechanical sound energy into electrical impulses (cochlear sensory system)
- Conduction of electrical impulses to brain (CN VIII, brainstem, thalamus and temporal lobe neural pathways).
CONDUCTION OF SOUND
Pinna
Pinna serves following functions because of its shape and location. It increases sound pressure level by 6 dB (two times)
- Collection: Gathers sound arriving from an arc of 135°
- Localization: Determines the origin of sound
- Concentration: Horn-shaped concha acts like a megaphone and concentrates the sound at the entrance of EAC.
External Auditory Canal
Along with pinna, it can increase sound pressure level at the TM by 15–22 dB at 4,000 Hz.
When the air-conducted sound travels to the cochlear fluids most of the sound energy is reflected away.4 Middle ear compensates for this loss of sound energy.
Middle ear converts sound of greater amplitude, but lesser force, to that of lesser amplitude and greater force.
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Fig. 51: Transformer function (ratio 18:1) of middle ear. Hydraulic effect of tympanic membrane (14:1) and lever action of ossicles (1.3:1)
Fig. 52: Ossicular and acoustic coupling pathways. Acoustic coupling is caused by the middle ear pressure that results from external auditory canal sound pressure and motion of eardrum. Ossicular coupling (coupled motion of tympanic membrane and ossicles including stapes footplate) is 60 dB more than acoustic coupling. Oval and round windows are spatially separated
This function of the middle ear is called impedance matching mechanism or the transformer action. The following are the different functions of various structures of the conducting mechanism of the hearing:
- Hydraulic action of TM: The area of TM is much larger than the stapes footplate. There is large hydraulic ratio between the TM and stapes footplate. The effective vibratory area of TM is about two-thirds. The effective areal ratio between TM and stapes footplate is about 14:1. This mechanical advantage is provided by the TM.
- Curved membrane effect: Movements of TM are more at the periphery than at the center, which provide some leverage.
- Lever action of the ossicles: Ossicular chain conducts sound from TM to the oval window. Lever action of the ossicles (handle of malleus is 1.3 times longer than long process of the incus) provides a mechanical advantage of 1.3.
Both oval and round windows provide free movement of cochlear fluids in scala vestibuli and scala tympani, respectively. Sound waves do not reach the oval and round windows simultaneously. The preferential pathway to oval window receives sound vibrations first and round window acts as a relief window. When the oval window is receiving wave of compression, the RW is in the phase of rarefaction.
- Acoustic separation of two windows: The sound should not reach to both oval and round windows simultaneously. An intact TM with the help of intact ossicular chain provides preferential pathway to oval window. The presence of air in the middle ear delays the pathway to RW. If the sound waves strike both the windows simultaneously, they would cancel each other's effect and there will not be any movement of the perilymph. This acoustic separation of two windows is provided by the TM and a cushion of air in the middle ear around the RW.
- Aeration: Patent Eustachian tube provides aeration to the middle ear.
Round Window Reflex
The RW membrane moves in response to the movement of footplate of stapes. When stapes is pressed, pressure is exerted on scala vestibuli perilymph which is transferred to scala media and then to scala tympani. The pressure is ultimately transferred to RW which bulges into middle ear.
Natural Resonance of External and Middle Ear
It is a tendency of a system to oscillate with greater amplitude at some frequencies than at others and is called natural resonance. Natural resonances of the external and middle ear allow certain frequencies of sound to pass more easily to the inner ear. The greatest sensitivity of the sound transmission is between 500 Hz and 3000 Hz (speech frequencies).
Following are the natural resonances:
- External auditory canal: 3000–4000 Hz
- Tympanic membrane: 800–1600 Hz
- Ossicular chain: 500–2000 Hz
- Middle ear: 800 Hz.
TRANSDUCTION OF MECHANICAL ENERGY TO ELECTRICAL IMPULSES
Organ of Corti (Fig. 53)
Pressure in scala media causes downward movement of basilar membrane. Along with the basilar membrane, organ of Corti moves up and down with sound stimulus. This causes a shearing action between tectorial membrane and the reticular lamina and results in bending of stereocilia.
Transduction
Transduction is the conversion of mechanical energy to electrical energy. Movements of the stapes footplate are transmitted to the cochlear fluids, which move the basilar and tectorial membranes differentially and set up shearing force that bends the stereocilia. Movement of stereocilia opens and closes ion channels and produces receptor potential in the IHCs. This cochlear microphonic (CM) triggers the nerve impulse by releasing neurotransmitters onto afferent nerve fibers (Flow chart 1).
- Traveling wave theory of von Bekesy: It hypothesizes that basilar membrane moves as traveling wave from the base of cochlea to its apex. Depending on the frequency, a particular segment of the basilar membrane achieves maximum amplitude. Each wave is weak at the onset but becomes stronger as it reaches its natural resonant frequency.
- Tonotopic gradient in cochlea: Tonotopic map of basilar membrane determines the site of largest peak of the wave. Higher frequencies are represented in the basal turn and the progressively lower tones toward the apex of the cochlea (Fig. 54). High frequency waves travel a short distance and die. Low frequency waves travel a long distance.
Functions of Hair Cells
- Inner hair cells: They are believed to be the classic auditory receptor cells which signal the brain about the presence of specific sound.
- Outer hair cells: They have been shown to shorten and lengthen when stimulated by sound. A protein called prestin provides OHCs their ability to contract. They are thought to have following functions:
- ▸ Amplification: OHCs amplify effect of sound stimuli to their adjacent IHCs.
- ▸ Sharpening: OHCs sharpen the frequency response of adjacent IHCs.
- ▸ Inhibitory: Efferent stimulation of OHCs may be responsible for decreasing the responsiveness of cochlea.
- ▸ Cochlear microphonics: OHCs are responsible for CM effect of electrocochleography.
- ▸ Otoacoustic emissions: OHCs produce otoacoustic emissions (OAE) that can be recorded and used to screen newborns for hearing loss. Details of OAE can be found in chapters “Hearing Evaluation” and “Hearing Impairment in Infants and Young Children”.
Fig. 54: Tonotopic gradient in cochlea. Higher frequencies are represented in the basal turn and the progressively lower tones toward the apex of the cochlea
Electrical Potentials
Endocochlear potential, cochlear microphonics (CM) and summating potential (SP) are from cochlea while the compound action potential (AP) is from the cochlear nerve fibers. Both CM and SP are receptor potentials similar to other sensory end-organs.
- Endocochlear potential: This resting potential of +80 mV direct current (DC) is recorded from scala media. This energy source for cochlear transduction is generated from stria vascularis by Na+/K+ -ATPase pump. Endolymph has high K+ concentration. It acts as a battery and helps in driving the current through the hair cells when they move after exposure to any sound stimulus.
- Cochlear microphonic (CM): It is an alternating current potential produced by outer hair cells. Basilar membrane moves in response to sound stimulus. Changes occur in electrical resistance at the tips of outer hair cells. Flow of K+ through the outer hair cells produces voltage fluctuations. CM has two elements—CM1 and CM2. CM1 is oxygen dependent and is abolished by lack of oxygen or by death of the individual. CM2 is about 10% of the CM and can be elicited for several hours after total oxygen deprivation or death.
- Summating potential (SP): SP is a DC potential, which may be either negative or positive. It is produced by hair cells. It follows the “envelop” of stimulating sound and is superimposed on cochlear nerve action potential. This is a rectified derivative of sound signal. Probably, it arises from IHCs with a small contribution from OHCs.
- Compound (auditory nerve) action potential: It is the neural discharge of auditory nerve. It follows all or none phenomena so has all or none response to auditory nerve fibers. Each nerve fiber has optimum stimulus frequency for which the threshold is lowest. Amplitude increases while latency decreases if the intensity is over 40–50 dB range. The following features differentiate it from CM and SP:
- ▸ No gradation (follows “are or none” law)
- ▸ Latency
- ▸ Propagation
- ▸ Post-response refractory period.
MEDIAL GENICULATE BODY AND TEMPORAL LOBE AUDITORY CORTEX
They are organized into isofrequency layers arranged tonotopically from low frequency to high frequency. Most cells respond to binaural stimulation. Their main function appears to be of sound localization. Neurons can either summate excitatory responses from both ears, or excitatory response from one ear and inhibitory response from other.
PHYSIOLOGY OF VESTIBULAR SYSTEM
Vestibular system is traditionally divided into two parts: peripheral and central.
- Peripheral vestibular system: It consists of semicircular ducts (dynamic labyrinth), utricle and saccule (static labyrinth) and vestibular nerve. Each vestibular receptor is precisely oriented to detect head movement in a specific direction or plane (Table 7). All receptors are tonically active.
- Central vestibular system: It includes vestibular nuclei and tracts that integrate vestibular impulses with other systems to maintain body balance.
SEMICIRCULAR CANALS’ FUNCTIONS
Semicircular canals respond to angular acceleration and deceleration. The three canals, which lie in three different planes, are situated at right angles to each other (Fig. 55). Any change in position of head can be detected. The one that lies at right angle to the axis of rotation is most stimulated. For example: the horizontal canal responds maximum to rotation on the vertical axis.
Nystagmus
The stimulation of SCCs produces nystagmus. The direction of nystagmus depends on the plane of the canal being stimulated.
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Fig. 56: Vestibular hair cells. Displacement of stereocilia toward kinocilium leads to depolarization (stimulation) and increases the vestibular nerve discharge rate
The nystagmus is horizontal from horizontal (lateral) canal, rotatory from the superior (anterior) canal, and vertical from the posterior canal.
- Flow of endolymph: The flow of endolymph displaces the cupula and stimulates the epithelial hair cells (Fig. 56) of crista in the ampulla of the SCC. The flow of endolymph toward the ampulla or utricle is called ampullopetal or utriculopetal. The flow of endolymph away from ampulla or utricle is called ampullofugal or utriculofugal. The quick component of horizontal nystagmus is always opposite to the direction of flow of endolymph in the horizontal SCC. In lateral SCC, ampullopetal displacement of stereocilia increases (stimulatory) the firing rate whereas ampullofugal displacement decreases (inhibitory) the firing rate. The opposite happens in posterior and superior canals.
- Rotating chair test: In the rotating chair test, when the patient is rotating to the right and then abruptly stopped, the endolymph continues to move to the right due to inertia. Here endolymph movement in the lateral SCC will be ampullopetal for left canal and the horizontal nystagmus will be directed to the left.
UTRICLE AND SACCULE FUNCTIONS
They respond to the linear acceleration and deceleration or gravitational pull during the head tilts. The sensory hair cells of the macula lie in different planes. Macula of the utricle is situated in its floor in a horizontal plane in the dilated superior portion of the utricle. Macula of saccule is situated in its medial wall in a vertical plane. During the head tilts, hair cells are stimulated by displacement of otolithic membrane. The functions of saccule and utricle are similar but the saccule is also seen to respond to sound vibrations.
- Saccular macula responds to the tilting of head. If the head is tilted to left side, left saccular macula is stimulated while right saccular macula will remain static.
- Utricular macula responds to forward and backward movement of head.
Striola
It is a narrow curved line in center that divides the macula into two areas. Type I cells are in higher concentration in the area of striola and change orientation (mirror-shaped) along the line of striola with opposite polarity. The kinocilia face striola in the utricular macula, whereas in saccule, they face away from the striola. The polarity and curvilinear shape of striola offer CNS a wide range of neural information of angles in all the three dimensions for optimal perception and compensatory correction. The bending of utricular hair cells away from striola causes depolarization (stimulation) whereas bending of saccular hair cells toward striola causes hyperpolarization (inhibition). During tilt, translational head movements and positioning, visual stimuli combined with receptors of neck muscles, joint and ligaments play an important part.
MAINTENANCE OF BODY EQUILIBRIUM
Sensory Component
The vestibular system records changes in the head position, linear or angular acceleration and deceleration and gravitational effects. The CNS receives information not only from the vestibular system but also from other sensory systems, which include visual, auditory and somatosensory (muscles, joints, tendons, and skin). All this information is integrated and utilized in the regulation of equilibrium and body posture. Cerebellum, which is connected to vestibular receptors further helps in coordinating muscular movements, which vary in their rate, range, force and duration.
Motor Component
The standing and walking not only need sensory integration (from vestibular, somatosensory and visual systems) but also motor commands, which are fine-tuned through the frontal cerebral lobes, cerebellum and basal ganglia. Disorder of any of these systems can lead to dizziness.
Push and Pull System
The balance system, which includes vestibular, visual and somatosensory organs, can be compared with a two-sided push and pull system. In a neutral position, push and pull of one side is equal to that of the other side. If one side is pulling more than the other, the body balance is disturbed. During the turning or tilting, a temporary change in the push and pull force of one system is taken care of by the appropriate reflexes and motor outputs to the eyes (vestibulo-ocular reflex), neck (vestibulocervical reflex) and trunk and limbs (vestibulospinal reflex), which maintains new position of head and body. If any component of push and pull system of one side is diseased, then it results in vertigo and ataxia.
- Example: Turning the head to the right direction produces an increase in the resting spontaneous outflow of AP in the nerve coming from right horizontal SCC. Simultaneously, there occurs a decreased activity in left vestibular nerve. The CNS compares the input coming from each vestibule. There is no sense of movement when input is equal. The CNS interprets asymmetric input not only as a head rotation but also generates compensatory eye movements and postural adjustment.
Diseases and Evaluation of Vestibular System
For the evaluation of nystagmus, dizzy patient and diseases of vestibular system, see following chapters in this book:
- Chapter 21 “Evaluation of Dizzy Patient”
- Chapter 22 “Peripheral Vestibular Disorders”
- Chapter 23 “Central Vestibular Disorders”.