Basic Principles

2 Basic Principles


Anatomy of the Temporal Bone and Adjacent Structures


Overview of Temporal Bone Anatomy


Anatomy of the External Ear and Eustachian Tube


Anatomy of the Middle Ear


Anatomy of the Inner Ear


Anatomy of the Internal Auditory Canal, Jugular Foramen, and Petrous Apex


Central Auditory and Vestibular Pathways


Physiology


Physiology of the Auditory System


Physiology of the External Ear


Physiology of the Middle Ear


Auditory Physiology of the Inner Ear


Central Auditory Pathways


Physiology of the Vestibular System


Balance Physiology of the Inner Ear


Central Vestibular Processing


Peripheral Nerves, CNS, and CSF


Physiology of Peripheral Nerves


Central Nervous System/CSF




Anatomy of the Temporal Bone and Adjacent Structures


A true three-dimensional understanding of the complicated anatomy of the temporal bone remains a challenging task. For the otologist, neurotologist, and skull base surgeon, a clear concept of this anatomy is fundamental. A written text, however, can never substitute for anatomical dissections in the human temporal bone laboratory. With some exceptions, this practice allows for a very realistic simulation of live temporal bone surgery, especially as it relates to the osseous details.


Overview of Temporal Bone Anatomy


Definition

Each temporal bone can be divided into four main bony divisions, which include the tympanic part, the squamous part (also termed squama), the mastoid process, and the petrous bone. The styloid process is typically classified as a separate bone but some authors view this as part of the temporal bone.


Closely Related Topics

• Imaging of the temporal bone—see p. 92


• Fractures of the temporal bone—see p. 247


• Neurotologic approaches to the medial temporal bone—see p. 338


Introduction (Fig. 2.1C)

The ear (excluding its neural connections), lies either attached to or within the confines of the temporal bone. It is divided into three parts: the outer ear, the middle ear, and the inner ear (Fig. 2.1C). The outer ear consists of the auricle and the external auditory canal, which terminates at the tympanic membrane. The middle ear starts with the tympanic membrane and medially ends at its medial bony walls, which are mainly formed by the cochlear promontory. By definition, the tympanic membrane itself is considered to be a part of the middle ear. The inner ear is surrounded by bone of the otic capsule.


Postnatal Developmental Anatomy

Specific features of developmental anatomical details will be discussed in the respective chapters; the overall postnatal growth of the temporal bones seems to be of surgical importance. It is clear that middle and inner ear structures have both completed development and growth long before birth. However, the dimensions of the temporal bone relative to the inner ear and ossicles continue to expand well into adolescence.


While changes in middle ear dimensions are modest in to adolescence, the mastoid process and tympanic bone continue to grow substantially. The mastoid process, for example, develops mainly after birth and thus the stylomastoid foramen and with it the facial nerve can be found quite superficially. The mastoid process typically develops inferiorly and laterally and the overall surgical “depth” of the mastoid cavity becomes greater with age. The mastoid tip develops quite independently from the petrous portion but this process occurs at variable time frames after one year of age.


Similarly to the lateral extension of the mastoid process, the tympanic ring also extends laterally to complete the formation of the bony external auditory canal. Therefore, the length of the external auditory canal is very short in the infant compared with the adult temporal bone. Also, this lateral growth changes the alignment of the tympanic membrane from an almost vertical alignment in the neonate into the typical adult angulated position.


Composition of the Temporal Bone (Fig. 2.1A, B, D, E)

The temporal bone is a complex conglomerate of different parts: the squamous part (or squama), the tympanic portion, the petrous bone, and the mastoid process (Fig. 2.1A). Some authors view the styloid process with its close relationship to the stylomastoid foramen as part of the temporal bone.


Squamous portion (Fig. 2.1A): The squama of the temporal bone forms the lateral bony boundary of the middle cranial fossa. It interfaces with surrounding bones of the skull in respective suture lines. On its medial surface the squamous portion carries a groove for the middle meningeal artery. Laterally, the temporalis muscle fans out broadly over its surface.


Mastoid portion (Fig. 2.1A, E): The mastoid process (surgically known as the mastoid) is a composite structure containing contributions from both the squamous and petrous portions. Both are separated by Körner’s (petrosquamous) septum. The sternocleidomastoid muscle attaches broadly along the tip and inferior surface of the mastoid and the posterior belly of the digastric muscle sits in the mastoid groove on the inferior and posterior surface of the mastoid process. Macewen’s triangle (or suprameatal triangle) is typically characterized by multiple small blood vessels and perforations in the temporal bone and serves as a landmark to begin mastoidectomy drilling. This triangle is located inferior to the temporal line, a bony posterior extension of the zygomatic root, and posterior to the osseous external auditory canal. The mastoid foramen is located posteriorly on the mastoid process and carries the mastoid emissary vein and the mastoid artery.




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Fig. 2.1 A–E


Tympanic bone (Fig. 2.1A): The tympanic bone forms the tympanic ring of the external auditory canal (EAC). It also forms the posterior bony wall of the glenoid fossa hosting the temporomandibular joint (TMJ). Several suture lines serve as landmarks within the EAC: the tympanomastoid suture lies posteroinferiorly and the tympanosquamous suture is situated superiorly within the canal. The petrotympanic suture line is a more medial structure within the middle ear proper and hosts the chorda tympani nerve and the anterior tympanic artery. All of these structures can serve as convenient landmarks to facilitate dissection in ear surgery.


Petrous portion (Fig. 2.1B, D): The petrous bone is named after its rocklike consistency, which hosts the sensory organs of the inner ear. On the superior surface, the arcuate eminence can be seen within the floor of the middle cranial fossa and roughly corresponds to the prominence of the superior semicircular canal. The greater superficial petrosal nerve can also be seen exiting the facial hiatus medial and anterior to the arcuate eminence. More anteriorly and laterally, the foramen spinosum is found, transmitting the middle meningeal artery from the infratemporal fossa to the middle cranial fossa.


Lateral View (Fig. 2.1A)

Viewing the temporal bone from lateral allows visualization of the tympanic, the mastoid, and the squamous portions. The close morphological relationship of the glenoid fossa and the EAC can readily be appreciated. Also, Macewen’s triangle and the suprameatal (Henle’s) spine can be seen. The zygomatic arch can be followed posteriorly into the temporal line. Contrary to common belief, the temporal line does not indicate the position of the middle cranial fossa plate (or floor). The squamous portion carries a sulcus for the medial temporal artery. The petrotympanic fissure hosts the anterior tympanic artery and chorda tympani nerve as they traverse to and from the middle ear, respectively.


Superior View (Fig. 2.1B)

The superior view of the temporal bone forms the floor of the middle cranial fossa. Landmarks are not very obvious and detailed knowledge and experience are typically required for middle fossa approaches to the temporal bone. Useful landmarks include the arcuate eminence, the foramen spinosum, the facial hiatus, and the petrous ridge.


Medial and Posterior Views (Fig. 2.1D, E)

This view features the porus acousticus of the internal auditory canal (IAC) and the internal carotid foramen (carotid canal). The sigmoid sinus and superior petrosal dural venous sinuses run in bony sulci, the former within the posterior fossa dura and the latter in the junction of the middle and posterior cranial fossae. The endolymphatic sac lies within a bony fossa on the posterior face of the petrous bone and the endolymphatic duct traverses the operculum to enter the vestibular aqueduct as it approaches the medial wall of the vestibule of the inner ear. The cochlear canaliculus (or aqueduct) forms a direct communication between the basal turn of the cochlea and the subarachnoid space.


Inferior Surface (Fig. 2.1E)

The inferior surface provides bony attachments for the deep muscles of the neck. The jugular fossa hosts the jugular bulb and is separated from the carotid canal by the jugulocarotid crest. This crest also hosts the inferior tympanic canaliculus, which is traversed by the inferior tympanic artery and the tympanic branch of CN IX (Jacobson’s nerve). Naturally, the stylomastoid foramen is located posterior to the styloid process.


Recommended Reading

Donaldson JA, Duckert LG, Labert PM, Rubel EW, eds.Anson-Donaldson: Surgical Anatomy of the Temporal Bone. 4th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 1992



Anatomy of the External Ear and Eustachian Tube


Definition

The external or visible part or the ear is the auricle or pinna. Its primary function is to capture sound waves and focus them toward the external auditory canal for the purposes of hearing. The auricle has great significance from an esthetic perspective, which can present a variety of surgical challenges for the reconstructive plastic surgeon.


Closely Related Topics

• General otologic/neurotologic physical examination—see p. 46


• Benign tumors of the external auditory canal—see p. 117


• Canaloplasty of the external auditory canal—see p. 277


The Auricle (Fig. 2.2A, B)

The auricle (or pinna) helps to focus and localize sounds. It is subjected to substantial anatomical variations in size, contour, and shape. This is in part due to its multicomponent embryologic origin. The lateral aspect shows various concavities and convexities (Fig. 2.2A), the concha being the largest concavity and also forming the entrance to the external auditory meatus.


The cartilage of the external auditory canal is contiguous with the conchal cartilage of the pinna (Fig. 2.2B). Thus, the pinna is securely attached to the tympanic bone. Furthermore, the pinna attaches via its skin, subcutaneous tissue, and ligaments to the skull. The typical numbness of the superior aspect of the pinna experienced after postauricular incision is mediated by the auriculotemporal nerve and/or the lesser occipital nerve. Also, the greater auricular nerve supplies the lower one-third of the lateral and the lower two-thirds of the medial surface of the auricle. Lymphatic drainage is mediated via pre- and post-auricular lymph nodes.


The posterior auricular muscle is typically the largest member of the rudimentary musculature of the auricle. It is classified as part of the facial musculature. It is typically supplied by the posterior auricular nerve, a branch of the facial nerve.


External Auditory Canal (Fig. 2.2C, D)

In the depth of the cavum conchae starts the external auditory canal. It measures ~3.5 cm in length and can be divided into three parts. The lateral one-third is cartilaginous, whereas the medial two-thirds are bony (Fig. 2.2C). Only the cartilaginous part possesses hair and sebaceous glands. Ceruminal glands can be found over the entire length of the EAC. In contrast to the skin of the cartilaginous portion, the skin overlying the bony EAC is very thin. Thus, the osseous portion of the EAC is most sensitive to examination and manipulation.


The cartilage of the EAC is a continuation of the cartilage of the pinna. Furthermore, the cartilaginous EAC does not form a complete ring since it is deficient superiorly at the incisura terminalis. On its anterior aspect, the cartilage has two or three variable fissures of Santorini. The endaural approach to the temporal bone utilizes this deficiency and the incision can thus be performed directly onto the bony EAC. The isthmus is the narrowest portion of the EAC and it lies directly medial to the junction of the medial and bony portions. In the child, up to two-thirds of the EAC can be cartilaginous. At the age of 6 years, half of the EAC still has cartilaginous walls.


The osseous EAC is formed by the tympanic portion of the temporal bone. Its medial-most boundary is the tympanic annulus and membrane. Notable landmarks include the tympanosquamous (superior) and tympanomastoid (posterior and inferior) suture lines. Visualization of the anterior aspect of the tympanic membrane can be obscured by an impression created by the temporomandibular joint fossa on the EAC. The temporomandibular joint can be dehiscent in the osseous portion of the EAC, especially in its medial aspect.


The auriculotemporal branch of the third division of CN V (V3), the auricular branch of the facial nerve (VII), the greater auricular nerve as a branch of CN II and III (C2 and C3), the lesser occipital nerve of C2 and C3, the auricular branch of CN X (Arnold’s nerve), and a small branch from the facial nerve all contribute to the sensory innervation of the EAC (Fig. 2.2D). Due to this rich sensory nerve supply, effective local anesthesia for the EAC can be obtained by subcutaneous infiltration of diluted lidocaine in all quadrants of the canal. When injecting the bony EAC, the locations of the tympanomastoid and tympanosquamous suture lines are usually evidenced by dense anchoring of the skin to the underlying bone.




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Fig. 2.2 A–F


The arterial supply to the EAC is mainly derived from branches of the external carotid artery: the posterior auricular artery and the superficial temporal artery. The posterior auricular artery supplies the inferior portion of the mastoid. The stylomastoid artery, a branch of the posterior auricular artery, enters the stylomastoid foramen and supplies the mastoid segment of the facial nerve. Small branches of the superficial temporal artery supply the anterior portions of the EAC.


Eustachian Tube (Fig. 2.2E)

The eustachian tube (pharyngotympanic tube, tympanic tube) is a 31–38 mm long multipurpose structure that connects the nasopharynx and the middle ear. It protects the middle ear from nasopharyngeal secretions and sound, clears the middle ear, and ventilates the middle ear. It has an anteromedial fibrocartilaginous part, which measures roughly two-thirds of its entire length, and posterolateral osseous part. The cartilaginous portion is composed of one main cartilage, which demonstrates great elastic potential. The cartilaginous portion is attached to the sphenoid sulcus and the posterior edge of the medial pterygoid plate.


The tympanic orifice is located 2–3 mm superior to the hypotympanic floor. The pharyngeal orifice protrudes into the nasopharynx ~10 mm above the plane of the hard palate. This protrusion is known as the torus tubarius and the indentation superior to that is the fossa of Rosenmüller.


There are four muscles associated with the eustachian tube: the tensor veli palatini, the levator veli palatini, the salpingopharyngeus, and the tensor tympani muscle. The tensor veli palatini seems to be the main player associated with the functional aspect of the tube. It is composed of two distinct bundles: the lateral portion originates from the scaphoid fossa and descends anteriorly, laterally, and inferiorly to converge in a tendon which then passes around the hamulus of the pterygoid plate and inserts on the horizontal process of the palatine bone and into the palatine aponeurosis of the velum. The medial portion is typically referred to as the dilator tubae and seems to be responsible for a direct opening of the fibrocartilaginous portion via inferolateral displacement. The remaining muscles are believed to provide minor support and opening function (Fig. 2.2E).


Developmental Remarks (Fig. 2.2F)

Both vertical development of the skull and increases in the angle of the skull base lead to a growth in length and more vertical angulation of the eustachian tube by age 7 years. The length of the eustachian tube is only ~13 mm at birth. Also, the angle between the skull base and the eustachian tube measures roughly 10° at birth.


This is in contrast to an overall length of 31–38 mm and an angle of 45° in adulthood (Fig. 2.2F).


It is believed that the childhood alignment of the eustachian tube provides adequate traction forces of the tensor veli palatini for efficient opening of the lumen and therefore pressure equalization of the middle ear. Also, the shorter length and overall smaller diameter might contribute to an overall poorer function of the eustachian tube in childhood. Recent studies have also suggested that adenoids play a minor role in the functional characteristics of the eustachian tube.


Normal Function

The normal eustachian tube is functionally collapsed at rest, which creates a slight negative pressure in the middle ear. After the tube briefly opens during swallowing, sneezing, and yawning, passive forces such as recoil of the elastic cartilaginous fibers lead to functional closure. A body of fat, the lateral fat pad of Ostmann, abuts the lateral aspect of the cartilaginous tube and aids in maintaining the functional closure at rest.


Recommended Reading

Bluestone CD. Pathogenesis of otitis media: role of eustachian tube. Pediatr Infect Dis J 1996;15(4):281–291



Anatomy of the Middle Ear


Definition

The middle ear is a noncollapsible gas pocket contained within the temporal bone extending from the tympanic membrane laterally to the cochlear promontory medially. The middle ear communicates with the nasopharynx anteromedially via the eustachian tube and with the mastoid air cell system through the antrum.


Closely Related Topics

• Physiology of the middle ear—see p. 27


• Assessment of middle ear function—see p. 58


• Congenital malformations of the middle ear—see p. 123


• Surgery for chronic otitis media—see p. 299


Introduction

The middle ear consists of the tympanic membrane and the spaces of the middle ear, which contain the auditory ossicles, two muscles, and their vascular supply and respective innervation. The tympanic cavity lies medial to the tympanic membrane and is in continuity with the eustachian tube anteriorly and with the mastoid air cell system via the aditus ad antrum. It is lined with a mucosal epithelium.


Tympanic Membrane (Fig. 2.3A–C)

The tympanic membrane has an irregular conical shape, the apex of which is formed by the umbo of the malleus. The tympanic membrane arises as a fusion of ectodermal, mesodermal, and endodermal elements. It can be separated into a superior pars flaccida, or Shrapnell’s membrane, above the short process of the malleus and the inferiorly located pars tensa. The pars tensa is a relatively rigid, trilaminar structure formed by all three embryological layers. By contrast, the pars flaccida lacks this middle layer, being only bilaminar and much less structurally rigid (i.e., flaccid).The lateral surface of the tympanic membrane is formed by squamous epithelium in continuity with the keratinizing epithelium of the external auditory canal and is devoid of glandular structures. The medial layer is a simple epithelium that is mucosal in nature, also devoid of true glands, and is in continuity with the mucosa of both the middle ear and mastoid. The middle layer is predominantly fibrous in nature and is devoid of elastic fibers.


The pars tensa is suspended in a fibrous annulus, which anchors the tympanic membrane in the tympanic sulcus. The tympanic membrane is also attached firmly to the malleus at the lateral (or short) process and the umbo. The plica mallearis connects the tympanic membrane to the rest of the manubrium of the malleus.


The overall diameter of the tympanic membrane is roughly 9 mm and the plane of the tympanic membrane forms an acute angle with the anterior bony wall of the external auditory canal and an obtuse angle with the posterior wall. Thus, the tympanic membrane, when viewed through the external auditory meatus is seen tangentially rather than en face.


Spaces of the Middle Ear and Mucosal Linings (Fig. 2.2B, E)

The tympanic cavity can be divided into several anatomically relevant compartments including the mesotympanum, the protympanum, the hypotympanum, and the epitympanum. The entire tympanic cavity is lined with a mucosal epithelium that includes a variety of cell types. Ciliated cells and secretory (or goblet) cells predominate close to the eustachian tube and anterior middle ear cavity and transition toward a flat, less ciliated epithelium posteriorly.


The mesotympanum (or middle ear) is bounded anteriorly, inferiorly, and posteriorly by the tympanic annulus and superiorly by the bony margin of the external auditory canal and notch of Rivinius. The lateral wall of the mesotympanum is the tympanic membrane and the medial wall is the promontory. The mesotympanum contains the manubrium of the malleus, long process of the incus, stapes and oval window, stapedius tendon, round window niche and membrane, and cochlear promontory. The space located anterior to the mesotympanum is the protympanum and includes the bony eustachian tube orifice, the carotid artery, and the semicanal of the tensor tympani muscle. The region superior to the mesotympanum is the epitympanum and is bounded superiorly by the floor of the middle cranial fossa or the tegmen tympani. This space includes the head and neck of the malleus, body and short process of the incus, the tympanic segment of the facial nerve, and the cochleariform process and tensor tympani tendon (Fig. 2.3B). The cog is a bony crest that runs vertically starting at the cochleariform process. It divides the epitympanum into a posterior region and an anterior region. The anterior portion is termed the supratubal recess, which is a difficult region for cholesteatomas. The posterior aspect of the epitympanum communicates with the mastoid via the antrum. The hypotympanum lies inferior to the mesotympanum and contains the bony canals for the jugular bulb posteriorly and the carotid artery anteriorly as well as the tympanic canaliculus that transmits Jacobson’s nerve for the jugular foramen.




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Fig. 2.3 A–F


The tympanic cavity region, posterior to the mesotympanum, remains nameless but is bounded posteriorly by the mastoid segment of the facial nerve, the ampullae of the posterior semicircular canal, and the chorda tympani nerve. Two notable pneumatized spaces in this region include the sinus tympani and facial recess. Due to its limited surgical accessibility, the sinus tympani is a difficult location for managing cholesteatomas (Fig. 2.3E). The sinus tympani is bordered by the ponticulus superiorly, the subiculum inferiorly, the facial nerve laterally, and the posterior semi-circular canal medially. The bony pyramidal process that contains the stapedius muscle also lies in this region.


The oval and round windows are in direct communication with the intralabyrinthine compartments that contain perilymph. The oval window contains the stapes footplate and the round window is covered by a fibrous membrane that moves in response to middle ear pressure changes or stapes compressions. Thus, these windows can be sites for leaks of perilymph following direct mechanical or barotrauma. The round window membrane is located medial to the variable bony niche overhang and allows direct access to scala tympani for cochlear implantation. The crista fenestra (semilunaris) is a small bony crest that can be seen inferiorly when looking at the true round window. It should also be mentioned that the true round window is most often obscured by a false membrane of mucosa and/or fibrous tissue.


Recognition of Prussak’s space is critical for understanding cholesteatoma formation. This space is located medial to Shrapnell’s membrane (or pars flaccida) and lateral to the head and neck of the malleus. It is also bounded by both the anterior and posterior malleal ligaments. The corresponding folds created by the ligaments create spaces anterior and posterior to the malleal head that promote the predictable spread of cholesteatoma.


Ossicles (Fig. 2.3C, D)

Three ossicles make up the ossicular chain: the malleus, incus, and stapes (Fig. 2.3C, D). They serve to conduct sound from the tympanic membrane to the cochlea. The lateral process of the malleus, which also has a head, a neck, and an anterolateral process, attaches to the pars propria of the tympanic membrane. The malleus is suspended via several ligaments: the anterior ligament passes through the petrotympanic fissure and attaches from the anterior (short) process.


The incus is the largest of the three ossicles. It has a body and a long, a short, and a lenticular process. The body of the incus articulates with the head of the malleus in the epitympanum. The incudal fossa anchors the short process posteriorly and the lenticular process is an extension of the long process inferiorly, which articulates with the stapes. The long process of the incus receives its blood supply from the body of the incus only, thus making it especially susceptible to bone erosion from either chronic ear disease or from a crimped prosthesis.


The stapes is the smallest ossicle. It articulates with the lenticular process of the incus and its footplate sits in the oval window. The footplate is surrounded by the annular (or stapediovestibular) ligament. The superstructure of the stapes (also known as the arch) has an anterior and a posterior crus. Also, the head of the stapes sits at the top of the arch where it articulates with the incus.


Middle Ear Muscles (Fig. 2.3B)

The middle ear has two muscles: the tensor tympani and the stapedius muscle. The latter receives its innervation from the facial nerve and the tensor tympani (TT) from the trigeminal nerve. The tendon of the TT muscle is anchored to the cochleariform process and attaches to the neck of the malleus. The stapedius muscle belly lies medial to the mastoid portion of the facial nerve. The tendon exits its bony sulcus at the pyramidal process to attach to the posterior crus of the stapes (Fig. 2.3B).


Vascular Supply and Innervation

The ascending pharyngeal artery (from the external carotid artery) gives rise to the inferior tympanic artery, which traverses the inferior tympanic canaliculus with Jacobson’s nerve. This artery is clinically relevant since it typically feeds tympanic paragangliomas. The anterior tympanic artery, the deep auricular artery, the mastoid artery, the stylomastoid artery, the superficial petrosal artery, and the tubal artery all contribute to the anastomotic vascular plexus of the tympanic cavity.


The glossopharyngeal nerve gives rise to its tympanic branch (Jacobson’s nerve) that enters the middle ear in the hypotympanum through the tympanic canaliculus between the carotid artery and jugular bulb with the inferior tympanic artery. This nerve provides sensory fibers to the middle ear as well as preganglionic parasympathetic fibers destined for the otic ganglion for salivary stimulation. Together with the caroticotympanic nerves (sympathetic innervation) from the pericarotid nervous plexus, they form the tympanic plexus. This plexus eventually gives rise to the lesser petrosal nerve, which travels along the floor of the middle cranial fossa.


Arnold’s nerve (i.e., auricular branch of the vagus nerve) arises from the jugular foramen and traverses the mastoid canaliculus to supply the temporal bone. This nerve may either join the sensory contributions from cranial nerve VII or innervate the external auditory canal directly. This nerve is thought to be responsible for reflex coughing that variably occurs upon sensory stimulation of the external auditory canal skin.


Pneumatization and Air Cell Tracts (Fig. 2.3F)

Several parts of the temporal bone can undergo pneumatization: the middle ear, the mastoid, the perilabyrinthine regions, and the petrous apex, as well as some accessory areas. The extent to which each part pneumatizes varies considerably among individuals. Typically, the extent of mastoid pneumatization is often used as a clinical marker for eustachian tube (dys) function. Pneumatization typically begins in the antrum, then carries on into the central mastoid area and then into the peripheral mastoid regions. The perilabyrinthine areas are subdivided into the supralabyrinthine and infralabyrinthine (retrofacial) air cell tracts. The apical and peritubal areas both comprise the petrous apex. Accessory regions include the zygomatic arch, the squama, the occipital bone, and the styloid area (Fig. 2.3F).


Recommended Reading

Maroldi R, Farina D, Palvarini L, et al. Computed tomography and magnetic resonance imaging of pathologic conditions of the middle ear. Eur J Radiol 2001;40(2):78–93


Palva T, Johnsson LG. Epitympanic compartment surgical considerations: reevaluation. Am J Otol 1995; 16(4):505–513



Anatomy of the Inner Ear


Definition

The inner ear is an osseous structure within the confines of the temporal bone that houses both the auditory and vestibular sensory mechanisms.


Closely Related Topics

• Auditory physiology of the inner ear—see p. 30


• Sensorineural hearing loss—see p. 158


• Surgery for cochlear implants—see p. 328


Overview (Fig. 2.4A, B)

The labyrinth is a fluid-filled system containing the inner ear sensory organs of hearing (located anteriorly, cochlea) and balance (located posteriorly, vestibule and three semicircular canals [SCCs]). The labyrinth has a layered structure: the bony labyrinth surrounds the fluid-filled spaces within and is lined with an endosteal membrane. The membranous labyrinth is located within the bony labyrinth and filled with endolymph (Fig. 2.4A, B). The space between bony and membranous labyrinths is filled with perilymphatic fluid.


Cochlea (Fig. 2.4C)

The cochlea has 2½ turns and spirals around the modiolus, its axis. The spiral ganglion cell bodies are located in the modiolus and the dendrites project to the organ of Corti via the cribrose area and the osseous spiral lamina. The apex of the cochlea is located medial to the cochleariform process and the tensor tympani muscle. The development of the cochlea has been completed long before birth. Thus, there is no postnatal growth, which is important for cochlear implantation. However, there are substantial interindividual differences in shape and size of the cochlea that deserve consideration.


The cochlea has three main fluid compartments: the scala tympani and vestibuli, which contain sodium-rich perilymph, and the scala media (also known as the cochlear duct), which contains endolymph and houses the organ of Corti. The organ of Corti rests on the basilar membrane, which spans between the osseous spiral lamina and the spiral ligament on the outer wall. The spiral ligament also contains the stria vascularis in its upper portion, which contains a rich vascular network and produces endolymph. Reissner’s (vestibular) membrane, is a fragile two-cell-layered structure, which separates the scala vestibuli and media. The more robust basilar membrane divides the scala tympani and media.


The organ of Corti contains the inner and outer auditory hair cells and thus is the central sensory element for hearing. Several other cells are part of the organ of Cori and they are illustrated in Fig. 2.4C.


Vestibule (Fig. 2.4D)

The vestibule is the central chamber of the inner ear. Its membranous components consist of the larger utricle and the smaller saccule. The utricle communicates with the endolymphatic duct via the utriculo-endolymphatic valve. The saccule connects to scala media of the cochlea via the ductus reuniens. The saccule and utricle contain the macula. The saccule lies in the spherical recess of the bony vestibule and the utricle in the elliptical recess. Similar to the cochlea, the bony vestibule has cribrose areas that are traversed by sensory nerve fibers of the vestibular nerve. Care must be taken during stapes surgery not to injure the membranous labyrinth (Fig. 2.4D).


Semicircular Canals (Fig. 2.4A, B)

The inner ear has three semicircular canals (SCCs): the superior (anterior), the lateral (horizontal), and the posterior canal (Fig. 2.4A). All three canals are aligned in mutual orthogonal relation. Each canal has a larger ampullated and a smaller nonampullated portion. The ampullae contain the sensory neuroepithelium within the cristae (i.e., crista ampullaris) and the bones of the ampulla show a cribrose area for the passage of vestibular nerve fibers. Of note, the superior vestibular nerve innervates the superior and lateral semicircular canals as well as the saccule. The utricle and the posterior semicircular canals are innervated by branches of the inferior vestibular nerve (Fig. 2.4B). The superior crus of the posterior canal and the posterior crus of the superior canal form the common crus. The lateral canal has an anterior crura and a posterior crura.


Blood Supply

Arterial supply of the inner ear is derived from the labyrinthine artery (internal auditory artery), a branch of the anterior inferior cerebellar artery (AICA) of the vertebrobasilar arterial system. The subarcuate artery (also a branch of the AICA) is commonly encountered during labyrinthectomy. Specifically, it is located between the anterior and posterior crura of the superior semicircular canal in the petromastoid canal. This artery arises from the anterior inferior cerebellar artery and does not supply the labyrinth.


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