temporal bone, external & middle ear - dr. sholley...temporal bone, external ear, and middle ear...
TRANSCRIPT
TEMPORAL BONE, EXTERNAL EAR, and MIDDLE EAR Dr. Milton M. Sholley
OBJECTIVES: Following this lecture, the student should be able to:
1. Locate the temporal bone and describe its position among the surrounding bones of the skull.
2. Locate and describe the parts of the temporal bone.
3. Understand the important relationships of nerves and vessels traversing the temporal bone.
4. Understand the relationships of the external auditory meatus to the temporal bone.
5. Define the boundaries of the middle ear cavity.
6. Identify the structures within the middle ear cavity.
7. Understand the function of the ossicular chain.
8. Understand the position of the middle and inner ear structures within the temporal bone.
Please study all references to Grant's Atlas, 12 th Edition.
I. OSTEOLOGY of the TEMPORAL BONE
A. In a lateral view (Grant's Atlas, Figs. 7.3A&B, pp. 612613) one can visualize by their separating sutures 3 of the 4 major divisions of the temporal bone:
1. Squamous part forms the roof of the external auditory meatus, the anterior part of the mandibular fossa, and gives off the zygomatic process.
2. Tympanic part is Ushaped, forming the anterior, inferior, and most of the posterior walls of the external auditory meatus, and gives off the styloid process.
3. Mastoid part is a large inferior projection located posterior to the external auditory meatus and contains the mastoid air cells.
4. Petrous part cannot be seen from the direct lateral view. In other views (Grant's Atlas, Fig 7.6A&B, pp. 618619) one can see that it forms the petrous pyramid, which contains the middle and inner ear structures.
B. Lateral bony landmarks
1. Bony external auditory meatus (Grant's Atlas, Figs. 7.3A, p. 612, and 7.40A, p. 664)
a. Its anterior, inferior, and most of its posterior walls are formed by the tympanic part of the temporal bone, while its roof and part of its posterior wall are formed by the squamous part.
b. It is closed medially by the tympanic membrane (eardrum), which is oriented obliquely so that it slopes medially from superior to inferior and also from posterior to anterior.
c. Posteriorly it is related to the mastoid part and out of the lower part of the tympanomastoid suture the auricular branch of the vagus (Arnold's nerve) emerges.
d. Anteriorly it is related to the mandibular fossa, which contains the mandibular condyle. Behind the condyle the upper tip of the parotid gland further fills the fossa.
2. Mastoid process (Grant's Atlas, Fig. 7.24B, p. 642)
a. If the lateral wall of the process were drilled away, one would see the contained mastoid air cells.
b. On the medial wall of the mastoid process there is a wide groove representing the sigmoid venous sinus. Thus, infection of the mastoid air cells can spread medially into the sigmoid sinus or posteriorly into the posterior cranial fossa, which also forms part of the mastoid's medial wall.
C. In an inferior view (Grant's Atlas, Figs. 7.5A&B, pp. 616617) one can visualize:
1. The beginning of the carotid canal entering the inferior surface of the petrous part. From this point the carotid canal passes anteriorly and medially parallel to the long axis of the petrous pyramid and then passes superior to the foramen lacerum. The carotid canal contains the internal carotid artery, its surrounding plexus of postganglionic sympathetic nerves, and some emissary veins. Superior to the foramen lacerum the artery exits the carotid canal and turns upward into the cavernous sinus.
2. Inside the beginning of the carotid canal, on its posterolateral wall, a small foramen which transmits a nerve into the middle ear cavity. This nerve is the caroticotympanic nerve, which is derived from the pericarotid plexus and provides sympathetic supply to the tympanic plexus. A blood vessel also uses the foramen.
3. Posterior to the beginning of the carotid canal the jugular foramen, the lateral part of which is formed by the petrous part. The jugular foramen contains the jugular bulb of the internal jugular vein. Also exiting through the jugular foramen are the inferior petrosal sinus and the glossopharyngeal, vagus, and accessory nerves. The jugular foramen is related to the floor of the middle ear cavity.
4. A flange of bone separating the carotid canal from the jugular foramen. On this flange there is the small inferior tympanic canaliculus, which transmits the tympanic branch of the glossopharyngeal nerve. This canaliculus conveys the tympanic branch through the floor of the middle ear cavity, where it contributes sensory and parasympathetic supply to the tympanic plexus.
5. The stylomastoid foramen, between the styloid process and the mastoid process. This foramen transmits the facial nerve as it exits the skull and the stylomastoid artery (from the posterior auricular artery), which enters and supplies the middle ear cavity.
6. The digastric and occipital grooves, which lie posterior to the stylomastoid foramen.
7. The mandibular fossa. A fissure runs coronally across the mandibular fossa, separating the larger anterior squamous part of the fossa from the smaller posterior tympanic part. Within the medial end of this fissure is a sliver of petrous temporal bone between the squamous and tympanic parts. Posterior to this sliver of petrous bone, i.e. through the petrotympanic fissure, the chorda tympani nerve enters the infratemporal fossa. At this point the chorda tympani has exited the anterior wall of the middle ear cavity and passed through a canal, the iter chordae anticus, just before passing through the petrotympanic fissure.
8. The bony auditory tube, which lies medial to the mandibular fossa between the tympanic and petrous parts. This bony tube connects the middle ear cavity to the cartilaginous part of the auditory tube, which opens into the nasopharynx.
D. In a superior view (Grant's Atlas, Figs. 7.6A&B, pp. 618619) one can visualize:
1. Primarily the anterior face of the petrous pyramid as it forms the posterior part of the middle cranial fossa. Note that the long axis of the petrous pyramid intersects the midsagittal plane at an angle of about 45 o .
2. The anterior end of the carotid canal just medial to the apex of the petrous pyramid.
3. The hiatus of the facial canal (Grant's, Fig. 7.6A, p. 618), through which the greater petrosal nerve emerges to lie on the surface of the anterior face of the pyramid (Grant's Atlas, Fig. 7.26A, p. 644) as it courses toward the pterygoid canal.
4. The hiatus for the lesser petrosal nerve, which emerges onto the anterior face of the pyramid lateral and anterior to the greater petrosal nerve (Grant's Atlas, Fig. 7.26A, p. 644).
5. The arcuate eminence (Grant's Atlas, Fig. 7.6B, p. 619), which is produced by the underlying superior (or anterior) semicircular canal (Grant’s Atlas, Figs. 7.77AC, p. 714). The eminence and canal are oriented in a plane perpendicular to the long axis of the pyramid.
6. The tegmen tympani, which lies anterolateral to the arcuate eminence and represents the thin roof of the middle ear cavity.
7. The roof of the external auditory meatus, which lies immediately lateral to the tegmen tympani.
E. In a posterior view (Grant's Atlas, Figs. 7.4C&D, p. 615, and Figs. 7.24B, p. 642) one can visualize:
1. Primarily the posterior face of the petrous pyramid, related to the pons medially and the cerebellum laterally.
2. The groove for the superior petrosal sinus along the superior petrosal ridge.
3. Part of the groove for the inferior petrosal sinus along the medial edge of the petrous apex.
4. The large groove for the sigmoid sinus indenting the posteromedial side of the mastoid part.
5. The internal auditory meatus, which conveys the facial nerve, the auditory nerve, and the labyrinthine artery.
6. The vestibular aqueduct, a slitlike bony orifice located superior and posterior to the internal auditory meatus. The vestibular aqueduct transmits the endolymphatic duct, which runs from the vestibule into the dura of the posterior cranial fossa where it ends as a blind sac (Grant's Atlas, Fig. 7.77AD, p. 714).
7. The bony cochlear canaliculus, which conveys the perilymphatic duct from the cochlea into the subarachnoid space in the region of the anterior margin of the jugular foramen (Grant's Atlas, Fig. 7.77B, p. 714).
II. EAR
A. For descriptive purposes the ear is divided into 3 parts:
1. External ear consisting of the auricle and the external auditory meatus, which is separated by the tympanic membrane from the middle ear cavity.
2. Middle ear cavity with its contained ossicles and their muscles, its connection with the pharynx through the auditory tube, and its various air cell expansions in the mastoid and petrous parts of the temporal bone. Its complex medial wall separates the middle ear cavity from the internal ear.
3. Internal ear consisting of a membranous labyrinth, which lies within an excavation (bony labyrinth) of the dense petrous bone medial to the middle ear cavity
B. External ear
1. Auricle (Grant's Atlas, Fig. 7.68AC, p. 703)
a. It consists of an irregularlyshaped plate of elastic cartilage covered by skin and connected to the external auditory meatus by ligaments and muscles.
b. The concha is the well of the auricle surrounding and posterior to the meatus. The helix forms the rim of the auricle and is continuous inferiorly with the fibrofatty lobule. The tragus is the tab that is anterior to the meatus.
c. Its skin is tightly adherent on the anterolateral surface but looser on the posteromedial side, so when the auricle is inflamed, swelling is more marked on the medial side.
Auricle Lateral view
Ear lobe
Antitragus Tragus
Helix
Tubercle
Concha Antihelix
Crus of helix
Crura of antihelix Triangular fossa
Scaphoid fossa
Grant’s Atlas, 12 th Ed. Fig. 7.68A, page 703
d. Its abundant blood vessels lie in a superficial exposed position, so the auricle is unusually subject to frostbite.
e. The inferior and posterior part of its lateral surface is innervated by the great auricular nerve. The superior and anterior part of the lateral surface is supplied by the auriculotemporal nerve (from mandibular division of V). The concha is supplied by the auricular branch of the vagus. The inferior part of the medial surface is supplied by the great auricular nerve and the superior part of the medial surface is supplied by the lesser occipital nerve.
2. External auditorymeatus (Grant's Atlas, Fig. 7.70A, p. 705)
a. It extends from the concha to the tympanic membrane and is about 2.5 cm long in adults.
b. Its outer 1/3 to 1/2 is formed by cartilage, which is continuous with that of the auricle.
c. When introducing a speculum into the external auditory meatus, it is important to recognize that the meatus is Sshaped. The cartilaginous portion has a convexity directed superiorly and posteriorly, while the bony part has a convexity directed inferiorly and anteriorly. The outer part of the canal can be straightened by lifting the mobile auricle superiorly and posteriorly with slight outward traction.
d. In newborn and young infants, the meatus is much shorter than adults, since the bony part of the canal is late to develop.
e. The meatus is constricted at the junction of the cartilaginous and bony parts and also at the isthmus, where the bony portion narrows down about 1/2 mm from the membrane.
f. Its anterior wall obtains sensory innervation from the auriculotemporal nerve and its posterior wall from the auricular branch of the vagus, supplemented posteriorly by branches from the facial and glossopharyngeal nerves.
3. Tympanic membrane (Grant's Atlas, Figs. 7.71AD, p. 706)
a. It forms the medial wall of the external auditory meatus and much of the lateral wall of the middle ear cavity (tympanic cavity).
Middle Ear Cavity Horizontal section
Middle ear cavity (Tympanic cavity)
Tympanic membrane (Eardrum)
Malleus
Incus
Lateral
Anterior
Chorda tympani
Facial nerve (in facial canal)
Similar to: Grant’s Atlas, 12 th ed. Figure 7.72A, p. 707
b. It is obliquely oriented, having a medial inclination from posterior to anterior and also from superior to inferior of about 40 o .
c. The plane of the middle ear cavity is similarly obliquely placed with respect to the usual anatomical planes.
d. It has a middle fibrous layer and inner and outer epithelial layers, continuous respectively with the epithelium of the middle ear cavity and the meatus.
e. It is convex toward the middle ear cavity and the point of maximum convexity is called the umbo.
f. The manubrium (handle) of the malleus is attached by its inferior tip to the inside of the tympanic membrane, forming the umbo invagination. The lateral (short) process of the malleus projects laterally from the superior end of the manubrium and evaginates the superior part of the membrane.
g. From the lateral process of the malleus, anterior and posterior mallear folds extend along the inner surface of the membrane to attach to their respective upper edges of the Ushaped tympanic part of the temporal bone. The large, more tense, thicker part of the membrane inferior to the lateral process and mallear folds is called the pars tensa. The smaller, thinner, looser superior part of the membrane is called the pars flaccida and attaches to the squamous part of the temporal bone.
h. Sensory innervation to the anterior half of the lateral surface is from the auriculotemporal nerve, while the posterior part of the lateral surface is innervated by the auricular branch of the vagus. The medial surface of the membrane is innervated by the glossopharyngeal nerve.
C. Middle ear cavity (Grant's Figs. 7.72 7.76, pp. 707713)
1. It has 6 walls and is basically shaped like a flat cigar box. Its walls are usually described as lateral, medial, anterior, posterior, a roof, and a floor. However, because of its oblique orientation, parallel to the tympanic membrane, these convenient names for the walls are only crude anatomical designations for the actual positions of the walls.
Diagram of Middle Ear Cavity Anterior wall removed
Malleu
s
Incus
Stapes
Tympanic plexus
MED
IAL W
ALL
LATE
RAL W
ALL
FLOOR Tympanic nerve
Lesser petrosal nerve
Facial nerve beyond stylomastoid foramen
Facial nerve in facial canal
Tympanic membrane
Chorda tympani
Grant’s Atlas, 12 th Ed. Figure 7.73A, page 708
ROOF
2. It is normally filled with air and open anteromedially to the pharynx through the auditory tube and posterolaterally through the aditus to the mastoid antrum and air cells.
3. The various structures within the cavity are all covered by reflections of mucous membrane from the walls.
4. Walls
a. Lateral wall
(1) Much of this wall is formed by the tympanic membrane.
(2) The middle ear cavity is divided into 3 regions according to its relationship to the tympanic membrane. The middle ear cavity proper (or mesotympanum) is at the level of the membrane, above the membrane is the attic (or epitympanum), and below the membrane is the hypotympanum.
Detailed Diagram of Lateral Wall of Middle Ear Cavity (Inside the cavity looking out)
Grant’s Atlas, 12th Ed. Figure 7.75D, page 711
Internal jugular vein Internal carotid artery
Bony auditory tube
Facial nerve
Aditus ad antrum
Tympanic canaliculus
Tensor tympani
Chorda tympani
Mastoid air cells
Tegmen tympani Attic
(3) The manubrium of the malleus is attached to the tympanic membrane, while part of the neck and all of the head of the malleus occupy the attic. The body of the incus, which articulates anteriorly with the head of the malleus, is also situated in the attic along with the short process of the incus, which is attached by a posterior ligament to the floor of the opening (aditus) to the mastoid antrum. Extending inferiorly from the body of the incus is its long process, which articulates at its tip with the head of the stapes. The long process of the incus is posterior and medial to the manubrium of the malleus. The anterior process, which arises from the neck of the malleus, is attached to the anterior wall of the middle ear by the anterior ligament of the malleus. The anterior ligament of the malleus and the posterior ligament of the incus establish
an axis around which these two ossicles oscillate. As sound vibrations displace the membrane inward, the manubrium of the malleus is moved medially, while its head and most of the body of the incus (which are superior to the axis) are displaced laterally. This causes a simultaneous inward displacement of the long process of the incus, which in turn moves the stapes medially into the inner ear fluids.
(4) The chorda tympani arises from the facial nerve in the descending part of its canal in the posterior wall of the middle ear. The chorda tympani then runs superiorly and anteriorly between the malleus and incus to exit the anterior wall of the middle ear cavity through the petrotympanic fissure.
b. Roof formed by a thin plate of bone, the tegmen tympani, which separates the middle ear cavity from the middle cranial fossa.
c. Floor a variably thick plate of bone separating the tympanic cavity from the jugular bulb in the jugular foramen below.
d. Anterior wall
(1) The superior part of the anterior wall is deficient where the auditory tube opens into the middle ear cavity. The posterior 1/3 of the auditory tube is osseous and the anterior 2/3 is cartilaginous. The cartilaginous part is shaped like an upside down J, with the tube's lateral wall largely deficient and closed only by a fibrous membrane. In adults the auditory tube is normally closed, with the mucosa of its walls approximated. Because part of the origin of the tensor veli palitini muscle arises from the lateral cartilaginous and membranous walls of the tube, contraction of this muscle effects an opening of the tube. Thus, any action which tends to tense the palate (eg. swallowing or yawning) will also pull inferiorly on the lateral tubal wall and open the tube, allowing equalization of middle ear pressure with that of the external environment. In infants and young children, the auditory tube is normally positioned more horizontally and open, explaining the increased incidence of middle ear infections secondary to upper respiratory tract infections in children.
(2) Superior to the auditory tube is the long thin tensor tympani muscle, which arises from the roof of the tube. The muscle lies in a bony canal. As its tendon emerges through an aperture at the posterior end of this canal it makes a right angle bend laterally to attach to the upper part of the manubrium of the malleus. The foramen ovale and the mandibular division of V, which innervates this muscle, are closely related to it.
(3) The inferior part of the anterior wall of the middle ear cavity is related to the carotid canal and the caroticotympanic nerves enter the middle ear cavity through this wall.
e. Posterior wall
(1) Its superior part has an opening (aditus) between the attic and the mastoid antrum, a large mastoid air cell connected to the others.
(2) Its inferior part contains the descending part of the facial canal. As the facial nerve comes off the medial wall, where it is posteriorly directed, it turns inferiorly into the posterior wall. Housed in this obtuse angle is the tiny stapedius muscle, situated inside a hollow bony pyramidal eminence. Its tendon emerges through an aperture in this eminence and runs anteriorly to attach to the neck of the stapes. This muscle is innervated by the closely related facial nerve. Both the stapedius and the tensor tympani act reflexly in response to loud sounds, thus dampening the oscillations of the ossicular chain and protecting the sensitive inner ear structures from excessive displacements.
f. Medial wall also is the lateral wall of the inner ear.
(1) The horizontal semicircular canal lies superiorly. It is tilted posteriorly about 30 o from the horizontal plane.
(2) Inferior to the horizontal semicircular canal lies the facial nerve (in the second part of its course) running posteriorly along the medial wall. Review the entire course of the facial nerve as it passes (1) laterally in a horizontal plane through the inner ear, (2) makes its genu (Grant’s Atlas, Fig. 7.74, p. 709) to run posteriorly in a horizontal plane in the medial wall of the middle ear, and (3) then runs inferiorly in a vertical plane in the posterior wall to emerge at the stylomastoid foramen.
(3) Inferior to the facial canal is the stapes, which has a head, two crura, and a footplate. The footplate of the stapes is fixed in the oval window, where its medial side comes into contact with the perilymphatic fluid of the vestibule.
(4) Inferior and anterior to the oval window is the rounded promontory, caused by the basal turn of the cochlea.
(5) The tympanic plexus of nerves ramifies over the promontory. The glossopharyngeal nerve provides the major sensory supply to the middle ear cavity, auditory tube, and mastoid air cells through its tympanic branch. The parasympathetic fibers brought to the plexus in the tympanic branch leave it as the lesser petrosal nerve. Sympathetics are supplied by the caroticotympanic nerves.
(6) Posterior and inferior to the promontory is the round window, closed by a membrane which separates the perilymph of the scala tympani of the cochlea from the middle ear cavity.
************MATERIAL BEYOND THIS POINT IS OPTIONAL.************ ******IT WILL NOT BE TESTED IN THIS COURSE.******
D. Internal ear (Labyrinth)
1. It consists of 2 parts:
a. Osseous labyrinth (Grant's Atlas, Fig. 7.77C, p. 714) a series of complex, interconnected cavities within the petrous part of the temporal bone.
b. Membranous labyrinth (Grant's Atlas, Fig 7.77E, p. 714) a series of communicating membranous ducts and sacs, which lie within the osseous labyrinth and largely conform to the shapes of their bony containers.
2. The osseous labyrinth is made up of 3 interconnecting parts: the vestibule, the bony semicircular canals, and the bony cochlea.
a. Vestibule is the central part of the osseous labyrinth. It is a small spherical cavity, which contains two smaller membranous sacs, the saccule and utricle, and their interconnecting duct. Between its bony walls and its contained sacs it is filled with perilymph. The vestibule is continuous posteriorly with the 3 bony semicircular canals and in front with the bony cochlea. Its perilymphatic fluid compartment is continuous posteriorly with those of each semicircular canal and anteriorly with that in the scala vestibuli of the cochlea. The oval window is in the lateral wall of the perilymphatic compartment of the vestibule and contains the stapes footplate, held in place by an annular ligament.
b. Bony semicircular canals contain correspondingly shaped membranous semicircular ducts surrounded by perilymph. Each canal communicates with the vestibule at both ends and is dilated to form a bony ampulla at one end. The whole semicircular canal system has its upper pole tilted posteriorly and inferiorly 30 o .
(1) Horizontal (lateral) canal posterior end tilted inferiorly 30 o , ampulla at the anterior end.
(2) Superior (anterior) canal vertically oriented and perpendicularly placed with respect to the long axis of the petrous pyramid. Its ampulla is at the anterior end. Its posterior end joins the upper end of the posterior canal to form a common crus, which then opens to the vestibule.
(3) Posterior canal vertically oriented parallel to the long axis of the petrous pyramid, ampulla at the inferior end.
c. Cochlea a bony compartment formed like a snailshell, with 2 3/4 coils around a central core called the modiolus. Its base is at the lateral end of the internal
auditory meatus and its apex points anterolaterally and inferiorly. The bony coils contain a membranous cochlear duct, which along with the osseous spiral lamina (protruding from the modiolus and to which the duct is attached), subdivides each bony cochlear coil into 2 separate perilymphatic compartments. One compartment is the scala vestibuli, which communicates at the cochlear base with the perilymphatic compartment of the vestibule and at the cochlear apex with the other perilymphatic cochlear compartment, the scala tympani. The scala tympani ends blindly in the cochlear base at the round window except for a small perilymphatic duct, which arises here and runs posteriorly to open into the subarachnoid space in the posterior cranial fossa.
3. Membranous labyrinth
a. Utricle and saccule The utricle receives each of the membranous semicircular ducts and the saccule communicates with the cochlear duct. The endolymphatic duct arises from the duct connecting the utricle and saccule and extends posteriorly to end blindly in the dura on the posterior face of the petrous pyramid.
(1) Both utricle and saccule contain a patch of sensory epithelium (macula) containing hair cells, the hairs of which are embedded in an overlying gelatinous otolithic membrane containing calcareous deposits (otoliths). This membrane is displaced upon these hair cells by gravitational force and particularly linear displacement of the head, thereby activating the hair cells.
b. Membranous semicircular ducts Each has an ampullated end containing a transverse thickening of the wall (crista) covered by a sensory hair cell type of epithelium. These hair cells are embedded in a gelatinous cupula, which is displaced by movements of the endolymphatic fluid. These are the receptor organs for angular displacements of the head.
c. Cochlear duct The basilar membrane wall of this duct supports the organ of Corti, which has receptor hair cells with cilia embedded in an overlying tectorial membrane. Movements of the stapes footplate are transmitted from the vestibular perilymphatic compartment to the cochlear perilymphatic compartments, resulting in a displacement of the basilar membrane structures against the tectorial membrane. Therefore, sounds impinging on the tympanic membrane are transmitted across the ossicular chain to the inner ear, where they activate cochlear hair cells.
d. The vestibular part of the auditory nerve receives sensory information from the utricle, saccule and semicircular canals and has a sensory ganglion upon it as it lies within the internal auditory meatus. The cochlear part of the auditory nerve receives auditory stimuli and has its sensory ganglion located within the modiolus and osseous spiral lamina of the cochlea.