 |  |  |  |  |  |
 |  |  |  |  |  |
Dissector Answers - Ear
| |||||||||||||
| |
| ||||||||||||
| |||||||||||||
Dissector Answers - Ear | |||||||||||||
| |||||||||||||
Learning Objectives:
Upon completion of this session, the student will be able to:
- Define the three parts of the ear and the function of each part.
- Describe each of the four walls of the middle ear cavity and identify deeper structures responsible for certain of their features.
- Describe the structure and actions of the tympanic membrane, the auditory ossicles, and the muscles of the middle ear.
- Trace the course of the facial nerve through the temporal bone and give the origin, course, and functional components of each of its intracranial branches.
- Identify the auditory tube and explain its function.
Learning Objectives and Explanations:
1. Define the three parts of the ear, and the function of each part. (W&B pp 302-307; N92, N93, N94A, N94B, TG7-65, TG7-68A, TG7-68B, TG7-70)The three parts of the ear are: external ear, middle ear and internal ear.2. Describe each of the four walls of the middle ear cavity and identify the deeper structures responsible for certain of their features. (N92, N94A, N94B, TG7-65, TG7-68A, TG7-68B)
Functions:
External ear: is comprised of the oval auricle and the external acoustic meatus. The external acoustic meatus is a canal, approximately 2.5cm long, which leads from the auricle to the tympanic membrane. The auricle is the cartilaginous portion of the outer ear, attached to the skull by ligaments and muscles, and covered by skin.
Middle ear: includes the tympanic cavity proper, the space directly internal to the tympanic membrane, and the epitympanic recess, the space above it. The middle ear is a narrow cavity in the temporal bone where the energy of sound waves is converted into mechanical energy through a chain of ossicles.
Internal ear: consists of the cochlea, for auditory sense, and a series of intercommunicating channels, the semicircular ducts, the utricle and the saccule, for the sense of balance and position. The internal ear provides the essential organs of hearing and of equilibrium.
Lateral wall: formed by the tympanic membrane which is set obliquely into the external acoustic meatus and faces outward, downward and forward. The tympanic membrane has three layers: 1) the outer surface is the modified skin of the external meatus 2) the intermediate layer is composed of radial and circular fibers which provide strength for the membrane and 3) the internal layer is composed of mucous membrane. The tympanic membrane is one cm in diameter and has a fibrocartilaginous ring at the greater part of its circumference which fixes it into the tympanic sulcus at the inner end of the external acoustic meatus. Where the membrane lacks this ring, at its upper limit, the membrane is termed the membrana flaccida. Membrana flaccida is one sixth the area of the total membrane and lacks the fibrous stratum which provides rigidity. The other five-sixths of the membrane, called the membrana tensa, has this strengthening fibrous layer. The head of the malleus is attached to the fibrous layer on the inner surface of the membrane and extends to a little below the center of the membrane. On N93, TG7-69A, TG7-69B, TG7-69C, TG7-69D, the head/handle of the malleus appears to make a projection into the membrane along its course. The most indrawn point of the tympanic membrane is termed the umbo. Additionally, the radial fibers which compose part of the intermediate layer of the tympanic membrane, diverge from the handle of the malleus. The circular fibers are more at the edge of the membrane.3. Describe the structure and actions of a) the tympanic membrane, b) the auditory ossicles, and c) the muscles of the middle ear. (N93,N94A, N94B, N96, TG7-68, TG7-69A, TG7-69B, TG7-69C, TG7-69D)
Medial wall: composed mainly of the promontory which is a bony eminence formed by the cochlea. The promontory is grooved because of branches of the tympanic plexus which lie under its mucous membrane. There are two fossa in the medial wall, formed by the oval vestibular window and the round cochlear window. The fenestra vestibuli (vestibular window or oval window) lies above the promontory, and is an opening in the medial wall which is actually closed in life by the stapes. The fenestra cochlea, or round window, is below and behind the promontory and is another opening which is closed in life, only this time by a membrane. This membrane yields to the surge of fluid in the closed system of canals of the inner ear produced by the piston-like action of the footplate of the stapes at the vestibular window. (WB 305) There are two prominences in the medial wall. One is the prominence of the facial canal and lies above the vestibular window. This carries the facial nerve and sometimes the bone over the facial nerve is quite thin. The other prominence is the prominence of the lateral semicircular canal and is the most superior in the lateral wall.
Anterior wall: divides the tympanic cavity and the carotid canal. The caroticotympanic nerves from the sympathetic plexus on the internal carotid artery perforate the anterior wall and join up with the tympanic plexus. The anterior wall is incomplete because at its superior edge are the openings for both the auditory tube and the semicanal for the tensor tympani muscle. These openings are separated by the septum canalis musculotubarii, a very long name for a very thin shelf of bone.
Posterior wall: at the top of the posterior wall is the aditus ad antrum, the entrance to the mastoid antrum, which is the common cavity in the mastoid bone into which the mastoid air cells open. Below the entrance of the mastoid antrum is the fossa incudis, the part of the posterior wall which receives the short process of the incus. Also below the aditus ad antrum is the pyramidal eminence, a conical elevation of bone which juts forward from the posterior wall in front of the vertical portion of the facial canal. The eminence is hollow and its walls give rise to the stapedius muscle. A small branch of the facial nerve goes through the stapedius muscle at the base of the pyramidal eminence.
[WB 305-306]a) Tympanic membrane: The structure of the tympanic membrane is described in some detail in the description of the lateral wall of the middle ear cavity in Objective 2 above. Sound waves travel through the external acoustic meatus and set the tympanic membrane into vibration.4. Trace the course of the facial nerve through the temporal bone and give the origin, course, and functional components of each of its intracranial branches. (N86, N94A, N94B, N97, N123, TG7-67, TG7-68A, TG7-68B, TG7-87)
b) Auditory ossicles: The auditory ossicles are three small bones, the malleus, the incus and the stapes. The three bones are united by true joints and form a lever system which converts the vibrations in air impinging on the tympanic membrane into mechanical energy to oscillate the footplate of the stapes in the vestibular window. This lever system causes a decrease in amplitude but an increase in power of the piston-like action of the stapes. Their fixation in space depends on the attachments of the malleus to the tympanic membrane and of the stapes in the oval window and on ligaments which suspend the bones from the walls of the cavity.
Malleus: Supposedly shaped like a hammer. Its handle, or manubrium, is attached firmly to the upper half of the tympanic membrane. Its anterior process arises from the neck of the malleus and projects forward toward the petrotympanic fissure, to which it is connected by ligamentous fibers. Its head projects upward into the epitympanic recess. The posterior aspect of its head receives the incus.
Incus: Supposedly resembles an anvil. Body of incus articulates with head of malleus. Short process of incus extends backward into fossa incudis and is attached there by posterior incudal ligament. The long process of incus descends vertically, parallel to handle of malleus and articulates with the stapes.
Stapes: Is supposed to and actually does kind of look like a stirrup. Its head is hollow and receives the end of the long process of the incus. The neck of the stapes is where the stapedius muscle (described next) inserts. Two crura diverge from the neck (which continues from the head) and then the crura are connected by a flattened oval plate, the base of the stirrup. The base of the stirrup is attached to the margin of the vestibular window ( or oval window) by ligamentous fibers. Additionally, the articulation between the malleus and incus is a synovial joint, a saddle-shaped articulation more specifically. And the articulation between the incus and the stapes is a synovial joint and more specifically, a ball and socket joint.
c) Muscles of the middle ear: The tensor tympani muscle arises from the septum canalis musculotubarii (that very thin shelf of bone in the anterior wall), the cartilaginous part of the auditory tube and part of the greater wing of the sphenoid bone. The muscle is 2 cm in length and seems to lie on the septum. Its tendon wraps around the end of the septum (at the processus cochleariformis) to enter the tympanic cavity. It inserts into the handle of the malleus at its root and acts to draw the handle of the malleus and the tympanic membrane toward the medial wall. This action increases the tension and dampens the vibrations of the membrane. The tensor tympani muscle is innervated by a twig of the medial pterygoid branch of the mandibular nerve. The fibers of the stapedius muscle arise from the hollow pyramidal eminence (part of the posterior wall.) The central tendon of the stapedius muscle inserts in the posterior surface of the neck of the stapes. The contraction of the stapedius muscle tilts the footplate of the stapes which tends to dampen its vibrations. It therefore seems to serve a protective function. Remember from the description of the posterior wall that a small branch of the facial nerve travels to the stapedius muscle at the base of the pyramidal eminence.
[Woodburne & Burkel 305 to 306]5. Identify the auditory tube and explain its function. (N55, N65, N67, N92, N94A, N94B, TG7-24, TG7-65, TG7-68A, TG7-68B)The facial nerve is composed of two unequal roots; the larger motor root supplies all the muscles of facial expression, the smaller root, the nervus intermedius, contains taste fibers from the anterior 2/3 of the tongue, fibers of general sensation from the external acoustic meatus, parasympathetic and visceral afferent fibers for the submandibular, sublingual, lacrimal, nasal and palatine glands.
The two roots enter the internal acoustic meatus with the vestibulocochlear nerve (CN VIII), and divide into branches. Facial roots pass lateralward in the meatus between the cochlea and semicircular canals and at the lateral end of the meatus they fuse and form the geniculate ganglion. The geniculate ganglion is the sensory ganglion of the facial nerve. It is located at the abrupt bend taken by the nerve as it turns from the acoustic meatus into the posteriorly directed facial canal.
Intracranial branches of the facial nerve: greater petrosal nerve, nerve to stapedius, and chorda tympani.
The greater petrosal nerve arises at the geniculate ganglion anteriorly, passes a short course in the bone, and emerges at the hiatus for the greater petrosal nerve, into the middle cranial fossa. After passing forward between the dura mater and the trigeminal ganglion, crossing the foramen lacerum lateral to the internal carotid artery, the greater petrosal nerve unites with the deep petrosal nerve (a sympathetic branch of the internal carotid plexus) to form the nerve of the pterygoid canal. Function: the greater petrosal nerve provides parasympathetic innervation of the lacrimal, nasal and palatine glands, and taste from the soft palate.
The facial nerve then enters the bony facial canal lateral to the geniculate ganglion, and passes posterolaterally in the medial wall of the tympanic cavity above the vestibular window, and then behind the window, where it curves nearly vertically downward along the posterior wall of the cavity.
The nerve to stapedius arises as the facial nerve descends past the stapedius muscle lying within the pyramid on the posterior wall of the middle ear cavity.
The chorda tympani arises 5 mm proximal to the stylomastoid foramen, recurs sharply upward and enters a canal in the bone which takes it into the tympanic cavity. The chorda passes forward over the medial surface of the tympanic membrane and under its mucous membrane, and leaves the tympanic cavity near the anterior border of the membrane. This anterior opening leads to the petrotympanic fissure, which is how the chorda exits the skull. Function of chorda tympani: taste from the anterior 2/3 of the tongue, parasympathetic innervation to the submandibular and sublingual glands. (W&B 309, 240, 287)
The facial nerve emerges from the skull at the stylomastoid foramen. As it leaves the skull, extracranial branches pass to stylohyoid and posterior belly of digastric muscle. It also gives off the posterior auricular branch that passes posterosuperiorly to supply the posterior auricular muscle and occipitalis portion of the epicranius muscle. Facial nerve then enters the deep portion of the parotid gland and splits into temporofacial and cervicofacial divisions which in turn give off its temporal, zygomatic, buccal, marginal mandibular, and cervical branches that supply the muscles of facial expression.
The auditory (eustachian) tube is a communication between the nasal portion of the pharynx and the tympanic cavity that allows for equalization of pressure on either side of the tympanic membrane. It is located in the anterior wall of the middle ear, and is comprised of bone (1/3 of the length) at the tympanic end, and cartilage (2/3 of it) on the pharyngeal end. Through the mucous membrane of the tube the pharyngeal mucosa is continuous with that lining the tympanic cavity and mastoid air cells, which allows passage of infectious material to the middle ear and mastoid area (otitis media).
Questions and Answers:
6. What is the primary function of the tensor veli palatini? (N68, N69, N72, TG7-24, TG7-65)The most important function of tensor veli palatini is to open the auditory tube by pulling its lateral wall inferiorly. This flushes the middle ear cavity with fresh air, thereby equalizing pressure within the middle ear.