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Basic Anatomy and Physiology of the Ear 11 1 BASIC ANATOMY AND PHYSIOLOGY OF THE EAR J. Irwin Introduction The ear is a small, complex series of interlinked structures that are involved in both maintenance of normal balance and the sense of hearing. In order to hear, the ear collects the sound waves that arrive as pressure changes in air and converts these into neurochemical impulses that travel along the cochlear- vestibular nerve to the brain. There are both active and passive mechanisms involved in this process.The prime function of the vestibular system is to detect and compensate for movement. This includes the ability to maintain optic fix- ation despite movement and to initiate muscle reflexes to maintain balance. For the purposes of describing structure and function the ear is usually split into four distinct parts. These are the outer ear, the middle ear and the audi- tory and vestibular parts of the inner ear (Figure 1.1). The outer ear This is sometimes known as the external ear and consists of the ear that is visible on the side of the head (the pinna), the external auditory meatus (ear hole) and the ear canal (external auditory canal) that leads to the eardrum (or tympanic membrane). The tympanic membrane has three layers and the outer layer is usually included as part of the outer ear. THE PINNA This is, for the most part, a piece of cartilage covered by skin (Figure 1.2). There is also a fatty earlobe in most people. The skin covering the cartilage is Infection and Hearing Impairment. Edited by V.E. Newton and P.J. Vallely © 2006 John Wiley & Sons, Ltd 2 infection and hearing impairment semicircular canals external auditory meatus incus malleus stapes round window pinna tympanic cochlea membrane Eustachian tube external ear middle ear inner ear …………………… …………… ………………. Figure 1.1. Diagrammatic representation of a cross-section through the ear Helix Triangular fossa Darwen’s tubercle External auditory meatus Concha Tragus Antitragus Inter-tragal notch Lobule Figure 1.2. The pinna (auricle) and the external auditory meatus of an adult male very thin with little subcutaneous structure. Any injury or infection can easily lead to deformity – cauliflower ear. The pinna’s shape enables it to funnel sound waves into the external audi- tory meatus. The various folds in the pinna’s structure amplify some high- frequency components of the sound.They also help in the localisation of sound in the vertical plane.As sounds hit the pinna from above and below, their paths to the external auditory meatus vary in length. This means that they take dif- ferent times to reach the meatus. Again, this is a feature of high-frequency sounds. The difference in time of arrival of the low-frequency and high- frequency components of the sounds allows for localisation. basic anatomy and physiology of the ear 3 The pinna is also involved in localisation of sound from in front and behind. As sound waves pass the pinna from behind they are diffracted around the pinna to the meatus whereas sounds from in front do not do this. The slight distortion produced allows for localisation. Localisation of sound in the lateral plane, i.e. left/right, is a function of the pinnas being on different sides of the head. A sound directly from the left reaches the left ear before the right. The sound is also quieter at the right ear because the head is between the sound and the ear – the head shadow effect. These two factors combine to allow localisation in this plane. THE EXTERNAL AUDITORY CANAL Because the middle ear structures are delicate the ear has evolved a mecha- nism to protect it. This is a tube leading from the external auditory meatus to the tympanic membrane. The middle ear is inside the temporal bone of the skull. This part of the bone is called the petrous temporal bone because of its stone-like density. The external auditory canal is approximately 25mm long and is tortuous in its path from the meatus to the eardrum. The canal has bends in both the ver- tical and horizontal planes. This means that it is difficult for anything poked into the meatus to hit the drum. Any trauma is likely to be to the walls of the canal. The canal is lined by skin throughout its length – it is the only skin-lined cul-de-sac in the body.The outer one-third is a cartilaginous tube and the skin here has ceruminous glands that secrete wax (cerumen), sweat glands and hairs. The skin of the inner two-thirds is closely adherent to the underlying bone and there is again very little subcutaneous structure. Skin is a living, growing tissue that is constantly renewed. On the rest of the body, skin grows vertically to the surface, and as it does so the skin cells flatten and die. Dead cells are constantly shed. These dead skin cells form the basis of house dust and sustain the house dust mites, which trigger allergies in some people. The skin of the external canal starts growing from a point near the centre of the eardrum called the umbo. As the skin cells are replaced by new ones they migrate out along the eardrum to the canal wall and then along the canal to the meatus. The wax and hairs have some protective properties by trapping air-borne particles before they get too deep into the canal. The wax also has some mild antibacterial properties and helps with moisture regulation in the canal: fresh wax is moisture giving and old wax absorbs water. As the skin cells migrate along the canal walls they carry the wax, and anything trapped therein, out of the ear. The outer ear is thus a self-cleaning system. The external ear canal has one other function. As a cylinder closed at one end it has a resonant frequency whose wavelength is four times the length of the canal or approximately 100mm. This equates to a sound of approximately 4 infection and hearing impairment 3kHz and the canal does contribute some amplification of sounds around this frequency. The middle ear This is an air-filled space within the petrous part of the temporal bone (Figure 1.3).The prime function of the middle ear is to transmit the vibrations of sound in air gathered at the tympanic membrane to the fluid of the inner ear at the oval window. It is easier to vibrate air than it is to vibrate fluid.There is greater impedance to vibration in fluid. It is difficult to transmit sound between areas of differing impedance – there is usually an echo. The middle ear functions as an impedance matching device to prevent such echoes. If it were not for the middle ear so much sound would be reflected at the air/fluid interface that there would be a hearing loss of 50–60dB. The tympanic membrane is the outer (or lateral) border of the middle ear and the air-filled space contains three small bones (ossicles) and the facial nerve and its chorda tympani branch.There are also two muscles and the open- ings of both the Eustachian tube and the mastoid air cells. The temporal lobe of the brain, surrounded by its meningeal lining, lies immediately above the roof (superior) of the middle ear and the jugular vein and internal carotid artery are below the floor (inferior). The mastoid air cells in the mastoid part of the temporal bone lie behind (posterior) and the inner ear is the inner (medial) wall. The middle ear space is lined with respiratory mucous epithelium. Most of the middle ear lies above the level of the external auditory canal and cannot be seen on otoscopy. This is the epitympanic or attic region. Ligaments Tympanic membrane Stapedius muscle Sound Perilymph input in scala vestibuli Origin of tensor Round window tympani muscle Eustachian tube Figure 1.3. Diagram of the middle ear showing the ossicles, muscles and Eustachian tube basic anatomy and physiology of the ear 5 THE TYMPANIC MEMBRANE This is a three-layered structure, the outer layer is skin, the middle layer is sup- porting connective tissue and the inner layer is respiratory epithelium. It is anchored into the external auditory canal by a thickened rim – the annulus (Figure 1.4). The annulus fits into a groove in the bony wall of the ear canal. A part of the first of the ossicles, the long process of the malleus, is embedded in the lower part of the tympanic membrane. This ends at a point known as the umbo. This is the point of origin of the skin that lines the external canal. The anterior (front) and posterior (back) malleolar folds divide the tym- panic membrane into two distinct parts: the upper pars flaccida and the lower pars tensa. It is the pars tensa that is involved in the transmission of sound. The vibrations are passed on to the ossicular chain via the malleus. THE OSSICLES These are the three smallest bones in the body (Figure 1.5). Their function is to transmit vibration into the inner ear at the oval window of the cochlea. The Figure 1.4. A diagrammatic representation of the left tympanic membrane in lateral view Malleus Incus Stapes Figure 1.5. The three ossicles of the middle ear 6 infection and hearing impairment long process of the malleus (hammer) is attached to the tympanic membrane and the footplate of the stapes (stirrup) sits in the oval window. Between these two bones is the incus (anvil). The bulk of the ossicular chain lies in the attic of the middle ear.
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