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II. GENERAL STRUCTURE AND DEVELOPMENT OF THE EXTERNAL EAR, MIDDLE EAR AND INNER EAR

Objectives:

Before delving into the details of auditory and vestibular transduction mechanisms, it is worthwhile to have an overview of the peripheral auditory and vestibular systems. Once the general plan of organization of the external, middle and inner ears is understood, then we can take up the various mechanisms that are involved in normal and abnormal hearing and balance.

Together, the external, middle and inner ears are derived from all three germ layers. Thus, knowing the embryonic origin of different parts of the ear is essential in understanding the sensory deficits that result from auditory or vestibular maldevelopment. In this section we seek a basic knowledge of the germ layers that give rise to specific structures of the external, middle and inner ears and how maldevelopment of these structures leads to sensory deficits.

At the end of this unit, each student should be able to:

1. Describe the basic structures of the external ear (including the pinna, external auditory, meatus, tympanic membrane)

2. Describe the basic structures of the middle ear (including the ossicles, muscles, Eustachian tube)

3. Describe the basic structures of the inner ear (including the osseous labyrinth and the membranous labyrinth)

4. Describe the structure of the membranous labyrinth (including the cochlea, ductus reuniens, utricle, saccule, semicircular canals).

5. Describe the fluid composition of the inner ear

6. Describe the blood supply to the inner ear

7. Describe the development of the external, middle and inner ear and state the contributions of each of the germ layers to the development of the ear.

GENERAL STRUCTURE OF THE EXTERNAL EAR, MIDDLE EAR AND INNER EAR

The mammalian statoacoustic organ may be divided into three parts: the external, middle and inner ears. Their structural relationships, highly schematized, are shown in Figure II-1.

Objective 1: The External (Outer) Ear

The external ear consists of the auricle (or pinna) and external acoustic meatus. The external acoustic meatus, lined with skin, leads inward from the bottom of the concha of the auricle to the tympanic membrane. The stratified epithelium of the skin in the canal is supplied with specialized ceruminous (wax) glands. The first part is supported by the cartilage of the pinna, while the medial 1.5 cm is supported by the temporal bone. Disorders of the external ear include inflammatory, traumatic, and neoplastic lesions. Congenital malformations are not uncommon.

Objective 2: The Middle Ear

The middle ear, or tympanic cavity, is a narrow, air-filled chamber lined with mucous membrane and is situated between the external acoustic meatus and the labyrinth. It communicates with the mastoid air cells and with the nasal pharynx via the Eustachian (auditory) tube. The auditory ossicles, forming a chain of three small bones, connect the tympanic membrane with the inner ear (Figure II-2). The manubrium (handle) of the malleus is attached to the tympanic membrane. The tensor tympani muscle, acting on the malleus, regulates the tension on the tympanic membrane, resulting in two identifiable regions of the tympanic membrane: pars tensor and pars flaccida. The incus is attached to the malleus and to the third ossicle in the chain, the stapes, which in turn is attached via its footplate to the oval window of the cochlea. The stapedius muscle regulates the range of motion of the stapes; the two muscles (tensor tympani and stapedius) thus regulate to some extent the amplitude sensitivity of the ear.

As a result of its development, the Eustachian tube connects the tympanic cavity with the pharynx, and thus provides an important mechanism for equalizing external and internal pressures acting on the tympanic membrane. It also provides a convenient pathway for infections of the pharynx to invade the middle ear.

The middle ear is susceptible to inflammatory disease, trauma, and neoplastic disease. It is also the site of the degenerative disease, otosclerosis. Congenital malformations of the middle ear frequently accompany those of the external ear.

Objective 3: The Inner Ear

The inner ear is called the labyrinth because of the complexity of its shape (Figure II-3). It contains six mechanoreceptive structures: three semicircular canals, utricle, and saccule, which serve the sense of equilibrium, and the cochlea, which is specialized for detection of sound waves. The inner ear consists of two parts: the osseous (or bony) labyrinth, a series of cavities within the petrous portion of the temporal bone, and a membranous labyrinth, which is a series of communicating sacs and ducts within the bony labyrinth.

The inner ear is easily damaged by intense sound, head injury, and ototoxic drugs. It can be affected by microorganisms and is susceptible to degenerative and metabolic disease. It may also suffer abnormal development.

Osseous labyrinth

The temporal bone shell of the inner ear is one of the hardest bones in the body. It is lined with periosteum and is filled with perilymph, a fluid closely resembling cerebral spinal fluid in its chemical composition. Midway between the semicircular canals and the cochlea is the vestibule. It is just medial to the tympanic cavity. The oval window, into which fits the footplate of the stapes, is the lateral wall of the vestibule. Note that motion of the stapes that results from sound waves striking the drum meets considerable resistance at this air (middle ear) - fluid (inner ear) boundary. Mechanisms by which this impedance (resistance) mismatch is overcome are covered in the later section on middle ear function.

The three semicircular canals open into the vestibule. The bony cochlea lies horizontally in front of it. The shape of the cochlea resembles that of a snail shell with two and one-half turns (in humans) and hence its name. The central conical core of the cochlea is called the modiolus. The outer wall of the modiolus forms the inner wall of a canal which spirals the full two and one-half turns around the central core. A thin shelf of bone, called the osseous spiral lamina projects from the modiolus and partially divides this canal into two parts along its entire length. From the free border of the osseous spiral lamina, a partition reaches across to the outer wall of the bony cochlea and, thus, separates the canal into two passages except for a small communicating opening between them at the apex. This opening is called the helicotrema. Thus, the cochlea can be seen as a long, coiled, fluid-filled tube (about 33 mm in humans) that is divided along its entire length by a partition. This is shown schematically in Figure II-4 below:

The cochlear partition is a complex structure of the membranous labyrinth that is described in a later section. At the basal end of the cochlea (that end nearest the vestibule) there are two openings to the tympanic cavity, one on each side of the cochlear partition, that are covered by membranes. One is called the oval window and, as we already mentioned, is in contact with the stapes foot plate. The other, called the round window, is just below the oval window and in contact with no structure. As we shall see in a later section this membrane yields under pressure developed at the oval window by stapes motion. The two channels formed by the cochlear partition are called the scala vestibuli and scala tympani, respectively. Again, they are filled with perilymph. A tiny canal, called the cochlear aqueduct (or perilymphatic duct), leads from the lowest turn of the cochlea through the temporal bone to the CSF-containing subarachnoid space at the base of the brain.

Objective 4: Membranous labyrinth

The membranous labyrinth lies within the bony labyrinth and, hence, takes on its general form. It is separated from the bony labyrinth by perilymph. The membranous labyrinth is filled with its own fluid, called endolymph. Endolymph is a fluid of somewhat higher specific gravity and different chemical composition than perilymph. That portion of the membranous labyrinth within the bony cochlea is called the cochlear duct or scala media. The receptor organ of hearing, the organ of Corti, is within the scala media. The scala media joins the vestibular organs of the vestibule, the saccule and the utricle through a small tube, the ductus reuniens. The membranous labyrinth continues as the semicircular canals, each of which has at its base a swelling, called the ampulla within which are the sensory epithelial cells. The membranous labyrinth connects to the endolymphatic sac within the cranium.

The six sensory structures (three canals, utricle, saccule and cochlea) are innervated by the distal process of bipolar neurons of the vestibular or cochlear divisions of the eighth cranial nerve. It should be clear that, because the vestibular and auditory receptor organs share the same continuous fluid environment, diseases of the inner ear often affect both systems.

Objective 5: Fluids of the inner ear

Perilymph and endolymph are of very different chemical composition. Under normal conditions they occupy separate compartments and, hence, do not mix. The distribution of these fluids with respect to the receptor cells may play an important role in inner ear transduction and thus may be a major factor in governing the great sensitivity of the mechanoreceptors of the inner ear. Studies of fluid composition and dynamics in the inner ear are technically challenging, and because of this definitive answers regarding the origin and flow of cochlear fluids remains somewhat controversial.

Endolymph

Endolymph is unlike any other extracellular fluid found in the body. Its predominant cation is potassium; sodium is very low. Like perilymph, it is generally believed that endolymph is not homogeneous in its ionic composition throughout the inner ear. The source of endolymph and its flow dynamics are still controversial. The source of K+ appears to involve active transport by the stria vascularis, although the precise cellular mechanisms by which this is accomplished are not understood. Evidence now points to ionic transport as a mechanism for maintaining constant the chemical composition of endolymph.

Perilymph

Perilymph resembles in its chemical composition other extracellular fluids that are characterized by high Na+ concentration. Osmolarity of perilymph is similar to that of plasma, hence it is in osmotic equilibrium with blood. The origin of perilymph is equally controversial. Two possibilities have been considered: 1) perilymph is derived from CSF, which enters the cochlea via the cochlea aqueduct 2) perilymph is produced locally in the cochlea by ionic or ultrafiltration mechanisms.

Inner ear disorders associated with disturbances of cochlear fluids

Membranes that separate the fluid compartments of the inner ear are permeable to ions. Ions may move passively between compartments along their electrochemical gradients. Thus, the maintenance of the high K+ composition of endolymph depends on active transport mechanisms, and these are believed to be operating in the stria vascularis of the cochlea. If the cochlea becomes anoxic or is treated with a Na/K transport inhibitor, endolymph begins to equilibrate with perilymph, intracochlear potentials fall, and hearing loss occurs.

Meniere's disease, which is characterized by tinnitus, fluctuating hearing loss and veritgo, is generally assumed to be the result of interuption in normal mechanisms of volume regulation of endolymph. The histological sign is expansion of the endolymphatic space, and endolymphatic hydrops.

Perilymphatic fistualae may occur between the perilymphatic scalae and the middle ear or between the perilymphatic and endolymphatic compartments. The origins of such fistuae are varied, and include mechanical trauma, congenital defects and bone erosion associated with cholesteotoma. When they occur perilymph escapes and is replaced by CSF. Clinical symptoms are similar to those seen in Meniere's disease.

Objective 6: Blood supply to the inner ear

The internal auditory artery, a branch of the basilar artery, supplies the entire membranous labyrinth. The artery passes through the internal auditory meatus and then divides into three branches. The first of these three branches, the vestibular artery, supplies the vestibular nerve and parts of the saccule and utricle, and semicircular canals. The second branch, the vestibulocochlear artery, supplies the basal turn of the cochlea, the saccule, utricle, and parts of the semicircular canals. The cochlear artery supplies the entire cochlea via the spiral arteries.

Infarction or acute ischemia of the cochlea and/or vestibular end organs can occur in humans. It has been proposed that symptoms of acute vestibular failure, e.g., vertigo, vomiting, unsteadiness, and nystagmus, may result from occlusion of the vestibular artery. Certain kinds of sudden deafness and tinnitus are common clinical problems and a number of etiological mechanisms have been suggested including cochlear ischemia.

Objective 7: Development of the external, middle and inner ear

The ear is a complex sensory organ of multiple embryonic origin. Different structures are derived from different germ layers. Understanding the origin of each structure is helpful in understanding and diagnosing the functional impairments associated with congenital malformations of external, middle and inner ears, and in knowing when and how to intervene and treat these disorders.

External Ear Development

The external ear is a modification of the surface ectoderm by which the skin is brought into functional relationship with the ossicles at the tympanic membrane. The pinna develops around the first branchial groove (Fig. II-5).

Six hillocks appear on the first (mandibular) and second (hyoid) branchial arches.; three on the facing border of each of the arches fuse to form the elevations, fossae and sulci of the adult pinna. You need not learn the names of these structural features of the auricle but you should know that they form resonance chambers that can profoundly alter the incoming sound waves.

The external acoustic meatus is a derivative of the first ectodermal groove between the mandibular and hyoid arches. The epithelium in the bottom of the first ectodermal groove is in contact with the endoderm of the first pharyngeal pouch. Connective tissue, derived from mesoderm is situated between the epithelial layers. It becomes the fibrous layer of the trilaminar tympanic membrane. Connective tissue around the margin of the membrane begins to ossify at about the third month. This tissue forms the tympanic ring which serves as circumferential support of the tympanic membrane.

Middle Ear Development

The middle ear is developmentally an air sinus that develops along with the Eustachian tube as an outgrowth of the first pharyngeal pouch and thus is lined with endoderm. The ossicular chain is developed from the upper ends of the first (mandibular) and second (hyoid) cartilages. Thus, the ossicles, formed from three condensations in the mesenchyme, come to be covered also with endodermal lining of the tympanic cavity. The tympanic membrane or ear drum is formed by the approximation of the ectodermal meatal plug and the endoderm of the tympanic cavity with intervening mesoderm. Thus, this thin membrane is derived from all three germ layers.

The nerve supply of the drum reflects its origin. The ectodermal (outer) surface of the tympanic membrane is supplied by the auriculotemporal branch of the trigeminal nerve and by the auricular branch of the vagus (Arnold's) nerve. The nerves that supply the drum also supply the external auditory meatus. Irritation of the auricular branch of the vagus nerve may cause reflex coughing or vomiting. Foreign bodies in the external ear may, therefore, simulate a thoracic condition. Pain that emanates from the eardrum when it is stretched or torn presumably results from activation of the trigeminal branch.

It should be clear how maldevelopment of the first and second branchial arches leads to developmental abnormalities involving both the external and middle ears. This, in turn, may result in a conductive hearing loss and a lifelong communication handicap.

Inner Ear Development

The membranous labyrinth is the fundamental part of the inner ear. Early in the life of the embryo, even before any other part of the inner ear develops, the peripheral processes of the acoustic nerve reach its membranous wall. In these areas the epithelium becomes modified into neuroepithelium for the end organs of hearing and equilibrium.

The primordium of the membranous labyrinth appears in human embryos of three weeks as a plaque-like thickening of the ectoderm on either slide of the head dorsal to the first branchial groove in the region of the hindbrain. This thickened plate of epithelium, the otic (auditory) placode, soon invaginates into the mesenchymal tissue to form the otic (auditory) pit. The invaginated portion then enlarges, and the mouth of the pit narrows by the growing together of the lips. When these meet and fuse, the otic pit is converted into a closed sac, the otocyst, or otic vesicle. In the 5-week embryo, the future membranous labyrinthine system is represented by an otocyst in which the parts have but lately become definitive. The developmental course whose progress is predicted in the 5-week stage is virtually completed in the ensuing 5-month period. During this period the receptor organs of the vestibular labyrinth and the cochlea are formed. It is now generally accepted that the vestibular (Scarpa) and cochlear (Spiral) ganglia arise from the otic placode.

The inner ear is very sensitive to viral and bacterial infection during the first tri-mester of uterine life. Drugs, including aminoglycoside antibiotics, salicilates, quinine, and diuretics, when administered during pregnancy (and afterward as well) can damage the receptor organs of the inner ear.

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