EAR

The ear consists of the external ear, the middle ear, and the inner ear. The external ear includes the auricle, external auditory meatus, and the tympanic membrane (ear drum). The middle ear is a cavity in which the ossicles or small bones of the ear are suspended. The inner ear encloses organs of hearing and balance.

External ear

The auricle is the visible external appendage of the ear. It is composed of yellow elastic cartilage covered by thin skin. The external auditory meatus extends from the auricle to the tympanic membrane. The external 1/3 is composed of elastic cartilage and the internal 2/3 is bony. The skin that lines the meatus has hairs, sebaceous glands, and ceruminous glands, which secrete the earwax, cerumen. This waxy substance keeps the skin moist and traps insects. The tympanic membrane is obliquely situated and has three parts: an outer covering of skin, a central core of connective tissue, and an inner lining of mucous epithelium. A part of one of the ossicles, the handle of the malleus, causes a bulge in the center of the membrane.

Middle ear

The middle ear is a cavity (the tympanic cavity), which communicates with the pharynx by the auditory tube and with the mastoid air cells in the mastoid process of the temporal bone. The cavity and its contents are lined mostly by squamous or low cuboidal epithelium. Three small ossicles (malleus, incus, and stapes) traverse the tympanic cavity. The malleus is attached to the tympanic membrane; the incus connects the malleus and the stapes; and the base plate of the stapes is attached to the oval window. Two small muscles, the tensor tympani, which inserts on the malleus, and the stapedius, which inserts on the stapes, dampen the movements of the ossicles so that high frequency vibrations do not damage the ear. The fenestra ovalis (oval window) separates the tympanic cavity from the vestibule of the osseous labyrinth and the fenestra rotunda separates the tympanic cavity from the scala tympani of the cochlea. The scalae are filled with perilymph. When movements of the stapes against the oval window compress this fluid, there is a corresponding bulge of the round window into the tympanic cavity. Thus, the round window serves as a safety valve for the expansion of the perilymph. The auditory tube, which extends from the tympanic cavity to the pharynx, consists of bony (tympanic 1/3) and cartilaginous (pharyngeal 2/3) portions. The epithelial lining of the auditory tube is ciliated columnar at the tympanic cavity and pseudostratified, ciliated columnar with goblet cells near the pharynx. The auditory tube is usually closed but opens during swallowing to equalize air pressure on both sides of the tympanic membrane.

The inner ear

Consider the inner ear as comprising two systems of canals and cavities. The osseous (bony) labyrinth is a system of canals and cavities within the petrous part of the temporal bone. This labyrinth is filled with perilymph. The membranous labyrinth can be thought of as floating within the osseous labyrinth. The membranous labyrinth contains endolymph.

The osseous labyrinth

Medial to the tympanic cavity, the osseous labyrinth is expanded into the vestibule. The fenestra ovalis is in the wall that separates the vestibule from the tympanic cavity. The semicircular canals, which are continuations of the osseous labyrinth, open into the vestibule. Near the opening of each canal into the vestibule is a swelling, the ampulla. There are three semicircular canals arranged at right angles to each other. They are designated superior, anterior, and inferior. The vestibule is also continuous with the bony cochlea. The cochlea is a spirally coiled tube that makes 2 and 3/4 turns. Its center support is the bony modiolus. The spiral lamina is a bony shelf that projects from the modiolus.

The membranous labyrinth

The membranous labyrinth is in some places adherent to the osseous labyrinth but in most places is separated from it by the perilymph. The utricle and saccule are two dilatations of the membranous labyrinth in the vestibule. The utricle is a connecting chamber that communicates with the semicircular canals. The saccule communicates with the utricle and the two combine to form the endolymphatic duct, a sac that ends blindly in the temporal bone. The saccule also communicates with the cochlear duct.

In the utricle and saccule are the maculae utriculi and maculae sacculi, respectively. These lie perpendicular to one another and each consists of supporting (sustentacular) cells and hair cells. The hair cells have long cilia that are embedded in an otolithic membrane. The otolithic membrane contains otoliths, which are small crystalline bodies of calcium carbonate. Position changes of the head stimulate the otolithic membrane and this stimulation is picked up by nerve endings between the hair cells. The maculae are concerned with the detection of linear movement of the head.

In each ampulla of the membranous semicircular canals is a crista ampullaris. The crista also contains supporting and hair cells. Cilia of the hair cells are embedded in the gelatinous cupula. The cristae ampullares function in maintaining balance. Over stimulation is believed to be the cause of motion sickness. See Figure 48 below.

Figure 48: Schematic diagram of the Crista Ampullaris. Taken from Wheater’s Functional Histology, a text and colour atlas, p. 405, Figure 21.28.

Cochlea

The cochlea is like a snail's shell, the cavity of which is divided by two membranes into three spiraling ducts. The modiolus forms the central bony support of the cochlea and from the modiolus projects the spiral lamina. The basilar membrane extends from the spiral lamina to the outer wall of the cochlea. This attachment is the spiral ligament. The vestibular (Reissner’s) membrane also extends from the spiral lamina to the outer wall. These membranes divide the cochlea into an upper scala vestibuli, a lower scala tympani, and a middle cochlear duct (scala media). The scalae vestibuli and tympani contain perilymph. The scala vestibuli is continuous with the vestibule and thus with the fenestra ovalis. The scala tympani is continuous with the fenestra rotunda. The scalae tympani and vestibuli communicate at the cochlear apex, the helicotrema. The spiral ganglion (CM VIII) lies in the modiolus and sends fibers to the organ of Corti, which lies on the basilar membrane. In the basilar membrane are basilar (auditory) fibers. The outer wall of the cochlear duct is lined by the stria vascularis and the epithelial layer is thought to secrete the endolymph.

The organ of corti is the organ of hearing and is located along the length of the cochlear duct (see Figure 49 below). It contains supporting and hair cells. There are several types of supporting cells and two types of hair cells. On considering a cross section of the cochlea and beginning at the lateral end of the basilar membrane the cell types are as follows. Cuboidal cells (cells of Claudius) are replaced medially by columnar cells (cells of Hensen). Outer hair cells are situated between outer pillar cells and outer phalangeal cells in rows of three to five. The apices of the outer phalangeal cells (Deiters) are cup-shaped and support hair cells. Inner and outer pillar cells diverge to form the inner canal. Inner phalangeal cells are similar to outer phalangeal cells. Inner hair cells are in a single row and are supported by pillar, phalangeal, and border cells. Border cells are slender columnar cells on the medial portion of the basilar membrane.

Figure 49: The Organ of Corti. Taken from Wheater’s Functional Histology, a text and colour atlas, p. 400, Figure 21.25b.

The hair cells are located on the basilar membrane and are the receptors of the cochlea. Their apical aspects are in contact with the tectorial membrane. Pressure waves enter the external auditory meatus and cause the tympanic membrane to vibrate. These vibrations are transmitted to the oval window by the ossicles. This results in vibration of the perilymph, which is in communication with the scalae. These waves pass through the scalae and vibrate the basilar membrane. Hair cells located at the apex of the cochlea are stimulated by low frequency sound while those near the base respond to high frequency sound.

Examine slide C93, internal ear, cochlea, (H&E). Using Figure 50 below and pictures in your atlas, identify the main structures shown in the slide. Be certain to identify the organ of Corti, inner and outer pillar cells, inner and outer phalangeal cells, inner and outer hair cells, border cells, and cells of Hensen. The inner and outer hair cells are similar to type I and type II hair cells of the maculae, but lack the lateral, non-motile cilia.

Figure 50: Diagrams of the inner ear.

Study slide BB-95, crista/macula, guinea pig, (H&E). This section is much like slide C93 except that it was selected to show a crista and a macula. Note: The neuroepithelium of both transducers look the same. Each has a coat or covering into which the hairs of the hair cells are inserted. The otoliths of the macula are very basophilic. The receptor cells appear empty. Compare your slide with Figure 51 below.

Figure 51: Diagram of a maculae. Taken from Junqueira and Carneiro, Basic Histology, a text and atlas, p. 484, Figure 24-21.