The+Histology+of+the+Ear

=**Objectives**=


 * 1. Identify the three components of the ear; i.e. outer, middle, and inner ear. Identify the boundaries and components of each. Relate structure to function.**

The ear is divided into three parts: the external ear, middle ear, and inner ear.

The external ear is divided into the auricle (pinnae), external auditory meatus, and tympanic membrane. The auricle is an irregular plate of elastic cartilage covered by thin skin and is involved in the localization of sound in space. The external auditory meatus is a tube connected to the temporal bone and functions in transmitting pressure waves and trapping foreign particles. The other 1/3 is elastic cartilage continuous with the auricle. The inner 2/3 is formed by the temporal bone and lined by thin skin and has ceruminous glands that secrete a brown waxy protective substance called cerumen. The tympanic membrane closes off the innermost aspects of the external meatus and functions in transmitting vibrations of sound pressure waves to the middle ear bones.

The middle ear is divided into the tympanic cavity, the ossicles, and the Eustachian tube. The tympanic cavity is an air-filled cavity deep in the external meatus that acts as a resonance chamber. It is bound by the tympanic membrane on its lateral wall, the bony labyrinth of the inner ear on its medial wall, and lined with a simple squamous epithelium. The Eustachian tube lies on the ventral surface of the middle ear and connects the tympanic cavity with the nasopharynx to equalize the pressure between the outside air and the middle ear cavity. It is made of cartilage covered by a pseudostratified columnar ciliated epithelium that beats towards the nasopharynx. This structure is clinically important because it provides a route for infection of the middle ear cavity, especially in infants, who have very short and straight Eustachian tubes. The ossicles are a series of three bones that span the tympanic cavity: the malleus, incus, and the stapes. The malleus is attached to the tympanic membrane and the incus while the incus is attached to the malleus and the stapes; both are suspended by small ligaments from the roof of the tympanic cavity. The stapes are attached to the incus and the membrane of the oval window. The ossicles act as a lever system to transmit tympanic membrane vibrations to the sensory apparatus of the inner ear by //impedance matching// to converts high amplitude, low force vibration into low amplitude, high force vibration in the inner ear. Because the inner ear is filled with non-compressible fluid, the forces transmited by the ossicles are releaved by a second opening in the medial wall of the tympanic cavity called the round window which is covered by the secondary tympanic membrane.

The inner ear is divided into the bony labyrinth and the membranous labyrinth. The bony labyrinth consist of the vestibule, the cochlear canal, and 3 semicircular canals. The membranous labyrinth is housed with the bony labyrinth and contains specialized sensory organs for hearing and vestibular sensation. See objective 3.


 * 2. Describe the structure of the tympanic membrane (ear drum).**

The tympanic membrane closes off the innermost aspects of the external meatus and consists of 2 layers of collagen (radial and circular) covered by thin skin on the external side, and simple squamous epithelium on the inner side. It functions in transmitting vibrations of sound pressure waves to the middle ear bones.


 * 3. Differentiate between the components of bony labyrinth and membranous labyrinth.**

The inner ear is divided into the bony labyrinth and the membranous labyrinth. The bony labyrinth consist of the vestibule, the cochlear canal, and 3 semicircular canals. The vestibule is a large irregular central cavity which has the oval and round windows on the lateral wall that lead into the adjacent cochlear canal. The cochlear canal is an anterior extending spiral tube. The canal extend 2 ¾ turns around a central axis called the modiolus and has laterally extending threads like a screw called the spiral lamina. The spaces within the bony modiolus houses blood vessels and cell bodies and processes of spiral ganglion cells and the acoustic branch of the vestibulocochlear nerve (CN VIII). The anterior, posterior, and lateral semicircular canals are attached to the vestibule by an ampulla. The bony labyrinth is filled with perilymph which resembles extracellular fluid and CSF in composition.

The membranous labyrinth is housed with the bony labyrinth and contains specialized sensory organs for hearing and vestibular sensation. It is composed of delicate connective tissue lined by simple squamous epithelium. The ducts of the membranous labyrinth are suspended within the perilymph of the bony labyrinth by thin strands of connective tissue containing blood vessels. The shape of the membranous labyrinth follows that of the bony labyrinth except at the bony vestibule region, where it is occupied by two membranous sacs called the utricle and the saccule. The membranous ducts are all interconnected ahs share a common fluid medium called endolymph which is similar to intracellular fluid in composition. The 3 semicircular ducts, utricle, and saccule are associated with the vestibular (balance) sense. The cochlear duct is associated with the auditory sense.


 * 4. Identify the region of the cochlea, including the basilar membrane, helicotrema, osseous spiral lamina, and spiral ligament.**

The cochlear canal is an anterior extending spiral tube which extends 2 3/4 turns around a central axis called the modiolus. It extends laterally so that it has a thin bony ridge like threads on a screw called the (osseous) spiral lamina. The spaces within the bony modiolus houses blood vessels and cell bodies and processes of spiral ganglion cells and the acoustic branch of the vestibulocochlear nerve (CN VIII).

The cochlear duct is part of the membranous labyrinth and is suspended medially with in the cochlear canal and follows its coiled path, ending as a blind sac at the apex of the cochlear canal. Viewed in cross section, the cochlear canal can be divided up into 3 roughly triangular sections. The scala media (or scala cochlea) is part of the membranous labyrinth (cochlear duct) and filled with endolymph that is flanked superiorly by the scala vestibuli and inferiorly by the scala tympani, both parts of the bony labyrinth and filled with perilymph.

The scala vestibuli and scala tympani are lined with thin connective tissue continuous with periosteum of the cochlear canal and communicate with each other across the //helicotrema//, a small aperture at the apex of the cochlear canal. The scala media (or scala cochlea) points its apex towards the bony modiolus.

The membrane between the scala media and scala vestibuli is Reissner’s membrane; the membrane between scala media and scala tympani is //Basilar membrane//. Reissner’s membrane is composed of two layers of simple squamous epithelium separated by a basement membrane. The basilar membrane is composed of amorphous ground substance with transversely oriented filaments on top of a columnar epithelium. The //basilar membrane// is thickest at its basal region (center of spiral) for responsiveness to low frequency sounds and thinnest at its apical region (edge of spiral) for responsiveness to high frequency sounds. The basilar membrane supports the auditory apparatus, the organ of Corti, which is anchored to the bony labyrinth by the //spiral ligament// on the outer end (away from bone), and the //osseous spiral lamina// on the inner end (next to bone).


 * 5. Describe the fluid that is found within the bony labyrinth (perilymph) and that found in the membranous labyrinth (endolymph) and how and where each is formed and removed.**

The fluid in the bony labyrinth (scala vestibuli and scala tympani) is called perilymph and resembles extracellular fluid (and CSF) in composition (high Na+ and low K+). The fluid of the membranous labyrinth (scala media or scala cochlea) is called endolymph and is similar in composition to intracellular fluid (low Na+ and high K+).

Endolymph is thought to be produced on the lateral wall of Reissner’s membrane, between the scala vestibuli and the scala media (scala cochlea). On the lateral wall of Reissner’s is a thin layer of connective tissue applied to the walls of the cochlear canal and covered by a highly vascularized epithelium called stria vascularis, which is believed to be the source of endolymph. Endolymph eventally drains into the endolympahtic duct at the utricle and saccule.


 * 6. Identify the three major divisions of the cochlear canal.**

See Objective 4.

Viewed in cross section, the cochlear canal can be divided up into 3 roughly triangular sections. The scala media (or scala cochlea) is part of the membranous labyrinth and filled with endolymph that is flanked superiorly by the scala vestibuli and inferiorly by the scala tympani, both parts of the bony labyrinth and filled with perilymph.




 * 7. Identify the boundaries of the cochlear duct.**

The cochlear duct is part of the membranous labyrinth and is suspended medially with in the cochlear canal and follows its coiled path, ending as a blind sac at the apex of the cochlear canal and points towards the bony modiolus. Its roof is bound by the Reissner’s membrane and its floor by the basilar membrane. The cochlear duct is filled with endolymph.


 * 8. Identify the components of the organ of Corti, including: tectorial membrane, outer and inner phalangeal cells, outer and inner pillar cells, and cochlear hair cells.**

The organ of Corti is the sense organ that senses vibrations in the cochlea. It consists of 1 row of inner hair cells and 3 rows of outer hair cells. Each of these hair cells has a phalangeal cell at its base that extends a cytoplasmic process that terminates in a plate-like expansion. The inner and outer phalangeal cells are two types of columnar supporting cells in the walls of the tunnel of Corti. The tectorial membrane is a thin gelatinous membrane composed of a "keratin-like" protein extending from the spiral laminae to overly the hair cells and interacts with the stereocilia of the hair cell to sense vibration. (See above diagram)

__Columnar Supporting Cells__ Pillar cells are cone shaped with basally placed nuclei. They are rigid due to numerous cytoplasmic microtubules. Phalangeal cells support the base of the sensory hair cells and have a slender cytoplasmic process, phalanx, which extends to the free surface of the organ of corti alongside the hair cells in a plate-like expalnsion. The lateral porders of adjacent phalanxes have well-developed tight junctions.

__Sensory Hair Cells__ Inner hair cells are short goblet shaped cells that are sensitive to small changes in sound intensity. They receive a //dense// innervation by fibers of CN VIII. Outer hair cells are cylindrical columnar cells with basally located nuclei that respond bewt to low intensity sounds. They have relatively few neuronal connections.


 * 9. Identify the spiral ganglion cells as they are embedded in the bony modiolus.**

The spiral ganglion cells are located in the center of the bony modiolus and extend dendritic processes to the hair cells, and their axons make up the the acoustic branch of the vestibulocochlear nerve (CN VIII).


 * 10. Identify the maculae of the utricle and saccule.**

The macula is a small patch of sensory epithelium of both the utricle and saccule that is responsible for the detection of head position and linear acceleration. The macula of the saccule is oriented in the vertical plane while the macula of the utricle is oriented in the horizontal plane. The apical surface of the sensory hair cells are covered by a gelatinous glycoprotein layer called the otolithic membrane. Suspended in the otolithic membrane are small (3-5 um) crystalline bodies composed of calcium carbonate called otoliths. During tilt of the head with respect to gravity or linear acceleration, the inertia provided by the Otoliths in the Otolithic Membrane cause bending of the stereocilia of the hair cells. This generates a depolarization of the sensory hair cells and initiates action potentials in the vestibular ganglion cells.


 * 11. Identify the components of cristae ampullaris, including the cupula, the vestibular hair cells, and the sustentacular cells.**

The Ampulla of each semicircular duct contains a raised transverse ridge, cristae ampullaris, which extends part way across the semicircular canal. The epithelium of the Cristae is composed of 3 cell types: sustentacular cells (tall columnar supporting cells), type I sensory hair cells (flask-shaped cells with apical stereocilia similar to the inner hair cells of the basilar membrane), type II (cylindrical cells with apical stereocilia similar to outer hair cells of the basilar membrane). The apical stereocilia of the sensory hair cells are embedded in the cupula, a gelatinous glycoprotein material which extends like a hinged flaps from the walls of the ampullae.