The ear diane L. M. Bick, PhD, faha reading



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THE EAR

Diane L. M. Bick, PhD, FAHA
Reading: Gartner & Hiatt, pp 397-399 & 402-418; Gartner, Hiatt and Strum, pp 337-342

Learning Objectives:

  • Describe the structural and histological components of the outer, middle, and inner ear.

  • Know the components of the membranous and bony labyrinths.

  • Describe the structures found in the 3 scala of the inner ear.

Key Words: Tympanic cavity, external auditory meatus, Eustachian tube, tympanic membrane, cupula, organ of Corti, perilymph, endolymph, ampulla, utricle, saccule, otoconia, crista ampullaris, macula, stria vascularis, vestibular membrane, basilar membrane, scalas.

BASIC EAR INFORMATION


  • Paired sensory organs that consist of the auditory system and the vestibular system.

  • Composed of external, middle, and inner ear.

THE THREE DIVISIONS OF THE EAR



EXTERNAL EAR

  • Auricle (or pinna) consists of elastic cartilage.

Covered by keratinized stratified squamous epithelium with hairs, sebaceous glands, and sweat glands.


  • External auditory meatus extends from the auricle to the tympanic membrane of the middle ear.

Outer one-third of wall composed of elastic cartilage that is continuous with auricle. Inner two-thirds of wall is formed by the temporal bone. Lined by stratified squamous epithelium with hairs, sebaceous glands, and ceruminous glands. Secretions of sebaceous and ceruminous glands combine to form cerumen.


MIDDLE EAR

Consists of tympanic cavity and its components - tympanic membrane (eardrum), 3 ossicles, and auditory or Eustachian tube. Tympanic cavity is an air-filled space.



  • Lined by simple squamous to low cuboidal epithelium. Cells become ciliated and cuboidal near the opening of the auditory tube.

  • Lateral wall is composed of the tympanic membrane; medial wall is shared by the inner ear.

  • Medial wall contains the oval window and the round window.


Auditory ossicles = malleus (hammer), incus (anvil), and stapes (stirrup).

  • Composed of compact bone joined by synovial joints\

  • Malleus is attached to inner aspect of tympanic membrane. Stapes is attached to the oval window of the inner ear by a fibrous ligament. Incus joins the malleus and the stapes.

  • Associated with 2 skeletal muscles, the Tensor Tympani (attached to the malleus), and the Stapedius (attached to the stapes).


Tympanic membrane is a thin, rigid, semitransparent membrane between external and middle ears.

  • Consists of a central core of connective tissue containing fibrous tissue and elastic fibers.

  • External surface covered by stratified squamous epithelium; inner surface covered by simple low cuboidal epithelium.

  • Divided into 2 parts. Lower four-fifths is the pars tensa and contains an organized core of connective tissue. Upper one-fifth is the pars flaccida (lacks a middle fibrous layer).


Auditory Tube (Eustachian tube) extends from tympanic cavity to nasopharynx.

  • Wall closest to the tympanic cavity is composed of bone. Is then replaced by elastic cartilage, except at the opening into the nasopharynx where it becomes hyaline cartilage.

  • Lined by ciliated epithelium ranging from simple low columnar to pseudostratified columnar. Towards the nasopharynx end, seromucous glands with goblet cells and diffuse lymphoid tissue are present.

  • Primary function is to equalize air pressure between the tympanic cavity and the external environment. The walls are normally apposed to one another; however, during swallowing and yawning, the walls separate, allowing air to enter the tympanic cavity.


INNER EAR

Consists of a series of membranous sacs and ducts: membranous labyrinth - encased within a series of bony cavities and canals, and bony labyrinth - located within the temporal bone. Bony labyrinth contains perilymph; membranous labyrinth contains endolymph.



Terminology Applied to the Inner Ear Labyrinths

Bony Labyrinth

Membranous Labyrinth

Vestibule

Saccule

Utricle


Semicircular canals

Semicircular ducts

Cochlea

Cochlear duct (Scala Media)



BONY LABYRINTH

  • Vestibule - central cavity of the bony labyrinth. Contains the utricle and the saccule.

  • Semicircular canals (superior, lateral, and posterior) extend posteriorly from the vestibule, oriented at right angles to one another. At one end of each is a dilatation, the ampulla.

  • Cochlea coils like a snail shell around a central pillar of bone, the modiolus. Projecting from the modiolus is a partial shelf of bone, the spiral lamina. Sitting on the spiral lamina is the cochlear duct.



MEMBRANOUS LABYRINTH


  • All epithelial-lined sacs and ducts communicate with each other, and contain endolymph. External to the membranous labyrinth is perilymph.




  • Are 5 sensory receptor regions associated with the vestibular system - one in the utricle, one in the saccule, and one each for the 3 semicircular ducts. There is a 6th sensory receptor associated with the auditory system located in the duct of the cochlea.


UTRICLE AND SACCULE
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