Histology_Ear

• Ear


  Three-chambered sensory organ that functions as an auditory system for sound perception and as a vestibular system for balance

  
• Divisions of the ear

  • External ear
  • Middle ear
  • Internal ear

  
• External and middle ear


  • Collect and conduct sound energy to the internal ear
  • Where auditory sensory receptors convert that energy into electrical impulses
• Vestibular system


  • Sensory receptors respond to gravity and movement of the head
  • Responsible for the sense of balance and equilibrium
  • Help coordinate movements of the head and eyes
• Ear development


  1. Invagination of surface ectoderm on each side of the myelencephalon forms the otic vesicle (otocyst)

  2. Otic vesicle sinks deep to the surface ectoderm into the underlying mesenchyme

  3. Otic vesicle serves as a primordium for development of the epithelia that line the membranous labyrinth of the internal ear

  4. First and second pharyngeal arch provide structures that augment hearing

  
• Tubotympanic recess


  Endodermal component of the first pouch that ultimately develops into the auditory tube (Eustachian tube) and the middle ear and its epithelial lining

  
• External acoustic meatus


  Ectodermal outgrowth of the first pharyngeal groove that gives rise to the external acoustic meatus and its epithelial lining
• Ossicles


  • Connective tissue part of the pharyngeal arches that produces the ossicles
  • Malleus and incus develop from the first pharyngeal arch
  • Stapes develops from the second pharyngeal arch
• Auricle


  External component of the ear that collects and amplifies sound

  
• Auricle


  • Oval appendage that projects from the lateral surface of the head
  • Internal supporting structure of elastic cartilage
  • Covered in thin skin with hair follicles, sweat glands, and sebaceous glands
  • Nearly vestigial structure in humans, but essential component in sound localization and amplification

  
• External acoustic meatus


  Air-filled tubular space that follows a slightly S-shaped course for about 25 mm to the tympanic membrane
• Lateral one-third of external acoustic meatus


  • Cartilaginous wall continuous with the elastic cartilage of the auricle
• Medial two-thirds of external acoustic meatus


  • Contained within the temporal bone
• Cerumen


  • Secretion from ceruminous glands that mixes with sebaceous gland secretions and desquamated cells
  • Lubricates the skin and coats the meatal hairs to impede the entry of foreign particles
  • Provides some antimicrobial protection
• Tympanic cavity


  Air-filled space within the temporal bone that contains the auditory ossicles

  
• Walls of the tympanic cavity

  • Roof
  • Floor
  • Anterior wall
  • Posterior wall
  • Lateral wall
  • Medial wall
• Mastoid process


  Spongy bone that contains the mastoid antrum and mastoid air cells
• Oval (vestibular) window


  Opening in the medial wall of the middle ear that is essential for converting sound waves into mechanical vibrations

  
• Round (cochlear) window


  Opening in the medial wall of the middle ear that is essential for converting sound waves into mechanical vibrations

  
• Tympanic membrane


  Separates the external acoustic meatus from the middle ear

  
• Tympanic membrane


  • Approximately 1 cm in diameter
  • Irregular cone shape with the apex at the umbo
  • Tilted anteriorly and inferiorly
  • Semitransparent in a light gray color
  • Has visible concavity toward the external acoustic meatus
  • Causes a triangular cone of light (light reflex) to radiate anteriorly and inferiorly from the umbo
• Layers of the tympanic membrane

  • Skin of the external acoustic meatus
  • Core of connective tissue with radial and circular collagen fibers
  • Mucous membrane of the middle ear

  
• Pars tensa


  Larger, lower part of the tympanic membrane that is tightly stretched and has a thick middle core of radial and circular collagen fibers
• Pars flaccida


  Smaller, upper part of the tympanic membrane that is loose and lacks a prominent middle fibrous layer
• Auditory ossicles


  Three small bones (malleus, incus, stapes) that connect the tympanic membrane to the oval window

  
• Auditory ossicles


  • Work like a lever system that increases the force transmitted from the vibrating tympanic membrane to the stapes by decreasing the ratio of their movement
• Tympanic cavity


  • Associated with mastoid air cells
  • Lateral wall formed by tympanic membrane
• Oval window


  • Opening to internal ear
  • Stapes footplate inserts here
• Facial nerve


  • Observed near oval window
• Cochlea, vestibule, lateral semicircular canal


  • Parts of bony labyrinth identified
• Cranial nerve VIII


  • Cochlear and vestibular nerves are divisions
• Tympanic membrane


  Landmarks include manubrium of malleus, umbo, lateral process of malleus, flaccid part, tense part, light reflex
• Tympanic membrane


  • Outer epithelial layer is stratified squamous, inner layer is simple cuboidal, middle layer is connective tissue
• Ossicles


  Convert sound waves to mechanical (hydraulic) vibrations
• Ossicles


  • Malleus, incus, stapes
  • Movable synovial joints connect them
• Tensor tympani muscle


  Increases tension on tympanic membrane
• Stapedius muscle


  Dampens movement of stapes at oval window
• Attenuation reflex


  Protective reflex that reduces transmission of vibrations to internal ear
• Auditory (Eustachian) tube


  • Connects middle ear to nasopharynx
  • Equalizes pressure in middle ear

  
• Mastoid air cells


  • Extend from middle ear into temporal bone
  • Allow infections to spread from middle ear