Hearing and Balance

Hearing and Balance

1. External ear (hearing)

   • Auricle
     • Fleshy part
     • Elastic cartilage covered in skin
     • Shape directs soundwave to external auditory canal
   • External auditory canal
     • Lined with hairs
     • Ceruminous glands
       • Cerumen - modified sebum (earwax)
     • Hairs + cerumen prevent foreign bodies reaching tympanic membrane
   • Tympanic membrane
     • Thin, semitransparent and nearly oval shape
     • Seprates external and middle ear
     • 3 layers
       • Simple cubodial epithelium (inner surface)
       • Thin stratified columnar epithelium (outer surface)
       • Connective tissue (middle)
     • Soundwaves cause it to vibrate
     • Rupture lead to hearing impairment

2. Inner ear (hearing and balance)

   • Sensory
   • Interconnecting, fluid-filled tunnels and chambers (within the petrous portion of temporal bone)
   • Bony labyrinth within the temporal bone
     • Lined with endosteum
       • Inner surface endosteum lined with perilymphatic cells
     • Membranous labyrinth
       • Outer surface of membranous labyrinth lined with perilymphatic cells
       • Filled with fluid - endolmyph
         • High [K+] and low [Na+] opposite of perilymph concentrations
     • Space between bony and membranous labryinth filled with perilymph
     • 3 regions
       • Cochlea
         • 3 parts of membranous labyrinth
           • Scala Vestibuli
             • Extends from oval window to helicotrema, back from apex, parallel to scala vestibuli to membrane of round window
           • Scala Tympani
           • Cochlear duct
       • Vestibule
         • Balance
       • Semicircular canals
         • Balance

3. Middle ear (hearing)

   • Air-filled space (within the petrous portion of the temporal bone)
     • 3 Auditory ossicles
       • Malleus
         • Vibration of tympanic membrane causes malleus to vibrate because malleus handle attachted to inner surface of tymoanic membrane
         • Malleus head attached by very small synovial joint to incus
       • Incus
         • Incus synovial joint with malleus and synovial joint with stapes
       • Stapes
         • Foot plate fits into oval window and held by flexible annular ligament
       • Transmit vibrations from tympanic membrane to oval window
       • 2 small skeletal muscles insert into auditory ossicles
         • Tensor Tympani attached to malleus and innervated by trigeminal nerve (V)
         • Stapedius muscle attached to stapes and innervated by facial nerve (VII)
   • 4 openings
     • 2 covered openings
       • Round and oval on Medial side
       • Seperate from inner ear
     • 2 additional openings
       • Opens into mastoid air cells
       • Auditory tube (pharyngotympanic tube)
         • opens into pharynx
         • Equalizes air pressure of outside air and middle ear cavity
           • Unequal pressure can distort tympanic plate, dampens its vibrations, associated pain fibres stimulated