Ear

3 Parts of the Ear: Outer I Middle I Inner 


Outer Ear: 

Composed of auricle + external auditory canal. Terminates at tympanic membrane. 

Auricle: 

  • Function = collect & direct sound to ear canal towards TM. 
  • Cartilagenous Subunits:
    • Helix
    • Antihelix
    • Concha
    • Tragus
    • Antitragus
  • Composed of elastic cartilage, that is continous w/ the cartilaginous EAC. 
  • Cartilage recieves its nutrients from the adherent perichondrium.
    • ⚠️ Auricular Hematoma occurs when trauma causes blood to accumulate between perichondrium & cartilage, creating a barrier to the perichondrial blood supply. Early drainage of the hematoma is essential to restore perfusion and prevent cauliflower ear. 
  • Skin of auricle is very thin and densely attached to the underlying perichondrium.
  • Innervation: 
    • Auriculotemporal (V3): Superiorly & medially
    • Great Auricular & Occipital N: Inferiorly 
    • Auricular Branch of Vagus: Concha & EAC  


External Auditory Canal: 

  • Typically 2.5 cm long in adults
    • Lateral ⅓ 
      • Cartilaginous
      • Has hair follicles w/ ceruminous & sebaceous glands →  produces ear wax!
      • Skin on top of fibrocartilagenous tissue, continuous w/ auricle.  
    • Medial ⅔ 
      • Bony 
      • Thin skin adherent to periosteum of temporal bone. Easily traumatized. 
      • Most medial portion = tympanic membrane
      • Inferior tympanic recess is a small depression in the inferior medial aspect, adjacent to TM.
        • debris can collect here and ↑ risk of infection. 
    • Isthmus
      • where the Cartilagenous & Bony portions of the EAC meet.
      • EAC is narrowest at this point 
      • Cerumen trapped medial to the isthmus = ↑ risk of becoming impacted and causing hearing loss. 
  • EAC is sigmoid shape in adults. 
    • Cartilagenous portion → angles posteriorly & superiorly
    • Bony portion → angles anterior & inferiorly 
  • Pulling the helix up and posteriorly straightens the EAC and allows for better visualization of TM. 
    • In infants, the EAC is nearly straight. Reaches adult size by about age 9. 
    • See GIF below. 
  • Innervation: 
    • Supplied by overlappping contributions from CN V, VII, IX, and X.
      • CN IX & X innervation is why speculum insertion can cause a patient to cough. 
output p4JuX2




Tympanic Membrane 

output V8DEpK


Tympanic Membrane: 

  • Is oval in shape, not circular. 
  • Has a depressed center - called the umbo - where the handle of malleus attaches to the membrane. 
  • Also attached to the TM is the lateral process of the malleus that appears as a small knob at the  anterosuperior margin of the malleus.  
  • Is composed of 3 Layers: 
    • Outer Epidermal Squamous layer - Ectoderm - 1st Brachial Cleft
    • Middle Fibrous Layer - Mesoderm
      • Can be further divided into radial outer and circular inner layers.
      • This layer is absent in the pars flaccida 
    • Inner Mucosal Layer - Endoderm - 1st Brachial Pouch
  • The lower 4/5’s of the TM is the pars tensa
    • this is the large, stiff, vibrating surface of the TM.
  • The upper ⅕ of the TM is the pars flaccida 
    • This area lacks the middle fibrous layer, thus it is less stiff.
    • Retraction pockets can develop here, which may develop into a cholesteatoma. 
  • The pars tensa and the pars flaccida are divided by the anterior and posterior malleolar folds. 


Middle Ear

Air-filled cavity that houses the ossicles, stapedius and tensor tympani muscles, and the chorda tympani nerve. It is continuous with the nasopharynx, via the eustachian tube, and the mastoid air cells, via the antrum. Medially, it is bound by the otic capsule (lateral wall of the inner ear). 


Sound travels from an air medium to a liquid medium (cochlea) which would cause dampening of the force. The body gets around this by creating a mechanical advantage. 

  • the Tympanic Membrane is about 17 times larger than the oval window, creating a reduction in the area of force distribution. 
  • In addition, the ossicles create a lever effect to further amplify the sound to almost completely overcome the ~30dB loss in energy when sound would have otherwise traveled between the air and liquid surface. 
ossicles


Ossicles

  • Malleus (Latin = hammer)
    • attached to the TM by its handle and umbo. 
    • tensor tympani muscle (CN V3) attaches to the malleus neck and manubrium by tendon from the cochleariform process. 
    • stabilized by well-developed anterior and posterior ligaments and various suspensory folds. 
  • Inucs (Latin = anvil)
    • least stable of the ossicles.
      • Skull trauma causing conductive loss usually indicates incus dislocation.   
    • articulates with the malleus head via a synovial joint and with the stapes via its long process. 
    • its posterior short process provides further ligament support. 
  • Stapes (Latin = stirrup)
    • smallest bone in the human body - roughly 3 x 2.5 mm 
    • the stapes footplate attaches to the bony margins of the oval window by the annular ligament. 
    • stapedius muscle (CN VII) attaches to the posterior neck from the pyramidal eminence. 

Eustachian Tube

  • Connects the middle ear to the nasopharynx.
  • One-third of the tube is bony and two-thirds are cartilaginous
    • bony part of the tube is located in the petrous bone, and the cartilaginous part continues onward to the pharynx. 
  • Functions:
    • Pressure Equalization
      • equalizes the air pressure on the two sides of the tympanic membrane.
      • This equalization is essential for maintaining normal tympanic membrane mobility and thus normal hearing. 
    • Drainage
      • Clears the middle ear of secretions and potential pathogens.
        • tube is lined with ciliated respiratory epithelium whose cilia beat toward the pharynx, thus inhibiting the passage of microorganisms into the middle ear. 
      • Dysfunction of this is implicated in the pathophysiology of otitis media.
    • Protection
      • When closed, protects from regurgitation with feeding.  
  • Associated muscles:
    • tensor veli palatini (CN V 3)  
      • predominant dilator
    • levator veli palatini (CN X)
    • salpingopharyngeus (CN X)
      • helps keep ET closed
    • tensor tympani (CN V 3 )
  • Variations:
    • in adults the eustachian tube is 45º from the horizontal axis.
    • in children, it is 10º from the horizontal axis and about ½ the length. 
      • This anatomical difference is unfavorable to drainage and more susceptible to regurgitation with feeding.  


Inner Ear

The inner ear is a system of passages that is dedicated to hearing and balance. It can be categorized by layers (bony and membranous) or regions (cochlear and vestibular). 


The membranous labyrinth is embedded in the bony labyrinth within the petrous bone. The membranous and bony labyrinth are separated by a space filled with perilymph, which has similar composition to extracellular fluid (High Na⁺ , Low K⁺). This is in contrast to endolymph, which fills the membranous labyrinth. Endolymph is high in K⁺ and low in Na⁺ . 


The bony labyrinth can be subdivied into three areas - the semi-circular canal system, the vestibule, and the cochlea. 

screenshot


The cochlea is a region within the bony labyrinth and contains three channels:

  • Scala vestibuli → bony → perilymph
  • Cochlear duct (scala media) → membranous → endolymph
    • makes 2 ½ spiral turns
    • the vestibular (Reissner) membrane lies between the cochlear duct and scala vestibuli. 
    • the basilar membrane lies between the cochlear duct and scala tympani. 
      • The basilar membrane and bony spiral lamina form the floor of the cochlear duct, upon which the organ of Corti is located.
  • Scala tympani → bony → perilymph
    • terminates at the round window, which is sealed by a mobile membrane and acts as a pressure valve for the incompressible perilymph. It is located inferior to the oval window. 
original-7
original-8



Tonotopic Organization of the Cochlea & Neural Pathway:  

Pressure at the oval window produces a “traveling wave” along the basilar membrane from the base of the cochlea to the apex. 

  • maximal amplitude of higher frequencies is at the base.
  • maximal amplitude of lower frequencies is at the apex.

Stimulation of hair cells from vibration of the basilar membrane stimulates the bipolar neurons of the spiral ganglion that form the cochlear division of CN VIII. The rest of the neural pathway can be rememberd by the mneumonic E COLI:  Eighth nerve → Cochlear nucleus → superior Olivary nuclei → Lateral Lemniscus → Inferior colliculus → Sylvian fissure of temporal lobe


Vestibular Apparatus

The vestibular apparatus includes the vestibular labyrinth with three semicircular canals, a saccule, and a utricle. While each of the membranous semicircular ducts is encased in its own bony shell, the utricle and saccule are contained in a common bony capsule, the vestibule.

  • The Utricle and Saccule contain otolith organs that detect linear acceleration.
    • Utricle detects horizontal acceleration.
    • Saccule detects vertical acceleration and changes in gravity. 
  • The semicirulcar canals consist of three perpendicular canals.
    • special receptors called the crista ampullaris reside in the ampulla of each semicircular canal.  
    • detect angular acceleration. 
Otolith organ of vestibular system
Inner ear's cupula transmitting indication of acceleration-2


Quiz

Bibliography: 

PW Flint, BH Haughey, VJ Lund, et al.: Cummings Otolaryngology: Head & Neck Surgery. ed 5 2010 Mosby Elsevier Philadelphia

KJ Lee: Essential Otolaryngology: Head and Neck Surgery. ed 8 2002 McGraw-Hill New York

R Pasha: Otolaryngology Head and Neck Surgery, Clinical Reference Guide. 2000 Singular San Diego

Scholes, M. A., & Ramakrishnan, V. R. ENT secrets. ed 4 2015 Elsevier Philadelphia

© Luke Sturgill 2016 I Disclaimer