ear

Anatomy & Physiology Overview - The Ear

Outer ear 

  • Pinna (auricle) - visible part of the ear outside of the head.  
  • External auditory canal 
  • Ceruminous glands - specialized sudoriferous glands (sweat glands) located subcutaneously in the external auditory canal. They produce cerumen (earwax) by mixing their secretion with sebum and dead epidermal cells.

 Middle ear: air filled 

  • Tympanic membrane -  vibrates in response to sound waves 
  • Malleus, incus and stapes - 3 small bones that transmit vibrations to each other

Inner ear: fluid filled 

  • Mechanoreceptor for hearing and balance 
  • Vesibular apparatus - balance
  • Semicircular canals 
  • Cochlea
  •  • Organ of Cor -  sensory epithelial cell

Cochlea

  • Perilymph = similar in compositon to plasma – Na+ 
  • Endolymph = high in K+ 
  • Organ of Cor: contains hair cells – move due to pressure waves 
  • 50-100 stereocilia on each cell 
  • Longest embedded in tectorial membrane 

In the cochlea that the vibrations transmitted from the eardrum through the tiny bones are converted into electrical impulses sent along the auditory nerve to the brain. 

  • The cochlea is a tapered tube which circles around itself 
  • The basilar membrane divides the tube lengthwise into two fluid-filled canals joined at the tapered end. 
  • ossicles transmit vibration to the cochlea where they attach at the oval window
  • resultant waves travel down the basilar membrane where they are “sensed” by  16-20,000 hair cells (cilia) attached to it which poke up from a third canal called the organ of Corti
  • Organ of Corti transforms the stimulated hair cells into nerve impulses 
  • Waveforms travelling down the basilar membrane peak in amplitude at differing spots along the way according to their frequency 
  • Higher frequencies peak out at a shorter distance down the tube than lower frequencies
  • The hair cells at that peak point give a sense of that particular frequency
  • The distance between pitches follows the same logarithmic distance as our perception of pitch i.e. the placement of octaves are equidistant.