What is the anatomy and physiology of the ear?

Anatomy and Physiology of the Ear

The ear is divided into three distinct parts—outer, middle, and inner ear—that work together to convert sound waves into neural signals perceived by our auditory cortex as sound. 1

Outer Ear

• The outer ear consists of the auricle (pinna) and the ear canal (external auditory meatus) 1
• The auricle captures sound waves and directs them into the ear canal 1
• The ear canal funnels sound waves toward the tympanic membrane (eardrum) 2
• The ear canal has unique anatomical adaptations including migrating epithelium and cerumen glands that protect it while maintaining its function as both a conduit and a cul-de-sac 2
• Defects in ear canal development or blockages can lead to conductive hearing loss 2

Middle Ear

• The middle ear is separated from the outer ear by the tympanic membrane (eardrum), a thin cone-shaped membrane 1
• The middle ear cavity contains three small bones called ossicles: malleus (attached to the tympanic membrane), incus, and stapes 1, 3
• The middle ear can be divided into five subspaces: mesotympanum (central portion), retrotympanum (posterior), epitympanum (superior), protympanum (anterior), and hypotympanum (inferior) 4
• The middle ear is connected to the nasopharynx by the Eustachian tube, which plays a crucial role in:
  • Ventilating the middle ear and equalizing pressure 5, 4
  • Protecting the middle ear from excessive sound pressure and nasopharyngeal secretions 5
  • Draining secretions from the middle ear 5
• The tensor veli palatini is the only active muscle that opens the Eustachian tube 5
• The middle ear is also connected posteriorly to the mastoid air cells 5

Inner Ear

• The inner ear connects to the middle ear via the oval window 1
• The inner ear includes the semicircular ducts (responsible for balance) and the cochlea (responsible for hearing) 1
• The cochlea contains the organ of Corti with hair cells that serve as auditory receptors 1
• These hair cells convert the mechanical energy of sound waves into electrical neural impulses 1

Physiology of Hearing

1. Sound conduction pathway:

   • Sound waves are captured by the auricle and directed into the ear canal 1, 6
   • The sound waves cause the tympanic membrane to vibrate 3
   • These vibrations are transmitted through the ossicular chain (malleus, incus, and stapes) 3
   • The stapes footplate vibrates against the oval window, communicating with the inner ear 3

2. Sound transduction:

   • The mechanical vibrations of the stapes against the oval window create fluid waves in the cochlea 1
   • These waves stimulate the hair cells in the organ of Corti 1
   • The hair cells convert mechanical energy into electrical neural impulses 1
   • These impulses are transmitted along the auditory pathway to the auditory cortex 1

3. Auditory pathway:

   • From the cochlea, signals travel through the auditory nerve to the brainstem 1
   • The pathway continues through the superior olive, inferior colliculus, and medial geniculate body of the thalamus 1
   • Finally, signals reach the auditory cortex in the temporal lobe where they are interpreted as sound 1

Clinical Implications

• Dysfunction of the Eustachian tube can lead to middle ear diseases including otitis media 5
• Abnormal bone formation that immobilizes the stapes (otosclerosis) can compromise hearing by preventing proper sound transmission 3
• Bacterial biofilms in the middle ear are associated with chronic suppurative otitis media and persistent otitis media with effusion 1
• Viral upper respiratory tract infections often precede acute otitis media by causing inflammation of the nasopharyngeal and Eustachian tube epithelium 1
• Understanding the complex anatomy and physiology of the ear is essential for diagnosing and treating ear disorders and performing ear surgeries 4, 6