The ear

Summary

The ear is the organ of hearing and balance. It is divided into three sections: the outer, the middle, and the inner ear. The outer ear comprises the auricle (pinna), external auditory meatus (auditory canal), and tympanic membrane (eardrum), which separates the outer ear from the middle ear. The middle ear is a hollow structure that comprises the tympanic cavity, the ossicles, and the eustachian tube. The eustachian tube connects the middle ear to the nasopharynx, equalizing the pressure between the middle ear and the atmosphere. The auricle captures sound waves and directs them through the external auditory canal towards the tympanic membrane, thus setting it in motion. The ossicles amplify the resulting vibrations and transmit them to the inner ear via the oval window. The inner ear is a fluid-filled cavity that contains the organ of Corti and the vestibular system. The organ of Corti is responsible for sound detection, and it transmits auditory information to the brain via the cochlear nerve, while the vestibular system is responsible for the registration of body movement and spatial orientation. It transmits information to the brain via the vestibular nerve.

Outer ear

The area behind the auricle and the external auditory canal are innervated by the glossopharyngeal and vagus nerve. Mechanical cleansing of the external auditory canal can lead to nausea and coughing!

Middle ear

Parts

Tympanic cavity

Ossicles

Eustachian tube (pharyngotympanic tube)

  • Definition: connects the middle ear cavity with the nasopharynx
  • Function:
    • Controls the pressure within the middle ear and allows the pressure to be equalized on both sides of the tympanic membrane (e.g., important for altitude compensation)
    • Usually closed but opens when chewing, swallowing, or yawning (ear clearing)

During and after a cold, the eustachian tube can become blocked by swelling of the mucosa and mucus impaction. This often results in dull hearing, pain, and a feeling of pressure or fullness in the affected ear.

Tensor tympani muscle

Stapedius muscle

  • The smallest skeletal muscle in the human body; attaches to the stapes

The stapedius muscle dampens transmission of loud noises to the inner ear (acoustic or stapedial reflex). Stapedius weakness (e.g., due to Bell palsy) can result in hyperacusis (increased sensitivity to environmental sounds, which can also result from ear trauma or middle/inner ear infection)

Mastoid process

  • Process of the temporal bone behind the ear
  • Becomes pneumatized (aerated) with age

A middle ear infection can spread to neighboring structures and result in perforation of the tympanic membrane, labyrinthitis, mastoiditis, cerebral venous sinus thrombosis, and meningitis.

Inner ear

  • Definition: : The innermost portion of the ear that contains organs of hearing (i.e., cochlea) and equilibrium (i.e., vestibule) and is situated within the petrous part of the temporal bone.

Parts

Labyrinth

Cochlea

Loop diuretics are ototoxic because they act on both the Na/K/2Cl carrier in the ascending loop of Henle of the kidneys and the same transporters in the stria vascularis of the cochlea. This effect can lead to edema of cochlear tissues, a decrease of the cochlear electrical potential, and temporary or permanent sensorineural hearing loss.

Vestibular system

References:[1]

Embryology

References:[2][3][4]

Clinical significance

  • 1. Lee SC. Vestibular System Anatomy. In: Meyers AD. Vestibular System Anatomy. New York, NY: WebMD. https://emedicine.medscape.com/article/883956-overview#a1. Updated June 27, 2016. Accessed April 18, 2018.
  • 2. Le T, Bhushan V,‎ Sochat M, Chavda Y, Zureick A. First Aid for the USMLE Step 1 2018. New York, NY: McGraw-Hill Medical; 2017.
  • 3. Myers JM. Inner Ear Anatomy. In: Meyers AD. Inner Ear Anatomy. New York, NY: WebMD. https://emedicine.medscape.com/article/1968281-overview#a1. Updated December 12, 2013. Accessed July 12, 2018.
  • 4. Chung KW, Chung HM. Gross Anatomy. Philadelphia, PA: Lippincott Williams & Wilkins; 2012.
  • Drake RL, Vogl W, Mitchell AWM. Gray's Anatomy for Students. Churchill Livingstone; 2005.
last updated 05/04/2020
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