The ear

Abstract

The ear is the organ of hearing and balance. It is classically divided into three components: the outer, the middle, and the inner ear. The outer ear is air-filled and comprises the auricle, the external auditory meatus, and the tympanic membrane, which separates the external ear from the middle ear. The middle ear is air-filled and comprises the tympanic cavity, the ossicles, and eustachian tube. The inner ear is fluid-filled and contains the organ of Corti (which is responsible for sound detection and connects to the brain via the cochlear nerve) and the vestibular system (which is responsible for balance and spatial orientation and connects 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

  • A skeletal muscle that attaches to the malleus
    • Origin: cartilaginous and bony margins of the eustachian tube
    • Insertion: : handle of the malleus
    • Action: : contraction of the muscle pulls the malleus medially. This action tenses the tympanic membrane and damps vibration in the ear ossicles, resulting in a reduction of the perceived amplitude of sound.
    • Innervation: : medial pterygoid nerve, a branch from the mandibular nerve (V3)

Stapedius muscle

  • The smallest skeletal muscle in the human body; attaches to the stapes
    • Origin: cartilaginous and bony margins of the eustachian tube
    • Insertion: : neck of the stapes
    • Action: : contraction of the muscle pulls the neck of the stapes laterally, thereby damping the vibrations of the stapes; and allowing it to control the amplitude of sound waves being transmitted to the inner ear.
    • Innervation: : the nerve to stapedius, a branch from the facial nerve

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.

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]

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. 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.
  • Chung KW, Chung HM. Gross Anatomy. Philadelphia, PA: Lippincott Williams & Wilkins; 2012.
  • 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.
  • Drake RL, Vogl W, Mitchell AWM. Gray's Anatomy for Students. Churchill Livingstone; 2005.
last updated 11/28/2018
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