Human Anatomy: The Ear

The Ear

• Maintains the balance of the body (vestibular) and perceives sound (auditory)
• Consists of the external ear, middle ear (tympanic cavity), and internal ear (labyrinth)

External Ear

• Comprised of the auricle and external auditory meatus
• Auricle:
  • Has a characteristic shape and collects air vibrations
  • Consists of a thin plate of elastic cartilage covered by skin
  • Possesses both extrinsic and intrinsic muscles supplied by the facial nerve
  • Divided into helix, antihelix, scaphoid fossa, concha, tragus, and antitragus
• External auditory meatus:
  • Curved tube that leads from the auricle to the tympanic membrane
  • About 2.5 cm in length and conducts sound waves to the tympanic membrane
  • Lined by skin and its outer third is provided with hairs, sebaceous and ceruminous glands

Tympanic Membrane

• Most medial portion of the external ear that separates it from the middle ear
• A connective tissue structure that is covered with skin on the outside and mucous membrane on the inside
• Composed of three layers: external layer (derived from skin), middle layer (fibrous), and inner layer (continuous with the mucous membrane of the middle ear cavity)
• Translucent and allows the structure within the middle ear to be observed during otoscopy
• Has a handle/manubrium of malleus and anterior and posterior malleolar folds
• Pars tensa is the tense portion of the tympanic membrane, and pars flaccida (Shrapnell's membrane) is the flaccid portion

Middle Ear

• Transmits sound vibrations from the tympanic membrane to the inner ear via the ear ossicles (malleus, incus, and stapes)
• Located mainly within the petrous portion of the temporal bone
• Resembles a biconcave lens in shape
• Composed of the tympanic cavity that connects anteriorly with the nasopharynx via the auditory tube (eustachian tube) and mastoid air cells posteriorly
• Contents:
  • Ear ossicles
  • Muscles: tensor tympani and stapedius
  • Nerves: chorda tympani, tympanic branch of CN IX, lesser petrosal nerve
  • Tympanic plexus: parasympathetic (CN IX), sympathetics (superior cervical ganglion via carotid plexus)

Ear Ossicles

• Malleus:
  • Largest ossicle that possesses a head, neck, long process/handle, anterior process, and lateral process
• Incus:
  • Possesses a large body and two processes (long and short)
• Stapes:
  • Smallest ossicle that possesses a head, neck, and two limbs that attach to the oval base

Muscles of the Ossicles

• Tensor tympani and stapedius muscles

Auditory Tube (Eustachian Tube)

• Connects the anterior wall of the tympanic cavity to the nasal pharynx
• Its posterior third is bony, and its anterior two-thirds is cartilaginous
• Serves to equalize air pressures in the tympanic cavity and the nasal pharynx

Mastoid Antrum

• Lies behind the middle ear in the petrous part of the temporal bone
• Communicates with the middle ear by the aditus
• Relations:
  • Anterior wall: related to the middle ear; contains the aditus
  • Posterior wall: separates the antrum from sigmoid venous sinus and cerebellum
  • Lateral wall: forms the floor of the suprameatal triangle
  • Medial wall: related to the posterior semicircular canal

Inner Ear

• Also known as the labyrinth
• Situated in the petrous part of the temporal bone, medial to the middle ear
• Consists of the bony labyrinth and the membranous labyrinth

Bony (Osseous) Labyrinth

• Located in the petrous portion of the temporal bone
• Surrounds the membranous labyrinth
• Contains perilymph
• Connects to the middle ear via the fenestra vestibuli and the fenestra cochlea
• 3 parts: vestibule, cochlea, and semicircular canals

Membranous Labyrinth

• Located within the osseous labyrinth
• Contains endolymph
• Divided into 4 parts: cochlear duct, saccule, utricle, and semicircular ducts

Cochlear Duct

• Spiral structure located within the cochlea
• Triangular in shape, with a base created by the endosteum of the canal known as the spiral ligament and the stria vascularis
• Contains sensory receptors (hair cells) and the organ of Corti

Saccule and Utricle

• Small structures located within the vestibule of the osseous labyrinth
• Connected to the utricle via the utriculosaccular duct and the endolymphatic duct
• Contain sensory receptors (maculae) that are sensitive to vertical and horizontal acceleration

Semicircular Ducts

• Correspond to the semicircular canals of the osseous labyrinth (anterior, posterior, and lateral)
• Open into the utricle via 5 openings
• Contain sensory receptors (crista) that are located in its ampullae