Ear Histology Overview

Ear Histology

• The ear is divided into three parts: external, middle, and inner.
• The external ear receives sound waves.
• The middle ear transmits mechanical vibrations to the inner ear.
• The inner ear transforms mechanical vibrations into nerve impulses to the auditory nerve, which then transmits the impulses to the central nervous system (CNS).

Learning Objectives

• Define the anatomical parts of the ear and the structural components of each part.
• Understand the structural components of the inner ear.
• Describe the structure and function of the membranous labyrinth.

External Ear

• Ear pinna: elastic cartilage covered by skin.
• External auditory canal: lined by hairy skin with ceruminous (wax) glands.
• Tympanic membrane (eardrum): a connective tissue core covered externally by epidermis and internally by simple cuboidal epithelium.

Middle Ear

• This is an air-filled cavity within the temporal bone, lined by simple squamous epithelium.
• It contains three bony ossicles: malleus, incus, and stapes.
• Muscles: tensor tympani and stapedius.
• Chorda tympani.

Inner Ear

• Located in the temporal bone, it has interconnected spaces called the bony labyrinth, which surround a smaller membranous labyrinth.
• Bony labyrinth: filled with perilymph
• Membranous labyrinth: filled with endolymph; contains:
  • Cochlea
  • Vestibule
  • Semicircular canals (SCC)s

Inner Ear Components

• Irregular central cavity (vestibule): Houses the utricle and saccule.
• Three semicircular canals (SCC): Contain three semicircular ducts.
• Bony cochlea: Contains the cochlear duct.
• Cochlea: 3.5 cm long, makes 2.75 turns around a bony core (modiolus).

Sensory Regions in Membranous Labyrinth

• Neuroreceptors, including hair cells, facilitate functions of the inner ear.
• Two groups of maculae for equilibrium
• Three crista ampullares for equilibrium
• Long spiral organ of Corti for hearing

Inner Ear Structure and Function

• Bony Labyrinth Component (containing perilymph and membranous labyrinth)
• Membranous Labyrinth Component (within the bony labyrinth & containing endolymph)
• Structures with Sensory Receptors
  • Vestibule: utricle, saccule, maculae: Detect linear movements and static position of the head.
  • Semi-circular canals: semi-circular ducts, cristae ampullares: Detect rotational movements of the head.
  • Cochlea: cochlear duct, spiral organ: Detect sounds.

Cochlear Duct

• Part of the membranous labyrinth, spirals within the bony cochlea.
• Consists of three parallel compartments:
  • Scala media: Filled with endolymph
  • Scala vestibuli: Separated from the scala media by Reissner's membrane, filled with perilymph.
  • Scala tympani: Separated from the scala media by the basilar membrane, filled with perilymph; cochlear duct is bounded by vestibular and basilar membranes and stria vascularis laterally.

Additional Inner Ear Structures

• Stria vascularis: Epithelial cells with an intraepithelial capillary loop; secretes endolymph.
• Organ of Corti: Sensory receptor for hearing; consists of hair cells and supporting cells resting on the basilar membrane.

Clinical Notes

• Otitis media: Viral or bacterial inflammation of the middle ear caused by the extension of upper respiratory tract infections via the Eustachian tube (more common in children due to the shorter and straighter tube).
• Vertigo: Dizziness caused by problems in the vestibular system; overstimulation of cristae ampullaris (e.g., spinning); overstimulation of the maculae of the utricle (repetitive linear acceleration changes).
• Meniere's disease: Episodes of vertigo, deafness, tinnitus due to increased pressure within the membranous labyrinth and endolymphatic dilation.

Deafness (Hearing Loss)

• Conductive: Outer ear (wax) or middle ear problems (otosclerosis, interfering with the vibrations of the ossicles); otitis media can also contribute to this type due to fluid buildup.
• Sensorineural: Defects in structures or cells in the cochlea and auditory centers in the brain.

Utricle and Saccule

• Part of the membranous labyrinth, sac-like structures in the bony vestibule; lined by simple squamous epithelium and filled with endolymph.
• Contain two neuroreceptors, perpendicular to each other, called maculae.

Semicircular Ducts

• Three membranous ducts in the bony semicircular canals, filled with endolymph.
• Each contains a neuroreceptor called crista ampullaris in its dilated part (ampulla).

References

• Junqueira's Basic Histology: Text and atlas, 13th Edition, 2013. Anthony L. Mescher.

Explore the anatomy and structure of the ear with this quiz on ear histology. Learn about the external, middle, and inner ear, including their parts and functions. Test your knowledge on the structural components and their significance in hearing.