Internal Ear Anatomy Quiz

Internal Ear Structure

• The internal ear, located within the petrous bone, is fully developed at birth.
• It comprises an osseous labyrinth, a bony cavity filled with a fluid, endolymph-containing membranous labyrinth.

Fluid Spaces

• The membranous labyrinth is smaller than the osseous, and is separated from the bone by perilymph.
• Endolymph and perilymph are distinct fluids, not communicating with each other.
• Perilymph may derive from cerebrospinal fluid or perilymphatic capillaries.

Parts of the Osseous Labyrinth

• The cochlear canal (cochlea), vestibule, and semicircular canals are parts of the osseous labyrinth.
• The cochlea is conical, with 2.75 turns.
• Promontory projects from the basal turn of the cochlea.
• The modiolus is the axial bony stem of the cochlea.
• Spiral lamina and hamulus are bone projections within the cochlea.

Parts of the Membranous Labyrinth

• Cochlear duct (for hearing): within cochlear canal,
• Utricle and saccule (for static balance): within vestibule,
• Semicircular ducts (for kinetic balance): within semicircular canals.

Cochlea Details

• The cochlear canal is partitioned by the spiral lamina and the membranous cochlear duct.
• Scala vestibuli and scala tympani are perilymph-filled spaces within the cochlea divided by the cochlear duct.
• Helicotrema is the point of communication between scala vestibuli and scala tympani at the apex of the cochlea.
• Round window and aqueduct of cochlea; the round window leads to the middle ear and the aqueduct leads into the cerebrospinal fluid.

Vestibule Structure

• The vestibule contains the utricle and saccule, small sacs with receptors for static balance.
• Cochlear duct, semicircular canal openings and internal acoustic meatus connection occur here.
• The vestibule's walls contain recesses (spherical and elliptical) housing the saccule and utricle, respectively.
• Cochlear recess and aqueduct of the vestibule open into the vestibule.

Semicircular Canals

• Three semicircular canals are oriented at right angles to each other in the petrous bone.
• Each canal is almost two-thirds of a circle, about 20mm long and 1mm in diameter except at the dilated ampulla.
• Ampullae contain sensory receptors for kinetic balance.
• The canals open into the vestibule via five openings.

Membranous Labyrinth Details

• It's a closed continuous cavity filled with endolymph, mimicking the osseous labyrinth.
• Three-layered: an outer fibrous and vascular layer, an intermediate layer similar to a basal lamina, and an inner epithelial layer with balance and hearing receptors.

Spiral Organ (Organ of Corti)

• Contains specialized hair cells (stereocilia) for hearing, arranged in rows in the cochlear duct.
• Hair cells are connected by the cochlear nerve.
• Hair cells are stimulated by vibrations from sound.

Saccule and Utricle

• Saccule (lower front vestibule): connected to cochlear duct via ductus reuniens
• Utricle (upper back vestibule): receives semicircular duct openings
• Endolymphatic duct, Y-shaped, that connects to the endolymphatic sac for endolymph absorption.
• Maculae (thickened walls of saccule & utricle): contain hair cells, otolithic membrane, which contains calcium carbonate particles (otoliths) for static balance.

Semicircular Ducts

• The membranous ducts are smaller than their bony counterparts, except at ampulla where they are similar in size.
• Sensory receptors (hair cells) for kinetic balance line the ampullae.
• Cristae, transverse crests, house the sensory hair cells of the ampullae.

Blood Supply of the Labyrinth

• Labyrinthine artery branches to cochlear and vestibular nerves
• Stylomastoid artery assists for blood supply
• Labyrinthine vein into inferior petrosal sinus.
• Other small veins into the superior petrosal sinus and aqueducts.

Eighth Nerve Distribution

• Cochlear nerve enters the internal acoustic meatus and reaches the hearing organ.
• Different parts of the vestibular nerve supply the utricle and saccule/semicircular ducts
• Cell bodies of cochlear/vestibular nerves found in cochlear/vestibular ganglia.