Equilibrium and Hearing in the Inner Ear

Overview of the Inner Ear

The inner ear is a complex structure responsible for both hearing and equilibrium. It is filled with perilymph and contains a maze of bony chambers within the temporal bone, including the cochlea, vestibule, and semicircular canals. The membranous labyrinth is suspended in perilymph and contains endolymph.

Organs of Equilibrium

Vestibular Apparatus

The vestibular apparatus consists of equilibrium receptors in the inner ear. It has two main functional parts:

• Static equilibrium

• Dynamic equilibrium

Static Equilibrium

Static equilibrium helps detect the position of the head relative to gravity and linear acceleration. The main receptors are the maculae located in the vestibule.

• Maculae report on the position of the head and send information via the vestibular nerve.

• Anatomy of the maculae:

  • Hair cells are embedded in the otolithic membrane.

  • Otoliths (tiny calcium carbonate stones) float in a gel around the hair cells.

  • Movements cause otoliths to bend the hair cells, generating nerve impulses.

Example: When you tilt your head forward, otoliths shift, bending the hair cells and signaling the brain about the new head position.

Dynamic Equilibrium

Dynamic equilibrium detects angular or rotary movements of the head. The main receptors are located in the crista ampullaris within the ampulla of each semicircular canal.

• Crista ampullaris:

  • Contains a tuft of hair cells covered with a gelatinous cap called the cupula.

  • When the head moves, the cupula drags against the endolymph, stimulating the hair cells.

  • An impulse is sent via the vestibular nerve to the cerebellum for balance coordination.

Example: Spinning in a chair causes endolymph to move, bending the cupula and activating the hair cells to detect rotation.

Organs of Hearing

Spiral Organ of Corti

The organ of Corti is the main hearing receptor, located within the cochlear duct of the cochlea.

• Receptors are hair cells situated on the basilar membrane.

• The tectorial membrane is a gel-like structure capable of bending the hair cells when sound vibrations occur.

• The cochlear nerve is attached to the hair cells and transmits nerve impulses to the auditory cortex in the temporal lobe of the brain.

Example: Sound waves cause the basilar membrane to vibrate, bending the hair cells against the tectorial membrane and generating electrical signals sent to the brain.

Summary Table: Comparison of Static and Dynamic Equilibrium