Ear Anatomy: Outer, Middle, and Inner Ear

Ear Anatomy

• The ear has three regions: external (outer), middle, and internal (inner).
• The external and middle ear are involved in hearing.
• The internal ear is involved in both hearing and equilibrium.

Outer Ear

• The outer ear consists of the pinna and the external auditory canal.
• The pinna collects sound waves and directs them into the external auditory canal.
• The pinna aids in sound localization.
• The external auditory canal amplifies sound waves.

Middle Ear

• The middle ear is an air-filled space behind the eardrum.
• It contains three small bones called ossicles: the malleus, incus, and stapes.
• The ossicles amplify sound waves and transmit them from the eardrum to the inner ear.

Inner Ear

• The inner ear is located deep within the temporal bone.
• It comprises the cochlea, vestibule, and semicircular canals.
• The cochlea is the main organ of hearing, converting sound waves into neural signals.
• The vestibule and semicircular canals are responsible for balance.

Cochlea Physiology

• The cochlea is 35 mm long with 2 and 3/4 turns.
• The Scala vestibuli and Scala tympani contain perilymph, which is rich in sodium ions (Na+).
• The Scala media is the middle cochlear chamber, containing endolymph secreted by the stria vascularis.
• Endolymph is rich in potassium ions (K+).
• The Scala media is electrically positive by 85 mV, known as endocochlear potential, relative to the Scala vestibuli and Scala tympani.
• The oval window separates the middle ear from the inner ear, allowing sound waves to enter the cochlea.
• Sound waves traveling through the cochlea cause the basilar membrane to vibrate.
• The basilar membrane is a thin, flexible structure running the length of the cochlea.
• Thousands of hair cells are embedded in the basilar membrane.
• Hair cells convert sound waves into electrical impulses.
• Hair cells are overlaid by the tectorial membrane, a gel-like structure.
• Vibration of the basilar membrane causes hair cells to move back and forth.
• Bending of hair cells triggers the release of glutamate.
• Glutamate stimulates auditory nerve fibers.
• Auditory nerve fibers transmit electrical signals to the brainstem.
• Different frequencies of sound waves stimulate different parts of the basilar membrane.
• The location of stimulated hair cells corresponds to the frequency of the sound waves.
• The cochlear amplifier involves outer hair cells that contract and expand in response to signals from the brainstem.
• Contraction and expansion of outer hair cells amplify basilar membrane vibrations.
• The cochlear amplifier enhances the sensitivity and selectivity of the cochlea to different sound frequencies.

Organ of Corti

• The organ of Corti contains the hair cells, which are the receptors for hearing.
• The resting membrane potential of hair cells is about -60 mV.
• Hair cells have motor protein called prestin.
• Hair cells have stereocilia and kinocilia.
• Stereocilia are hairlike protrusions.
• Kinocilia are specialized single long cilium originally but during mammalian development it disappears in maturation.
• Stereocilia are arranged in rows of progressively decreasing height.
• Tip links connect stereocilia to each other.
• Tip links contain mechanically sensitive cation channels.
• Cation channels allow positively charged ions (potassium and calcium) to flow into the cell.
• When sound waves cause stereocilia to bend, tip links stretch and open cation channels.
• Deflection of stereocilia toward the kinocilium opens potassium channels, depolarizing the hair cell.
• Influx of calcium stimulates the release of glutamate.
• Glutamate stimulates afferent neurons to transmit neural impulses to the auditory cortex.
• At rest, potassium channels are partially open.
• Deflection of stereocilia away from the kinocilium closes potassium channels.
• Closing potassium channels inhibits signal transmission by afferent neurons.

Outer and Inner Hair Cells

• Hair cells in the organ of Corti are arranged in four rows: three rows of outer hair cells and one row of inner hair cells.
• There are approximately 20,000 outer hair cells.
• There are approximately 3,500 inner hair cells.
• Inner hair cells transmit most of the sensory input to the auditory nerve fibers.
• Outer hair cells amplify sound signals before they're transmitted to the auditory nerve fibers.
• 90-95% of sensory neurons innervate the inner hair cells.
• 5-10% of sensory neurons innervate the outer hair cells.

Spiral Ganglion

• The spiral ganglion contains the cell bodies of sensory neurons that innervate the hair cells.
• Sensory neurons are called spiral ganglion neurons.
• Spiral ganglion neurons form synapses with hair cells.
• They transmit electrical signals to the brain.
• Spiral ganglion neurons are bipolar neurons.
• One dendrite synapses with hair cells, and one axon extends to the brainstem.
• Spiral ganglion neurons are essential for transmitting sensory information to the brain for interpretation as sound.

Role of Basilar Membrane in Hearing

• The basilar membrane is critical for frequency analysis of sound waves.
• It separates the fluid-filled chambers of the cochlea.
• Sound waves cause the basilar membrane to vibrate.
• The basilar membrane is stiffer at the base of the cochlea and more flexible towards the apex.
• Different frequencies cause maximum displacement at different points along the membrane.
• Higher frequencies cause displacement near the base.
• Lower frequencies cause displacement near the apex.
• The frequency analysis allows the auditory system to distinguish between different frequencies and identify pitch.
• Hair cells on the basilar membrane respond to mechanical vibrations by generating electrical signals.
• Hair cells at the base of the cochlea respond to high-frequency sounds.
• Hair cells at the apex respond to low-frequency sounds.

Traveling Wave Theory of Von Bekesy

• Sound waves create a traveling wave along the basilar membrane.
• The wave moves from the base to the apex of the cochlea.
• The wave peaks at a specific location corresponding to the frequency of the sound wave.
• Higher frequencies cause maximum displacement near the base; lower frequencies near the apex.
• As the traveling wave moves along the basilar membrane, it causes hair cells to bend and generate electrical signals.
• Outer hair cells amplify the traveling wave, enhancing sensitivity and selectivity.