Guyton and Hall Physiology Chapter 53 - The Sense of Hearing

The Inner Ear

• The cochlea is a fluid-filled, spiral-shaped organ that houses the organ of Corti.
• Fluid waves travel along the basilar membrane within the cochlea.
• The basilar membrane is a fibrous, elastic membrane that separates the scala media from the scala tympani.
• The basilar membrane contains 20,000 to 30,000 basilar fibers that project from the modiolus towards the outer wall.
• Basilar fibers are stiff, elastic, reed-like structures with varying lengths and stiffness across the membrane.

Vibration Patterns of the Basilar Membrane

• The basilar membrane vibrates in response to fluid waves generated by stapes movement.
• Different frequencies of sound waves cause different patterns of vibration along the basilar membrane.
• High-frequency sounds cause maximal vibrations at the base of the basilar membrane near the oval window.
• Low-frequency sounds cause maximal vibrations at the apex (helicotrema) of the basilar membrane.
• The resonance frequency of the basilar membrane is dependent on the stiffness and length of its fibers.

The Organ of Corti

• The organ of Corti is located on the basilar membrane and is the sensory organ of hearing.
• It contains specialized hair cells that are responsible for converting mechanical vibrations into neural signals.
• Hair cells have stereocilia that extend from their apical surface and are embedded into the tectorial membrane, a gelatinous structure overlying the organ of Corti.

Sound Transduction

• Hair cell bending in one direction depolarizes the hair cell, while bending in the opposite direction hyperpolarizes it.
• When the basilar membrane vibrates, the hair cells are bent, causing the opening of potassium channels in the stereocilia.
• The influx of potassium ions into the hair cell causes depolarization, which triggers the release of neurotransmitters at the base of the hair cell.
• The neurotransmitters stimulate auditory nerve fibers, which transmit the signal to the brain.

Sound Intensity and Frequency Coding

• The intensity of sound is determined by the amplitude of the vibration of the basilar membrane.
• The frequency of sound is determined by the location on the basilar membrane that is most strongly vibrated.
• The place principle for the determination of sound frequency refers to the fact that different frequencies activate specific regions of the basilar membrane.
• The auditory pathway from the cochlea to the cerebral cortex maintains a tonotopic organization, meaning that neurons responding to specific frequencies remain organized in a specific pattern along the pathway.

Auditory Pathway

• Auditory nerve fibers from the spiral ganglion of Corti project to the cochlear nuclei in the medulla.
• From the cochlear nuclei, second-order neurons project to the superior olivary nucleus (SON) on both sides of the brainstem.
• The auditory pathway then continues through the lateral lemniscus, the inferior colliculus, and the medial geniculate nucleus before reaching the auditory cortex in the temporal lobe.
• The auditory cortex is where more complex auditory information is processed and interpreted.
• The auditory cortex is located in the superior gyrus of the temporal lobe.

Sound Intensity

• Sound intensity is measured in decibels (dB).
• A 10-fold increase in sound energy corresponds to a 1 bel increase, which is equivalent to 10 decibels.
• The human ear can detect a very wide range of sound intensities.
• The decibel scale is logarithmic, meaning that a small increase in decibels represents a large increase in sound energy.
• The threshold for hearing sound varies with frequency.

Impedance Matching

• Tympanic membrane and ossicular system act as an impedance matching system between air and fluid in the cochlea.
• This system effectively matches sound waves in air to sound vibrations in the cochlear fluid, allowing for approximately 50% to 75% efficiency in the frequency range of 300 to 3000 cycles/second.

The Cochlea

• The cochlea is a fluid-filled, coiled structure crucial for hearing.
• It comprises three fluid-filled tubes: scala vestibuli, scala media, and scala tympani.
• Reissner's membrane separates scala vestibuli and scala media, while the basilar membrane separates scala tympani and scala media.
• The organ of Corti, located on the basilar membrane, contains hair cells, the sensory receptors for sound.

Sound Transmission in the Cochlea

• Stapes movement against the oval window generates pressure waves in the cochlea, causing the basilar membrane to vibrate.
• This vibration creates a "traveling wave" along the membrane, with different frequencies resonating at specific locations.
• High frequencies resonate near the base of the cochlea, while low frequencies resonate near the helicotrema.

Basilar Membrane Resonance

• The basilar membrane is a flexible structure composed of fibers of varying stiffness and width.
• Stiffer fibers vibrate at higher frequencies, while the wider and more flexible fibers resonate at lower frequencies.

Auditory Pathway

• Auditory nerve fibers from the hair cells of the organ of Corti transmit signals to the cochlear nuclei in the medulla.
• From the cochlear nuclei, second-order neurons primarily project to the superior olivary nucleus on the opposite side of the brainstem.
• The auditory pathway continues through the lateral lemniscus to the inferior colliculus, where most auditory fibers synapse.
• The pathway then ascends to the medial geniculate nucleus, where all auditory fibers synapse.
• Finally, the pathway projects to the auditory cortex in the temporal lobe.

Sound Intensity

• Sound intensity is measured in decibels (dB), a logarithmic scale representing a 10-fold increase in energy for every 10 dB increase.
• The ear can perceive a change of approximately 1 dB in sound intensity.

Hearing Threshold

• The minimum sound pressure required to elicit a response in the ear is known as the hearing threshold.
• Hearing threshold varies with frequency, with the human ear most sensitive to sounds between 2000 and 5000 Hz.

Middle Ear

• Malleus, incus, and stapes are the three bones in the middle ear
• The malleus is connected to the incus by ligaments
• The incus articulates with the stapes
• The stapes pushes forward on the oval window when the incus moves forward
• The stapes pulls backward on the oval window when the incus moves backward
• The tympanic membrane and ossicular system help to match impedance between sound waves in air and sound vibrations in the fluid of the cochlea
• The ossicular system provides impedance matching of about 50-75% for sound frequencies between 300 and 3000 cycles/sec

Cochlea

• The cochlea is a coiled tube consisting of three tubes: scala vestibuli, scala media, and scala tympani
• The scala vestibuli and scala media are separated by Reissner’s membrane
• The scala tympani and scala media are separated by the basilar membrane
• The organ of Corti contains hair cells which are the receptive end organs that generate nerve impulses in response to sound vibrations
• The endolymph in the scala media contributes to the high electrical potential of the hair cells

Sound Frequency Determination

• The place principle is used by the nervous system to detect different sound frequencies
• Low-frequency sounds activate the basilar membrane near the apex of the cochlea
• High-frequency sounds activate the basilar membrane near the base of the cochlea
• Intermediate-frequency sounds activate the membrane at intermediate distances between the two extremes

Auditory Pathways

• The auditory pathways from both ears are transmitted through the pathways of both sides of the brain, with a preponderance of transmission in the contralateral pathway
• Crossing over between the two pathways occurs in three places in the brain stem
• Many collateral fibers from the auditory tracts pass directly into the reticular activating system of the brain stem

Neural Firing Rate

• Single nerve fibers entering the cochlear nuclei from the auditory nerve can fire at rates up to at least 1000/sec
• The firing rate is determined mainly by the loudness of the sound.

The Organ of Corti and Hair Cells

• The organ of Corti contains hair cells, which are responsible for the sense of hearing
• Hair cells are located in the scala media between the tectorial membrane and the basilar membrane
• The tectorial membrane presses against the stereocilia (hairs) on the hair cells
• Bending of the stereocilia in one direction depolarizes the hair cells, and bending in the opposite direction hyperpolarizes them
• This bending of the stereocilia is caused by vibrations in the basilar membrane
• Depolarization of hair cells leads to the release of glutamate, an excitatory neurotransmitter, which excites the auditory nerve fibers
• The endolymph has a high electrical potential, which sensitizes the hair cells and increases their ability to respond to sound

The Place Principle and Determination of Sound Frequency

• The place principle explains how the brain determines the frequency of a sound
• Different frequencies of sound cause different parts of the basilar membrane to vibrate
• Higher frequency sounds cause vibrations closer to the oval window
• Lower frequency sounds cause vibrations farther away from the oval window
• The hair cells that are stimulated by the vibrations in the basilar membrane send signals to the brain
• Different hair cells send signals for different frequencies, so the brain can decode the frequency of the sound
• This allows the ear to detect a wide range of sound intensities

The Auditory Pathway

• The auditory nerve fibers from the spiral ganglion of Corti synapse in the cochlear nuclei in the medulla
• Second-order neurons from the cochlear nuclei mainly cross to the opposite side of the brainstem and travel to the superior olivary nucleus
• The auditory pathway then ascends through the lateral lemniscus to the inferior colliculus
• From the inferior colliculus, the pathway goes to the medial geniculate nucleus
• The auditory pathway finally reaches the auditory cortex, which is located in the superior gyrus of the temporal lobe

Sound Localization

• Sound localization relies on the difference in the time of arrival of sound waves at each ear
• Neurons in the medial superior olivary nucleus are sensitive to these time differences
• When a sound comes from directly in front of the head, there is no time lag
• When a sound comes from a specific location, one ear hears the sound slightly before the other
• The medial superior olivary nucleus uses these time differences to determine the direction of the sound

Auditory Cortex

• The auditory cortex is responsible for processing and interpreting sound information
• Lesions of the auditory cortex can impair the ability to discriminate and understand sound patterns
• The auditory association areas help interpret the meaning of sounds

Audiometer and Hearing Tests

• An audiometer is used to measure hearing ability
• An audiometer emits pure tones of different frequencies and intensities
• The volume control on the audiometer is calibrated so that zero decibels represent the level of sound that a normal ear can just barely hear
• A hearing loss is defined as the number of decibels above normal that a sound must be increased for it to be heard
• Hearing tests are used to evaluate the different frequencies of sound a person can hear

Explore the fascinating structures of the inner ear, including the cochlea and basilar membrane. This quiz covers the mechanics of sound vibrations and how they affect our hearing. Test your knowledge on the roles of fluid waves and vibration patterns in auditory perception.