2.3 Hearing and Vestibular Sense

Louder Sounds

Have higher amplitudes.

Deeper Sounds

Have longer wavelengths.

Sensorineural Hearing Loss

Hearing loss caused by damage to the cochlea's receptor cells or to the auditory nerves; also called *nerve deafness*. These patients are candidates for cochlear implants.

Scala Vestibuli

Leads from oval window to apex of cochlea; filled with perilymph.

Incus

Lies between the malleus and stapes, it is also known as the anvil.

Directions in auditory space include...

Elevation, distance, and azimuth. Loudness is not a direction in auditory space.

Endolymph vs. Perilymph

*Perilymph*: Much like cerebrospinal fluid, it has a high concentration of sodium ions and low concentration of potassium ions. *Endolymph*: Uniquely, it has a high concentration of potassium ions, and low concentration of sodium ions.

Cochlear Implant

*Receiver* goes to a *stimulator*, which reaches the cochlea. *Receiver* receives information from a *transmiter*. The *transmitter* gets information from the *speech processor*. The *speech processor* gets information from a *microphone*.

Scalae

3 chambers of the cochlea.

Scala Tympani

Terminates at the round window; filled with perilymph.

Cochlea

A coiled, bony, fluid-filled tube in the inner ear through which sound waves trigger nerve impulses.

Round Window

A membrane-covered hole in the cochlea that allows the perilymph to actually move within the cochlea.

Basilar Membrane

A thin flexible membrane that the organ of Corti rests on.

Place Theory

Accepted theory of sound perception. It states that the location of a hair cell on the basilar membrane determines the perception of pitch when that hair cell is vibrated.

The cone of confusion arises from an ambiguity in the sound information that reaches the ear. What is the cause of this ambiguity?

All of the points on the cone of confusion have the same interaural level difference and interaural time difference.

Pinna

Also called the auricle, it is the cartilaginous outer portion of the ear.

Stereocilia

As vibrations reach the basilar membrane underlying the organ of Corti, these sway back and forth within the endolymph.

How do the ossicles help to amplify the sound that reaches the tympanic membrane?

By transferring vibration from a membrane of large area, the tympanic membrane, to a membrane of small area, the oval window.

High-Frequency Sound Waves

Cause the tympanic membrane to vibrate faster against the ossicles.

Low-Frequency Pitches

Cause vibrations at the apex, away from the oval window.

High-Frequency Pitches

Cause vibrations of the basilar membrane very close to the oval window.

Organ of Corti

Composed of thousands of hair cells that are bathed in endolymph.

Tectorial Membrane is Immobile

Connect to the stereocilia of hair cells involved in hearing. As pressure waves displace the basilar membrane, the hair cells move in relation to the stationary tectorial membrane, and the stereocilia consequently bend.

Eustachian Tube

Connects the middle ear to the nasal cavity.

Otoliths

Cover the modified hair cells of the *utricle and saccule*. As the body accelerates, these resist that movement. This bends and stimulates the underlying hair cells, which send a signal to the brain.

External Auditory Canal

Directs sound waves to the tympanic membrane.

Membranous Labyrinth

Fill the cochlea, vestibule, and semicircular canals.

Scala Tympani and Scala Vestibuli

Filled with perilymph, they surround the hearing apparatus and are continuous with the oval and round windows of the cochlea.

External Auditory Meatus

Formal name for the external auditory canal, sound funneled from the outer ear travels through it to reach the tympanic membrane.

Function of the Pinna

Funnels sound waves into the external auditory canal.

Intensity

Higher for louder sounds, as it corresponds to an increased amplitude.

Bony Labyrinth

Houses the inner ear, which includes the cochlea, vestibule, and semicircular canals.

Scala Media

Houses the organ of Corti.

Kinocilium Location

If the stereocilia bend towards the kinocilium, it is excitatory because it stretches the tip links and opens the K+ channels. If they bend towards the shorter stereocilia, it is inhibitory because it compresses the tip links.

Tectorial Membrane

Immobile membrane on top of the organ of Corti.

Superior Olive

Localizes the sound.

Oval Window

Located on the cochlea, it is the entrance to the inner ear. The stapes is attached to it, and it vibrates back and forth.

Inferior Colliculus

Involved in the startle reflex and in keeping the eyes fixed when the head is turned.

Stapes

It receives vibrations coming from the incus. Also called the stirrup, the base plate rests on the oval window of the cochlea.

Vestibulo-Ocular Reflex

Keeps the eyes fixed when the head is turned.

Sound Waves

Longitudinal waves of compressions and rarefactions in air as the medium, thus, creating areas of high and low pressure.

The kinocilium attaches to a potassium channel in the hair bundle. What type of gate is the potassium channel?

Mechanical gate.

Audition

Our sense of hearing sound.

What forms the border between the middle and inner ear?

Oval Window

Loudness is dependent on both sound pressure and frequency.

Points along the equal loudness curve are all perceived at the same volume. Therefore, all points on the threshold of feeling would be at the threshold of pain. Point A is below the threshold of hearing and would be inaudible. Points B and D both fall along the same equal loudness curve. C is the loudest.

Vestibule

Portion of the bony labyrinth that contains the utricle and the saccule.

Vestibular Sense

Senses linear and rotational acceleration using the hair cells.

Semicircular Canals

Sensitive to *rotational acceleration*. They're perpendicular to one another and end in a swelling called an ampulla, where the hair cells are located.

Ossicles

Smallest bones in the ear.

Tonotopic Mapping

Spatial arrangement of where sounds of different frequency are processed in the cochlea allows the brain to distinguish sounds of different frequencies. The brain then processes these sounds in different parts of the primary auditory cortex.

Perilymph

Suspends the membranous labyrinth in the bony labyrinth. It also transmits vibrations while cushioning the inner ear structures.

Ampulla

Swollen end of the semicircular canals where hair cells are located.

Spiral Ganglion Cell

The action potential caused by the voltage-gated calcium channels opening activates this type of cell, which then activates the auditory nerve.

These are reasons why low tones are represented near the apex, while high tones are represented near the base of the cochlea.

The base of the basilar membrane is narrower than the apex. The base of the basilar membrane is much stiffer than the apex. Pressure changes in the cochlea cause a peak amplitude at a certain area of the basilar membrane as a function of the frequency of the sound.

Fluid Movement in the Cochlea

The fluid moves away from the oval window in response to vibrations, and it meets the center of the cochlea then travels back the other direction until it meets the round window. The movement of fluid causes the mechanoreceptor hair cells on the organ of corti to trigger action potentials.

Malleus

The hammer bone that is affixed to the tympanic membrane.

Researchers showed participants a video of two objects that appeared to be moving toward one another at a 90 degree angle. More than 88% of the participants saw the objects pass each other and continue on their original course. In a second condition, participants were played a click sound when the objects appeared to be close together. This time 63% perceived the objects as bouncing off each other in opposite directions. What impact did sound have on visual perception?

The input from the auditory system modulated visual perception.

Membranous and Bony Labyrinth

The membranous labyrinth is filled with endolymph (blue). The membranous labyrinth is suspended within the bony labyrinth which is filled with perilymph (purple).

Basilar Tuning

The method by which the cochlea distinguishes between types of sound during auditory processing. Hair cells at the start of the cochlea are activated by high-frequency sounds, and those at the apex are activated by low-frequency sounds.

Vestibulocochlear Nerve

The pathway of hearing to the brain goes from the organ of Corti => Superior Olive => Inferior Colliculus => Medial Geniculate Nucleus => Auditory Cortex.

Endolymph

The potassium-rich fluid that bathes the membranous labyrinth.

Which of these events is caused by the stapes pushing against membrane covering the oval window?

The pressure change causes the organ of Corti to vibrate vertically.

Hair Bundle

The stereocilia of one inner hair cell, and it is made of filaments.

Stereocilia Mechanics

The swaying back and forth causes the opening of ion channels, which causes a receptor potential.

The tympanic reflex helps prevent damage from loud sounds. How does the tensor tympani work to protect one's hearing?

The tensor tympani dampens loud sounds by tightening the tympanic membrane. This slows down transduction.

Utricle and Saccule

These *otolithic organs* are sensitive to *linear acceleration*, so they are used as part of the balancing apparatus to determine one's orientation in three-dimensional space.

Primary Auditory Cortex

Think of tonotopic mapping.

Kinocilium

This is a large hair cell important in the detection of the head's position. Whether the displacement of the stereocilia is towards or away from this determines whether it is excitatory or inhibitory.

Endolymph in the Semicircular Canals

This resists the motion when the head rotates, bending the underlying hair cells, which sends a signal to the brain.

Pressurized Sound Wave and Hair Cell

Two requirements for the process of audition.

Tympanic Membrane

Vibrates in phase with the incoming sound waves. High-frequency sound waves make the membrane vibrate faster.

Charge Influx

When stereocilia bend towards the tallest member and the tip links pull open cation channels, this depolarizes the membrane. This results in the voltage-gated calcium channels opening.

Tip Links

When the stereocilia are bent toward the tallest member of a bundle, these fibrous connections pull open mechanically gated cation channels, and the resulting charge influx depolarizes the membrane. This opens voltage-gated calcium channels near the base of the cell, which triggers neurotransmitter release.

Tonotopically Organized Cochlea

Which hair cells are vibrating gives the brain an indication of the pitch of the sound. Determining the pitch of the sound allows the brain to know where to process the sound within the primary auditory cortex.