36.4 Hearing and Vestibular Sensation

Sound and Hearing

Sound waves shape our auditory world through two key properties: amplitude and frequency. Amplitude is the maximum displacement of a wave from its resting position, measured in decibels (dB), and it determines how loud a sound seems. A jet engine is loud because it produces high-amplitude waves. Frequency is the number of wave cycles per second, measured in Hertz (Hz), and it determines pitch. High-frequency waves sound high-pitched (like a whistle or bird chirp), while low-frequency waves sound low-pitched (like a bass drum or thunder).

Journey of Sound Through the Ear

Sound travels through a series of structures that progressively convert air vibrations into electrical signals the brain can interpret. Here's the path, step by step:

1. Outer ear (pinna) and ear canal. The pinna collects sound waves and funnels them into the ear canal. Its curved shape helps amplify sound and gives you some ability to localize where sounds are coming from.

2. Tympanic membrane (eardrum). Sound waves traveling down the ear canal strike the tympanic membrane, causing it to vibrate.

3. Ossicles (middle ear bones). Those vibrations pass to three tiny bones: the malleus (hammer), incus (anvil), and stapes (stirrup). The ossicles amplify the vibrations and transmit them to the oval window of the inner ear. This amplification is necessary because the vibrations are moving from air into fluid, which requires more energy.

4. Cochlea (inner ear). Vibrations at the oval window create pressure waves in the fluid-filled cochlea, a snail-shaped structure. Inside the cochlea sits the organ of Corti, which rests on the basilar membrane and contains specialized hair cells.

5. Frequency sorting along the basilar membrane. Different regions of the basilar membrane respond to different frequencies. Hair cells near the base of the cochlea respond to high-frequency sounds, while hair cells near the apex respond to low-frequency sounds. This spatial arrangement is called tonotopy.

6. Mechanotransduction by hair cells. When pressure waves move through the cochlear fluid, tiny projections called stereocilia on the hair cells bend. This bending opens ion channels, which triggers electrical signals.

7. Signal transmission to the brain. Those electrical signals travel along the auditory nerve to the auditory cortex, where the brain processes and interprets them. This is how you recognize a familiar voice or distinguish a guitar from a piano.

Vestibular System

The vestibular system is responsible for your sense of balance and spatial orientation. It sits in the inner ear, right next to the cochlea, and has two main components: the otolith organs and the semicircular canals. Each detects a different type of motion.

Otolith Organs: Detecting Linear Motion and Gravity

The two otolith organs, the utricle and saccule, detect linear acceleration (like moving forward in a car) and the position of your head relative to gravity.

• They contain hair cells embedded in a gelatinous layer topped with tiny calcium carbonate crystals called otoconia.
• When your head tilts or accelerates in a straight line, the otoconia shift due to gravity or inertia. This bends the hair cells, generating electrical signals sent to the brain.
• That's why you can sense when you're tilting your head back even with your eyes closed.

Semicircular Canals: Detecting Rotation

The three semicircular canals detect rotational acceleration (turning or spinning your head).

• They are arranged roughly perpendicular to each other (anterior, posterior, and horizontal), so together they can detect rotation in any direction across three-dimensional space.
• Each canal is filled with a fluid called endolymph and has an enlarged region called the ampulla, which contains hair cells.
• When your head rotates, the endolymph lags behind due to inertia, pushing against the hair cells in the ampulla and bending them. This generates electrical signals.
• If you spin in circles and then stop, the endolymph keeps moving for a moment, which is why you feel dizzy afterward.

Integration and Balance

The brain doesn't rely on the vestibular system alone. The vestibular nerve carries signals from both the otolith organs and semicircular canals to the brainstem, where that information is combined with visual input (what you see) and proprioceptive input (sensory feedback from muscles and joints about body position). This integration is what lets you keep your balance while walking on uneven ground or keep your eyes focused on an object while your head is moving.

Vestibular Disorders

When the vestibular system malfunctions, common symptoms include dizziness, vertigo (a spinning sensation), and balance problems. One specific sign of vestibular dysfunction is nystagmus, an involuntary, rhythmic movement of the eyes that occurs because the brain is receiving abnormal balance signals and trying to stabilize vision.