Sound Detection and Auditory Pathways

Sound is a pressure disturbance involving alternating areas of high and low pressure.
This disturbance is produced by a vibrating object and propagated by molecules of the medium (usually air).

Sound waves are created when an object moves.
Air molecules displaced by the object's movement are pushed forward, compressing them together and creating an area of high pressure.
When the object returns to its original position, it leaves an area with fewer air molecules, creating an area of low pressure (rarefaction).
Sound waves consist of alternating compressions and rarefactions.
The vibrating object causes waves to move outward in all directions.
Sound waves can be illustrated as an S-shaped curve, or sine wave, with compressions as crests and rarefactions as troughs.

Hearing involves the reception of an air sound wave, which is converted to a fluid wave.
This fluid wave stimulates mechanosensitive cochlear hair cells.
These hair cells send impulses to the brain for interpretation.

Sound waves vibrate the tympanic membrane (eardrum).
Auditory ossicles (malleus, incus, stapes) vibrate, amplifying the pressure.
The stapes pushes on the oval window, creating pressure waves in the scala vestibuli.
These waves can take two paths:
- 4a: Sounds with frequencies below hearing range travel through the helicotrema and do not excite hair cells.
- 4b: Sounds within the hearing range go through the cochlear duct, vibrating the basilar membrane and deflecting hairs on inner hair cells.

The wave ends at the round window, causing it to bulge outward into the middle ear cavity.
The tympanic membrane is about 20 times larger than the oval window, so the vibration transferred to the oval window is amplified by approximately 20 times.

As the basilar membrane vibrates, the hair cells move with it.
Movement of the basilar membrane deflects the hairs of inner hair cells.
These “hairs” are microvilli called stereocilia.
Stereocilia project into K+-rich endolymph.
Stereocilia vary in length, being longest on one side of the cell and shortest on the other.
The longest stereocilia are enmeshed in the gel-like tectorial membrane.
Sterocillia are connected to each other via fibers called tip links.
Tip links are connected to mechanically gated ion channels.
Pulling on tip links opens ion channels, leading to increases in action potentials traveling along the cochlear nerve axon to the brain.
As liquid pressure waves move the basilar membrane up and down, changes occur in the bend of the stereocilia of the hair cells.
This results in alternating increases and decreases in action potentials along the cochlear nerve.
The brain interprets these changes as sound.

Resonance is the movement of different areas of the basilar membrane in response to a particular frequency.
The basilar membrane varies along its length:
- Fibers near the oval window are short and stiff.
- Fibers near the cochlear apex are longer and floppier.
Different frequencies vibrate the basilar membrane at different places.
Low-frequency sounds can’t move the short, stiff fibers at the base; they continue to the longer, floppier apex fibers.
Medium-frequency sounds vibrate the basilar membrane near its middle.
High-frequency sounds vibrate the basilar membrane near its base.
Each frequency that we hear corresponds to a specific place on the basilar membrane.
The brain determines the frequency of a sound wave by the location of the hair cells activated by the vibrating basilar membrane.
Low frequency = low pitch
Medium frequency = medium pitch
High frequency = high pitch

Frequency (Hz)

Nerve fibers coiled around hair cells of the outer row are efferent neurons that convey messages from the brain to the ear.
Outer hair cells can contract and stretch, changing the stiffness of the basilar membrane.
This ability serves two functions:
- Increase “fine-tuning” responsiveness of inner hair cells by amplifying the motion of the basilar membrane.
- Protect inner hair cells from loud noises by decreasing the motion of the basilar membrane.

The auditory pathway involves a complex series of signals moving to the brain.
Key structures include:
- Cochlear nuclei.
- Superior olivary nucleus (pons-medulla junction).
- Lateral lemniscus.
- Inferior colliculus.
- Medial geniculate nucleus of the thalamus.
- Primary auditory cortex in the temporal lobe.
Like the visual system, signals from one ear can cross over and provide signals to the opposite side of the brain.
Some fibers cross over, while others do not, ensuring that both auditory cortices receive input from both ears.

Perception of pitch: Sound waves of different frequencies activate hair cells in different positions along the length of the basilar membrane.
- Impulses from specific hair cells are interpreted as specific pitches.
- When a sound is composed of tones of many frequencies, it activates several populations of cochlear hair cells and cortical cells simultaneously, leading to the perception of multiple tones.
Detection of loudness: Louder sounds cause larger movements of the tympanic membrane, auditory ossicles, and oval window, resulting in pressure waves of greater amplitude in the fluids of the cochlea.
- These larger waves cause larger movements of the basilar membrane and larger deflections of the hairs on the hair cells.
- This causes larger graded potentials in the hair cells, leading to the release of more neurotransmitter and the generation of more frequent action potentials.
- The brain interprets more frequent action potentials as greater loudness.
Localization of sound depends on the relative intensity and relative timing of sound waves reaching both ears.
- If the sound source is directly in front, in back, or overhead, the intensity and timing cues are the same for both ears.
- When sound comes from one side, it activates the receptors of the nearer ear slightly earlier and more vigorously.