Auditory and Somatosensory Systems Notes

Auditory System & Somatosensory System

Auditory System (Hearing)

Functions of Hearing (Audition):
- Detecting sounds.
- Determining the location of sound sources.
- Recognizing the identity and meaning of sound sources.

What is Sound?

Sound as Vibration:
- Vibrating objects create alternating waves of compression and rarefaction in a medium.
- These waves propagate away from the source like ripples in water.
- Sound intensity decreases with distance from the source.
Pure Tone:
- A pure tone is a single sinusoidal waveform.
  - Pressure is plotted as a sine wave.
  - Frequency is measured in Hertz (Hz), representing cycles per second.
  - Amplitude is the magnitude of the wave, measured in decibels (dB).
Physical vs. Perceptual Dimensions of Sound:
- Frequency corresponds to pitch (high or low).
- Amplitude corresponds to sound intensity.
Complex Sounds:
- Natural sounds are complex patterns of vibrations.
- Complex sounds can be deconstructed into sine waves.

Range of Human Hearing

Frequency Range: Humans can detect frequencies from 20 Hz to 20 kHz.
Intensity Range: Humans can hear a range of intensities.

Sound Intensity Levels (dB)

Harmful: 140 dB
Pain Threshold: 120 dB
Risk of Hearing Loss: 100 dB
Common Sounds: 80 dB
Soft Music: 60 dB
Whisper: 40 dB
Hearing Threshold: 0 dB

Anatomy of the Ear

Outer Ear:
- Pinna: external ear structure.
- Ear canal: auditory canal.
- Tympanic membrane: eardrum.
Middle Ear:
- Ossicles: malleus (hammer), incus (anvil), stapes (stirrup).
- Oval window.
- Eustachian tube: connects to the throat.
Inner Ear:
- Cochlea: contains hair cells.
- Auditory nerve.
- Vestibule.
- Round window.
Overall Process: Air pressure is converted to mechanical movement, then to electrical activity.

Path of Sound in the Ear

Sound waves enter the auditory canal and vibrate the tympanic membrane (eardrum).
This vibration sets the ossicles in motion, triggering vibrations of the oval window.
Vibration of the oval window sets the fluid in the cochlea in motion, which is possible due to the compensating movement of the round window.
Hair cells in the organ of Corti transduce mechanical pressure into glutamate release.

Function of the Ossicles and Round Window

Inner ear fluid is not compressible like air.
The round window allows movement of the fluid in the cochlea.
The oval and round windows have membranes that move, so no liquid escapes the cochlea.

The Cochlea

The stapes vibrates against the membrane behind the oval window.
The basilar membrane is inside the cochlea.
A specific region of the basilar membrane flexes in response to a particular frequency.

Basilar Membrane

The basilar membrane vibrates in response to sound hitting the eardrum.
It is narrower and stiffer at the basal end and wider and less tightly stretched at the apical end.
The frequency of the sound determines which portion of the basilar membrane vibrates maximally.

Organ of Corti

Composition:
- Basilar membrane: auditory receptors (hair cells) are mounted here.
- Tectorial membrane: rests on top of the hair cells.
Mechanotransduction:
- Produced by the movement of hair cells against the tectorial membrane.

Hair Cells and Mechanotransduction

Basilar membrane vibration causes deflection of hair bundles.
Tip links: filaments that physically connect hair cell cilia together.
Shearing movement of basilar and tectorial membranes deflect hair cells.
Tension on tip link filaments causes the opening of spring-gated ion channels.
Calcium and potassium ions flow in and depolarize the hair cell (receptor potential).
Glutamate is released onto the spiral ganglion cell.

Auditory Pathway

The base of each hair cell is contacted by a process from one or more spiral ganglion cells.
The somas of the ganglion cells are in the center of the cochlea, and the axons form the auditory nerve (8th cranial pair).

Mechanotransduction and Coding

Deflection of hairs on inner hair cells.
Generation of receptor potential.
Release of glutamate onto the spiral ganglion cells.
Small amounts of K^+ and Ca^{2+} enter the ion channel to larger amounts based on force applied and open probability.

Sound Encoding (Sound Recognition)

The frequency of the sound is encoded by the place on the basilar membrane that is maximally vibrated by the sound (Place Coding).
Tonotopy:
- Spatial arrangement of where sound is received, transmitted, and perceived.
- The basilar membrane is narrower and stiffer at the basal end, and it is wider and less tightly stretched at the apical end.

Linking Basilar Membrane Motion to Place Coding

Low frequencies activate the apex of the cochlea.
High frequencies activate the base of the cochlea.

Review

Inner ear fluid is not compressible like air.
The round window allows movement of the fluid in the cochlea.
The oval & round windows have membranes that move, so no liquid escapes the cochlea.
Basilar membrane – auditory receptors (hair cells) are mounted here.
Tectorial membrane – rests on top of the hair cells.
Mechanotransduction is produced by the movement of hair cells against the tectorial membrane.
Deflection of hairs on inner hair cells, generation of receptor potential, release of glutamate onto the spiral ganglion cells.

Auditory Pathway Steps

Sound hits the eardrum.
Vibrations reach the oval window.
Inner ear fluid is displaced.
The basilar membrane vibrates.
The hair-like structures (cilia) of the inner hair cells deflect against the tectorial membrane.
Tension pulls the tip links and opens spring-gated ion channels.
The inner hair cells depolarize.
Glutamate is released on the spiral ganglion cells.
Auditory nerve produces action potentials.

Transduction vs. Encoding

Inner Hair Cells - mechanotransduction
Spiral ganglion cells - sound encoding

Auditory Pathways

Monoaural pathway (one ear): recognition of sound
Binaural pathway (both ears): localization of sound
- Requires computation of differences in time of arrival and sound intensity between both ears.
- Each hemisphere of the brain receives information from both ears but primarily from the contralateral one (medulla).

Auditory Pathway Structures

Medulla
- Ventral cochlear nucleus
- Dorsal cochlear nucleus
- Superior olivary complex
- Trapezoid body
Midbrain
- Lateral lemniscus
- Inferior colliculus
Thalamus
- Medial geniculate nucleus
Cerebrum
- Auditory cortex

Tonotopic Mapping in the Cortex

Frequency is mapped tonotopically in the cortex.

Auditory Cortex

Organization:
- Hierarchical organization:
  - Primary (core)
  - Secondary (~7 regions including belt & parabelt)
- Parallel processing:
  - Anterior stream:
    - Pathway to prefrontal cortex
    - Identifying sounds (what)
  - Posterior stream:
    - Pathway to posterior parietal cortex
    - Locating sound (where) and sound movement
    - Preparing for action
    - Integrating vision information

Effects of Damage to the Auditory System

Deafness in Humans:
- Total deafness is rare due to multiple pathways.
- Types:
  - Conductive deafness: damage to ossicles.
  - Nerve deafness: damage to cochlea.
- Partial cochlear damage results in loss of hearing at particular frequencies.
Cochlear Implant:
- A microphone and processor that transmits signals to the implant, placed along the basilar membrane.
- Best in young children & people deaf in adulthood
Cocktail party problem:
- Difficulty in filtering background noise

Cortical Lesions in Humans

Auditory cortex damage impairs complex sound perception.
Patients:
- Patient FD: Difficulty recognizing environmental sounds but able to identify sound location & movement.
- Patient CZ: Able to recognize environmental sounds but unable to identify sound location & movement.
- Patient MA: Not deaf, but trouble recognizing environmental sounds & identifying sound location & movement.

Processing Visual and Auditory Information

**Dorsal Stream (

Dorsal Stream (Where):- Processes the spatial aspects of sounds (location and movement). - Pathway to posterior parietal cortex. - Ventral Stream (What):- Processes complex characteristics of sounds. - Pathway to the prefrontal cortex.