Auditory Anatomy/Phisiology

The Peripheral Auditory system includes:

• outer ear

• middle ear

• inner ear

Sound localization

auditory system's ability to pinpoint the location of a sound source

• intensity and phase (time) difference

The Central Auditory system includes:

• Auditory Brainstem (AB)

• Auditory forebrain (AF)

The Auditory Forebrain includes:

• Medial geniculate body

• auditory cortex

Auditory brainstem includes:

• cochlear nucleus

• superior olivary complex

• inferior colliculus

Medial geniculate body (MGB)

Processes and relays specific detailed

auditory information to the auditory cortex

• AF

Auditory Cortex consists of:

primary auditory cortex (AI) and

secondary auditory cortex (AII).

Neurons in the Auditory cortex detect

Complex sound features: speech, pitch, and rhythm

Frequency Modulations of AC responds to

Changes in pitch over time (rising or falling tones)

Temporal Modulation of AC responds to

Changes in timing or rhythm of sound.

Superior olivary complex (SOC)

• Receive bilateral inputs

• Localize sound

• AB

What does binaural hearing in the auditory cortex allow?

Combines input from both ears to localize sound.

Cochlear nuclei (CN)

• the first stop for auditory nerve fibers after they leave the cochlea.

• AVCN, PVCN and DCN

• AB

Nuclei of lateral lemniscus (NLL)

Helps with processing timing and temporal patterns

• AB

Inferior colliculus (IC)

Combine the analysis of complex sound and the direction in space simultaneously

• located in midbrain

Function of the Outer Ear

Funnels sound waves to the ear drum using localization

• provides intensity and phase difference

• Pinna

• External ear canal

Eustachian Tube

Connects middle ear to the nasopharynx

• Equalizes air pressure between the middle ear and the atmosphere.

Sound pressure gain

• amplification peaked around 2.5 kHZ

• primary contribution from concha and outer ear canal (act as a resonator)

• degrees is the angle faced

Pinna

Sound localization is in the midplane of the head

External Ear canal

The pathway for sound waves

• tube shaped

• protects ear canal by trapping dust

Middle Ear Includes

• Tympanic Membrane

• Tympanic Muscles

• Auditory Ossicles

• Eustachian Tube

Function of the Middle Ear

• Transduction of sound: transferring sound energy from air (low impedance) to the fluid of the cochlea (high impedance).

• The stapes pushes in and out on the oval window (membrane covered opening) creating a pressure wave in the fluid-filled cochlea

Acoustic Impedance

determines how much sound is reflected and transmitted at the boundary between two mediums

Problem with Sound Transmission

A large amount of acoustic energy will be reflected because the difference of acoustic impedance between the two sound media.

Solution for Sound Transmission

solutions: Increase pressure/force at the oval window,- Impedance Mismatch problem (makes sure sound enters instead of reflecting off the oval W)

Acoustic Reflexes in the Middle Ear

After receiving intense sounds, the two middle ear muscles are contracted to lower the sound transmission in the middle ear, providing a protection to the inner ear

Tensor Tympani

Contracts and increases tension on the tympanic membrane with intense sounds

• in the middle ear

• a tympanic muscle

Stapedius

Contracts and works with the tensor tympani during high intensity sound to limit the motion of the ossicles and protect the inner ear

• in the middle ear

• a tympanic muscle

Inner Ear Includes

• Vestibular Apparatus

• Cochlea

Vestibular Apparatus

responsible for balance and spatial orientation, not hearing

Cochlea

• coiled shape around modiolus

• three scalae: scala vestibuli, Scala media (cochlear duct), Organ of Corti, Scala tympani

• fluid is produced by vibration of stapes

Basilar Membrane

• uses movement of fluid to vibrate up and down

• Separates scala media from scala tympani

• housed in cochlea

Reissners Membrane

• Separates scala vestibuli and scala media.

• Helps keep the two fluids (perilymph and endolymph) apart so their chemical balance stays correct

Scala Vestibuli

• first chamber that recieves the vibration

• filled with perilymph fluid, and the wave starts traveling through it

Scala Media (cochlear duct)

• “middle” chamber

• organ of corti

• important for working with basilar membrane

Organ of Corti

• contains hair cells (15,000) that detect vibration

• convert sound vibrations into electrical signals that are transmitted to the brain for interpretation

Scala Tympani

• “bottom“ chamber: filled with perilymph

• After the wave travels through the cochlea, it exits through this chamber and releases pressure at the round window

Helicotrema

joint opening at the apex of the cochlea for scala vestibuli and tympani.

• allows fluid to move between them so low-frequency (bass) sounds can travel all the way to the top of the cochlea.

Supporting Cells

Structurally and metabolitically support the outer and inner hair cells

Frequency Selectivity

• why we can hear a range of noise

• BM tuned to a specific pitch

• shown on ftc

Where each auditory nerve fiber is most sensitive to a particular sound frequency

nonlinearity

to prevent distortion and damage, hair cells adjust loud and soft sounds to either amplify or dampen

Auditory Nerve (AN)

A direct synaptic connection between the hair cells of the cochlea and the cochlear nucleus

• Type I and Type II spiral ganglion cells

Spontaneous Firing Rate (AN)

baseline electrical activity of neurons, which occurs without external stimulation

• Low, Medium, High

• Higher spontaneous rate = lower threshold (more sensitive to quiet sounds)

Type 1

90 - 95% of spiral ganglion cells, connected to IHC 

• (20 fibers to one IHC in human)

• responsible for transmitting the majority of auditory information to the brainstem

Type 2

5 – 10% of spiral ganglion cells, connected to

OHC, one to many

• (one fiber to 10 OHC in human).

• form widespread connections with outer hair cells

Frequency Threshold Curve (FTC)

• x-axis: sound frequency (Hz)

• y-axis: sound intensity (dB SPL)

• It shows how much sound level is needed for that nerve fiber to respond to each frequency.

Tonotopic organization

the spatial arrangement of the basilar membrane where different regions respond to specific sound frequencies

• High freq: Base of cochlea (near stapes)

• Low freq: Along basilar membrane (apex of cochlea)

Intensity resolution

The ability to detect different sound levels

Threshold

The lowest sound level that causes a nerve fiber to start responding

• for intensity resolution

Saturation

The highest sound level where the neuron’s firing rate stops increasing

• for intensity resolution

Dynamic Range

range of sound intensities between threshold and saturation

• 20-50 dB: fiber can accurately represent sound intensity changes

Phase locking

Neurons fire at a consistent phase of the sound wave

• Frequency limit: up to about 4 – 5 kHz

• Temporal Pattern: Spike timing matches the wave’s period