Auditory System

Sound is a fluctuation of air pressure in [[blank_start]Pa[blank_end]].

The [blank_start]sound pressure[blank_end] is a local pressure deviation of a compressible sound transmission medium which occur by spreading of sound.

The human hearing threshold is approximately

The human pain threshold is approximately [blank_start]1.000.000[blank_end] times higher than the hearing threshold.

The sound [blank_start]frequency[blank_end] is the number of pressure fluctuations per second. Unit: [[blank_start]Hz[blank_end]]

Speed of sound: c = [blank_start]lambda[blank_end] * [blank_start]f[blank_end]

The speed of sound [blank_start]increases[blank_end] with the density of the medium.

In transition from one medium to another, sound can be

Sound intensity level: L = [blank_start]20[blank_end] * [blank_start]log10[blank_end]([blank_start]p[blank_end] / [blank_start]p0[blank_end]) Doubling of p results in an increase of sound intensity by [blank_start]6[blank_end] dB. Loudness is perceived in a [blank_start]logarithmic[blank_end] gradation.

Humans have their highest sensitivity between [blank_start]1[blank_end] and [blank_start]4[blank_end] [blank_start]kHz[blank_end]. Human speech is roughly between [blank_start]1[blank_end] and [blank_start]2[blank_end] [blank_start]kHz[blank_end].

The communication channel correlates with the [blank_start]body volume[blank_end].

Hearing range: Babies: [blank_start]20[blank_end] - [blank_start]20.000[blank_end] Hz Young adult: ... - [blank_start]15.000[blank_end] Hz Older people: ... - [blank_start]5.000[blank_end] Hz

The external ear consists of the [blank_start]pinna[blank_end] to capture sound and the [blank_start]ear canal[blank_end] to transmit sound.

Pinna and ear canal amplify frequencies between [blank_start]2 and 4 kHz[blank_end] by a factor of [blank_start]8[blank_end]. This [blank_start]is not[blank_end] uniformly effective for every direction.

External ear and middle ear are separated by the [blank_start]tympanum[blank_end].

The middle ear is connected to the [blank_start]pharynx[blank_end] (jaw) by the [blank_start]Eustachian tube[blank_end].

The middle ear

The middle ear is filled with [blank_start]air[blank_end]. It works as an impedance [blank_start]converter[blank_end] to balance different air pressures between outer and inner ear. It also allows impedance [blank_start]matching[blank_end] of sound traveling in air to acoustic waves traveling in a system of fluids and membranes in the inner ear. The area of the tympanum is much [blank_start]bigger[blank_end] than the stapes footplate and the chain of ossicles (malleus, incus, stapes) work as a [blank_start]lever[blank_end].

Besides the sense of hearing the inner ear contains two more organs of perception: - Sense of linear [blank_start]acceleration[blank_end] - Sense of [blank_start]rotation[blank_end] They all use [blank_start]hair cells[blank_end] in [blank_start]fluids[blank_end] to detect sound or balance.

The labyrinth in the inner ear: The membranous labyrinth is filled with [blank_start]K+[blank_end] rich [blank_start]endolymph[blank_end]. The bony labyrinth is filled with [blank_start]Na+[blank_end] rich [blank_start]perilymph[blank_end]. The hair cells are contained in the [blank_start]membranous[blank_end] labyrinth.

The three scalae of the cochlea

Inner hair cells: - [blank_start]1[blank_end] row - [blank_start]ca. 3.500[blank_end] cells - provide [blank_start]neural output[blank_end] Outer hair cells: - [blank_start]3[blank_end] rows - [blank_start]ca. 12.000[blank_end] cells - provide [blank_start]amplification[blank_end]

There are roughly [blank_start]30.000[blank_end] spiral ganglion cells, which belong to the [blank_start]peripheral[blank_end] nervous system. Afferent SGC lead to the [blank_start]cochlear nucles[blank_end]. Most afferent SGC innervate [blank_start]exactly 1 IHC[blank_end]. Efferent SGC come from the [blank_start]superior olivary complex[blank_end]. Most efferent SGCs innervate [blank_start]multiple OHCs[blank_end].

[blank_start]Outer[blank_end] hair cells possess a unique motor protein called [blank_start]prestin[blank_end], which causes them to contract every time they are depolarized.

The mechanical frequency analysis in the cochlea is done via the basilar membrane. The base is [blank_start]100µm[blank_end] wide, [blank_start]thick and taut[blank_end] and sensitive to [blank_start]high[blank_end] frequencies. The apex is [blank_start]500 µm[blank_end] wide, [blank_start]thin and floppy[blank_end] and sensitive to [blank_start]low[blank_end] frequencies. Different stiffness at different points results in different frequencies creating vibration maxima at different points! This is a [blank_start]passive[blank_end] frequency analysis. There are also [blank_start]more[blank_end] hair cells (and therefore a [blank_start]higher[blank_end] resolution) at positions corresponding to low frequencies.

The hair cells' organelles that respond to fluid motion are called [blank_start]stereocilia[blank_end]. Hair cells cannot create APs but they can induce APs in nerve cells from mechanical energy. When the stereocilia are lifted up [blank_start]K+[blank_end]-channels open and the cells are depolarized causing transmitter release at the synapse to the SGC.

Ascending pathway

The descending pathway of the auditory system is possibly involved in selective attention. It uses [blank_start]inhibition[blank_end] by negative feedback. The pathway is called the [blank_start]olivocochlear[blank_end] pathway.

Cell types of the cochlear nucleus: [blank_start]Bushy[blank_end] cells receive input via large [blank_start]excitatory[blank_end] [blank_start]Endbulbs of Held[blank_end] and send output to the [blank_start]superior olivary nucleus[blank_end] via large synapses, called [blank_start]Calyx of Held[blank_end]. Bushy cells have the largest synapses in the brain, which leads to [blank_start]great[blank_end] temporal precision and an [blank_start]exact[blank_end] resolution (1 spike for 1 spike).

Tonotopy is not only found in the cochlear nucleus or the superior olivary complex but preserved all the way to the primary auditory cortex.

Two cues are used for sound localization: ILD: [blank_start]Interaural level difference[blank_end] ITD: [blank_start]Interaural time difference[blank_end] ILD works best for [blank_start]low[blank_end] frequencies ITD works best for [blank_start]high[blank_end] frequences

Which brain regions are important for sound localization?

Neurons in the lateral superior olive (LSO) are most sensitive to [blank_start]high[blank_end] frequencies and [blank_start]mainly[blank_end] responsible for [blank_start]ILD[blank_end] detection. They integrate [blank_start]excitatory[blank_end] signals from the ipsilateral ear with [blank_start]inhibitory[blank_end] input from the contralateral ear. The ILD [blank_start]can[blank_end] be analyzed frequency-specific. Neurons in the medial superior olive (MSO) are most sensitive to [blank_start]low[blank_end] frequencies and responsible for [blank_start]ITD[blank_end] detection. One model to explain this mechanism is the [blank_start]Jeffress Model[blank_end].

The Jeffress Model Which neuron will fire if the sound comes from midright?

Sound localization: There is strong evidence for the Jeffress Model for ITD processing in [blank_start]birds[blank_end]. In mammals it is called into question, e.g. by the fact that [blank_start]the MSO receives also inhibitory input[blank_end].

Primary auditory cortex (A1): Brodman [blank_start]41[blank_end] and [blank_start]42[blank_end]

Auditory belt areas (including secondary auditory cortex A2) are [blank_start]less[blank_end] precise in their tonotopic organization. They process [blank_start]combinations of[blank_end] frequencies and temporal sequences of sound. A2 includes [blank_start]Wernicke's area[blank_end].

During development the synaptic density peaks at [blank_start]2-4[blank_end] years. After that the brain needs to specialize its functions. In juveniles synaptic potentials have a [blank_start]longer[blank_end] duration and synaptic plasticity is [blank_start]higher[blank_end].

Continuous noise input during the critical period of development result in a [blank_start]disrupted[blank_end] tonotopic organization and a degraded [blank_start]frequency response selectivity[blank_end] for neurons in the adult auditory cortex.

There are two types of hearing loss: (1) [blank_start]Conductive[blank_end] hearing loss: - damage of tympanic membrane - occlusion of the ear canal (2) [blank_start]Sensory-neural[blank_end] hearing loss: - damage to hair cells - damage to auditory nerve

The most common cause for hearing loss is

Early hearing loss leads to (1) [blank_start]Delayed[blank_end] development with synaptic [blank_start]overshoot[blank_end] (2) [blank_start]Increased[blank_end] elimination of synaptic function

Patients with a cochlear implant can often get the [blank_start]rhythm[blank_end] of a piece of music but have great difficulty recognizing a [blank_start]melody[blank_end].

The problem with bilateral cochlear implants is (a) [blank_start]their limited range[blank_end] which limits ILD coding (b) [blank_start]lack of synchronization of the implants[blank_end] which limits ITD coding

Components of a cochlear implant: [blank_start]Microphone[blank_end] --> [blank_start]Sound processor[blank_end] --> [blank_start]Transmitter[blank_end] --> [blank_start]Receiver[blank_end] --> [blank_start]Electrode array[blank_end]

The ratio of tympanum vs. [blank_start]stapes[blank_end] foot plate is [blank_start]17[blank_end]:1. The leverage effect is [blank_start]1.3[blank_end].

The cochlea has [blank_start]2.5[blank_end] coils and its length can vary between [blank_start]28 and 41[blank_end] mm.

Outer hair cells [blank_start]contract[blank_end] upon depolarization due to their motor protein [blank_start]prestin[blank_end]. Inner hair cells release [blank_start]glutamate[blank_end] upon depolarization.

The Organ of Corti is located in the scala [blank_start]media[blank_end] and contains the [blank_start]basilar[blank_end] and the [blank_start]tectorial[blank_end] membrane (alphabetic order). The hair cells are located on the [blank_start]basilar[blank_end] membrane.

The hair cells are [blank_start]logarithmically[blank_end] distributed on the [blank_start]basilar[blank_end] membrane. More hair cells are located towards the [blank_start]apex[blank_end] where [blank_start]low[blank_end] frequencies are coded.

Inner hair cells contain stretch-activated [blank_start]K+[blank_end] channels. Upon depolarization voltage-gated [blank_start]Ca2+[blank_end] channels open and lead to the release of [blank_start]glutamate[blank_end].

Cochlear hair cells have [blank_start]v[blank_end]-shaped [blank_start]tuning[blank_end] curves that describe their best frequencies.

Bushy cells improve [blank_start]phase locking[blank_end] compared to the signals that come from the auditory nerve.

The Jeffress model contains a [blank_start]delay-line[blank_end] and a [blank_start]coincidence[blank_end] detector. It is called into question because the medial superior olive receives inhibitory [blank_start]glycine[blank_end] input.

There are studies about the regeneration of hair cells upon the [blank_start]adenoviral[blank_end] expression of Atoh1/Math1.