Alternative waves that lead to changes in air pressure (Longitudinal Wave - It is fastest in Solid>Liquid>Air.)

(Transferred to in Ear tunnel translated into waves in fluid sensed by receptors generate AP travel to integration sites for perception and finally to primary auditory cortex)

Properties of Sound1. Amplitude

Height of wave – perceived as loudness (0- 85dB) Amplitude is directly proportional to intensity.

* There is approximately 1012 difference in resolution.2. Frequency

# of waves per second(hertz) Perceived as pitch (20hz-20,000hz).

Other mammals E. whales and bats higher (ultrasound) and dogs even lower (infrasound).

Another property of sound nuances is Doppler shift, Important in animal echolocations (E. Bats, Dolphins)

If the distance between the source and the detector is decreasing then the perceived frequency becomes higher and if the distance is increasing between the detector and the source the perceived frequency will decrease.

Animal Echolocations -

▪ Involves static localization and Doppler shift localization.

Outside to Inside1. Funnel shaped pinna localize sound to

the tunnel 2.5 cm/ 1 inch that leads to the inner ear receptors that sense mechanical energy as a stimuli before it is a connective tissue called the tympanic membrane and beyond it is the middle ear.

2. Large out ward appearance also amplify sound waves.

3. Certain Animal pinna are more flexible and under cortical (voluntary) muscular control to localize a certain angle or direction of a source. (Ex. dogs, Bats…)to the ear tunnel

- Consists of a small air filled chamber, tothe other side of the tympanic membrane;it is equalized to the atmospheric pressureby the Eustachian tube that connects themiddle ear to the pharynx. (Ex. Altitudeclimb.)

- The tympanic membrane is connectedfrom the medial surface to a series ofthree bones called ossicles that transferthe movement or (sound vibration) to ahole in the bone of the skull called theoval window. Behind the oval window isthe fluid-filled cochlea, which contains theapparatus for transforming the physicalmotion of the oval window membraneinto a neuronal response.

COMPONENETS- (3 - bones)1. First bone (Malleus)2. Second bone (Incus)3. Third bone (Stapes)

A protective neuronal reflex pathway against load noises that forms a loop in the brain stem nuclei before reaching the cortex via (descending) corticofugal pathway.

Causes tightening (contraction) of tensor tympani (i.o cavity/malleus) and stapedius muscles (i.ocavity/staples) and creates a rigid contact to protect inner ear from extreme load stimuli.

*Not proof against sudden loadstimuli but important in censoringspeech. (High freq. is more discernedin low freq. environment.)

ANATOMY OF COCHLEA

spiral shape resembling a snail’s shell filled with three fluid filled chambers and receptor cells located in the organ of corti are submersed in the fluid thus movement in fluid causes a response in sensory neurons.

Three Scala-

1) S.Vestibular filled with perilymph. (high concentration of Na+ [140mM] and low K+[7mM] similar to CSF)

2) S.Media filled with endolymph.(high concentration of K+ [150mM] and low Na+[1mM] due to active pumps of striavestibularis.

3) S.Tympani filled with perilymph.The Ressner membrane is between SV and SM.

The Basilar membrane is between the SM and ST.

*(Difference in [ions] is important in endocochlearpotentials – later seen in physiology of cochlea.)

Anatomy Continued …

1. Organ of Corti is hanging over the tectorial membrane and sit below (BM).

2. At the apex of the cochlea the scalamedia is closed off and the scalatympani becomes continuous at a hole: Helicotrema. (includes Ion movements)

3. Scala vestibuli meets oval window, since fluid has nowhere to go it’s propagated in a wave because the basilar membrane is more wider and flexible at the apex. (E. Think flippers in water.)

The distance traveled by the wave is dependent on frequency.

The higher the frequency the less distance traveled up to apex because the stiff end vibrates.

Depending on the mammalian anatomical specification, (>20KHz) Ultrasound and Infrasound (<20Hz) can be sensed, stiff vs. longer BM, respectively.

Stiffness (HF) v. length [LF] ?

Organ of Corti- consists of hair cells (inner - 3.5K, 1 row; and outer - 15-20K, 3rows) , rods of corti (structural supporters) and various supporting cells (spiral ganglion).

1. Reticular lamina (RL) lies between tectorial membrane (TM) and basilar membrane (BM).

2. Hair cells are between (RL) and (BM) have sterocilia (actin filament stimuli sensing extensions) above RL either into TM (outer hair cells) or just below (inner hair cells).

HOW IT WORKS - 1) Movement of BM is sensed by stereo-cilia of hair cells (receptor cells) 2) send activation to spiral ganglion (bipolar cells) inside modulus. 3) Transferred message gets sent by auditory vestibular nerve cranial nerve (VIII) via

ascending corticofugal pathway.

Movement can be of two kinds…1) One cause is for the basilar membrane to move down that causes stereocilia to away from tectorial membrane.2) Or another cause is for the basilar membrane to move toward tectorial membrane causing stereocilia to induce more contact with stereocilia.

Physiology of Cochlea

Recording done by (A. J. Hudspeth et all) in 1980s at CIT, indicated that bending of hair cells in one direction caused depolarization while the other direction, hyperpolarization from resting potential of -70mV. See Fig.

Recently, TRPA1 channel has been discovered on tips of sterocilia, opening and closing ion conductance due to mechanical-stimuli.

TRPA1 channels are linked to tip links (elastic filament) in adjacent cilia allowinf small leak of potassium (endocholear potential) that triggers voltage gated Calcium release and Neurotransmitter mobilization and thus release, in signal transduction.

1) As we can see here in the fig 11.15, Half of the cell (stereocilia) is in endolymph separated by reticular lamina and half is inside perilymph.

2) Opening of (TRPA1) potassium ion channels from mechanical bending of sterocilia causes potassium influx from endolymph.(endocochlearpotential)

1) Thus, calcium voltage gated channels let Calcium influx from perilymph (similar [ion] , cerebral spinal fluid).

1) NT release causes spiral ganglion cells to initiate, signal transduction up the ascending corticofugalpathway.

1) Convergence – (35,000 - 50,000) spiral-ganglions

communicate 95 % with innerhair cells and 5 % outer hair cells.

2) Sensitivity – Ironically, outer hair cells are

more sensitive to low stimuli and research shows they are cochlear amplifiers.

*The reason: They are attached to basilar membrane and reticular lamina they are more influential on shapes of the membrane.

Treatment with Furosemide decreases hair cell transduction consequently there is a decrease in the amplification effect of flexing Basilar membrane thus proving that outer hair cells are involved in amplification of sound due to their location.

Sound localization is important part of sound nuances, creates a better understanding of environment (E. survival cues, interactions)

1) HORIZONTAL PLANE - Usually requires both ears and it is a spacialauditory imagery created in brain. (E. closed eye localizing sound source)

A and B make the duplex theory of sound localization.

a. Interaural time delay - Time difference in arrival in each ear due to location of being source being closer to one ear. (ears are 20cm apart and messages from each are independently analyzed of delay between each messages)

b. Interaural intensity difference – is another method of horizontal sound localization for high frequency. It works because the brain casts a shadow, creating an intensity difference between shadow casted ear and direct detection. It does not work well on low frequencies because sound diffracts around the head and is ineffective.

1) VERTICAL PLANE –

Usually doesn’t require bi-aural discrimination and it’s due to the reflection patterns and the sensory discrimination of pinna.

• (Other animals, like certain species of owls don’t require pinnae to be excellent vertical plane sound localizers and use same horizontal discrimination techniques namely inter-aural time differences.)

• Most aurally adaptive animals like bats and owls can discriminate inter aural delay time differences about 2 or 3 orders of magnitude. (E. Bats [10-

5]; average humans only [10-2] )

Auditory Processing –

• Starts with spiral ganglion Brain stem nuclei medial

geniculate nuclei primary auditory cortex A1 in temporal lobe.

• In one sense, the auditory pathway is more complex than the visual pathway because there are more intermediate stages between the sensory receptors and cortex. However, the systems have analogous components. Each starts with sensory receptors and ends in the primary sensory cortex.

Once a neural response to sound is generated in the inner ear, the signal is transferred to and processed by a series of nuclei in the brain stem.

Out- put from these nuclei is sent to a relay in the thalamus, the medial genic- ulate nucleus (MGN). Finally, the MGN projects to primary auditory cortex, or A1, located in the temporal lobe.

In one sense, the auditory pathway is more complex than the visual pathway because there are more intermediate stages between the sensory receptors and cortex. However, the systems have analogous components. Each starts with sensory receptors and ends in the primary sensory cortex.

Moredetails– intermediatebrainstemnuclei –

1. All ascending pathways converge on to the inferior colliculus subsequently (IC) to MGN and then A1.

2. There are cortifugal feedback in these pathways that contact (modulate) each recipient and sender (including hair cells) via descending corticofugalpathways.

3. Each cochlear nucleus receives input from just one ear on ipsilateral (same side), all other auditory nuclei in the brain receive input from both ears.

Brain stem nuclei converge at Inferior colliculus (IC) then MGN and then A1.

Inferior colliculus can send tractsto SC (superior colliculus) to getintegrated with visioninformation at cerebellum.

This part of the auditory imagerycreated in the brain, as far asspactial and temporal acuity weare concerned about.

FREQUENCY AND AMPLITUDECODING -

1. Amplitude is coded withincreased basilar membranevibrations, as a result, nerve fibersfiring rate increases to increasedrate of AP stimulus from from haircell activation in the cochlea.

2. Frequency is coded-

The graph represents the firing ratein response to sounds at differentfrequencies

The neuron is most responsive tosound at one frequency, called theneuron’s characteristic frequency,and it is less responsive atneighboring frequencies. This typeof frequency tuning is typical ofneurons at each of the relays fromcochlea to cortex.

--------- NOW TO ARTICLES --------

1. Huffman, R. F., & Henson, O. W., Jr. (1990). The descending auditory pathway and acousticomotor systems: connections with the inferior colliculus. Brain Res Brain Res Rev, 15(3), 295-323.

2. Cant, N. B., & Benson, C. G. (2003). Parallel auditory pathways: projection patterns of the different neuronal populations in the dorsal and ventral cochlear nuclei. Brain Res Bull, 60(5-6), 457-474.

3. Xiao, Z., & Suga, N. (2002). Modulation of cochlear hair cells by the auditory cortex in the mustached bat. Nat Neurosci, 5(1), 57-63. doi: 10.1038/nn786

4. Suga, N., & Ma, X. (2003). Multiparametric corticofugal modulation and plasticity in the auditory system. Nat Rev Neurosci, 4(10), 783-794. doi: 10.1038/nrn1222

Background –

Bats are only legally flying mammals but some can be legally blind also…

Macro bats have well developed visual cortex and have visual acuityfor most of their flying while micro bats like the mustached bats usewell developed corticofugal (descending and ascending pathwayswith multiple brainstem nuclei involved in processing beforesending to thalamic relay (MGN) to primary auditory sensory cortexfor sound processing also then to auditory association areas tocreate a clear image acuity of their environment.

Animal echolocation in bats is optimized at 20KHz – 61KHz formustached bats that prey on small insects: can generate up to 212KHzin actuality. (E. Doppler shift and Localization) … So how???

So how does the corticofugal auditory system modulate signal processing??

Since 1950s, many studies have been done in thalamic and collicular neurons of anaesthetized animals. The results was strong/high activation or inactivation of primary AC evoked excitation/ or inhibition of these sub cortical (brainstem) nuclei.

These physiological data were contradictory some found only or predominantly , inhibitory activity or predominantly excitatory of facilitatory activity 4.

* The contradiction between the studies might beresolved if the frequency dependence of excitation andinhibition, and the relationship in tuning betweenstimulated and recorded neurons, are considered.

To resolve -

1. Researchers started using awake animals, first obtain tuning curves of both the cortical neurons to be stimulated and the subcortical neurons that respond to this stimulation.

2. After stimulation from pharmacological or electrical routes

3. They focused on the relationship in tuning between them, and on the frequency- dependence of facilitation and inhibition. Using this experimental system, they have found that corticofugal modulation occurs in a specific and systematic way…

The best frequency is the frequency at which neuron responds at maximal magnitude of activation (depolarization).

Here we see a map of brain stem nuclei of best frequency at the :

* VA ventroanterior area, SCsuperior colliculus, IC inferiorcolliculus , SOC superior olivarycomplex VF ventrofringe and VPventroposterior areas.

Centripetal shifts vs Centrifugal shifts

The later is a shift away from BF while the earlier is a shift toward the BF. (best frequency).

1. These two types of shift occur in specific spacial pattern along the frequency axes of the IC and AC and the shift is essentially the same in both structures.

2. In AC of big brown bat when big brown bat and Mongolian gerbil and in the IC of the two species and the house mouse, centripetal shift happen in large areas and centrifugal BF shifts happen in narrow zones that surround the centripetal area (in between areas)

3. But the specialized area of AC of moustached bat – the Doppler shftedconstant frequency (DSCF) area centrifugal BF shifts occur in large area that surrounds matched neurons.

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1. Huffman, R. F., & Henson, O. W., Jr. (1990). The descending auditory pathway and acousticomotor systems: connections with the inferior colliculus. Brain Res Brain Res Rev, 15(3), 295-323.

2. Cant, N. B., & Benson, C. G. (2003). Parallel auditory pathways: projection patterns of the different neuronal populations in the dorsal and ventral cochlear nuclei. Brain Res Bull, 60(5-6), 457-474.

3. Xiao, Z., & Suga, N. (2002). Modulation of cochlear hair cells by the auditory cortex in the mustached bat. Nat Neurosci, 5(1), 57-63. doi: 10.1038/nn786

4. Suga, N., & Ma, X. (2003). Multiparametric corticofugal modulation and plasticity in the auditory system. Nat Rev Neurosci, 4(10), 783-794. doi: 10.1038/nrn1222

5. Bear, M., & Connors, B. (2007). Neuroscience: Exploring the brain (3rd ed.). Philadelphia, PA: Lippincott Williams & Wilkins.