The Auditory System PHYSIO: #3 DR.LOAI ZAGOUL 24/3/2014 Refer to the slides for some photos. Before we talk about the auditory system we will talk about the sound and waves All waves have basic characteristics: Amplitude Frequency Wavelength These properties have particular names with reference to sound Amplitude = loudness or volume Sound Frequency = pitch it could be high frequency or low frequency and it doesn t depend on amplitude so high frequency could have low or high amplitude Range: 20 Hz to 20,000 Hz Pitch: High pitch = high frequency; low frequency = low pitch Intensity: High intensity louder than low intensity
Anatomy of the ear External ear: Auricle that function to collect sound and define the vertical angle (distance) of the sound. Cerumen gland that secret cerumen (ear wax). Tympanic membrane converts sound waves to vibration. Middle ear: Its basic function is to transmit vibrations of the tympanic membrane to the internal ear. It accomplishes this through three interconnected but movable bones that bridge the space between the tympanic membrane and the internal ear. These bones are the malleus, the incus, and the stapes attached to the lateral wall of the internal ear at the oval window. Also the three bones amplify the vibration 10-100 folds to overcome difference in resistance between the air vibrations (tympanic vibration) to the fluid vibration (inside the cochlear). In addition the amplification helps in the difference in the surface area between the tympanic membrane (big) while the oval window is much smaller. Inner ear: Contain Cochlea that s filled with fluid, have 2 windows: Oval window (connected with stapes) rounded window: Vibrations enter through the oval window and exit through the rounded window
Inside the cochlea we have compartments, it s divided into three compartments two connected with each other s but the middle is separated and has different fluid. The content is different in the outer we have perilymph high in Na+ and low in K+ The middle part have endolymph high in potassium K+ The sensory organ of hearing is Organ of Corti and composed of hair cell and tectorial membrane and basilar membrane.
How vibration (mechanical energy) converts to sensory perception (electrical)? Vibration in the cochlear fluid will move with the basilar membrane up and down and the hair cell that attached to the basilar membrane to be pushed against the tectorial membrane function of the hair cell: they are mechanical receptor converts mechanical to electrical signal, if the hair cell move to the big hair they will produce depolarization if the cell moves toward the short hair it will produce hyper polarization. The reason is that on the hair there is a mechanical ion channels (gates) which will open/close depending on the direction the hairs bend to.
When bending to the long ones the channels will open when bending toward the short ones the channels will close. The channels is for K+ ions (it s located in the endolymph so there s high K+ concentration which will allow the K+ to enter) When K+ enters there will be elevation of the membrane potential, causing polarizing that will open voltage gated calcium to cause exocytosis. Every sound wave (vibration) will produce excitation and inhibition to the hair cell because sound wave will make an up and down like a wave, if the wave was slow it will make slow vibration. But that doesn t make us recognize all the sounds because normally human recognize frequency as low as 20 Hz to as high as 20 KHz. Main differentiation of wave frequency is by basilar membrane. Basilar membrane presents inside the cochlea and is attached in one side (the beginning of the cochlea) and free on the other (the end of the cochlea). Also its base is narrow and the apex is wide. When the vibration moves in the cochlea it moves the free edge.
So when a low frequency sound enters, the apex part will move only and the hair cell in this part only will get excited. But high frequency sound will move the base of the membrane and the hair cell in this part only will get excited. This called place differentiation of frequency, that s mean depending on the site of the hair cell that have been stimulated the mind will know the exact frequency. The first order neurons in the auditory system is the bipolar cells cell body of the bipolar exist in the spiral ganglia Bipolar cells start from the organ of corti (take information from hair cell) and terminate to give the cochlear ganglion. The cochlear ganglion divided mainly into 2 parts: Posterior cochlear nucleus Anterior cochlear nucleus (bigger and more important) Because the main feature of the sound is frequency this information must be separated and preserved in the same order along the way to the cortex, that s way all the parts of the cochlear nucleus and the cochlear nerve will have the frequency separation until we reach the cortex.
When information get out of the cochlear nucleus : Mainly we have 2 pathways for hearing: Monaural: will take the information from one ear Binaural: information will come from both ears Bi pathway The information will come out of the anterior cochlear nucleus and go to the superior olive. The superior olive is divided into: medial and lateral. When I say 2 ears I want to compare between the two, I do that for knowing the plane and orientation of the sound especially in the horizontal plane. We said that the auricle gives us some information about the orientation of the vertical plane and vertical axis.
Superior olive help to determine the location of the sound in the space: Medial part: compare depending on the time difference between the two ears, used mainly in low frequency Lateral part: depend on difference in intensity, used mainly in high frequency. Medial superior olive complex differentiate the orientation When a sound comes from the right side it will reach the right ear before the left one Coincidence detection of the superior olive: we have a set of difference of time when the difference as low as 10 microsec a neuron will fire; if the difference is 20 microsec here the next neuron will be firing 30,40,50,60. (explained in the next page and the video) http://auditoryneuroscience.com/topics/jeffress-model-animation Lateral part: depend on difference in intensity (also coincidence) because the ipsilateral side will be activated directly from the cochlear nucleus while the contralateral part will be inhibited via trapezoid body nucleus. when we say intensity, if the right ear s intensity is bigger than the left, activation will happen to the right and inhibition will happen to the left
Notice that: because the sound is from front it will arrive to both ears at the same time But here it will arrive to the right first and the right ear will produce AP first The neural coincidence detector (NCD) receive inputs from right and left ears, and when there is a sound both ears produce AP that input to all the NCD but only when both AP from the right and the left ears hits the NCD simultaneously the NCD make AP (it depend on the summation of the left and right ears AP to reach threshold)
Trapezoid body: inhibitory relay nucleus between information of the ipsilateral and contralateral side that comes to lateral superior olive. From the superior olive, auditory pathway will continue to lateral laminuscus >>> inferior Colliculus >>> medial geniculate nucleus >>> end in primary auditory cortex. This is all Bi pathway: But there is information I don t want it to be Bi: Because in Bi the lateral superior olive compared between the two intensities (coincidence) and give the cortex the difference, but the brain also need the absolute intensity to determine if the sound is loud or not. In addition the intensity overtime is important because the sound is not one frequency, the complex sound is frequency over time and the frequency differs with the every syllabus. Frequency is preserved because we stayed the same (preserved) line from the basilar membrane to the cochlear nucleus to all the parts of the pathway.
Q: If the sound is from the front exactly what will happen? It will reach the brain with the same intensity from the two side but I don t only care about difference in intensity, I also care about intensity over time, so Bi pathway is good but it doesn t give all information, it will not make me differentiate between 3 syllabus that s why I need other pathway to give me intensity over time. Now the superior olive compares the intensity and that s good. But now regarding the intensity over time this is done by monaural pathway which mainly originate from the dorsal cochlear nucleus and bypass all these structures even the lateral laminiscus and go to the inferior colliculus and from there it goes to the medial geniculate nucleus and from there it goes to cortex. This is the difference between the monaural and binaural. Good Luck DONE BY: AMMAR AL-HABAHBEH