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Hearing system
Physiology lecture # 45
8th.May.2011
last lecture we started talking about the hearing system, and we know that what we hear are sound waves which have pressure "intensity" as they press the air while moving through it, and since the pressure is directly proportional to the resistance, the movement of the sound in the air doesn’t require as much pressure as required if the sound is being transmitted through fluid since the resistance of air is less than the resistance of fluid.
- this is the main principle of the tympanic membrane function in concentrating the sound waves "increasing their intensity" and contributing to the Impedence matching . *discussed shortly*
*slide 5 / page 3
- intensity<=> pressure of the sound "this is at the scale of loudness measurement becauseactually Intensity=(pressure)2"
- Frequency : how many waves occur in one second.
- amplitude = intensity in dB
*slide 5,6 / page 3
two different frequencies with the same intensity; different amplitude.
-Intensity is a logarithmicmeasurewith a unit of (dB), and we use the "Log" to get rid of the power thus increasing the intensity scale which is (Zero-120dB).
-Decible : is a measure of sound intensity, note that its is more convenient to measure the sound pressure rather than the sound intensity to give an indication of the decibel or the loudness.
*Remember that sound intensity = (sound pressure)2 => but both give an indication of the sound loudness that’s why we use them interchangeably, so we use either formula but as we said using the pressure is more convenient :
dB = 10Log(I/IR) or dB = 20Log(P/PR)
-"PR" the relative pressure => it’s the threshold for the sound meaning the least amount of sound that you can hear.
- we know also that the frequency of sound that we can hear is between 20-20000Hz , but note that a frequency of the appropriate range is not enough to hear the sound, we also need an appropriate intensity of the sound wave in the range of (Zero-120dB) to hear it.
- since the intensity is the threshold it decides weather you can hear the sound or not while the frequency determines the sound pitch.
structures of the ear
- External ear
1-ear pinna
2-audotary tube
3-tympanic membrane
- Middle ear => 3 ossicles
1-Malleus; which lies on the tympanic membrane.
2-Incus
3-Stapes; which lies on the oval window (a membranous structre the closure of scala vestebuli of the cochlea)
4-two muscles :
-tensor tempani: attached to the malleus which lies on the tympanic membrane, when it contracts its pulls the tympanic membrane and the malleus inward.
-stapedius: attached to stapes which lies on the oval window , when it contracts it pulls stapes outside.
> the contraction of both muscles causes the ossicles to be so rigid as they become very close to each other making them harder to be vibrated, this occurs as a protective mechanism when there is a very loud sound that may injure the cochlea "Attenuation reflex" as the rigidity of the ossicles help attenuating the conduction of loud sounds to the cochlea via them by the prevention of their vibration , this reflex also occurs when your are speaking because you don’t need to hear what you are saying in order to understand what your talking about "its already represented on the cortex" so that your sound wont interfere with your own thoughts.
- Inner ear
1-oval window : which stapes lies on (closure of scala vestibuli)
2-Round window : where the sound gets dissipated after conducting through the entire cochlea.
3-three scala
-scala vestibuli
-scala media
-scala tympani
* scala vestibuli is separated from scala media, by Rissener's membrane (vestibular membrane) which is very thin.
* scala media is separated from the scala tympani by the Basillar membrane.
* on top of the Basillar membrane is the area for the receptors of hearing which make up whats called (Organ of Corti).
* Vestibular membrane is so thin and has no resistance so any vibrations of the fluid in scala vestibuli are going to be conducted through the vestibular membrane to the scala media causing the basilar membrane to vibrate up and down.
*components of these Scala:
1 - both scala vestibuli and scala tympani contain a perilymph similar to the ECF => rich in Na+
> if we stretch the cochlea we will figure out that scala tympani is actually a continuation of scala vestibuli at an area know as Helicotremia justifying the fact that both of them have the same kind of fluid.
2- scala media contains an endolymph similar to ICF => rich in K+
Impedance Matching
matching the pressure of sound moving in the air with the pressure of sound in the fluid of cochlea, this occurs by the amplification of the intensity of the sound in the air by a factor of 22 times to match the intensity in the fluid which is high due to the higher resistance of it than the air, two contribution to this mechanism occurs:
1- a factor of 17 times results as the external ear "tympanic membrane" which has a larger surface area compared to the stapes and the oval window, so it acts to collect the sound waves and then concentrate them at the oval window increasing by that their intensity( x17 times).
2- a factor of 1.3 times result as the middle ear ossicles act as a leaver system to increase intensity ( x1.3 times).
> 1.3x17 = 22 times.
> by this mechanism sound waves can move the fluid inside the cochlea.
* Now before we continue check slide 25 / page 13 and note :
1-the area of the cochlea at the oval window is called the base.
2-the area of the cochlea at the helicotremia is called the apex.
3-Bassilar membrane which has the hearing receptors on top of it "organ of corti" all through the way extending from the base to the apex.
4-Bassilar membrane is narrow at the base area and large at the apex.
5-Bassilar membrane has lots of fibers (about 30000) which are short and thick at the base but long and thin at the apex, these fibers don’t vibrate except with certain frequencies:
- fibers at the base vibrate with high frequency sounds.
-fibers at the apex vibrate with lowfrequency sounds.
-fibers at the middle moderatefrequency sounds.
*Note that as we move from the base to the apex the basilar membrane gets larger the fibers gets thinner but longer.
*Organ of Corti
-it is supplied by both afferent and efferent fibers from vestibulocochlear nerve.
efferent fibers are corticofugal fibers which to increase sensitivity, intensity and suppression inhibition.
-consists of 3 rows of outer hair cells about (12000 cells) and one row of inner hair cells (4000cells) supplied by both sensory and motor (efferent and afferent) these are the receptors for hearing .
> these cells have hair like structures at their apices "hair bundles" short ones are called (steriocillia) and long ones are called (kaniocillia)
on top of these hair bundles there is another membrane called the tectorial membrane a little bit heavy.
*Note that:
-Auditory signals are transmitted by the inner hair cells to =>afferent fibers
-outer hair cells may control the sensitivity of the inner hair cells for different sound pitches => receives efferent fibers (corticofugal)
*Hearing process
1- sound waves are collected by the tympanic membrane then transmitted to the malleus which is attached to the membrane -as to the other two ossicles- causing them to vibrate with the same frequency.
2-as these vibrations reach the stapes it vibrates inside and outside through the oval window causing the fluid inside the scala vestibuli to vibrate.
3-as we know the vestibular membrane is thin and has a low resistance thus scala vestibuli fluid vibrations are going to be conducted through this membrane to the scala media and then these vibrations will cause the basilar membrane to vibrate up and down.
4-as the basilar membrane vibrates upward that causes the steriocillia of the inner and outer hair cells to touch the tectorial membrane then bend to one side , when they do so they get stretched that’s why in fact hearing receptors are stretch receptors, this stretch causes the opening of Stimulus gated channels then depolarization inside the cells.
*Note before that the ions involved in the receptor potential of these cells are different than the ions involved in other receptors , here they are K+ channels rather than Na+ channels, so they Stimulus gated K+ channels.
> the explanation:
as we know the hair cells in organ of corti of the basilar membrane are surrounded by an endolymph which is similar to ICF rich in K+ rather than an ECF , so the membrane potential difference between the ICF outside the cell and the ICF inside the cell is around -150mV.
so if there is depolarization there is an influx of K+ ions from the endolymph to the inside of the cells until K+ reaches its equilibrium potential (-90) while hyperpolarization involves the efflux of k+ ions.
5-depolarization in the form of "generator/receptor potential" because these cells are receptors you wont have an actual action potential ! so this depolarization will cause the release of Neurotransmitters (the one involved here is mostly glutamate) which will act on the afferent fibers causing action potential which reaches specific areas on the cerebral cortex "temporal lobe".
*Remember that glutamate in the visual system is inhibitory while here in the hearing system its excitatory , so excitatory or inhibitory is determined by the receptor rather than the N.t .
6-afterwards the vibrations continue to travel in the cochlea through the scala tympnani until it reaches the round window where the sound waves are going to be dissipated.
*Note that the round window is continuous with the nasopharynx through the Eustachian tube and this is important in equilibrating the pressure between the ear and the nasopharynx.
*Determination of sound frequency and amplitude "sound intensity"
we have three mechanisms to tell our system what is the frequency of the wave sound and another one to determine intensity
- determination of frequency
1- when the basilar membrane vibrates upwards depolarization occurs and when it goes downwards repolarization occurs and so on, its like an (on/off) mechanism, a cycle of continuous depolarizations and repolarizations that can be used to calculate the frequency of sound waves (how many de/re we get).
2-Place principle theory : frequency can be determined acoprding the area of the basilar membrane that is stimulated.
we already know that if afferent fibers that are stimulated are coming from:
-Base => High frequency.
-Apex => Low frequency.
-In between => moderate frequency.
*note that the tonotopic organization in the hearing system starts from the basilar membrane . discussed shortly
3-Phase-locked (volley) :
we know that sound waves have different phases to fit an area but with different discharge rates. **
> at each phase there is a stimulation of a certain area , and this mechanism occurs specially in low frequencies in the range of 20-200Hz but this is a very small area , in this case differentiation depends on the different rates of discharge at that area.
** ok now me my self I didn’t get exactly what the doctor was trying to explain here o.O' , so if u understand it this is perfect ,and for those who still have some trouble absorbing the principle you will find what's written in guyton about the whole "frequency determination" thing at the last paper of this sheet , I recommend you to read it both way in order to ensure the information :D
- determination of intensity
the amplitude of the wave "intensity" is determined by how far the basilar membrane is displaced according to the frequency of the sound waves and the number of nerve fibers stimulated.
*Tonotopic Organization of the hearing system
you remember the retinotopic organization of the visual system => each site on the retina has its own site on the cortex, meaning that the information that comes from different sites via the afferent fibers go to specific area on the cerebral cortex.
tonotopic organization of the hearing system is similar to that (each tone has its own site on the cortex) adding on top the fact that this tonotopic organization starts from the basilarmembrane till it reaches the cortex because as you know each frequency has its own site on the basilar membrane it self even before reaching the cortex.
*Threshold of hair cells
remember if we are concerned about threshold then its intensity we are talking about here rather than frequency.
> now look at slide 31 page 16
* Note that
-threshold is at its lowest value around 2000-3000Hz frequency => so you can hear the Best around that range.
-lower frequency sounds have higher threshold so they are harder to be heard ; you need the sound to be higher in terms of dB to hear it.
*Central Auditory pathway
you should know that the pathway for hearing is very special, different and more complicated than vision because of:
- what you hear by one ear goes to both cortexes .
- starting from the basilar membrane fibers of vestibulocochlear nerve leaves the ear and then enter at the junction between pons and medulla , then goes to both dorsal and ventral cochlear nuclei at the same side.
> so if you have a patient that has a hearing deficit and disequilibrium and he has a tumor , you should expect that this tumor will be at the junction between pons and medulla.
- from both dorsal and ventral cochlear nuclei fibers go to both right and left Superior Olivary nucleus (1st sight of crossing).
- from each sup olivary nucleus fibers ascend upward to reach both right and left Inferior colliculi (2nd sight of crossing).
> remember the importance of the colliculi is moving the head in response to sound.
- from both inf colliculi to the medial geniculate nucleus at the same side (remember in vision to the lateral geniculate nucleus).
- from medial geniculate fibers to the cortex.
*** after this explanation the doctor read the two slides page 18 make sure to read em.
*Note that : the area around the primary auditory area is called the association area which helps to explain and understand the meaning of the sound upon past saved experiences , any lesion will defiantly lead to word deafness.
*Determining the Direction of sound
it occurs at the level of sup olivary neucleus as follows:
- Medial Olivary Neucleus detects direction by the time lag between sound waves entering the tow ears as the ear closer to the sound wave side receives it before the other one.
- Lateral Olivary Neucleus detects direction by the difference in sound intensities between the tow ears.
Finally please keep in mind that starting from the basilar membrane all through the way till reaching the cortex the hearing system is tonotopically organized "the doc kept repeating this sentence in the lecture so its pretty important don’t forget it :D"
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Finally the sheet is done al 7amdulelah :D I hope it has been easy for all of you to study it and plz sam7oni if there are any mistakes and if you’ve got any corrections plz share with us they are more than welcomed :D
I want to dedicate this sheet to
-all of our great doctors, thank you very much !
-to my sweet friends Enas and Tasneem I luv u both <3
-to all those who has been working so hard for our own good since the beginning of this year.
-to all of you my colleagues , study hard ! we r almost done :D
ed3oleeee <3 mwf2een jme3an ^_^
Done by ur colleague
Furat Atharbeh
If nothing changed, we wouldn't have butterflies, would we? Appreciate change ; it is the only thing that creates progress.
:)
**Guyton "an extra reference"
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Determination of Sound Frequency—
**The “Place” Principle
From earlier discussions in this chapter, it is apparent that low-frequency sounds cause maximal activation of the basilar membrane near the apex of the cochlea, and high-frequency sounds activate the basilar membrane near the base of the cochlea. Intermediate frequency sounds activate the membrane at intermediate distances between the two extremes.Furthermore, there is spatial organization of the nerve fibers in the cochlear pathway, all the way from the cochlea to the cerebral cortex. Recording of signals in the auditory tracts of the brain stem and in the auditory receptive fields of the cerebral cortex shows that specific brain neurons are activated by specific sound frequencies. Therefore, the major method used by the nervous system to detect different sound frequencies is to determine the positions along the basilar membrane that are most stimulated. This is calledthe place principle for the determination of sound frequency.
**The "Volley" principle
the distal end of the basilar membrane at the helicotrema is stimulated by all sound frequencies below 200 cycles per second. Therefore, it has been difficult to understand from the place principle how one can differentiate between low sound frequencies in the range from 200 down to 20. It is postulated that these low frequencies are discriminated mainly by the so called volley or frequency principle. That is, low frequency sounds, from 20 to 1500 to 2000 cycles per second, can cause volleys of nerve impulses synchronized at the same frequencies, and these volleys are transmitted by the cochlear nerve into the cochlear nuclei of the brain. It is further suggested that the cochlear nuclei can distinguish the different frequencies of the volleys. In fact, destruction of the entire apical half of the cochlea, which destroys the basilar membrane where all lower-frequency sounds are normally detected, does not totally eliminate discrimination of the lower-frequency sounds.