 auditory pathway starts from hair cells which are present in cochlea. These hair cells are of two types that is the outer hair cells and there are inner hair cells and both of them receive the afferent and efferent. However, major afferents come from inner hair cells and the major efferents go to the outer hair cells but both have the efferents and the efferents. Now the afferents from these outer and inner hair cells form the auditory component of the vestibulo cochlea nerve and these are basically the dendrites of the bipolar neurons which are present in the spiral ganglion. These are bipolar neurons because they have a long dendrite process and then there is axon which goes to the medulla. So these afferents form the auditory component of the vestibulo cochlea nerve. Now this spiral ganglion is also present in the cochlea where it is present it is present in the modulus of cochlea. Fine. Now at medulla they actually synapse all the fibers of this axon synapse and they synapse in nuclei in the medulla known as ventral and dorsal cochlea nuclei. Ventral and dorsal cochlea nuclei so first synapse is occurring in the cochlea nuclei. From ventral and dorsal cochlea nuclei there is diversion of the pathways and the auditory pathway goes in four direction. So this is the ventral and dorsal cochlea nuclei. From here most of the pathways actually go and synapse on the nuclei present in the medulla and this is the superior olivary nuclei. Again superior olivary nuclei are two there is medial and lateral superior olivary nuclei. So most of the fibers go synapse on the opposite side. So suppose this is the midline okay and I am drawing pathway of only one side remember. So there will be ventral and dorsal cochlea nuclei this side also. For simplicity I am drawing only one side. So remember most of the pathways travel on the contralateral side going to the superior olivary nuclei. Then some fibers also go to the ipsilateral superior olivary nuclei so that is the second pathway. Third from here some of the fibers directly go to the nucleus of the lateral leminescus nucleus of the lateral leminescus and here it forms the the bundle of fibers forms a lateral leminescus. So that is the third pathway and also some of the fibers go to the reticular formation reticular formation in the pons. So all these group of fibers have different processing going on remember that is why there is so much diversion of the pathways. We will see what is the processing little bit later. Let us move on further with the auditory pathway. So from superior olivary nuclei then again the fibers ascend and they reach to another nucleus of the midbrain that is the inferior colliculus. Again from here they reach to the inferior colliculus okay and here also nucleus the fibers which have synapse in the lateral leminescus they also reach to the inferior colliculus. So these fibers which reach to the inferior colliculus they are third order neurons okay. First order neuron synapsed in the ventral and dorsal cochlear nuclei second order neuron mostly the synapses is occurring in the superior olivary nucleus and from there the third order neuron it is reaching to the inferior colliculus. From inferior colliculus these fibers reach to the thalamus and there is nucleus medial geniculate body. So there again there is synapsing of the neurons and from medial geniculate body there are auditory radiations okay auditory radiations and then from there they reach to the auditory cortex and where is this auditory cortex present? It is present in the superior temporal gyrus. Okay with this now let us see what is going on at different levels in the auditory pathway. You see in the cochlea what is going on in the inner hair cells there is basically discrimination between the pitch and loudness okay. So in the cochlea along the length of the cochlea there is basically the fibers which respond to different pitch at the base of the cochlea the fibers respond to higher pitch and at the apex of the cochlea the fibers respond to lower pitch. So at the level of the cochlea itself there is coding going on for the pitch of the sound and loudness of the sound for this I have made another video that is theories of hearing for pitch discrimination you can have a look on that. So from the cochlea that discrimination between the pitch has started and the loudness is also coded. So this information reaches to the ventral and dorsal cochlear nuclei. Now from the ventral and dorsal cochlear nuclei the fineness between this pitch discrimination starts. So here itself there is something known as lateral inhibition okay. So all the fibers are reaching there but you might be aware that in sensory system lateral inhibition is one concept which is important for increasing the contrast between the different sensations coming from different areas basically. So information from one area inhibits the information from just a nearby area so that is lateral inhibition that fineness of information starts in ventral dorsal cochlea nuclei especially for pitch discrimination and why it is traveling to so many different areas you see the information of pitch directly travels to the lateral discrimination. So here whatever fineness of information has happened that goes directly to the lateral lemniscus but the information also goes to the superior olivary nucleus where now the second level processing starts that is for the localization of the sound localization of the sound. So what happens you see that this superior olivary nucleus receives information from both sides okay. So from this side also the information will cross and reach to the opposite superior olivary nucleus. Now depending on from where the sound is coming say suppose the sound is coming from the center both of these superior olivary nuclei will receive the information together. However if the sound is coming from say somewhat left side what will happen that left ear superior olivary nucleus will receive same information little bit earlier while right superior olivary nucleus will receive same information little bit later and we are saying that the information is going to the opposite side also. So from left ear this superior olivary nucleus will receive the information first and from right ear same superior olivary nucleus will receive the same information little bit later. So there is something known as time lag okay and there is also intensity lag okay because the sound has to travel little bit more distance so little bit less intensity happens from the information which is reaching from the ear of the opposite side. So this information that is the difference in the time lag and difference in the intensity lag reaching to the superior olivary nucleus helps in localization of the sound. Again a detailed video on this localization of the sound I have already made you can have look on that as well. Then moving on further I said that information is also going to the reticular formation and what is reticular formation responsible for? Reticular formation is important for activation of the nervous system because from here information goes up also and there is generalized information which is going to activate the nervous system and goes down into the spinal cord as well. So your awake state alert state it is caused by activation of the reticular formation. So our responses to loud sound which make us alert to the sound. Suppose if we are sleeping and there is some loud sound suddenly we wake up that is being caused by the information which is going to the reticular formation fine. Next going into the inferior colliculus that is the tectum what is happening here. See suppose you are focusing somewhere you are maybe reading or watching this video suddenly you hear a loud sound somebody claps and you turn your head towards that loud sound that is happening because of the information going to the inferior colliculus. So our head and eye movements which occur towards the loud sound for that the reflex movement for that this tectal information is important. Then the information goes to the medial geniculate body and relays here and you know that thalamus is the relay center because lot of information is being communicated within the thalamus itself and lot of processing is going on here and why is it important thalamic relay center? Because it is the gateway of the information the information how much information is going to the auditory cortex and how much information is also coming from the other sensory systems or what is the state of the cortex that all decides that how much auditory information is going to the cortex say suppose for example you are sleeping in that case you see the responses to loud sound are intact because the information is going to the lower centers as well but how much you are able to appreciate the intensity of the sound while sleeping it will be very difficult so initially maybe you might become aware of the sound you will become awake and there will be postural movement but actual discrimination of the sound you cannot make because the information has not gone to the auditory cortex that well because our gateway has little bit to stop that information some processing has happened there so that is the mean thalamic nuclei function then the information goes to the auditory cortex now in auditory cortex there are tonotopic maps what are these tonotopic maps as we know that in other sensation also there are maps which are basically for the localization of the sensation from different areas so in auditory system there are tonotopic maps that is the maps which are based on pitch okay and mostly the maps which are for low frequency they are basically arranged laterally and laterally actually and then as we move posterior medially the maps are for high frequency so there is arrangement like this for the maps and why i'm saying maps are not a single map because actually there are six maps in the auditory system six maps and why do we need six maps well there are different maps for different characteristics of the sound for example we need to find about the pitch okay then we need to localize the sound then we need to find out how complex the sound is so all that we have six different maps in the auditory cortex then from this auditory cortex which is basically the primary auditory cortex which is the area number 41 and 42 the information also goes to association area association auditory cortex and all the association areas of the cortex basically receive information from other sensory areas and that is very important for interpreting any information because from the environment we interpret information in a holistic way we don't separate all the information okay this is the sound this is the vision when we are supposed watching a movie everything we interpret in a holistic way so that is going on in the association area and very important in case of audition is the verni case area which is important for interpreting the meaning of the sound so that is the fundamental of the auditory pathway which goes from bottom to the top but there are some top-down pathways as well so before going into that little bit just we will summarize this so what is happening from outer hair cells and inner hair cells there is the auditory component of the vestibulo cochlear nerve which is the bipolar cells located in the spiral ganglion in the cochlea then information goes to the ventral and dorsal cochlear nuclei which is present in the medulla then there is crossing over lot of crossing over most of the fibers cross over to the superior olivary nuclei of the other side again present in the medulla some fibers go to the superior olivary nucleus of the same side from here information is going to the inferior colliculus which is present in the mid-brain okay and that is via the later leminescus pathway that is the some fibers here also go to the nucleus of the later leminescus some are basically synapsing and here is the later leminescus then from inferior colliculus the fibers go to the medial geniculate body okay so that is medial geniculate body that is located in the thalamus and then from here the fibers are going to the auditory cortex so this is a simplified auditory pathway and what is the mnemonic to remember it i remember it like collic m a collic m n what is this there is cochlear nuclei c o is cochlear nuclei o l is olivary nucleus superior olivary nucleus before it because it is coming first so i remember it as a superior superior olivary nucleus then there is later leminescus l l then there is inferior colliculus and m is the medial geniculate body a is the auditory cortex so that is a mnemonic to remember the auditory pathway let's go into some more details about the auditory pathway that is the top down pathway some information is also coming from top to down and why it is important again i'll give you the example of focusing suppose you are focusing on something watching movie or reading what happens during that time our sensitivity to other sounds diminishes so you can try this basically sit in an environment where fan is on you will hear the environmental sounds as well so try to focus on the environmental sounds now try to focus somewhere else where you start reading say suppose and you will notice that the other sounds basically have disappeared now you are not able to even listen to that sound so how it is happening that is because of some thalamic processing yes and also because of some top down pathway which is causing the information to change which is coming from i h c n o h c so this outer hair cell basically there is change in the length of this outer hair cell and because of this change in the length of the outer hair cell the sensitivity of the basilar membrane of the cochlea to respond to sound changes then also the efferents which are going to the inner hair cell from here it basically it impinges on these afferents only and this decreases the response of this afference to the stimulus so this is top down pathway very important for sensitivity to various sounds in our environment we are basically living in a noisy world so we need to choose where we focus and which sounds we should hear however any dangerous sound i said that any dangerous loud sound even then your cortex can respond because of the activation of the reticular formation fine now we will end the session with some lesions if you have understood the auditory pathway you will understand that if there are lesions at various places what can be the problems so suppose there is lesion of this hair cells what will happen see not all the hair cells will die immediately there will be progressive loss of the hair cells and that happens in old age or when there is consistent exposure to loud sounds so in that case also there is hair cell damage so there will be progressive hearing loss progressive hearing loss okay so that is one then what will happen if there is damage to the ventral and dorsal cochlear nuclei in the medulla the information will not go further isn't it so there will be hearing loss okay then what will happen if there is damage to the superior olivary what is the function of superior olivary nuclei the processing for the localization it starts here right so this superior olivary nuclei if that is damaged we will have problem with the localization of the sound but can but can we determine the pitch and loudness of the sound yes we can determine because this is another pathway which is going via the lateral leminescus and synapsing in the nucleus of the lateral leminescus and reaching to the inferior colliculus so yes we can determine the pitch and loudness but we will not be able to localize the sound properly because the localization pathway travels from here to the auditory cortex and has a different map so that is not happening then what will happen if there is a damage to the auditory cortex will we able to hear yes if there is damage to auditory cortex or one side only you see we are telling that most of the fibers cross and reach to the auditory cortex so we will be able to hear properly only little bit hearing loss might be there but otherwise not much problem happens because auditory information travels bilaterally understanding however we will not be able to localize the sound properly there will be some problem in determining the complexity of the sound differentiating very fine differentiation of the pitch that may be problem yes somewhat crude differentiation we will be able to do the responses to the sound will be intact the reflex responses to the sound will be intact so hearing loss full hearing loss will not occur when there is damage to the auditory cortex and what will happen if there is damage to the varni case area that is then interpretation of the speech problem for that again I have made another video please have a look on that as well so that was all about the fundamentals of the auditory pathway thanks for watching the video if you liked it do press the like button please share the video with others and don't forget to subscribe to the channel physiology open thank you