Structure of Ear & Mechanism of Hearing | Nervous System Part 8 | Biology Class 11 | Hindi


topic of this presentation is Structure of Ear & Mechanism of Hearing let’s first discuss structure of ear in brief External ear, middle ear and internal ear- we divide ear in these three parts pinna, External auditory canal (meatus) and tympanic membrane these are three parts of external ear Eustachian tube and three ear ossicles together form middle ear bony labyrinth and membranous labyrinth – these two parts together constitute internal ear we will discuss these parts in detail in the presentation ahead before that we will learn some more important facts related to ear Internal ears evolved first and occur in all vertebrates from Pisces to Mammals. Then evolved the middle ears that occur from Amphibians to Mammals but not in Pisces. And lastly evolved the external ears that occur in Mammals only. it means all those animals who have pinna – external ears – are mammals but exceptions are there Aquatic Mammals do not have external ears Whale, dolphin and porpoise are some of the aquatic Mammals we will study different parts of ear with the help of this diagram by now we have learnt that pinna, auditory canal and tympanic membrane together constitute external ear 3 bones – malleus, incus and stapes are shown in middle ear and this eustachian tube also lies in the middle ear this is the cavity of middle ear called tympanic cavity this eustachian tube connects tympanic cavity with oral cavity and equalizes the air pressure between these two cavities you might put some ear drop in the ear and have felt the bitterness of that medicine in the mouth perhaps you might be surprised that you put medicine in the ear and why you felt bitterness in the mouth we feel this because of eustachian tube some medicine comes in the oral cavity through eustachian tube, the bitterness of which is felt in mouth here we have shown internal ear It consists two main divisions this is spiral shaped cochlea and this is the vestibular apparatus we will discuss this vestibular apparatus in detail ahead there are two window like structures in the internal ear this oval shaped window is called oval window and circle shaped window is called circular window internal ear communicates with middle ear with these two windows these are the parts of ear, we will discuss more details about these now we will summarize some important points related to external ear these points are important if you are preparing for some medical entrance tests first, Pinna consists of elastic cartilage many a time questions related to this point have been asked in PMTs next is, External auditory canal (meatus) lies in temporal bone of skull The skin of external ear has (ear) wax secreting sebaceous glands some chemicals in this wax prevent growth of pathogens and in this way prevent various infection in our ear now we will discuss about Three Ear Ossicles found in the middle ear with the help of this diagram Three ear ossicles are shown in the diagram – malleus , incus and stapes one important point here – stapes is the smallest bone of our body observe the location of these bones in the middle ear malleus bones in contact of tympanic bone and lies just against the tympanic membrane or ear drum whereas this stapes lies against oval window of internal ear in this way these three bones are situated in the middle ear here in this diagram we hvae shown only these three bones we compare malleus with hammer it resembles the shape of hammer that’s why compare malleus with hammer we call incus ‘anvil’ you must have seen this device at shoemaker’s shop incus resembles the shape of this device and stapes is also called stirrup we have shown a stirrup here people use it as support to get upon a horse and people rest their feet in these stirrups while seated upon horse back these are some of the important points related to Three Ear Ossicles one more important point here the joints between these ear ossicles are synovial joints it could be an important piece of information if you are preparing for PMTs now we will discuss about structure of internal ear by now we have learnt that internal ear consists of two components bony labyrinth and membranous labyrinth Bony labyrinth is a series of bony cavities in the temporal bone of skull highly delicate structure called membranous labyrinth lies in the cavities of this bony labyrinth here we are repeatedly using this term labyrinth labyrinth means maze internal ear is highly complicated structure that’s why we call it labyrinth Membranous labyrinth consists of two important components – vestibular apparatus which is a balancing organ and cochlea it is a hearing organ we further divide vestibular apparatus into two components three semicircular canals and an otolith organ we must tell you meaning of ‘otolith’ ‘oto’ OTO oto it means something related to ear and ‘lith’ means stones crystals of calcium carbonate are found in this organ that’s why this organ is aka otolith organ we further divide otolith organ into two parts – utricle and saccule we will discuss in detail all these components, so ,these are the components of internal ear we will understand components of internal ear in detail with the help of this diagram before starting this discussion, I want to tell you one more point that two types of fluids are found in internal ear perilymph and endolymph – characteristics of these fluids is a bit diffrent we will not discuss those differences for now know this thing that two types of fluids occur in internal ear perilymph and endolymph so far we have discussed that it is a bony labyrinth and we can this highly delicate structure called membranous labyrinth these are two important components of membranous labyritn – cochlea which a hearing organ and this whole structure is called vestibular apparatus we further divide vestibular apparatus in two parts we can see these three semi circular canals and this structure is called otolith organ two components of this otolith organ are utricle and saccule if you observe these three semi circular canals carefully all these semi circular canals are situated at right angle to each other these are perpendicular to each other one end of each semi circular canal is swollen and this swollen end is called ampulla these are some of the components of internal ear here we shown a highly simplified diagram of this internal ear let’s do a quick revision of components of internal ear this this whole internal ear and it is divided into cochlea and vestibular apparatus these are three semi circular canals in vestibular apparatus and this is the otolith organ this is the utricle part and this is the saccule part of otolith organ observe carefully, semicircular canals are connected to utricle whereas cochlea is connected to saccule so this is the structure of internal ear when this structure is clear to us, you will ahead that learning mechanism of hearing is a cakewalk we have just discussed the structure of internal ear and we have known that cochlea and vestibular apparatus are two important components of the internal ear and cochlea is a coiled structure now visualize that if uncoil this coiled structure, we unfold it then we will get a stretched tube like structure and if we see its longitudinal section then we will get this structure now let’ do quick revision this is an uncoiled cochlea this is saccule this is utricle and these are semicircular canal this is the oval window and this is round window these tow are components of internal ear now we study longitudinal section of cochlea we find three tube like structure in cochlea we call the upper most tube like structure, scala vestibuli the middle one is called scala media and the lower one is known as scala tympani we told you about two types of fluids scala vestibuli and and scala tympani are filled with perilymph whereas scala media is filled with endolymph scala media is filled with endolymph there is narrow passage like structure at the end of cochlea which we call helicotrema helicotrema connects these two tube – Scala vestibuli and scala tympani now see the membranes found between these tubes Reissner’s membrane lies between scala vestibule and scala media whereas basilar membrane separates scala media from scala tympani an Organ of Corti rests on the basilar membrane we have shown here Organ of Corti in a very simple way we have drawn these small line in the organ of Corti – these are the hair cells which are in the contact with this tectorial membrane so, this is the structure of longitudinal section of cochlea here we have shown a section of organ of Corti these are the hair cells and these are their stereo cilia which touch the tectorial membrane if the diagram is so clear to us then we will understand the mechanism of hearing easily now we will study transverse section of cohlea when we observe transverse section of cochlea then we see these three cavities upper most cavity is scala vestibuli middle one is scala media and the lower most is called scala tympani Reissner’s membrane lies between scala vestibule and scala media whereas whereas basilar membrane separates scala media from scala tympani. a hearing organ is found on the basilar membrane which is called organ of corti we can also see the tectorial membrane just above the organ of corti so this is the transverse section of cochlea now we will discuss Mechanism of Hearing with the help of this diagram these are the sound waves shown in the diagram which hit the tympanic membrane and generate the vibrations in the tympanic membrane and these vibrations reach oval window through ear ossicles and now oval window starts vibrating these vibrations will now proceed ahead and will reach perilymph of the scala vestibuli as a result, perilymph will start vibrating now these vibrations will be picked up by Reissner’s membrane these vibration will proceed ahead and reach scala media and endolymph will start vibrating when endolymph vibrates then the stereocilia of the hair cells these start rubbing against the tectorial membrane the stereo cilia rubbing against the tectorial membrane due to vibrations in the endolymph then action potential is generated and impulse is generated this nerve impulse now reaches a particular region in our brain through cochlear or auditory nerve and brain analyses that impulse and tells us what er are hearing I must tell one more point here whatever vibrations are generated in the fluids these create a kind of pressure in the fluids of the internal ear and to release this pressure is very important otherwise this press will keep on increasing and parts of our internal ear can be damaged now we will how this pressure is released the vibration which are generated in the perilymph of the scala vestibuli these vibrations proceed to reach perilymph of the scala tympani now these vibrations will hit against the round window and will generate vibrations in the round window now this pressure will be released in this way, the pressure generated in the fluids is released by hitting against the round window so this is mechanism of hearing we have understood with the help of this is diagram

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