this lecture covered the anatomy of the ear, the sound transduction mechanism of the cochlea, and the auditory pathways of the CNS. the ear can be roughly divided into three sections. external ear is the auricle (the skin covered elastic tissue which includes the helix/antihelix, tragus/antitragus, concha and lobule), up to the tympanic membrane. the middle ear contains the ossicles, which transmit sound from the tympanic membrane to the inner ear. the pathway for vibration goes from the tympanic membrane to the malleus, then the incus, then the stapes. the stapes transfers its vibrations through the oval window into the cochlea. a few other middle ear notes: the tegmen tympani is the bony membrane that separates the CNS from the middle ear (middle ear infections can thus lead to meningitis by spreading through the tegmen typmani). the tensor tympani attaches to the malleus, is innervated by V3, and absorbs low frequency vibrations from the malleus. the stapedius is a muscle that is innervated by the facial nerve which absorbs extreme vibrations from the stapes.
the stapes transfers its vibrations into one of the two channels within the helical bony structure of the cochlea, the scala vestibuli (the other being the scala tympani). separating these two channels is the cochlear membrane which contains the organ of corti, which contains the sensory receptors in the form of ciliated receptor cells. the organ of corti is covered by the basilar membrane, which contacts the scala vestibuli, and the tectorial membrane, which contacts the cilia of the receptor cells. vibrations pass through the oval window into the perilymph fluid in the scala vestibuli, which causes a distortion of the basilar membrane- this causes a shift in the ciliated cells via the tectorial membrane, which produces a receptor potential. along the length of the cochlea, the basilar membrane gets wider and the cilia cells get longer, such that higher frequency vibrations are absorbed closer to the oval window. each sound thus produces a characteristic deflection pattern along the length of the basilar membrane.
the cochlea is innervated by the cochlear portion of CNVIII, the vestibulocochlear nerve, via the spiral ganglion. from there, the auditory pathway goes to the cochlear nuclei in the medulla, to the superior olive nuclei in the medulla (which receives input from both ears), to the inferior colliculus in the midbrain, to the middle geniculate nucleus of the thalamus via the lateral lemniscus, to the primary auditory cortex in the temporal lobe. from there, the information can be sent to the auditory association areas, such as wernicke's area for language comprehension or the parietal areas for reading/writing. music comprehension is divided between the hemispheres; the left hemisphere processes rhythm and the right processes melody.
questions
ear anatomy, innervation...
1. describe the transduction of sound from pressure waves in the air to the inner ear.
2. what is the auricle and what are its components?
3. what type of glands are around the external auditory meatus?
4. what is the umbo of the tympanic membrane?
5. what are the landmarks of the ear visible with an auriscope?
6. what are the sensory nerves to the ear?
7. which nerves innervate the tympanic membrane?
8. where is the chorda tympani and what does it convey?
middle ear...
9. tympanic cavity contains...
10. the auditory tube connects tympanic cavity to...
11. what is the tegmen tympani?
12. what is an otitis media? what might it lead to?
13. what do the ossicles do?
14. what is the tensor tympani and what does it do? what is it innervated by?
15. what is the oval window?
16. what does the stapedius do? what is it innervated by?
inner ear...
17. describe the shape of the cochlea.
18. what are the two channels in the cochlea? what are they separated by? where do they meet?
19. what is endolymph? perilymph?
20. describe the different roles of the oval and round windows.
21. describe the role of the basilar and tectorial membrane in the inner ear.
22. describe how the basilar membrane encodes a specific pitch.
23. closer to the apex, the basilar membrane becomes...
24. closer to the apex, the hair cells become...
innervation and auditory pathways...
25. describe the sensory innervation of the cochlea.
26. describe the auditory pathway from the cochlea to the primary auditory cortex.
27. disparity in time and intensity between right and left sounds...
28. how are low frequency sounds localized as opposed to high frequency sounds?
29. what are the three types of deafness and what are they caused by?
30. what are the auditory association cortices?
31. describe where rhythm and melody are processed in the brain.
answers
1. pressure waves enter the external ear and vibrate the tympanic membrane, which transmits vibration to the middle ear: malleus, incus, stapes, which translates the vibration into the fluid filled cochlea.
2. the elastic cartilage around the external ear which is covered by skin. has the helix/antihelix, tragus/antitragus, concha and lobule.
3. ceruminous.
4. the central depression formed by tension from the malleus.
5. umbo, malleus, incus, stapes, cone of light, flacid/tense portions of tympanic membrane.
6. greater auricular / lesser occipital, auriculotemporal (V3), facial (VII), glossopharyngeal (IX), vagus (X)
7. V3 and vagus innervate lateral side, glossopharyngeal innervates medial side.
8. deep to the tympanic membrane, conveys taste from anterior 2/3 tongue, parasympathetic to submandibular and sublingual glands via submandibular ganglion.
9. ossicles and their muscles.
10. nasopharynx.
11. the thin bone between the tympanic cavity and the brain.
12. a middle ear infection that can spread through the tegmen tympani to cause meningitis or brain abscesses.
13. transmit the vibrations from the wide tympanic membrane through the narrow base of the stapes.
14. the muscle that attaches to the malleus and dampens extremely low frequency vibrations. innervated by V3.
15. the area which the stapes transmits vibrations into the cochlea.
16. dampens extreme vibrations of the stapes. innervated by facial nerve.
17. a helical bony canal.
18. scala vestibuli and scala tympani: separated by cochlear duct, meet at the helicotrema at the apex.
19. endolymph is the fluid in the cochlear duct which has high K+. perilymph is the fluid in the scala vestibuli and tympani which has high Na+.
20. oval window is the area through which the stapes triggers fluid vibrations in the perilymph, and the round window is the area which absorbs the outward displacements of fluid vibrations.
21. vibrations of perilymph in the scala vestibuli are translated to the basilar membrane, which causes a distortion of the organ of corti. tectorial membrane is the membrane that contacts the cilia of the auditory receptor cells and causes the actual auditory signal.
22. the pitch of an incoming sound is encoded by the stretch and width of the basilar membrane from the oval window to the helicotrema. each sound or pitch has a maximum amplitude at a characteristic point along the basilar membrane.
23. wider
24. taller
25. provided by cochlear branch of vestibulo-cochlear nerve (CNVIII). synapses at spiral ganglion.
26. cochlea, cochlear nerve, cochlear nuclei in medulla, superior olive nuclei (each nuclei receives from both ears), inferior colliculus, medial geniculate nucleus of thalamus via lateral lemniscus, primary auditory cortex.
27. localizes object in space.
28. low frequency sounds are localized via time differences and high frequency sounds are localized via intensity differences.
29. conductive deafness due to damage of tympanic membrane or ossicles. sensorineural deafness due to damage of cochlea or cochlear nerve. central deafness due to damage of central auditory pathways.
30. wernicke's area on the left side of the temporal lobe (language comprehension), parietal areas for reading and writing.
31. rhythm is processed on the left side, melody on the right.
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