Introductory Neurobiology Week 9 Day 1 Notes
Introductory Neurobiology Week 9 Day 1 Notes Biol 3640
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This 7 page Class Notes was uploaded by lucy allen on Thursday March 10, 2016. The Class Notes belongs to Biol 3640 at University of Denver taught by Dr. John C Kinnamon in Fall 2016. Since its upload, it has received 9 views. For similar materials see Introductory Neurobiology in Biology at University of Denver.
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Date Created: 03/10/16
room 105 310 56pm review session final is cumulative most of the material 70 is on material since second exam The Auditory System the scream represents the universal anxiety of modern man What is Sound rarefaction and compaction of air molecules if a tree falls in the forest and nobody is around to hear it there still is a sound have varying frequencypitch ow frequencypitch long wavelength esser compaction and rarefaction of air molecules high frequencypitch short wavelength greater compaction and rarefaction of air molecules 1012 between a soft whisper and a loud rock concert eardrums motion can be as little as 103910 inches for soft sounds for loud sounds it can be enough to cause physical pain frequency acuity can be as detailed as 01 sounds are measured in decibels 1 decibel db 20 x log10pressure of soundminimum detectable sound a sound 10x the minimum is 20 x log1010 20 x 1 20 db a sound 100x the minimum is 20 x log10100 20 x 10 200 db arbitrary base reference level considered quiet 40 db busy traffic intersectionalarm clock considered annoying 80 db ampified rock music 110130 db can damage hearing after about 375 minutes to 30 minutes of exposure shotgun 130 db can damage hearing after about 4 minutes of exposure Sound Range of Animals humans with perfect hearing can detect sounds of about 20 Hertz up to about 20000 hertz presbycusia loss of hearing as one ages dogs have good range of hearing 5H2 to over 50000 Hz whaesdophins have impressive hearing as well birds and frogs have a limited range of frequencies they can hear Structure of the Ear sound enters through the auditory canal where it strikes the eardrum tympanic membrane then there are three ossicles malleus incus and stapes eardrum ossicles convert vibrations into movements of the oval window pushing on the uid lled cochlea maleus incus and stapes have muscles connecting them to the middle ear which are important for maintenance of functioning of ossicles in the middle of their dynamic range in a loud environment muscles contract to make it more dif cult for the muscles of the ossicles to move a protective method chalenge must have an ef cient transfer of vibration of air molecules to the movement of the stapes pressing on the oval window accompished by two processes area of the eardrum is large compared to the area of the open window 1875 times as large forces are concentrated over a smaller area increasing the pressure on the oval window lever action of the ossicles arm of the incus is shorter than that of the malleus ampli cation factor of about 21 times impedance matching cochea like a snail tube wrapped around itself has two windows one is the oval window where the stapes pushes on the oval window and since the cochlea is lled with incompressible perilymph there must be another exible to window to push out if the oval window is pushed in the round window accomplishes this cross section through the cochlea three chambers top schalae vestibulae bottom schalae tympani movement of uid causes the basilar membrane to move up and down in the organ of corti where the action takes place transduction movement upwards of basilar membrane causes receptor cells in organ of corti to push on the tectorial membrane note that the uid in the perilymph is high in sodium whereas that of the endolymph is high in potassium schaae tympani and vestibulae are high in potassium other is high in sodium basiar membrane at the bottom of the organ of corti is what moves up and down in response to pressure eon the oval window 39unroing39 the cochlea and turning it into a straight tube and looking at basilar membrane shows that it is not homogeneous in nature tip is narrow and stiff apex of the basilar membrane near the hole which connects the scala vestibulae with the tympani is wide and softer base of the basilar membrane is narrow and stiff other part is wide and soft the base is therefore vibrating due to higher frequencies example of frequency tuning organ of corti is between vestibulae and tympani high frequency causes a wave at the base low frequency causes it at the apex know slide 19 for the exam base is narrow stiff and responds to high frequencies apex is wide soft and responds to lower frequencies The Organ of Corti stapes push on perilymph vibration causes movement of basilar membrane up and down causes the receptor cells in the region of the organ of corti to push up into the tectorial membrane tectoria membrane is like jelly an a receptor cell is there which terminates in hairs basiar membrane pushes up and hairs push into tectorial membrane and hairs are de ected which stimulates the auditory receptor cells two types of hair cells single row of inner hair cells three rows of outer cells inner single row are the receptor cells not neurons specialized receptor cells terminates in a collection of microvilli named stereocilia but NOT CILIA they are lled with actin laments not microtubules at the end of one side of tuft of stereocilia is a true cilium outer cells are what are referred to as the cochlear ampli er making the inner hair cells act more ef ciently when inner row of hair cells and their stereocilia push out on the tectorial membrane is where sensory transduction occurs initiates a response onto the hair cell pushes one way when going up another way when coming down bends the sensory hairs to the right when pushing up to the left when the basilar membrane goes down side 26 inner hair cell shown row after row of stereocilia all not the same height progressively taller until the last hair is reached which is the kinocilium someone with microscissors cut away the stereocilia in a living hair cell while recording its responses as stereocilium were cut and removed the response of the hair cell decreased just removing the kinocilium showed no change in function at all so the belief is that the stereocilia perform the sensory transduction when stereocilia are de ected in the direction of the kinocilium there is a depolarization and action potentials when stereocilia are de ected away from the kinocilium there is a hyperpolarization and inhibition of sensory neuron side 27 inner or outer hair cell look similar could be either kinocilium can end in a spherical balllike structure stereocilia have ion channels in their tips these channels are connected by little springs made out of protein to the adjacent stereocilia these channels are potassium channels scaa media where these reside is high in potassium when stereocilia de ected towards kinocilium spring pull on the potassium channels activating them and potassium comes in and depolarizes the receptor cell first example of potassium being used to depolarize a cell hyperpolarization can occur because when we are at rest and no de ection is occurring approximately 15 of the potassium channels are open potassium channels can be closed when de ected away from the kinocilium allowing the 15 of open channels to close allowing the hyperpolarization to occur tip link potassium channels are here connects stereocilia Inner Hair Cells and Outer Hair Cells inner hair cells receptor cells produce a receptor potential and if it is of sufficiently high magnitude in the afferent axon then we get an action potential outer hair cells ook similar have stereocilia and kinocilium do not function in signaling or sensing the vibrations of the basilar membrane they tend to be organized in chevrons Adaptation we can hear a range of 1012 we can do this because of the receptor cells and the inner hair cells they can remain in the middle of their dynamic range through adaptation the spring connected to the longer stereocilium quotwalksquot down and takes tension off of the spring allowing potassium channels to close and it can respond once again if the stereocilia are de ected even more tip link connected to an actin lament inside the stereocilium and using an actinmyosin motor it 39walks down39 to take the tension off of the spring and allow closure of channels etc tension taken off of the spring quiet room by the motor moving in the opposite direction marching up and putting more tension on the spring keeping the sensory transduction apparatus in the middle of its dynamic range Frequency Tuning mechanical tuning physical nature of basilar membrane apex is wide and oppy moves in response to low frequencies base is narrow and stiff moves in response to high frequencies nature of stereocilia themselves high frequency cells on the base of the basilar membrane have shorter stiffer stereocilia cels at the apex of the basilar membrane tend to be longer and oppier and respond better to low frequency stimulation electrical tuning ifl present a low frequency cell movementstimulus a large response in the low frequency cell is seen and a smaller response in the high frequency cell ifl present a H frequency stimulus there is a large response in the high frequency cell and not as much of a response in the low frequency cell as internal calcium levels increase they act on calcium activated potassium channels allowing potassium to leave hyperpolarizing the cell and allowing closure of potassium channels then the internal calcium level decreases and the cycle conUnues depoarizing and hyperpolarizing phase distance between calcium and potassium channels determines frequency type of cell close together high frequency cell Cochlear Ampli er Importance of Outer Hair Cells when outer hair cells are functioning a response is seen such as slide 56 after treatment with a drug outer hair cells are knocked out and the response is not seen therefore they are responsible for cochlear ampli cation outer hair cell has an apex with stereocilia and kinocilium touching the tectorial membrane sound is presented causing stimulation of the outer hair cell changing its shape longer pushing on the tectorial membrane the downward part of the sound shortens the length of the outer hair cell and pulls down on the tectorial membrane emphasizes movement of the tectorial membrane by changing shape of the outer hair cell accentuates stimulation of inner hair cells video 2 electrophysiological recording of an outer hair cell in response to a sound presentation Tonotopy we previously talked about somatotopy in somatosensory system orderly mapping of stimuli onto the brain homuncuus simiar mapping in the auditory system low frequency sounds are anterior and orderly progression of sound occurs until the posterior side is reached where high frequency sounds are interpreted over 30000000 Americans have signi cant hearing problems sensorineural hearing loss nerve deafness due to damage to inner hair cells each cochlea normally contains about 16000 of these receptor cells simiar hair cells involved in vestibular system balance causes variable genetic de cits ear infections can damage the inner hair cells or the outer hair cells ototoxic drugs aspirin and aminoglycosides presbycusia acoustica trauma gunshots etc aminogycoside ototoxicity outer hair cells damaged hurts ability to distinguish between frequencies due to damage to cochlear ampli er apparatus Eskimos ive in quiet environments hunt naturally quiet Eskimos will sit at an opening in the ice waiting for a seal to come up to breath they then shoot them muscles in their ears become very relaxed then when they shoot the seal there is a loud sound from the gun it is common for Eskimo seal hunters to go deaf due to the immediate sudden loud noise
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