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60 Cards in this Set
- Front
- Back
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Inner Hair Cell # per ear and apex and base of cochlea
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Total per ear: 3500
Number at Base:80/mm Number at the Apex: 115/mm |
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physical characteristics of IHC (shape, rows, supporting cell, location, body, covering)
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Shape: Droplet
Rows at Organ of Corti level: 1 Support Cells:Go all the way up Location:OSL Body Length:Stays the same Covering Covered with plasma membrane Has a cuticular plate, basal body (kinocilium), cell body and stereocilia |
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IHC stereocilia characteristics
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Stereocilia: Attached to cuticular plate, 2 wavy rows Parallel to axis of BM, 48 cilia per hair cell
Longest cilia at apex and outside Move as a unit (linked) stiff and resilient coarser Has rootlet and joined by bridges No touching tect. membrane |
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Outer Hair Cell # per ear
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13,500
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physical characteristics of OHC (shape, rows, supporting cell, location, body, covering)
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Shape: Cylinder
Rows: 3 at base, 5 at apex Supportin Cells: Deiter’s shorter Location: BM Body Length: Gets longer from base (20 micron) to the apex (50 micron); longer at apex Covered with plasma membrane Has a cuticular plate, basal body (kinocilium), cell body and stereocilia |
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OHC Stereocilia
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Rows form a V or W, with the base directed toward the spiral lamina
70 cilia/OHC at apex 130 cilia/OHC at the base Respond to lateral/radial shearing Longest at apex and outside Move as a unit (linked) stiff and resilient Has rootlet and joined by bridges Tallest touch tect. membrane |
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Unique characteristics of OHC
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-Specialized Endoplasmic reticulum including lateral subsurface reticulum cisternae
-Motility -LATERAL WALLS act more like MOTILE CELLS |
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Nerves of IHC and OHC?
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IHC: Afferent ONLY (efferent connect through afferent) touch the base
OHC: Afferent and Efferent touch the base Any one neuron supplies but one inner hair cell, IHCs are represented point for point in the cochlear nuclei. |
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Role of basilar membrane
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Properties of the basilar membrane ARE OF THE UTMOST IMPORTANCE responsible for translation of mechanical vibration of stapes into the neural responses
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Looks and Composition of Basilar membrane
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Basilar Membrane is wider, but flaccid at the APEX, and stiffer and thinner at the BASE. There is GRADUAL CHANGE IN STIFFNESS FROM THE BASE TO THE APEX.
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What can the BM do with vibrations and frequencies?
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Frequency-dependent maximum of membrane displacement is a clear indication that the coachlea performs as a mechanical frequency analysis. Basilar membrane can decompose a complex sound into its component frequencies. Here is why: Different stimulus frequencies cause a maximum vibration amplitude at different points along the BM. In other words: the locus of the maximum disturbance of the basilar membrane is frequency dependent. Narrow and stiff like a piano high string at the base and wide and floppy like a low string at the apex. Because each portion of the basilar membrane vibrates maximally for a particular frequency of sound. High frequency sounds maximally displace the hair cells toward the base and the low frequency sounds maximally displace hair cells toward the apex.
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What movement is permitted on the BM based on its attachments and how impact cilia of hair cells?
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Attached at two edges (OSL and Spiral Ligament) and is displaced or bent in a transverse or radial fashion as well as longitudinal fashion.
Basilar Membrane - radial movement responsible for hair cell response, not the longitudinal |
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What causes the BM to move?
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Vibratory action of stapes is coupled to basilar membrane. Motion of stapes causes compression and rarefaction which generates pressure that causes RM and BM to bend toward (compressions) and away (rarefactions) from ST. (compressions cause round window to go out
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Place Theory
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Each point along the membrane develops a maximum of excitation that is associated with a specific frequency of excitation.
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How was traveling wave discovered and by whom?
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Discovery
-Bekesy -Prevalent belief from Late 50’s-70’s -Used models and actual cochleas to test the vibratory responses of the BM. He found they were constant even when the following are changed: 1) elasticity of round window, 2) length of cochlear canal, 3) altered position of the stapes, 4) nature of fluid. -He found that the motion of the basilar membrane was the form of a traveling wave. |
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Characteristics of traveling wave
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Always starts at the base
Amplitude changes at it traverses the length of the cochlea Position along the BM at which its amplitude is highest DEPENDS ON THE FREQUENCY of the STIMULUS |
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Traveling Wave Paradox
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Simple resonance cannot account for basilar membrane displacement. The velocity and wavelength decrease with distance from the stapes. In other words, it doesn’t matter what happens the TRAVELING WAVE PARADOX says the wave always travels from the base to apex.
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What is the envelope and what is important about it?
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-A curve that shows the amplitude of traveling wave along each point of the basilar membrane.
-It is not the instantaneous stimulus maximum that is of ultimate importance, but the maximum point of displacement of the entire envelope. |
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Why can low frequencies mask high frequencies?
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Because low frequencies displace larger segments of the basilar membrane (@ apex is wider), low frequencies can mask high frequencies.
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2 Major Ways Hair Cells are Excited?
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2 Major Ways Hair Cells are Excited:
1) Changes in radii during compression and rarefaction 2) Off set pivot points between the Tectorial Membrane and the Basilar Membrane This accounts for the shearing mechanism associated with the cilia at the apexes of the outer hair cells. |
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Helmoltz
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(Pre 1940s)
Cochlea somewhat passive, but due to Mechanical Resonant Elements the cochlea performs spectral analysis of the incoming signal to distinguish different incoming sounds. |
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Bekesy
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(1940-1970)
Traveling Wave Theory Cochlea/Inner ear (Bekesy Licked Pink Butts) 1) Linear (input = output) Amplitude of incoming sound interpreted by CNS in linear fashion 2) Passive 3) Broadly Tuned (analog radio station) |
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Gold
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(late 1940s)
Puzzled by high sensitivity, sharp frequency tuning, wide dynamic range of hearing. Conclusion: Cochlear must possess a SHARPLY tuned system. Cochlea is NOT PASSIVE; there is a POSITIVE INNER EAR FEEDBACK ELEMENT Theory Suggestions (Aging nerds need pocketprotectors): ● Active Elements (hair cells are not passive) ● Narrow Frequency Response ● Non Linearity ● Positive Feedback System (OtoAcousticEmissions) His ideas were rejected until 1978 |
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David Kemp:
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1978
Published a paper on Evoked OtoAcousticEmissions (EOAE), now called the Motor Theory of Hearing |
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MOTOR THEORY OF HEARING
Sequence of events: |
1) Traveling Wave produces radial shearing between the RETICULAR MEMBRANE and the TECTORIAL MEMBRANE.
2) Between these 2 membranes, the CILIA are STIMULATED. 3) The OUTER HAIR detects this signal because only the OHC CILIA TOUCH THE TECTORIAL MEMBRANE and then: A) CHANGE their SHAPE: The OHC can change their shape because they have several layers of SUBSURFACE MEMBRANES (like CONTRACTILE CELLS) which have MITROCHONDRIA that produce ACTIN and ENERGY, these plates slide back and forth and up and down much like an ACCORDION allowing the walls to ELONGATE and SHORTEN. This accounts for the OHC’s MOTILITY. B) ALTER the MICRO MECHANISMS in the Organ of Corti so as to intensify the stimulus signal enough to stimulate the IHCs. (Cochlear Amplifier) C) OHCs use local energy source (ELECTRICAL POTENTIAL) to amplify the wave induced deformation. 4) IHCs respond to the amplifed stimulus and elicit ACTION POTENTIALS in the afferent neural fibers at their base. 5) The nerve fibers carry this signal to the BRAIN. |
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Motor Theory of Hearing Research shows:
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-Outer Hair Cells are able to act as motor cells as well as sensory cells. This sensory plus motor action is unique amongst cells in the body
-OHC move passively and react actively to vibrations |
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Afferent Fibers
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95% Afferent (up to the brain) and they connect with IHC (95 is an A).
Only 5% of Afferent connect with OHC. 98% of total auditory tract is afferent. |
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Efferent Fibers
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80% of Efferent fibers (down from the brain) go to OHC. 20% Efferent go to IHC (connect through afferent). 2 percent of total system are efferent
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explain what ELECTRICAL POTENTIALS are and name the 4
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electrical signals which can be monitored by electrodes: Cochlear microphonic, summating potential, resting potential, action potential
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Cochlear Microphonic
-faithful- |
● electrical response to an acoustic stimulus-Faithfully represents the stimulus waveform.
● alternating electrical current (A/C) ● due to radial bending or shearing of stereocilia of cochlear hair cells ● if hair cells are destroyed, the CM is no longer present, but will still exist if the VIII nerve is severed ● more sensitive than the SP |
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Summating Potential (SP)
-typography- |
● not an A/C response, but rather a shift in the baseline of the CM usually in the negative direction, but can be in the positive direction
● is DIRECT current (D/C) response that follows the envelope pattern of the acoustic wave ● Takes 20-30 dB more intensity to measure the SP than the CM ● due to the LONGITUDINAL bending or shearing of primarily IHCs in the direction of the length of the cochlear partition |
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Resting Potential
-normal polar bears- |
● normal D/C charge of an area or structure under non-stimulable conditions
● 2 types: ○ Intracellular polarization: negative 80 millivoltelectrical charge or present in most body cells and tissues ○ Postive polarization or Endolymphatic Potential of plus 80 millivoltelectrical which is unique to the endolymph of the scala media. Thought to be due to the stria vascularis. |
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Action Potential (AP)
-gun- |
● usually considered the discharge pattern, or overall action pattern of the cochlear partition of the VIII nerve
● Sudden short change in charge which moves at a constant amplitude along the length of the nerve at great speed ● obeys all or none law, either nerve fibers fire at max limit or not at all, also demonstrates absolute and relative refractory periods ● when a single unit of AP is measures, the response from the nerve fiber is called SPIKE. ● Spikes occur in an irregular pattern when the electro-chemical status of the neuron is suitable for firing However, when a timed stimulus such as a click is presented, it is possible to observe spikes occurring at regular intervals |
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One may expect the helicotrema to offer an alternative route but....
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cochlear mechanics are such that the helicotrema acts as though it were CLOSED
• The helicotrema functions as a static balance for perilymph |
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-- Cytoplasm –
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substance of a cell – exclusive of the nucleus --
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Reticulum
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a fine network formed by cells
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Cisternae
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plural for cisterna, refers to the ultra microscopic spaces occurring between the membranes of the flattened sacs of the cytoplasmatic reticulum.
-- |
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Actin
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Actin protein found in cell areas where there is tissue movement. The mitochondria produce ACTIN.
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Act as micro-levers transmitting vibrations to the
sensory-receptor cells and modify local electrical current that flows into these cells |
Cilia
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Describe the 1st order neuron
(components and general connections going backwards from the hair cell) |
OSL contains DENDRITIC EXTENSION that terminates at BASE of hair cell.
SPIRAL GANGLION contains CELL BODY CENTRAL AXONAL EXTENSION moves AWAY from the cell body TO the brain stem (first junction is COCHLEAR NUCLEI – VENTRAL/DORSAL) NOTE: The DENDRITIC EXTENSIONS may contact MANY hair cells and EACH hair cell may have MORE THAN ONE peripheral extension at its base. (MANY TO MANY RELATIONSHIP) |
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Describe nerve supply of cochlea (# and types of fibers as well how fibers ascend and descend through cochlea)
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FIBERS coming from the APEX of the cochlea descend through the core of the modiolus and EXIT through the FORAMEN CENTRALE (core). LOW FREQUENCIES stimulate the fibers here.
FIBERS coming from the BASE of the cochlea descend though the MODIOLUS and exit through the TRACTUS SPIRALUS (periphery). HIGH FREQUENCIES stimulate the fibers here. NERVE SUPPLY to COCHLEA – Around 30,000 Afferent Fibers and 1800 Efferent Fibers 98% of total auditory tract is AFFERENT |
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Why does the destruction of the nerve pathway on one side NOT result in complete deafness for the corresponding ear?
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Nerve impulses from each ear reach both the left and right temporal lobes of the auditory cortex of the brain, hence destruction of the nerve pathway on one side does not result in complete deafness for the corresponding ear.
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Describe cochlear afferent innervation
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The 8th nerve afferents are Bipolar cells that have their cell bodies in spiral ganglion in the modiolus and send one process to synapse on the hair cells and a longer process (axon) to the cochlear nuclei. The Central (Axon) Processes of Spiral Ganglion cells pass to the CORE of MODIOLUS where they form the COCHLEAR BRANCH of the AUDITORY NERVE
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Describe the 8th nerve.
how is it created, where start, where go.... |
AUDITORY NERVE/8th nerve is only 5 mm long, it is made up of the COMBINED VESTIBULAR (goes to the vestibule) and COCHLEAR (goes to the cochlea) nerves. Nerve fibers from the cochlea enter the IAM from the modiolus. The IAM is about 5 mm across. The IAM then turns toward the brain stem. 2 branches of the vestibular nerve enter the IAM at the corner where it turns (basal turn). Hence the vestibular and cochlear combine into the auditory nerve/8th nerve at the basal turn of the Internal Auditory Meatus (IAM). The IAM ends at the Brain stem where the 8th nerve ENTERS the BRAIN STEM. It ENTERS the MEDULLA OBLONGATA (medulla oblongata is the lower half of the brainstem) laterally at the level of the lower PONS (where cochlear bundle goes into the cochlear nucleus and then divides into the VENTRAL/DORSAL nuclei). Then the nerve acends from the lower pons on up to the CNS/auditory cortex. The last fibers (4th and 5th Order) end below the FIssure of Sylvius.
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Describe the % of afferent fibers that go to the IHC and OHC
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Afferent fibers synapse primarily on IHCs
95% Afferent (up to the brain) associate with IHC (95 is an A) 5% Afferent OHC |
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Describe the % of efferent fibers that go to the IHC and OHC
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Efferent fibers synapse primarily on OHCs
80% Efferent (down from the brain) associate with OHC 20% Efferent associate with IHC |
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How much of the total auditory tract is afferent?
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98% of total auditory tract is afferent.
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How many IHC do afferent neurons typically supply?
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1 IHC per afferent fiber
Any one neuron supplies but one inner hair cell, but any single hair cell is supplied by about 8 neurons. This means that the IHCs are represented point for point in the cochlear nuclei. Another name for this is many (neurons) to one (hair cell) innervation. This tonotopographical arrangement tells us that the basilar membrane, the inner hair cells, and the neurons that supply them constitute a frequency dependent sensory system. |
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Describe how afferent and efferent nerve fibers contact the base of OHC and IHC
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Note that on the outer hair cell, an afferent and an efferent fiber connect directly to the base. On the inner hair cell, the efferent fiber does not connect directly to the cell base but instead it connects to the afferent fiber.
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Describe the IPSLATERAL ASCENDING (AFFERENT) PATHWAY
1. 1st Cold Noodles Deter Vermin 2. 2nd like Donning SOCks, before going getting NaiLLs done. Others along with 3rds skip the SOCks and get NaiLLs done. 3. After getting NaiLLs done, 2nd, 3rd and 4th go get Ice Cream 4. After Ice Cream we drink MuGs of Beer. 5. Aftern MuGs of Beer, a 3rd has a nIght CAP at apartment 41 and 42 |
1) 1st Order Neurons of the VIII nerve divide and then enter the DORSAL or VENTRAL COCHLEAR NUCLEI. They SYNAPSE at this level.
2) 2nd Order travel up to CANS via two routes: a) from DORSAL COCHLEAR NUCLEUS, some travel to the SOC, then from SOC, 2nd and 3rd order NEURONS travel to the NUCLEUS of the LATERAL LEMNISCUS (NLL) (some do not synapse at SOC) b) from DORSAL COCHLEAR NUCLEUS, some travel to NLL 3) from the NLL, 2nd, 3rd, and 4th Order NEURONS travel to the INFERIOR COLLICULUS (IC) (some synapse) 4) from the IC, fibers travel to the MEDIAL GENICULATE BODY (MGB) where all fibers terminate and synapse 5) from the MGB, fibers of at least 3rd order travel through the INTERNAL CAPSULE to areas 41 and 42, these areas are located in the SUPERIOR TEMPORAL GYRUS, the fibers end BELOW the FISSURE of SYLVIUS (division of temporal lobe and frontal lobe). |
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Where are areas 41 and 42 located?
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areas 41 and 42, these areas are located in the SUPERIOR TEMPORAL GYRUS, the fibers end BELOW the FISSURE of SYLVIUS (division of temporal lobe and frontal lobe).
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Where does Contralateral crossover happen in the ascending tract?
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CONTRALATERAL CROSSOVER
At least at the NLL and IC, may happen at other areas |
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What is another way that auditory information can enter the cortex?
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Some neurons also connect to the reticular formation (forms the core of the brain stem and where some auditory information does enter the cortex) at various levels. Latency (delay) through the reticular formation appears to be much longer than through classic auditory tracts.
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Describe the CONTRALATERAL ASCENDING (AFFERENT) PATHWAY
1. Dirty CavemeN send Dinky Sailboats and Monster Sailboats across trapezoid to get Cozy SOCks 2. Voluptous CavemeN send large Voluptous Sailboats across trapezoid to get Cozy SOCks 3. They all get NaiLLs done. (some skip icecream because caveman are lactose intollerant and go for MuGs of Beer.) 3. Some get Ice Cream then drink MuGs of Beer. 5. Aftern MuGs of Beer, they have a nIght CAP at apartment 41 and 42 |
1) From the DORSAL COCHLEAR NUCLEUS, 2 pathways cross the trapezoid body to the CONTRALATERAL SOC. These two fiber tracts are the DORSAL STRIA and the MEDIAL STRIA.
2) From the VENTRAL COCHLEAR NUCEUS, 1 pathway cross the trapezoid body to the CONTRALATERAL SOC, this pathway is called the VENTRAL STRIA, which is the largest of the 3 strias. 3) From the SOC the fibers go to the NUCLEUS of the LATERAL LEMNISCUS (NLL). 4) From the NLL, fibers lead to the MEDIAL GENICULATE BODY (MGB) or to the INFERIOR COLLICULUS and then to the MEDIAL GENICULATE BODY. 5) From the MGB, fibers travel through the INTERNAL CAPSULE to areas 41 and 42, these areas are located in the SUPERIOR TEMPORAL GYRUS, end BELOW the FISSURE of SYLVIUS (division of temporal lobe and frontal lobe). |
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Describe the DESCENDING OR EFFERENT PATHWAY
Brain, IceCream, NaiLLs, SOCks, 8 day Vacation at the OC |
1) Fibers descend from the cerebral cortex to the INFERIOR COLLICULUS (IC).
2) From the IC, they go to the LATERAL LEMNISCUS (LL). 3) From LL, they go to the SOC and travel to the basal turn of the Organ of Corti via the vestibular branch of the VIII nerve. |
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Names for Efferent Fiber Bundles
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Bundle of EFFERENT FIBERS are AKA BUNDLE OF OORT, or RASMUSSEN’S BUNDLE, or the OLIVO-COCHLEAR BUNDLE.
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What may the descending efferent pathway interact with and also what effect does it have on cochlea output?
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The descending/efferent pathway may interact with the ascending/afferent pathway at several of the nuclei mentioned. (An example of this is why the fibers have a habit of going to the SOC.)
The descending/efferent pathway has an inhibitory/regulatory effect on the cochlea output. |
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Who has more fibers ascending? Contralateral or Ipsalateral?
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Note there are more fibers ascending via the contralateral route.
CONTRALATERAL ASCENDING pathway carries around 2/3 of the fibers, while only 1/3 of the fibers ascend IPSILATERALLY. So the right ear is represented in the left brain by more fibers, the left ear is represented in the right brain by a greater ascending neural tract. Each ear is represented on both sides. The crossed or contralateral fibers seem to be much more responsible for speech intelligibility than the ipsilateral fibers. |
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Who has more fibers descending? Contralateral or Ipsalateral?
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Note there are more fibers descending via the contralateral route. CONTRALATERAL DESCENDING pathway carries around 4/5 of the fibers (reach the cochlea from the superior olivary complex from the contralateral side) , while only 1/5 of the fibers descend IPSILATERALLY.
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Where do most efferent fibers synapse in the cochlea?
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Most EFFERENT fibers synapse at the OHC level or on the nonmyelinated dendritic portions of the AFFERENT FIBERS in the IHC.
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