Presentation on theme: "CSD 3103 anatomy of speech and hearing mechanisms Hearing mechanisms Fall 2008 Central Pathways."— Presentation transcript:
1 CSD 3103 anatomy of speech and hearing mechanisms Hearing mechanisms Fall 2008 Central Pathways
2 Cells of the nervous system The primary cell of the nervous system is the neuronNon-replaceableBefore we look at the central pathways, we need to discuss the cells of the nervous system in general, and their characteristicsThe neuron is the primary cell of the nervous system.One important characteristics of nerve cells is that that can’t (at this time) be replaced.
3 Typical neuron Important Parts Cell body Dendrites Axon Fig 7-1 shows a typical neuron--they come in varying shapes and sizesImportant parts• cell body• projections or nerve fibers--primary means of communication- dendrites--short projections that conduct nerve impulses in the direction of the cell body- axon--a single long process that conducts the nerve impulse to other neurons
4 Neuron specialization The three major types of neurons, depending on their specialization:Sensory NeuronsMotor NeuronsInterneuronsThere are 3 types of neurons, depending on their specialization:• sensory neurons• motor neurons• interneurons
5 Sensory neuronsSensory neurons conduct nerve impulses from the ear and deliver sensory information to the brain for processing and interpretationAfferent refers to this direction of travel and this kind of pathway or systemThe type of neurons making up the central auditory pathways are sensory.These neurons conduct nerve impulses from the ear and deliver sensory info to the brain for processing and interpretation.In general, sensory impulses travel from the periphery to more central structures of the nervous systemAfferent refers to this direction of travel and this kind of pathway or system.
6 Neuron classification by structure Neurons are alos classified by their structure:• monopolar• bipoloar• multipolarFig 7-2 shows examplesMost of what’s in the CAP are bipolar and multipolar
7 How neurons communicate Communication between neurons is achieved by the release of small packets of neurotransmitters into the synapseIf the release of neurotransmitters reaches a critical level to the receiving neuron, it will cause an action potential to be generated in the cell body“All-or-none” behaviorHow neurons communicate:communication between neurons is achieved by the release of small packets of chemicals into the synapse (space between 2 neurons)The chemicals are called neurotransmitters.If the release of neurotransmitters reaches a critical level to the receiving neuron, it will cause an action potential to be generated in the cell body.
8 How neurons communicate The action potential is an electrical eventThe action potential travels down the axon to reach another neural cell bodyNeurotransmitters are released at the synapse and the process is repeated in a new neuronThis is an electrical event--and the action potential travels down the axon to reach anotherneural cell bodyNeurotransmitters are released at the synapse and the process starts over in a new neuron.
9 The viii. Cranial nerve (vetibulocochlear) Sensory neuronAcoustic PortionVestibular PortionThe VIII cranial nerve (vestibulocochlear) is entirely sensory and has 2 parts:• the acoustic part--transmits auditory impulses from the cochlea• the vestibular part--transmits vestibular impulses from the sensory cells of the vestibular system (saccule, utricle, ampullae of semicircular canals, etc..).Fig 2.11 shows the first order neurons leaving the inner ear. You can see the major branch of the vestibular portion of the VIII N. Joining the auditory portion of the nerve. Notice where the facial nerve is as well.We will only follow the auditory portion of the VIII N.
10 The central auditory pathways Landmarks:Auditory nerveCochlear nucleusSuperior olivary complexLateral lemniscusInferior colliculusMedial geniculate bodyAuditory cortexFig 14-1 shows a schematic of the whole pathway.Without exception, every anatomical structure on one side of the brain has an identical structure on the opposite side.
11 The auditory nerve VIII cranial nerve Bipolar neurons Cell bodies from these neurons lie right outside the cochlea and form the spiral ganglionOne end innervates the individual inner and outer HCs of the cochlea and the other end synapses with the neurons of the cochlear nucleusThese first order neurons are bipolar. The collection of their cell bodies form the spiral ganglion. The second set of axons make their way and synapse on the cochlear nucleus.
12 The auditory nerve Auditory Portion--30,000 fibers Vestibular Portion--20,000 fibersEvidence of tonotopic organizationSpiral GanglionCell bodies of the first order neuronsBipolarCerebello-Pontine angle--where the cerebellum, medulla oblongota and pons meetThe major structures:• auditory nerve--fibers pass from the modiolus of the cochlear thru the internal auditory canal. This IAC also carries the vestibular portion of the VIII N. There are about 30K separate fibers from the auditory portion and about 20k from the vestibular portion. Talk about the spiraling characteristic of the auditory nerve within the IAC and tonotopic organization (low freqs inside, high freqs outside).- spiral ganglion--these are the cell bodies of the first order neurons of the auditory portion of the VIII N. These single fibers are bipolar neurons.The auditory nerve extends mm beyond the IAC, where it attaches to the brainstem where the cerebellum, medulla oblongata and pons (all parts of the CNS) join to form the cerebello-pontine angle. It is here where the auditory and vestibular portions of the VIII N separate. One part of the cochlear bundle descends to the dorsal cochlear nucleus, and the other ascends to the ventral cochlear nucleus.
13 The auditory nerveEvidence of tonotopic organization
14 The auditory nerveThe individual fibers pass from the modiolus of the cochlea through the internal auditory meatus, which exits at the base of the brainThe IAM also carries fibers from the utricle, saccule, and semicircular canals that form the vestibular portion of the VIII nerveThe vestibular and auditory portions of the VIII N. separate at the cerebellopontine angleThe branch of the facial nerve that courses through the middle ear also exits herethe auditory nerve (eighth cranial nerve)--these fibers, which innervate individual hair cells pass from the modiolus of the cochlea thru the internal auditory canal, which exits at the base of the brain. The IAC also carries the fibers from the utricle, saccule and semicircular canals and form the vestibular portion of the eighth nerve. Talk about how the cochlear fibers are arrayed as they exit the cochlea and define tonotopic organization (low freqs in the center, high freqs on top). Vestibular portion and auditory portion of the eighth nerve separate at the level of the cerebellopontine angle.
15 The cochlear nucleus Two major parts Tonotopic organization Dorsal and ventralTonotopic organization“must-synapse” station--second order fibersPreserves, but does not enhance, information received from the auditory nerveThe Cochlear Nucleus--Each division of the cochlear nucleus is organized tonotopically. The nucleus is made up of a variety of different cell types. It’s thought that the cochlear nucleus preserves, but doesn’t enhance info it receives from the auditory nerve. All fibers arising from the cochlea synapse here.
16 Superior Olivary Complex Most of the fibers from the cochlear nuclei cross and project to the contralateral SOCPlays a role in the acoustic reflexAnalyzes intensity and time-of-arrival differences between the two ears to help with localization tasksThe Superior Olivary Complex--Most of the fibers from the cochlear nucleus project to the SOC. Most fibers from one ear cross and synapse on the contralateral SOC.The SOC plays a role in the acoustic reflex (as we have discussed) and analyzing intensity and time of arrival differences between the 2 ears for localization
17 The superior olivery complex Most (about 80%) of the fibers from the cochlear nuclei cross and project to the contralateral SOC via the trapezoid bodyEvidence of tonotopic organizationPlays a major role in the acoustic reflexAnalyzes intensity and time-of-arrival differences between the two ears to help with localization/lateralization tasksThe Superior Olivary Complex--Most of the fibers from the cochlear nucleus project to the SOC. Most fibers from one ear cross and synapse on the contralateral SOC.The SOC plays a role in the acoustic reflex (as we have discussed) and analyzing intensity and time of arrival differences between the 2 ears for localization
18 The lateral lemniscusHighway of axons that arise from the SOC and terminate in the midbrainLateral lemniscus—major brainstem pathway for the aud system. There are some nuclei, but most the LL is a pathway formed by fibers arising from the contralateral cochlear nucleus that combine with fibers running from the ipsilateral SOC. About 3mm in length, so this is a large tract
19 The lateral lemniscus Tonotopic organization is evident Nuclei within the lateral lemniscus have a large proportion of cells that are sensitive to interaural time differences, binaural input, and interaural intensity differencesLateral lemniscus—major brainstem pathway for the aud system. There are some nuclei, but most the LL is a pathway formed by fibers arising from the contralateral cochlear nucleus that combine with fibers running from the ipsilateral SOC. About 3mm in length, so this is a large tractView shows upper pons and lower midbrain. 2 represents the LL tract terminating to the inferior colliculi (1). 6 is the thalamus
20 The inferior colliculus “must-synapse” station at the level of the midbrainHighly tonotopicFirst evidence of neurons that are sensitive to sound durationActive in binaural processingThe inferior colliculus is a key auditory region in the midbrain and is an obligatory connection for fibers arising from lower brainstem structures.It’s also the largest structure in the auditory brainstem pathway
21 Auditory CortexAreas of auditory reception are in the temproal lobes on both sides of the cerebral cortex in an area called the superior temporal gyrus or Heschl’s gyrusThe Auditory Cortex--the areas of auditory reception are in the temporal lobes on both sides of the cerebral cortex in an area called the superior temporal gyrus or Heschl’s gyrus.At one time it was believed that the cortex was the only center of auditory discrim. It’s now known that many discrims may be mediated subcortically. Perceptions of pitch and loudness can be maintained without the cortex.
22 The medial geniculate body Located in the auditory thalamusLast subcortical relay in the pathwayEvidence of tonotopic organizationVery active in localization and lateralizationThe Medial Geniculate Body--located in the auditory thalamus is the last subcortical relay station of the pathway. At this point, nerve fibers fan out as the auditory radiations and ascend to the cortexWe’re not sure how information is organized and represented at this level.
23 The medial geniculate body Pathways of neurons projecting from the medial geniculate to cortical areasFibers from the MGB fan out into radiations to inervate the cortex in the auditory areas
24 The human brainstem Structures: Cochlear nuclei (1) Lateral lemnisci (2)Inferior colliculi (3)Superior colliculi (4)Medial geniculates (6)Auditory thalmi (7)Fibers from the MGB fan out into radiations to inervate the cortex in the auditory areas
25 The auditory cortexAreas of auditory reception are in the temproal lobes on both sides of the cerebral cortexThe Sylvian or lateral fissure is the focal pointPrimary and secondary auditory areas are above and below this pointAt one time it was believed that the cortex was the only center of auditory discrim. It’s now known that many discrims may be mediated subcortically. Perceptions of pitch and loudness can be maintained without the cortex.
26 Central Auditory Pathways Fig 17-1 is another diagram of the auditory pathways.
27 The corpus callosumLarge fiber tract that connects the two hemispheres of the brainAllows information (like auditory) to be transferred from one side of the brain to the otherVery important for normal dichotic listening and pitch pattern perception
28 Behavior of the auditory nerve look at neural responses of the auditory system. How is information coded here??We know that a certain region of the cochlea is innervated by a single auditory nerve fiber. This fiber makes its way out of the cochlea. What kind of info does it carry??let’s imagine we were able to put an electrode on just one of these fibers and measure its electrical activity---observe when it firesOne thing we would notice is that even in the absence of any auditory input, the nerve fiber would be activerandomly delivering APs or discharging---key word here is random.the discharge patterns would not be periodic, nor would they be tied to any auditory event.If we observed this fiber for a period of time and counted up the number of random firings, we could calculate a spontaneous discharge ratethink of this as the resting activity level of the nerve fiber.Let’s pretend that for our imaginary fiber, the spontaneous discharge rate is 75 spikes/sWhat do you think will happen to the fiber in response to a sine wave it can respond to?Fig shows how the single neuron responses are tied to the characteristics (temporal) of the signal. Refer to this as “phase locking”.This figure shows how the single neuron responses are tied to the temporal characteristics of the signal“phase-locking”
29 Input-output function discharge rate will increase above 75 spikes/s and will be tied to the presentation of the signal.as the intensity of the signal increases, the discharge rate will increase.Fig 6-120
30 Input-output function and histogram Fig 2-41 shows the same input-output function of a single fiber. Point out dynamic range of fiber and saturationThis figure shows the input-output function of a single fiber. Notice that the firing rate of the neuron increases as the stimulus intensity increases within its dynamic range, and eventually plateaus.
31 Single fiber tuning curve Characteristic frequency (tip)TailDifference in slope between the low frequency and high frequency sidesShaded area is the response areaAfter observing the change in discharge rate from our one fiber in response to increasing the intensity of our pure tone or sine wave, we are going to arbitrarily decide that 100 spikes/s represents the threshold level of responding for this fiber.in other words, when the discharge rate is 100 spike/s or higher, the fiber has been “captured” by a pure toneif the discharge rate is less than 100 spikes/s, then the fiber is not responding to the toneNow we’re going to do a little experiment.We’re going to present a variety of different pure tones to the ear of this system.with each pure tone we present, we will keep an eye on the discharge rate of the fiber.for each pure tone presented, we will find the intensity of the pure tone that will give us a discharge rate of 100 spikes/s---criterion discharge rateWe’ll do this running through a range of frequencies from HzPlot the response---single fiber tuning curvedB SPL for criterion discharge rate as a function of frequencypoint out tip (characteristic frequency of fiber)point out low frequency tailmake high frequency slope very steepfig 8.5tuning curve of auditory fiber with cf=10,000 Hzimportant points:one fiber has the capability of responding and providing info about a wide range of frequencies---if the intensity is high enougheach fiber responds “best” to a very narrow range of frequencies.shaded area:if frequency and intensity of pure tone falls within the response region, the fiber will respond at or above criterion discharge rateif frequency and intensity of pure tone falls outside of the response region, the fiber will not respond.
32 Single fiber tuning curve Fig 8.6: tuning curves of fibers with 3 center frequenciesPortions of response areas for each of three cochlear neurons with CFs of 100, 1000, and 10,000 Hz
33 Single fiber tuning curves of many frequencies Fig 2-42 tuning curves of neurons with a variety of cfs.Discuss issue of tonotopic organizationcertain regions of the BM vibrate with greatest displacement depending on frequencycfs of fibers represent an orderly display along length ofBM--fibers with high cf are near base, fibers with low cf are near apexas fibers exit cochlea, high frequencies are on top, low frequencies are inside.This kind of organization continues all the way up.How else is information about a pure tone coded at the peripheral neural levels?Tuning curves of auditory neurons with a variety of characteristic frequencies
34 Post-stimulus time histogram Here’s the situation:we’ve isolated a single fiber auditory neuron with cf of 1600 Hz.we present this fiber with a 1000 Hz pure tone at an intensity high enough to be within the fiber’s response area.the tone will be on for 1 s.during the presentation of the tone, we are going to keep track of when, after the onset of the tone the fiber responds, each time it respondskeep track of the length of time between one AP to the next (intervals)We’re going to repeat this many times and develop a histogram of how many times the fiber responds to the tone and what the interval duration is from the last firing.number of times as a function of duration of intervalsPST (post stimulus time) histogramswhat we find is that some durational units are more likely tohave events in them than other durational unitsthere seem to be some “prime times” when the fiber will jump in and start firing.the time between these high probability times is equal to the period of the stimulating pure tone.go through fig 8.15 to illustrate---phase locking behavior of neural fibers. This will come up again.
35 Pst histogramsSelected histograms showing that neural firing during a pure tone is timed to the period of the tone. The dots along each x-axis correspond to the multiples of the period of the tone.Fig 2-44single fiber with cf=1600 Hzpure tones of 412 Hz, 600 Hz, 900 Hz, 1000 Hzpresented all at 80 dB SPL for 1 sNotice that the overall number of spikes increases as you get closer to the fiber’s CFImplications: single fiber with cf not equal to stimulating tone can provide frequency (period) info about stimulating tonesystem looks at which fiber is firing maximally as well as temporal characteristics of discharge of other fibers.