Peripheral Auditory Mechanisms, part 3

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來自 Duke University 的課程

Medical Neuroscience

995 個評分

Duke University

Medical Neuroscience

995 個評分

Medical Neuroscience explores the functional organization and neurophysiology of the human central nervous system, while providing a neurobiological framework for understanding human behavior. In this course, you will discover the organization of the neural systems in the brain and spinal cord that mediate sensation, motivate bodily action, and integrate sensorimotor signals with memory, emotion and related faculties of cognition. The overall goal of this course is to provide the foundation for understanding the impairments of sensation, action and cognition that accompany injury, disease or dysfunction in the central nervous system. The course will build upon knowledge acquired through prior studies of cell and molecular biology, general physiology and human anatomy, as we focus primarily on the central nervous system. This online course is designed to include all of the core concepts in neurophysiology and clinical neuroanatomy that would be presented in most first-year neuroscience courses in schools of medicine. However, there are some topics (e.g., biological psychiatry) and several learning experiences (e.g., hands-on brain dissection) that we provide in the corresponding course offered in the Duke University School of Medicine on campus that we are not attempting to reproduce in Medical Neuroscience online. Nevertheless, our aim is to faithfully present in scope and rigor a medical school caliber course experience. This course comprises six units of content organized into 12 weeks, with an additional week for a comprehensive final exam: - Unit 1 Neuroanatomy (weeks 1-2). This unit covers the surface anatomy of the human brain, its internal structure, and the overall organization of sensory and motor systems in the brainstem and spinal cord. - Unit 2 Neural signaling (weeks 3-4). This unit addresses the fundamental mechanisms of neuronal excitability, signal generation and propagation, synaptic transmission, post synaptic mechanisms of signal integration, and neural plasticity. - Unit 3 Sensory systems (weeks 5-7). Here, you will learn the overall organization and function of the sensory systems that contribute to our sense of self relative to the world around us: somatic sensory systems, proprioception, vision, audition, and balance senses. - Unit 4 Motor systems (weeks 8-9). In this unit, we will examine the organization and function of the brain and spinal mechanisms that govern bodily movement. - Unit 5 Brain Development (week 10). Next, we turn our attention to the neurobiological mechanisms for building the nervous system in embryonic development and in early postnatal life; we will also consider how the brain changes across the lifespan. - Unit 6 Cognition (weeks 11-12). The course concludes with a survey of the association systems of the cerebral hemispheres, with an emphasis on cortical networks that integrate perception, memory and emotion in organizing behavior and planning for the future; we will also consider brain systems for maintaining homeostasis and regulating brain state.

從本節課中

Sensory Systems: Audition, Vestibular Sensation and the Chemical Senses

Our survey of the sensory systems continues as we now turn our attention to the auditory system, the vestibular system, and the chemical sensory systems. As you study this content, notice the similarities and the differences that pertain to the general mechanisms of sensory transduction and the broad organization of the central pathways in each of these sensory systems. In particular, note the similarity in transduction mechanisms for audition and vestibular sensation; and note the “logic” of sensory coding in the chemical sensory systems.

Peripheral Auditory Mechanisms, part 118:33

Peripheral Auditory Mechanisms, part 213:57

Peripheral Auditory Mechanisms, part 310:06

Central Auditory Processing, part 114:01

Central Auditory Processing, part 211:16

Central Auditory Processing, part 315:00

與講師見面

Leonard E. White, Ph.D.
Associate Professor
Department of Neurology, Department of Neurobiology, Duke University School of Medicine; Department of Psychology & Neuroscience, Trinity College of Arts & Sciences; Director of Education, Duke Institute for Brain Sciences; Duke University

So, what we see here is with displacement of the Stereocilia on the apical surfaces

of the hair cell. There is the Cyclical depolarization and

Hyperpolarization that happens as the Stereocilia are moved back and forth or

up and down. In the direction that is parallel to the

axis of orientation from smallest Stereocilium to largest Stereocilium.

And if one were to move these hair cells in the Orthogonal direction, that is

rather back and forth from the access of shortest to longest, then what we would

find is basically no change in membrane potential.

Because we would neither be stretching those tip links nor would we allow them

to relax back. So If we were to look down on the hair

cell, what we would find is that there is a very special type of Stereocilium

called a Kinocilium. We're actually born with this, but for

most Inner hair cells that Kinocilium is lost in life.

Nevertheless there is this gradient from shortest Stereocilium to tallest

Stereocilium that is present next to where the Kinocilium once was.

And so if we move these Stereocilium along this axis from smallest to longest

and back and forth, that's where we find the greatest amplitude of Oscillation in

membrane potential. If we were to move In the Orthogonal

axis, basically we wouldn't be stressing out those tip links.

And consequently, we would have no change in membrane potential.

Now let's look at how good these hair cells actually are at faithfully

reproducing the cycles. Of pressure, that cause these traveling

waves to be set up along the length of the Basilar membrane.

If we stimulate these hair cells at a frequency of let's say 300 hertz, 300

times a second, we see a beautiful modulation In the membrane potential of

that hair cell. And as we advance the frequency what we

find is that these hair cells can faithfully represent the temporal aspects

of the stimulus all the way up through at least a kilohertz.

So here's a thousand hertz a kilohertz. Of modulation and we can still detect a

beautiful Sinusoidal type cycle in the membrane potential of that hair cell.

But once we get above about a kilohertz or so we begin to lose the ability to

follow the frequencies of that pressure wave.

And you know, that's a pretty amazing accomplishment that we can record with

fidelity, the movements of these hair cells up to more than a thousand cycles

per second. But, at some point, we just can't operate

as quickly as the energy and the environment.

But we can still encode that energy, because there is a DC component to the

response, that is, the hair cell on average becomes depolarized.

Even though the modulations, the AC component now basically vanishes as we

get above about 2 Kilohertz. So as we continue to increase in pitch,

in the frequency of the stimulus we can still activate hair cells in an

appropriate zone at the Basilar membrane. But we can no longer produce an AC

component of modulation that faithfully captures the temporal dynamics of the

stimulus. Let's back up for just a moment, and I'll

remind you then that at the Basal region of the hair cell, we find Synaptic

Vesicles. So for an action potential to be fired in

the afferent nerve fiber neurotransmission must take place.

And a sufficient depolarization to active Voltage gated Sodium and Potassium

Channels must be generated here in the afferant nerve fiber.

Now I'd like to invite you to pause this tutorial.

And look at your tutorial handout for a couple of weblinks to some really

excellent, resources that have been generated.

That summarize, these aspects of Auditory Physiology that relate to these

Peripheral Structures found in the middle and the inner ear.

The first is an Animation found on the website that supports our textbook, and

you can navigate there on your own or you can simply click the link in your

tutorial notes. The second is really one of my favorite

Animations in all of Neuroscience, it's just a wonderful tour through the

biomechanics of the Basilar membrane created by one of the real giants in the

field of Auditory Neuroscience Dr.James Hudspeth.

Well, he didn't actually do the Animation, but he commissioned it and

oversaw the production of this really beautiful presentation.

That I think really makes very clear how the Basilar membrane can decompose

complex sound even Bach's Toccata and Fugue, into component frequencies, that

are then encoded in the Oscillations of membrane potential in the hair cells that

happen to be at each location a long the length of the[UNKNOWN] membrane.

So please go watch these two Animations, and then come back and we will conclude

this tutorial. Okay well I hope you enjoyed that I know

i sure did. Now I want to just conclude by talking

about Sensory Coding in the Auditory nerve.

If we look at the cycles of stimulation that are going to be transduced by our

hair cells, what we'll find is that for any given Axon in the Auditory nerve,

there is a barrage of action potentials that can be recorded.

With some predictable temporal relationship to the stimulus.

In this case, it's a fairly low frequency stimulus and what we see is a faithful

and highly reproduceable barrage of action potentials that is elicited at a

particular phase relationship to the stimulus.

If one were to look across a family of axons within the Auditory nerve and plot

the firing rate that is observed for any given frequency of stimulation, what we

would be able to do is to plot a tuning function for that Axon.

And so, what we're looking at here is a plot of the intensity of the stimulus

that's necessary to elicit action potential firing in the Axon.

And what we find, for example this Axon that is plotted here in purple above, is

that the threshold is lowest at a particular frequency.

Meaning it takes less amplitude of modulation to elicit actual potential

firing of that Axon. well a different axon is going to have a

different BESS frequency, and so on. So, here we have examples from 6

different Axon's, recorded in an Auditory nerve, and they all have a different BESS

frequency. And this figure is illustrated in a way

that, is meant to organize these best frequencies across that tonotopic map

that we find established in the Basilar membrane.

And sure enough, the Auditory afferents that innervate the base of the Cochlea.

Are going to have their best frequency near the higher range of what can be

heard and transduced. So in this case this gold colored plot is

showing the best frequency around ten kilohertz for this particular Axon that's

innervating the base of the Cochlea. meanwhile, way at the other end of the

Cochlea, here is an Axon that's innervating some domain of the Basilar

membrane that's closer to the Apex. And we find a much lower best frequency.

much less than one kilohertz. Well, I should have made this point

earlier, but for us humans, we're sensitive to sound.

Over a range from about 20 Hz to 20 kHz. So if we were to be at the base of the

Cochlea, that's roughly where we would find our maximum frequency that could be

represented. acro-, across our Basilar membrane

whereas out near our helocrotrema, near the apex of the cochlea that's were we

would find the lowest range of frequencies found.

Okay, well in our next tutorial, I'd like to discuss with you how this information

That is generated here in the inner ear is processed in the central nervous

system. So, I'll see you back for that

discussion.

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