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.