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Lecture 7

CNS sensory and motor (Lecture 7).docx

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Department
Physiology
Course
PHGY 209
Professor
Erik Cook
Semester
Fall

Description
CNS sensory and motor (Lecture 7): Motion of basilar membrane is frequency dependent • It turns out that the basilar membrane is critical for the transduction process. This membrane vibrates up and down, while the oval window is being pushed back and forth on the top and the round window moves correspondingly. • The location of vibration on the basilar membrane is a function of the frequency of the sound. The basilar membrane is stiff near the oval window and as we get farther away from it, the membrane gets more flexible. • Low frequencies vibrate towards the end of the basilar membrane, away from the oval window. High frequencies, on the other hand, vibrate closer to the oval window. • If we have a mixture of frequencies, we have vibrations at different places. • What the basilar membrane is doing is extracting the frequency of the sound. In a 1kHz sound, the time these pressure waves flow by your head is every 1 milisecond. • Remember this is the cochlea which we’ve unrolled. The basilar membrane in action Video of a computer model of the basilar membrane. The model was made by a researcher named Dr. James Hudspeth, from Rockefeller University. Over the last 20 years, he has been one of the leaders at revealing the transduction process of the auditory system. Basilar membrane motion is converted into neuronal activity at the organ of Corti • Cochlea, all round up, is presented in blue. If we take a slice through the cochlea, we see three fluid-filled chambers. The middle one, the cochlear duct, has the organ of Corti sitting on the basilar membrane. This is where the transduction process occurs. • So, what happens when the basilar membrane moves up and down? Deflection of basilar membrane produces shearing of hair cell sterocelia: • We first have to notice that there a four rows of specialized hair cells, which are the receptor cells where the transduction process occurs. • These four rows run down the length of the cochlea. There are only about 50 000 hair cells per cochlea, which there are 100 million photoreceptors in the retina. • These cells are called hair cells, because they have a little tuffed stereocelia that sticks out at the top. These hair bundles (sterocilia) stick up into the tectorial membrane, sitting right on top. • There are three rows of outer hair cells. • Inner hair cells & outer hair cells have slightly different functions, but we will not distinguish between them. • As the basilar membrane goes up and down, the motion between the basilar membrane and the tectorial membrane is a little bit different. This difference produces a shearing force that causes the stereocelia (hair bundles) to move back and forth. When the basilar membrane goes up, stereocelia bend one way. When the basilar membrane goes down, they move the other way. • At a given frequency of sound, the basilar membrane is vibrating at one location, so we are only wiggling the stereocilia of a few hundred hair cells. Hair cells contain mechanoreceptors: • Through electrophysiology, by putting tiny electrodes on the surface of hair cells, we push the stereocilia back and forth and try to figure out what’s going on. Movement of hair cell stereocilia: • Dr. David Corey, Harvard Medical School • Movement of stereocilia back and forth with a tiny electrode. • First image: hair bundles. The stereocilia are short and get longer and longer. • Where are the ion channels? Maybe at the base of the stereocilia? It turns out they are at the tip of the stereocilia. • Notice in the video that as you push the hair bundle in one direction, the distance between the tips gets closer together, and then farther apart when pushed back. • There is something about the stereocilia (+ the fact that we’re going from short to long) which is the key to the transduction process. Tip links connect each stereocilia: • Between the tip of the shorter stereocilia and the longer one, there were tiny and fragile molecular strings connected the different hair bundles (seen on second image). These tiny molecular threads are called tip links. • It was noticed that as the hair bundles (the tips) go in one direction and the distance between the tallest and shorter stereocilia get farther apart these tip links get pulled. • As hair bundles move in the other direction and the tips get closer together, there is less tension on the tip links. Tip links gate ion channels in the stereocilia: • In the auditory system, the vestibular system has a mechanically-gated transduction. • The molecular string (represented as springs being stretchy) is attached to ion channels. • As the hair bundles move towards the tallest stereocilia, the tip link gets pulled and stretched, ultimately opening one ion channel at a time. • There are perhaps 20 stereocilia per hair cell/bundle, so there isn’t a lot of tip links and ion channels. • The reason it’s designed this way is because, when we are born and the hair cells are new (we haven’t listened to our ipod yet), you can detect pressure waves that are just above the ambient random motion of the air molecules. => This mechanism was devised by nature to be super sensitive. • Because pressure waves fly past our head very quickly at 1kHz per ms (distance between the peak at high pressure and the next high pressure). We could also hear 10kHz in less than a ms. • So you don’t need a G-protein cascade which takes 10s or 100s of miliseconds to get activated the way the visual system is. We need something faster and this is why we have this pushing and pulling of the hair bundles and tip links. Mechano-transduction at tip link activates afferent neurons: • As we push the hair bundle towards the tallest stereocilia, the tip links pull open the channels and potassium enters which causes a depolarization of the hair cells. This triggers the normal processes associated with depolarization: voltage-gated channels allow calcium to come in, which releases neurotransmitters, and then there are afferent neurons that then fire theAPs. Note that the hair cells do not fireAP. • When the stereocilia go in the other direction, towards the shorter one, they come closer together and the tip links become floppy, and these mechanically-gated ion channels close andAP stop. • Something is wrong: potassium is entering the stereocilia. That doesn’t happen: potassium flows out of neurons. When it flows out, it causes the hyperpolarization and not depolarization (?). • It turns out that the cochlear duct (middle compartment) has extracellular fluid that is much different from regular extracellular fluid. Specialized fluid with a high potassium concentration. o Normally, in extracellular space, there is more potassium inside the cell than outside, but here the potassium concentrations are reversed. (REVERSAL POTENTIAL = Nerst potential). Same reversal potential. • Ears are ringing after going out of a club: this might be because you blew up some of the stereocilia, so tip links have become disconnected and start flopping around. Then, stereocilia grow back and tip links “sort of” reconnect, but not perfectly.As you keep blowing up tip links, it is thought they don’t reconnect properly anymore – they quit function. Or the stereocilia, being fragile, become broken and disconnected. • The things that make our auditory system so sensitive are what you damage under loud noise conditions. • There exists a condition where the ringing doesn’t stop. Some people have to have their afferent neuron to stop it. We don’t know what causes it? Is it due to central mechanisms in the brain stem or due to factors associated with the transduction in hair cells? Loud noises or certain antibiotics can kill hair cells! Visual versus auditory transduction: • Photons are very high to catch but have a lot of energy, so we need trillions of opsin molecules stacked up on these discs in photoreceptors just to catch one! Once you get one, you can use its energy to trigger a G-protein cascade. • Sound waves are low energy: just above the random movement of molecules, but they are all around. Pressure waves are easy to catch. So we just need a few hundred thousand tip links in the cochlea to hear as opposed to trillions of opsin molecules. • In the visual system, the photoreceptors use a G-protein cascade that slowly closes the sodium channels (takes 10 to 100 ms = ok, since we are not moving at 500 miles per hour and visual information is relatively constant on the time scale of a few hundred ms). • However, pressure waves fly past our head very fast and they’re gone. So, we need something very fast that would directly activate channels.
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