Electric Signal Neurons and Action Potentials

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Electric Signal Neurons and Action Potentials

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University of Southern California
Bisc 307l - General Physiology
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BISC 307L 2nd Edition Lecture 6 Current Lecture Recording from Single K+ Channels This is the Patchclamp method, and it measures current flowing through a single ion channel. The basic definition of current = movement of charge through space. So ions carrying their +/-, and moving through aqueous solution, are a current. The way it works: Take a glass pipette with a polished tip and press it against the membrane of a neuron or any cell. The binding of the charges in the glass to the charges in the membrane will be very tight, and if there happens to be an ion channel in the patch of membrane underneath the open tip of the pipette, and if the seal between the glass and membrane is good, then any current flowing through the channel will follow into the electrode and ultimately into an amplifier where it is measured. im = measure of current through a single channel. From the recordings, Patchclamp was able to plot data graphs of current vs. time. Most of the time, there was 0 current flowing, but at random intervals, a small current of 10 pA occurred rapidly(instantaneous – faster than our current technology can measure) and then shut off just as quickly. There abrupt transitions represent the opening and closing of a single ion channel. They observed that the time at which the channel opens is random, but you can determine the probability of it being in an open or closed state. Depolarization affects Single Channel Current Current = i, average current that flows when the channel is open. With this apparatus, they could utilize a depolarization on a patch of membrane, and they found recordings like the one on the bottom. The channel opens more frequently (still random though, but probability has increased) and the individual single current amplitude is bigger(i is bigger). In general, this directly demonstrates that this channel is voltage gated. The second effect can be understood by looking at Ohm’s Law, which relates current to voltage to resistance. Rate at which charge moves past a point in space = voltage divided by resistance. Current is movement of ions through channels. Lipid bilayer membrane itself has a very high resistance. Ions are hydrated in real life, surrounded by a shell of water. Lipid bilayer has a high, close to infinite resistance, BUT open ion channels give it a finite resistance.

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