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Chapter 4 notes

by: Joshua Notetaker

Chapter 4 notes NROSCI 1017

Joshua Notetaker
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About this Document

Helps with neurophys basics
Synaptic Transmission
Stephen Meriney
Class Notes
Ion Channels and Signaling, Neurophysics




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This 4 page Class Notes was uploaded by Joshua Notetaker on Tuesday January 12, 2016. The Class Notes belongs to NROSCI 1017 at University of Pittsburgh taught by Stephen Meriney in Fall 2016. Since its upload, it has received 111 views. For similar materials see Synaptic Transmission in Neuroscience at University of Pittsburgh.


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Date Created: 01/12/16
Chapter 4: Ion Channels and Signaling 1/12/16 3:04 PM Chapter 1 covers the introduction slide if you want to read it, but any notes would most likely be repeats of notes taken in intro to neuro. As it happens, notes from Dr. Meriney may also be similar to those taken from these chapters, but as he said, the language and presentation will be different. Key Terms: Graded Potential: Potentials specific to regions of a cell membrane Action Potential: Potential propagated along the entire neuron. Resting Membrane Potential: The electrical potential the cell reaches when it stabilizes and is not propagating a signal. Negative Potential: Inside of the cell is negative compared to the outside. Depolarization: Membrane potential is less negative. Hyperpolarization: Membrane potential is more negative. Transport Molecules: (pumps and transporters) move substances across the membrane against their electrochemical gradients. Vestibule: the open ends of ion channel proteins. Selectivity Filter: A ring of charges in the plane of the membrane that constricts ion channels. Negative to promote cations and positive to promote anions. Agonist vs. Antagonist: Agonist means the molecule’s presence increases the effects of a neurotransmitter while antagonists decrease effects of a neurotransmitter. Outside-in Membrane Patch: Records section of membrane Outside-out Membrane Patch: Records whole cell potential. Current: Flow of ions. Depends on the channel as well as the membrane potential. Key Concepts: Mean Open Time (t): While open time seems to vary randomly, on average certain channels will stay open at different lengths of time. (Individual movements are in the order of 1E-12 meters with frequencies close to 1E13 HZ.) An example of this variability is determined with frequency of finding channels open and closed. K and Cl channels associate with resting membrane potential are found to be open more often than others. Note: Activation and deactivation of a channel means increase or decrease in the probability of the channel opening, not an increase or decrease in the mean channel open time. Modes of Activation: Voltage-sensitive channels respond to physical changes while ligand-activated channels respond to chemical agonists. The voltage- sensitive sodium channel for example, depolarizes the cell, but only after the rising phase of the action potential in order to regenerate. (covered more in chapter 7) Examples (not rigid): Voltage include stretch activated while Ligand include post - synaptic and pre-synaptic bindings. Measurement of Single-Channel Current Studies: Ling and Gerard 1949 was the first study to record membrane potentials or currents from whole cells. They used really small electrodes to get the currents inside cells. Katz and Miledi 1970 recorded noise produced by acetylcholine (Ach) inside frog muscle fibers. These channels were ligand-gated in the postsynaptic membrane. When Katz and Miledi applied Ach to the area they were testing, they noticed “noisy” depolarization, meaning that there was variation in the opening of channels. They applied noise analysis which works on the principles: 1. Single channel currents are large 2. Channels that open for a long time will produce low- frequency noise. Take-away: this was the first study that could publish information on the functioning of an individual ion-channel. Erwin Nher and Bert Sakmann created patch clamp recording in 1978 which essentially sucks part of the membrane into a pipette in order to receive current only from that part of the membrane. The benefits are clear rectangular currents signaling the opening and closing of individual channels, as well as being able to record extremely small currents (courtesy of the superior seal formed by suction). Driving force is represented by V-V(0) meaning the voltage now, minus the voltage at which the current is 0. This difference “V-V0” displays the effect voltage has on current while conductance represents the channels influence on current. Channel Conductance: The time between open and closed states provides information about steps involved in activation, while the channel current is a direct measure of the speed at which ions move through a channel. Ohm’s law is I=y(V-V(0)) or Current=conductance*driving force In summary: Current= The attributes of the channel*The voltage situation. There are 2 relevant attributes of a channel: 1. the ease at which ions pass through when unhindered, and 2. the concentrations around the channel at a given time. Text book gives this relationship: Open channel!permeability ; Permeability + ions! Conductance Equilibrium Potential: An ion current through a channel depends on both electrical potential and electrochemical gradient (AKA concentrations gradient). Potassium equilibrium potential, E(K) is when the electrochemical gradient is such that the concentration of K is balanced. When the membrane potential is at E(K) the driving force for K is 0. This means that when the K ions are at E(K) the driving force is V-E(K). The Nernst Equation is similar to equations seen in chemistry, except the constant k is equal to RT/zF. F is the Faraday or the number of coulombs of charge in one mole of monovalent ion. The Nernst equation for potassium would be: E(k)= k(ln[k]0 - ln[k]i ). In english this reads, the equilibrium potential for potassium = the constant for potassium*(log of potassium’s concentration outside Minus the log of potassium’s concentration inside). -You can see why they use symbols. One caveat provided by the book is that ions interact with other particles, so concentrations are not the best measure. Activity is more exact, but as is explained in Chapter 6, concentration and activity are fairly close to each other. Non-linear I; V relationships (current; voltage): As a cell depolarizes away from equilibrium the outward current increases more and more rapidly. As a cell hyperpolarizes the inward current increases more slowly. This is because the the [K+] is much higher inside the pipette than in the external solution, there are more ions available to carry outward current than inward current. This difference is made more evident as membrane potential goes to either extreme. Inward Rectifier Channels do not experience the expected relationship because the channels do not permit outward movement when potential is reversed, (remember inward is hyperpolarizing aka more negative. Ion Permeation (permitting) is not just through diffusion, ions are selectively brought in by binding to active sites within the channels as well. The electrochemical and concentration forces bring particular ions to the sites, and incoming ions will settle into active sites pushing previous ions through the channel.


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