Ion+Channels+Glossary-1 - This is from the Ion Channels...

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Unformatted text preview: This is from the Ion Channels course at Cold Spring Harbor Lab, and is a useful reference even if all the terms may have not been discussed in class. Ion Channels Glossary Nelson Spruston, Mark Farrant, Jeff Diamond I. Basic electricity Charge: The fundamental property of matter that is responsible for electrical phenomena. Charge (Q) is measured in Coulombs (C). Elementary charge, e = 1.602 x 10 -19 Coulombs (C) Valence: a term to describe the charge of a particle. Na+ and Ca++ have positive valence, while Cl- has negative valence. Moreover, Na+ is monovalent, and Ca++ is divalent. In the Nernst equation and the GHK voltage equation, valence is represented by the variable z. Anion: a negatively charged particle. Cation: a positively charged particle. Current: The rate of charge movement. Current (7) is measured in amperes (A), which is equivalent to coulombs per second (C/s). I = !Q/!t = Coulombs/sec = Amps (A) Voltage: The force created on a charge caused by the separation of charge. Voltage (F) is measured in volts (V). Voltage is equivalent to potential difference. Potential Difference: The same as voltage. Another definition is the difference in potential energy experienced by a charged particle in two locations (the work required to move a charge from point A to B). Potential difference (E) is measured in volts (V). E = Joules/Coulomb = Volts (V) Conductance: The ability of something (e.g. a wire or an ion channel) to pass current. Conductance (G) is measured in Siemens (S). Resistance: The inverse conductance. That is, the ability of something to impede current. Resistance (R) is measured in Ohms (!). Conductivity: The ability of a substance to pass current. For a membrane, the conductivity (Gm) is measured in units of S/cm2 and G=Gm*area. For a cable (e.g. axon or dendrite), the conductivity (G,, for intracellular conductivity) is measured in units of S/cm and G=Gi*length/area (i.e. cross-sectional area). Resistivity: The inverse of conductivity. Membrane resistivity (Rm) has units of !cm2. Intracellular resistivity (Ri) has units of !cm. 1 Ohm's law. Describes the relationship between voltage, current, and resistance, V=IR or R = V/I = Ohms (!) I = g V or g = I/V = Siemens (S) Rectification: The electrical property that causes current to flow more readily in one direction than another. This occurs when conductance is dependent on voltage. II. Properties of Ion Channels Gating: Conformational changes in an ion channel induced by an extrinsic source (i.e., voltage, ligands, stretch, etc.). Such conformational changes are responsible for functional properties like activation and inactivation. Activation: Opening of a channel due to the presence of a gating signal. Deactivation: Closing of a channel due to removal of the gating signal (i.e. the opposite of activation). Inactivation: Closing of a channel in the continued presence of the gating signal. The term "inactivation" is usually only applied to voltage-gated channels, whereas "desensitization" describes the analogous process for ligand-gated channels. Desensitization: Closing of a ligand-gated channel despite the presence of a bound activating ligand. For example, glutamate receptors desensitize in the continued presence of glutamate. Selectivity: The degree to which a channel allows a specific ion to pass while excluding others. Selectivity is often expressed as permeability ratios. For example, K+ channels pass K+ and exclude Na+ with a permeability ratio of about 105. Permeability: The ability of an ion to move through a channel. Different types of K+ channels can have different K+ permeabilities. Permeability is related to conductance, but not identical. Conductance depends both on permeability and the availability of the selective ion to carry charge. (See also GHK rectification). Rectification (of channels): A characteristic of an ion channel, generally independent of gating, that biases the preferred direction of current flow to either the inward or outward direction. Rectification can be due to an intrinsic property of the channel or be conferred by voltage-dependent block by an extrinsic agent. For example, the relatively high concentration of K+ ions inside a cell can cause outward rectification of some K+ channels, because more K+ ions are available to carry outward current than the number available to carry inward current. This is called GHK rectification. Another type of rectification is caused by polyamines. These charged molecules are only present inside cells and at depolarized potentials they move into the pore of some voltage-gated and ligand-gated channels, thus limiting outward current (i.e. inward rectification). These factors cause the channel conductance to be voltage dependent, thus resulting in rectification. Modulation: Alteration of channel properties by some chemical interaction or modification. For example, phosphorylation results in the modulation of many kinds of channels. 2 III. Special electrophvsiology terms. Tail current: A current that flows during the repolarizing phase of an action potential or voltage command. K+ tail currents can be used to determine the reversal potential of voltage-gated K+ currents. In a physiological context, tail currents are often carried by Ca2+ ions and result from the increased driving force as the action potential repolarizes. Access resistance: the electrical resistance between the inside of the patch pipette and the inside of the cell during a whole-cell recording. Compromises recordings by introducing a voltage-divider error and slowing the response time of the voltage clamp. Access resistance can be reduced by using larger patch pipettes and can be compensated electronically with the patch clamp amplifier. Gating current: a current resulting when charged residues within an ion channel protein move through the electric field. In voltage-gated channels, a change in membrane potential causes the protein to move; this movement gives rise to the gating current. Markov model: A probabilistic process over a finite set of states, often used to described channel behavior. Transitions between states are determined by rate constants. A zero-order Markov process has no memory; a first-order Markov process has a memory of one step, i.e., the possible states that a channel can occupy at time t depends on which state it was in at time t-1. Space clamp error: Neurons do not behave as single electrical compartments, making it impossible to clamp the entire cell membrane at the same membrane potential. This causes problems in two ways. First, some of the charge entering though an ion channel out on a dendrite leaks across the membrane resistance, such that the amplifier receives less charge than actually entered the cell. Second, some of the charge delivered by the amplifier to clamp the membrane potential leaks out on the way back through the dendrite. As a result, the current delivered from the amplifier that reaches the membrane surrounding the ion channel is not sufficient to offset the original current. The charge difference is deposited on the membrane capacitance, which causes a change in the membrane potential (commonly called an "escape potential"). Voltage escape further complicates bad voltage-clamp recordings because it changes the amount of current that flows relative to the perfectly clamped case. Voltage clamp: a rude imposition perpetuated by the experimenter to hold the membrane potential invariant, thereby eliminating current and allowing ioiic currents to be studied in isolation. 3 ...
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This note was uploaded on 02/02/2012 for the course BIOLOGY 222 taught by Professor Nagaya during the Winter '11 term at University of Michigan.

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