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37 Cards in this Set
- Front
- Back
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Electrical Signals of Nerve Cells |
- Receptor potential (graded) - Synaptic potential (graded) - Action potential (all-or-none) |
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Receptor Membrane Potential |
- an intrinsic property of the cell - can be calculated if you know channel permeability and ion concentration in/out of the cell |
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Current |
- the movement of charge in a given period of time - units are amperes (A) where 1A = 1C/second |
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(2) Factors Determining Size/Amplitude of Currents |
- the potential difference between the electrodes - the electrical conductance of the medium between them |
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Electric Potential |
the amount of potential energy per unit of charge at a given location |
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Potential Difference |
the difference in potential between 2 locations (the work needed to move a test charge between points)
units are in volts (V) |
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Conductance |
the ease of flow of current between two points
units are in siemens (S) |
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Resistance |
the inverse of conductance
units are in ohms (Ω) |
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How to Calculate Conductance and Resistence |
- can be calculated if potential and current are known - use Ohm's Law |
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Ohm's Law |
V = IR or I = gv
where V is voltage, I is current, R is resistance and g is conductance |
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Cell Membranes and Capacitors |
the cell membrane is a capacitor: - two conductors (outside and inside of cell) - separated by an insulator (the membrane)
when charge is introduced onto 1 of the conductors, it pushes similar charges away from the other conductor as they repel. Thus, a separation of charge occurs. |
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Capacitance |
a measure of how much charge (Q) needs to be transferred from 1 conductor to another to set up a given potential difference
units are farads (F) where C=Q/V
a 1F capacitor will be charge to 1V when +1C of charge is on one conductor and -1C on the other |
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Cell Membrane Capacitance |
cell membranes have a capacitance of approximately 1 µF/cm² so the capacitance of a cell increases with membrane surface area |
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Ion Flux |
current can travel across the membrane through conductors (or resistors) in the membrane (ie. ion channels) |
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Channel Permeability |
- channels are selectively permeable to specific ion species |
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Flow of Ions... |
creates currents
(no ion gradient = no voltage difference and no current flow) |
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Ion Transporters |
- actively move ions against concentration gradient - create ion concentration gradients |
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Ion Channels |
- allow ions to diffuse down concentration gradient - cause selective permeability to certain ions |
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Cloride Intracellular Concentrations in Mammalian Neurons |
varies considerably more than other ions (7-50 mM) |
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Equilibrium Potential |
- the potential at which the net flux of ions into and out of the cell are equal - the current for an ion at the equilibrium potential will be 0 |
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The Nernst Equation |
V or Ex = (RT)/(zF) ln[CO/CI]
V or Ex: the equilibrium potential C: the concentration of the ion (I=inside, O=out) R: the gas constant [2 (cal)/(mol K)] T: temperature (K) F: Faraday's constant [23000 (cal)/(V mol) z: the charge of the ion |
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The Nernst Equation at 37° C for a Monovalent Cation |
V or Ex = 61.5 log[CO/CI] |
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At a Given Temperature... |
RT/F can be treated as a constant |
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Extracellular/Intracellular Concentrations of Na(+) |
extracellular = 145mM intracellular = 5-15mM |
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Extracellular/Intracellular Concentrations of K(+) |
extracellular = 5mM intracelluar = 140mM |
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Extracellular/Intracellular Concentrations of Ca(2+) |
extracellular = 1-2mM intracellular = 0.0001mM (high levels are toxic) |
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Extracellular/Intracellular Concentrations of Cl(-) |
extracellular = 110mM intracellular = 4-30mM (sits near resting membrane potential, either above or below, carefully dependent on concentrations - very important signalling molecule) |
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Direction of Flux |
For (+) charged ion: - out of the cell: if membrane potential is more (+) than the equilibrium - into the cell: if the membrane potential is more (-) than the equilibrium
reverse is true for (-) charged ion |
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Exceptions to Ohm's Law |
If V= the equilibrium potential for K(+) there is no K(+) flowing but there should not be a voltage if I=0 |
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Modification to Ohm's Law |
(accounts for concentration gradient)
V - Veq = IR (V: voltage, Veq: equilibrium potential)
Thus V - Veq = 0 at the equilibrium potential and the further from the equilibrium potential, The larger the current will be for that ion |
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Current Voltage (IV) Plot |
lines cross the x-axis at the equilibrium potential for the ion that is mediating the current |
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Effects of Changing Ion Concentrations on the Current Voltage (IV) Plot |
changing the concentration of an ion on either side of the membrane will change the equilibrium potential but not the slope |
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Goldman-Hodgkin-Katz Equation |
Vm = (RT/F) ln[ (PNa[Na(+)]O + PK[K(+)]O )/( PNa [Na(+)]I + PK [K(+)]I )
modification of the Nernst equation to include the relative permeabilities (P) of each ion (at rest relative permeability of K(+) = 40, Na(+) = 1 -- resting membrane potential is dominated by the K(+) equilibrium potential |
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At Rest the Resting Membrane Potential is Determined By: |
K(+) distribution |
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(3) Determinants of Ion Distribution Across the Membrane |
1. the chemical gradient 2. the electrical gradient 3. permeability (conductance) |
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Maintaining the Resting Membrane Potential |
- an active process (requires energy) - maintains gradients using sodium-potassium pump (Na(+)/K(+) ATPase): 3 Na(+) in, 2 K(+) out - not an equilibrium potential because it relies on the constant expenditure of energy (ATP) by ion pumps for its maintenance |
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What Results in Action Potentials? |
action potentials are the result of increasing a cell's permeability to Na(+) and K(+)
(molecular properties of these ion's channels underlie the shape of action potentials) |
