Resting membrane potential 1 mV= 0.001 V membrane separates intra- and extracellular compartments inside negative (-80 to -60 mV) due to the asymmetrical.
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Resting membrane potential 1 mV= 0.001 V membrane separates intra- and extracellular compartments inside negative (-80 to -60 mV) due to the asymmetrical distribution of ions across the cell membrane AND the differential permeability of the membrane to these ions
Channels allow ions to diffuse across membranes Voltage-gated: Na + channels, K + channels, Ca 2+ channels Ligand-gated: neurotransmitters (acetylcholine, glutamate)
Figure 5-34c Resting membrane potential is due mostly to high potassium permeability
The Nernst equation describes an ion’s equilibrium potential where: R is the gas constant (8.314 X 10 7 dyne-cm/mole degree), T is the absolute temperature in o Kelvin, z is the charge on the ion F is the Faraday (the amount of electricity required to chemically alter one gram equivalent weight of reacting material = 96,500 coulombs).
A simpler version of the Nernst equation At 37ºC : When ions can move across a membrane, they will bring the membrane potential to their equilibrium potential.
Calculating the membrane potential for a cell that is only permeable to K + [K + ] out = 5 mM [K + ] in = 150 mM E k = 61 x (-1.5) = -92 mV
Sodium Equilibrium Potential E Na = 61 x 1 = +61 mV
The Na + -K + -ATPase (“sodium pump”) works to keep intracellular K + high and Na + low
The membrane potential can be described by the relationship between ion permeabilities and their concentrations The Goldman equation: V m = P Na [Na + ] out + P K [K + ] out + P Cl [Cl - ] in Predicting the membrane potential (V m ) P Na [Na + ] in + P K [K + ] in + P Cl [Cl - ] out 61 log At the resting potential a. K + is very close to equilibrium. b. Na + is very far from its equilibrium. c. P K >> P Na
Real neurons and “Dynamic Polarization” Pyramidal cell Layer V neocortex Purkinje cell Cerebellum Axon Dendrites Santiago Ramon y Cajal, 1900 Axon collaterals Collateral branch Input Output
Electrical Signals: Ion Movement Resting membrane potential determined by –K + concentration gradient –Cell’s resting permeability to K +, Na +, and Cl – Gated channels control ion permeability –Mechanically gated –Ligand gated –Voltage gated
Current flow through ion channels leads to changes in membrane potential Ohm’s Law: V = I * R V = voltage, I = current (Amps), R = resistance (Ohms) I = V/R or I = V * G G = conductance (Siemens) For current to flow, there must be a driving force (V m - E ion ) > or < 0, thus I = (V m - E ion ) * G If current flows across a resistance--the cell membrane acts like one--there is a change in voltage (membrane potential).
Graded potentials can be:EXCITATORYorINHIBITORY (action potential(action potential is more likely)is less likely) The size of a graded potential is proportional to the size of the stimulus. Graded potentials decay as they move over distance. Graded Potentials
Graded potentials decay as they move over distance.
Patch-clamp recording Giga=10 9 Mega= 10 6 vs. sharp microelectrode Pros: high resistance seal & low resistance electrode better for recording small currents and injecting large currents Cons: disrupt (“dialyze”) cellular contents
Single channel recordings “stochastic behavior” Characterize channels by their: conductance (pS) selectivity kinetics
Whole-cell recording of different types of K channels