Membrane Potentials I and II Flashcards
What is the typical membrane potential of a cell?
-60 mV
two basic functions of membrane potentials
storage of energy
generation of action potentials for communication
driving force behind influx of calcium
extracellular/intracellular concentrations of Na+
145/10 mM
extracellular/intracellular concentrations of K+
5/140 mM
extracellular/intracellular concentrations of Cl-
116/4
causes of membrane potential
membrane is selectively permeable to K+
K+ leaks out of cells down its concentration gradient
leakage of K+ leaves a net negative charge inside the cell and a net positive charge outside
this generates an electric field, which opposes the opposite of K+
ionic equilibrium potential
the membrane potential at which there is no net flux of the ion across the membrane
different for each type of ion
due to a balance of electrical and chemical forces
defined by the Nernst equation
Nernst equation
Ei = (RT)/(zF)ln[(Ciout)/(Ciin)]
R = Gas constant (8.314)
T = temperature in K
z = number of elementary charges of the ion in question
F = Faraday constant (96,485)
Nernst potential for Na+
+67mV
Nernst Potential for K+
-84 mV
Nernst potential for Cl-
-84 mV
Nernst potential for Ca2+
+125 mV
Goldman-Hodgkin-Katz equation
sum of equilibrium potentials for major ions
What maintains the membrane potential?
the Na+/K+ ATPase moves 3 Na+ and 2 K+ in per cycle
resting potential occurs when ATPase activity counter-balances the passive fluxes
How quickly does potential across a passive membrane change? Why?
exponentially because the membrane acts as a resistor and a capacitor
Equivalent Circuit Model
modeling a membrane as a capacitor and a resistor (conductor) in parallel
conductance equation
Gion = Iion/(RP - Eion)
equation to calculate resting membrane potential using the electric circuit model
equation to calciulate membrane potential in terms of permeability
permeability
a measure of how many channels are open, does not depend upon ion concentrations
response time
rate of change of Vm
has a delay due to Cm
time constant = Rm x Cm
stimuli that can alter membrane potential
natural an artificial stimuli
natural stimuli
neurotransmitters
hormones
toxins
artificial stimuli
elevated extracellular K+ concentrations
intracellular Na+ injection
pharmaceutical agens that are analogs of natural stimuli
action potential
a membrane depolarization that reaches a theshold and elicits additional depolarization without further stimulation
properties of an action potential
rapid, transient depolarization of the membrane potential that exceeds 0 mV and is followed immediately by a rapid hyperpolarization
during of aobut 1-100 msecs
absolute refractory period (ARP)
the short period of time after an action potention where a second one cannot be evoked
relative refractory period (RFP)
the period of time from the end of the absolute refractory period to the time when new action potentials can be generated
Why does the Na+ permeability of the membrane change with depolarization?
Due to the opening of depolarization-gated Na+ channels
activation of Na+ channels produces a regenerative increase in Vm
What gives rise to the rapid fall of the action potential?
the opening of potassium channels that allows efflux of potassium
slightly slower than sodium channels
leads to the hyperpolarization of the membranes because it makes the membrane much more permeable to potassium than sodium
drugs that modify or block action potentials
Na+ channel blockers
k+ channel blockers
Ca2+ channel blockers
