NBB 301 Lecture Notes - Lecture 6: Equivalent Circuit, Potassium Channel, Ion Transporter

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Lecture 6
A. A neuron is in an
B. Equivalent circuit model of the passive ion fluxes across the membrane
a. In the model the battery for Na is oriented where the + is on the cytoplasmic side because
Na wants to move inside (positive charge is moving inside)
b. K is the opposite
c. Chloride will move inside to the cytoplasmic side and as the negative ions move in, it causes
positive ions to go out resulting in the outside is becoming more positive so positive faces
the extracellular side
d. When calculating membrane potential --> look at conductances
e. + and - values for current are the movement of positive ions
i. + meaning moving out
ii. - meaning moving in
C. Equivalent circuits: pups and capacitance
a. Ion pumps are active current generators, not conductances
b. Pump is critical in maintaining the concentration gradient
D. You have a neuron with Ek = -80 mV, and Ena = +55 mV and a Vrest = -65 mV. An experimenter
injects negative current into the neuron, resulting in a new Vm = -90 mV. Calculate the new driving
force for K and Na . What effect will this have on the direction of ion flux for each ion?
a. Driving force
i. -65 - -80 = 15 for K
ii. -65- 55=-120 for Na
b. New driving force with new Vm = -90
i. -90 - -80 = -10 for K
ii. -90- 55=-145 for Na
c. Direction of K is changed and going into the cell instead of going out. Direction of Na did not
change.
E. Equivalent circuits and 'driving force'
a. For any ion channel, current through this open channel is determined by 2 things:
i. The single-channel conductance
ii. The driving force
b. Modified ohm's law
F. Draw an I-V plot for a potassium channel with single channel conductance (gamma) = 100 pS and
Ek = -90 mV. Assume the channel has a linear I-V relation. Include data points on your plot for the
following membrane voltages ( in mV) : -130, -110, -90, -70, -50.
a. 100x10^-9*(-130--90 X10^-3) = -4x10^-12
b. 100x10^-9*(-110--90 X10^-3) = -2x10^-12
c. 100x10^-9*(-90--90 X10^-3) = 0
d. 100x10^-9*(-70--90 X10^-3) = 2x10^-12
e. 100x10^-9*(-50--90 X10^-3) = 4x10^-12
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