PHSI2005 Study Guide - Midterm Guide: Reversal Potential, Relative Permeability, Cell Membrane
15 May 2018
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Department
Course
Professor
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Cellular Neurophysiology (L6-10)
Membrane potential
Imbalance of charge across the plasma membrane of a motor neuron
K/Na ATPase pump Na into cell (depolarise, positive ions into cell) & K+ out/Cl- into cell
(hyperpolarise, positive ions out of cell)
*Na +, K more -ve
ECF (conductor): high concentration of sodium ions, balanced by chloride ions
ICF (conductor): high conc of K ions, balanced by organic anions (e.g. protein, ATP)
Type of ion channels
Non-gated (permeable to Na/K)
Voltage-gated (permeable to Na/K/Cl/Ca)
Ligand-gated (permeable to Na/K/Cl/Ca)
Selective diffusion of a few K+ ions generates membrane polarity
Only a tiny fraction of K+ ions need to diffuse across the membrane to generate
membrane polarity (-60 mV)
K+ leakage channels allow K to slowly diffuse across the membrane, driven by its
chemical driving force (non-gated K selective channel)
Electrical forces on ions
Cations attracted to areas of net negative charge
Anions attracted to areas of net positive charge
The net force acting on an ion is the sum of the chemical & electrical driving force
vectors (chemical – electrical)
Equilibrium/ Nernst potential
When electrical = chemical driving force, = equilibrium/Nernst potential
R=gas constant, T=temp in kelvin, F=faraday constant, Z=valance of ion (chemical driving force
e.g. +1)
Ek ~ -75 mV
ENa ~ +55 mV
ECl ~ -60 mV
Relative permeability to ions
When membrane is permeable only to K+, the membrane potential (Vm) will be equal
to the Nernst potential for K+ (Ek)
When membrane is permeable only to Na+, the membrane potential (Vm) will be equal
to Nernst potential for Na+
When membrane is permeable to both K+ & Na+, then Ek < Vm < ENa
*resting membrane is 20x more permeable to K+ than Na+ (thus resting MP is closer to Ek)
Buffering of membrane potential (Vm)
9
Vm rise due to synaptic input reduced electrical force increase outward flow of K &
reduce inward flow of Na
Vm falls increased inward electrical driving fore drives more Na in through leakage
channels to bring Vm back to resting value
Action potential
Each motor neuron receives many excitatory synaptic inputs (bouton synapses)
Electrical signal fade with distance
oMost central neurons are contacted by other neurons that form excitatory
synapses on their dendrites
oDepolarising inward sodium currents carry signals in neurons
oGraded depolarizations diminish in amplitude with distance from the source
oAmplification is therefore needed
Electrical signals
Axons and dendrites have electrical properties that slow the spread of changes in
membrane potential
oConducting fluid inside – axoplasm, has resistance that impedes the depolarising
current down the membrane
oConducting fluid outside – ion channels in the membrane have resistance but
allow leakage of local circuit currents out of the membrane
oInsulating membrane separating them- electrical capacitance
Voltage gated Na+ channels amplify depolarisations
The Hodgkin Cycle
Positive feedback loop in which an initial membrane depolarization leads to
uncontrolled deflection of the membrane potential to near VNa
oStimulus depolarisation opens activation gates of a small fraction of Voltage-
Gated Na+ channels (VGSCs)
oIncreased inward Na+ current through these activated VGSC’s further
depolarises the membrane
oAs membrane polarises more, activation gates open on a greater fraction of the
VGSCs
Collective behaviour of VGSCs
The Hodgkin Cycle
Higher the membrane density of VGSCs the lower the threshold potential (trigger zone)
Threshold potential helps gating of information flow
Close packing of VGSCs increases current density & lowers threshold for an action
potential
Voltage-gated K+ channels rapidly repolarize
Properties of VGKCs
Many VGKCs scattered through axon membrane
Most VGKCs are closed at resting Vm
Fraction of channels open increases proportionately with depolarisation
Opening & closing of VGKC activation gates is slow
Document Summary
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