PHYL3001 Lecture Notes - Lecture 6: Lysine, Potassium Channel, Hyperkalemic Periodic Paralysis
LECTURE SIX: Electrophysiology Revision
Membrane Potentials and Ion Channels:
• Vital for cell function
• Membrane potential = major force on ions and molecules in all cells
• Osmotic balance in cells → ions are most abundant in dissolved solutes
• Ion flows and voltages can control fluid flows in specialized epithelia →
secretion and absorption
• Important for sensory signaling, force generation/motility, intracellular
enzyme cascades, gene expression, cell growth and cell death
• Membrane potentials are generated by diffusion of ions through
selectively permeable membrane
• Selective diffusion results in separation of opposite charge
• Voltage generated is proportional to concentration gradient
• Voltage is generated without significance in concentration gradient
• Capacitance → capacity to store charges of opposite sign on conducting
plates separated by an insulating layer (Farads/cm2)
Electrochemical Driving Force:
• Electrochemical potential energy difference → chemical potential energy
difference – electrical potential energy difference
• Ex → the value Vm would have to have for ion X to be in electrochemical
equilibrium
• If Vm -/- Ex → ion X cannot be in electrochemical equilibrium
• Net flow measured as flow of ions/current (I) → Ix = Gx(Vm-Ex)
• IV curve → relationship between Vm and current flow
• Ohms law → I = V/R
Dependence of Vm on Concentration Gradient:
• Vm recordings from glass microelectrode (3 M KCl)
• Dependence of membrane potential on [K+]o
• Deviation from linearity due to Na+ permeability
• Resting Vm = steady state, not equilibrium
IV Curve for Na+ and K+:
• At rest PK >>> PNa → Vm is close to EK
• During AP PNa >> PK → Vm is close to ENa
• Depolarization/repolarization reflect transient reversal of the K/Na
conductance’s
Voltage-Clamping:
• Allows control of voltage and measurement of current
• Use of channel blockers reveal transient inward Na current and delayed
outward K current
• Na current is transient due to channel inactivation
• 40% of Na channels are inactivated at normal resting potential
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