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Summary The membrane action potential
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Week 9 CTL NotesMembrane Potential & Excitable Tissues
Membrane Potential:
• Voltage across membrane
• Cause: unequal distribution of ions (K+ & Na+)
• Result of selective/differential permeability of specific plasma membrane proteins. (mainly ion
channels)
• Evident in all living cells (ranges between -20 & -200mV)
• In excitable tissues – stimulation causes change in Membrane Potential.
o Results in activation of the cell.
o Nervous & Muscle Tissues = Excitable Tissues.
• Membrane Potential: Depends on:
o Relative permeability of PM to ions
o Each ion’s concentration gradient.
o Electrochemical gradient
• Resting Membrane Potential:
o Stable membrane potential of cells when unstimulated.
o For Nerve Cells – approx -70mV
Ion Distribution Across Plasma Membrane
• K+: [greater inside cell]
o At rest, membrane is much more permeable to K+ than to Na+.
o Ie. K+ diffuses out of the cell through leakage channels down its conc. gradient.
§ Therefore, loss of positive charge from cell makes:
• Inside the cell negative
• Outside the cell positive
§ Eventually the negativity of the inner membrane face attracts K+ back into the cell.
• Therefore, conc. Gradient drives K+ out and is equally opposed by electrical
gradient. (equilibrium potential has been reached)
+
• Na : [greater outside cell]
o Membrane has much lower permeability to Na+.
o Negative inner membrane-face attracts Na+ into cell, but is opposed by low permeability.
o Therefore low diffusion of Na+ into cell.
**Simply: Resting MP is established due to greater diffusion of K+ out than Na+ in**
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, The Na/K ATPase: Maintaining the Resting Membrane Potential.
• Na passively diffuses into cell and K passively diffuses out.
o Why doesn’t the chemical & electrochemical gradients dissipate?
• The Conc. Gradients for both Na & K are maintained by the Na/K ATPase.
Excitable Tissues (Nerves/Muscle)
• In excitable cells, stimuli can alter the permeability of the membrane to K+ and/or Na+.
o Via opening/closing gated ion channels (ligand/chemically gated, voltage gated,
mechanically gated, vibration gated, temperature gated)
• This changes the membrane potential.
• If the membrane potential is sufficiently altered, an action potential is initiated.
• Action potential: an electrical impulse generated and conducted along a nerve’s axon in response to
stimuli.
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