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Lecture
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Human Physiology
Monday, September 28, 2009
Resting Membrane Potential
•Resting membrane potential
Membrane permeable to a variety of ions, so can’t use the Nernst equation
Each ion affects the RMP depending on its permeability & concentration gradient
Therefore use the Goldman equation
•Goldman equation
No valence in the equation because the charge is taken into account based on the position of the
ion in the equation
Shows
Important ions (K, Na, and Cl) and their concentration gradients
Degree of importance of each ion at a particular time is proportional to membrane
permeability
•RMP = -70mV (can vary from cell to cell)
This potential maintains the concentration gradient of chloride
Some potassium will leak out because the RMP isn’t strong enough to counter the movement of
potassium
Potassium moved back in using the Na/K pump
Sodium (whose equilibrium potential is much higher than the RMP) theoretically should move
down its gradient, but membrane is less permeable to sodium than potassium
Cells in the heart don’t have an RMP
Ionic Basis & Propagation of the Action Potential
•Neuron
Excitable cells (i.e. capable to self-generating electrochemical impulses)
Parts
Soma/cell body (contain all the organelles)
Dendrites (receive signals from other neurons)
Axon (send signals to other neurons/muscles via the nerve terminals)
•The action potential
Initiated at the axon hillock by a depolarizing stimulus
Not completely dependent on each other because one can function without the other
Basically a rapid reversal of the membrane potential
Voltage-gated ion channels (only found on the axon)
Sodium & potassium
•Both channels have activation & inactivation gates
Open when the membrane is depolarized
Different from leak channels/pores in that they have gating mechanisms
Sodium channel
At rest, activation gate is closed
Stimulate the cell to cause a depolarization (i.e. the cytosol becomes positive relative to
the outside)
Activation gate opens [instantaneously; briefly ~0.1ms] & sodium can enter the cell
Each channel allows about 10 million ions through per second
After ~0.1ms, the inactivation gate closes, preventing the passage of more ions into the
cell (channel is now called inactivated)
1-2ms later, channel returns to its resting state
Potassium channel
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