Neurophysiology Study Guide
1. Define "equilibrium potential". Why is the resting potential closer to the potassium equilibrium potential (EK+) than the sodium potential (ENa+)?
The equilibrium potential is the point at which the force exerted on an ion by electrostatic and concentration gradient forces are balanced, and there is no net movement of that ion. The resting potential is closer to EK+ than ENa+ because the cell membrane is more permeable to potassium than sodium.
How would ENa+ change following an increase in the external concentration of sodium? Would the resting potential be affected significantly?
If the external concentration of sodium was increased, the concentration gradient driving the movement of sodium would be increased more sodium would diffuse into the cell and would reduce some of the negative charge on the inner surface of the lipid bilayer. This would serve to decrease (make less negative) the resting potential, but would not affect it significantly since the membrane is much more permeable to K.
Does any net Ionic current flow at the resting potential? What are the relationships among “passive" and “pump”ionic currents at the resting potential?
There is no net ionic current flow at the resting potential. However, since neither Na+ nor K+ is at equilibrium, there is a net flow of each across the membrane. The Na-K pump generates the concentration difference that sustains the resting potential: but it is the passive ionic flow that creates the resting potential. Pump currents balance passive currents (diffusion) at the resting potential.
Define "electrogenic" and "neutral” pumps. What role does the Na-K pump play in the resting potential?
An electrogenic pump creates a potential difference across the membrane - positive and negative charges are not transported across the membrane in equal amounts. Neutral pumps are balanced according to charge - there is no potential difference created. The Na-K pump generates the concentration difference that sustains the resting potential.
5. Describe the sequence of ionic events and their effect upon the resting potential following
blockade of the Na-K pump, or following an increase in extracellular potassium or
When the Na-K pump is blocked by an agent such as oubain or digitalis, Na+ and K+ will continue to diffuse passively across the cell membrane down their respective gradients. After a while, intracellular [Na] will increase and intracellular [K] will decrease. The reduced [K] gradient will reduce K efflux through nongated channels; therefore, the resting potential will depolarize. The reduced [Na] gradient will have little effect on the resting potential. If extracellular [K+ ] is increased, the driving force for potassium diffusion out of the cell will decrease, and the resting potential will become less negative (depolarized). If extracellular [Na+ ] is increased, the driving force for sodium movement into the cell will increase with a resultant slight depolarization, but there will be no significant change in resting potential.
\Na+ -K+ ATPase pump activity Y [[Na+]in Y \[Na+]O/ [Na+]i Y\ ENa+ Y
\(EM - ENa+) Y \ INa+ Y \ Conduction velocity and Depolarized threshold
Explain the interrelationships between ionic and capacitive currents during postsynaptic potentials or during an action potential.
Ionic current is the flow of ions through channels into or out of the cell. Capacitive current is the movement of charges, usually positive, up to or away from the cell membrane. During an excitatory postsynaptic potential or an action potential, there is an initial ionic current flow into the cell, followed by a capacitive current out of the cell, which serves to depolarize the membrane potential at a distance. The current...
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