Resting and action potentials Flashcards
Why do ions migrate at different rates?
According to the size of their hydrated molecules
How are migration rate of ions measured?
Under standard conditions
How are migration rates expressed?
In terms of the mobility of an ion
What is another way of saying mobility potentials?
Diffusion potentials
What does the magnitude of diffusion potentials depend on?
Ion salts
What is the difference in diffusion potentials with KCl and NaCl?
KCl- barrier removal will cause a slight change in potentials, lasting until all ions are dispersed evenly
NaCl- barrier remval will cause a much larger potential
Why does NaCl cause a larger potential than KCl?
As there is a bigger difference in mobility’s of Na and Cl ions
What is a semi-permeable membrane?
A membrane that is selectively permeable to some ions, but impermeable to others
What is the mobility of an impermeant ion?
Zero
In an electrical membrane potential what direction and charge is on the K ions?
Down their concentration gradient
Carry positive charge
In an electrical membrane potential what direction and charge is on the Cl ions?
They do not move as they are impermeant
What does the electrical membrane potential of K and Cl result in?
Positive charge build up in the right compartment
What then steps in?
Electrical gradient which opposes the movement of K ions
What is the result when there is no further net movement of K ions?
Electrical gradient equals the concentration
The system is in equilibrium
What does Em stand for?
Electrical membrane potential
Does the Em remain after the system is back in equilibrium?
Remains indefinitely since the unequal distribution of ions remain indefinitely
What are the major impermeant anions in cells?
Proteins carrying net negative charge, including neurons
What is the resting membrane potential inside the cell?
-73mV
What are the most permeant through the membrane of a neurone at rest?
K+
What is the permeability of K+ due to?
The existence of ion channels in the membrane that are open in the resting state and are most selective for K+ ions
What are these selective channels called?
Leak channels
What do leak channels allow?
A very small number of Na+ ions to pass through
How quickly does K+ diffuse?
Very quickly
How do the concentration and electrical gradient for K+ ions act?
In opposite directions
When is an equilibrium struck between the concentration and electrical gradients?
When these two forces exactly match each other
How do the electrical and concentration gradients reach equilibrium?
Electrical gradient must increase slightly from the resting Em
What is the value of the equilibrium potential for K+?
-74 mV
Why is the inside of the cell negative?
Na+ and K+ pump
3 Na+ pumped out of the cell
2 K+ pumped into the cell
Accumulates to give more positive ions outside the cell, and more negative ions inside the cell
Why would sodium like to move down it’s concentration gradient but can’t?
As there is a higher number of Na+ inside the cell and it would like to go into the cell but can’t because of the membrane
What is the concentration and electrical gradient for Na+ ions?
Initially act in the same direction
How should sodium enter the cell?
Very quickly
Why doesn’t sodium move very quickly into the cell?
Permeability for Na+ is very low
When is a Na+ equilibrium reached?
When the cell interior becomes sufficiently positively charged to exactly counteract the inward concentration gradient
What is the equilibrium potential for Na+?
ENa= + 54mV
Why must the resting membrane potential be a K+ potential?
As the resting intracellular membrane potential is -73mV and with K+ its -74mV whereas with Na+ it’s 54mV meaning it had to be a K+ potential
Where does the Na+/K+ exchange pump act?
Across the membrane
What happens to Na+ and K+ ions in this exchange pump?
Ions are moved aross their concentration gradients
How does the exchange pump consume energy?
As the exchange of ions is coupled to the splitting of ATP
Why is the sodium potassium pump electrogenic?
As it produces a change in the electrical potential of a cell.
What is intracellular recording?
Measures the voltage across or passing through membranes by inserting an electrode inside the cell
What is extracellular recording?
Measures electrical activity between two points outside the cell
What is the resting potential mainly dictated by?
K+
What is the threshold in an action potential?
The level of depolarisation required to cause an increase in Na+ permeability (pNa)
Beyond this point the action potential is an ‘all-or-none’event
What is the rising phase of an action potential?
The rapid depolarisation occurs as the large driving forces causing Na+ entry (negative potential inside and concentration gradient) come into play
What is the overshoot in an action potential?
The inside of the neurone becomes negative to the outside, as the system is driven towards ENa
What is the falling phase in an action potential?
The switch from high pNa to high pK allows the driving forces causing K+ exit (+ve potential inside and concentration gradient) to come into play
What is the undershoot in an action potential?
The system is driven towards EK, before returning
to its resting state
