Lecture 3: Cellular Neurophysiology Flashcards
Define chemical gradient (concentration gradient)
the _______ provided by the difference in ________ across the _____ ________
the energy provided by the difference in concentration across the plasma membrane
Define electrical gradient (potential difference)
the energy associated with moving charged molecules across the membrane, when a membrane potential exists
What is the charge difference between the two sides of the membrane (resting membrane potential)?
-70mV
There is a high concentration of Na+ ions inside or outside the cell?
outside
There is a high concentration of K+ ions inside or outside the cell?
inside
There is a high concentration of Cl- ions inside or outside the cell?
outside
The cell membrane is most permeable to which ion?
K+
What is the electrical gradient of K+ ions and why?
Since K+ ions are positively charged and the inside of the cell is negatively charged (-70mV), the electrical gradient drives K+ ions into the cell
What is the electrical gradient of Na+ ions and why?
Since Na+ ions are positively charged and the inside of the cell is negatively charged (-70mV), the electrical gradient drives Na+ into the cell
What is the chemical gradient of K+ ions and why?
There is a higher concentration of K+ ions inside the cell than outside the cell. As ions flow down their concentration gradient, the chemical gradient drives K+ outside the cell
What is the chemical gradient of Na+ ions and why?
There is a higher concentration of Na+ ions outside the cell than inside the cell. As ions flow down their concentration gradient, the chemical gradient drives Na+ inside the cell
What are electrochemical gradients set up by?
the ionic distribution across the membrane
What is the electrochemical gradient used for?
It is used to quantitate the driving force acting on a molecule to cause it to move across a membrane
What is the electrochemical gradient a measure of?
the free energy available to carry out the useful work of transporting the molecule across the membrane
What are the two components of the electrochemical gradient and what do these represent?
- the chemical gradient which represents the energy in the concentration gradient for an ion across the membrane
- the electrical (potential) gradient which represents the energy associated with moving charged molecules across the membrane when a membrane potential exists
What are the three definitions for equilibrium potential (reversal potential)?
- the energy of a concentration gradient of an ion
- the electrical potential that exactly balances the concentration gradient of an ion
- the charge pushing an ion in one direction balancing the concentration gradient pushing it in the other direction
What is the Nernst equation and what does it calculate?
E(ion) = (60/Z)log(Co/Ci)
This calculates the equilibrium potential for the ion
What does the Z in the Nernst equation represent?
the valence (charge of the ion)
What is the base for the log in the Nernst equation?
base 10
What does the Co in the Nernst equation represent?
the concentration of the ion outside the cell
What does the Ci in the Nernst equation represent?
the concentration of the ion inside the cell
How is the resting membrane potential of -70mV maintained?
by the movement of K+ and Na+ ions against their concentration gradient via ATPase
What are local potentials?
Small ion fluxes (channel openings) across the membrane which change the membrane potential in one area that cannot spread far from the site of stimulation
Define depolarisation
becoming more postive
Define overshoot
becoming more positive than zero
Define repolarisation
becoming more negative but still above RMP
Define hyperpolarisation
move negative than RMP
Describe a local potential (5)
- it is graded in ____
- the _______ decreases over _______
- it is _____ in ____ and ____
- can ________ and ________
- influences the generation of an _______ ______
- it is graded in size
- the amplitude decreases over distance
- summated in time and space
- can depolarise and hyperpolarise
- influences the generation of an action potential
What is the difference between an action potential and a local potential?
Local potentials are passive changes in membrane potential by flow of current across the membrane by opening of channels when synapses are active. Local potentials decrease in amplitude from the point of origin. This differs from an action potential which are an actively propagated depolarisation that spreads from the point of origin and typically down the axon to trigger communication with the next neuron in the sequence
What is the length constant?
the distance over which the voltage in a local potential decays to 37%
What is the axial resistance?
the resistance along the axis
What happens when the resistance is low?
the voltage decays much quicker
What are some features of action potentials?
4
- they are actively propagated from the point of origin
- a local potential is generated in that region which pushes it to threshold
- all or nothing so once threshold is reached, an action potential occurs
- regenerative
Describe the process of an action potential
- A local potential lifts the membrane potential to threshold and then an action potential is generated
- this overshoots beyond zero towards the equilibrium potential for sodium
- then repolarisation and afterhyperpolarisation
An action potential is regenerative. What does this mean?
it is propagated without decreasing amplitude
