Membrane Potential and Action Potentials Flashcards
What is flux?
The number of molecules that cross a unit area per unit
of time (number of particles). i.e. molecules.m−2.s−1
Why is diffusion useful?
Useful for transport over short distances
Spontaneous
No energy input required
What are the properties of ions? (3)
Charged molecules
Opposite charges attract
Like charges repel
What is voltage?
Voltage = Potential difference
Unit: Volts
Generated by ions to produce a charge gradient (i.e. like a chemical battery)
What is current?
Current
Unit: Amps
Movement of ions due to a potential difference
What is resistance?
Resistance
Unit: Ohms
Barrier that prevents the movement of ions
How do you calculate voltage?
V= I R
current x resistance
How do you measure membrane potential?
To measure membrane potential a reference electrode is placed outside the cell. This is the zero-volt level.
Another electrode is placed inside the cell. It measures a voltage that is negative compared with the outside (i.e. reference).
All cells have a membrane potential. The difference in voltage between the inside and outside.
What is the cell membrane?
Lipid (hydrophobic) cell membrane is a barrier to ion movement and separates ionic environments.
The cell membrane can selectively change its permeability to specific ions.
What are ion channels?
Permeable pores in the membrane (ion channels) open and close depending on transmembrane voltage, presence of activating ligands or mechanical forces.
Ion channels can be selective for different types of ion (K+, Na+, Cl-, Ca2+).
When does movement across a membrane occur and when does it stop?
Movement across the membrane will occur when the concentration of the ion is different on one side of the membrane and will stop when equilibrium is reached.
Can some ions be pulled back through ion channels while diffusing to an area of lower concentration?
Yes
What is electrochemical equilibrium?
electrical forces balance diffusional forces
A stable transmembrane potential is achieved
What is equilibrium potential?
The potential at which electrochemical equilibrium has been reached. It is the potential that prevents diffusion of the ion down its concentration gradient.
How do you calculate equilibrium potential (E)?
The Nernst equation
How can the Nernst equation be simplified?
Na+ and K+ are the most important ions at determining what?
The resting potential of neurones
What is the extra and intra cellular concentration of Na+?
150mM extra
10mM intra
What is the extra and intra cellular concentration of K+?
5mM extra
150mM intra
What are the concentrations of other ions?
What are Ek and Ena?
EK and ENa are theoretical values. In reality biological membranes are not uniquely selective for an ion. Membranes have mixed and variable permeability to all ions (but, for neurones at rest K+»_space; Na+).
A typical resting membrane potential (Em) is -70 mV and not - 90 mV which is EK.
Each ion’s contribution to membrane potential is proportional to how permeable the membrane is to the ion at any time.
What is the GHK equation?
Goldman-Hodgkin-Katz
The GHK equation describes the membrane potential (Em) more accurately; P is permeability or channel open probability (0 = 100% closed, 1 = 100% open, 0.5 = open 50% of time), Subscript on P indicates the ion, [K+], [Na+] and [Cl-] represent concentration and the subscript i or o indicates inside or outside the cell.
What influences an ion’s diffusion?
It’s charge
What does the GHK equation allow?
Membrane potentials being estimated in more complex systems (many ions and variable permeability)
What is the resting membrane potential almost entirely due?
the movement of K+ ions out of the cell
What does depolarisation mean?
Membrane potential becomes more positive towards zero mV
What is repolarisation?
Membrane potential decreases towards resting potential