Membrane potentials and action potentials Flashcards
What is an ion flux?
Number of molecules that cross a unit area per unit of time
What are the properties of ions?
Charged molecules
Opposite charges attract
Like charges repel
Define voltage
Voltage = p.d.
Generated by ions to produce a charge gradient
Define current
Movement of ions due to a p.d.
Define resistance
Barrier that prevents the movement of ions
How do you measure membrane potential?
Reference electrode is placed outside cels (zero-volt level)
Another electrode is placed inside cell, measure a voltage difference that is negative compare with the outside
How do ions cross the cell membrane?
Through selectively permeable pores called ion channels in, which open and close in response transmembrane voltage, presence of activating ligands or mechanical forces
What is an electrochemical equilibrium?
Electrical gradient is balancing the concentration gradients. Stable transmembrane potential is achieved
Define equilibrium potential
potential at which electrochemical equilibrium has been reached. The potential that prevents diffusion of the ion down its concentration gradient
What equation can be used to calculate the equilibrium potential?
Nernst Equation
What are the most important ions for the resting potential for neurones?
Na+ and K+
Why does the Goldman-Hodgkin-Katz (GHK) equation describe membrane potential more accurately?
Because it takes into account the permeability of the membrane to the particular ions
Define depolarisation
Membrane potential becomes more positive towards zero
Define repolarisation
Membrane potential decreases towards resting potential
Define overshoot
Membrane potential becomes positive
Define hyperpolarisation
Membrane potential decreases beyond resting potential
What can cause a change in membrane potential?
External stimulation or neurotransmitters
How is change in membrane potential graded?
Type or strength of stimulation; i.e. weak or strong
What happens to graded potentials after they’ve moved a short distance (i.e 1mm)
Decay over the length
Why do graded potentials decay down the length of the axon?
Charge leaks from the axon and the size of the potential change decreases along the axon
What happens when a graded potential reaches a threshold for the activating of Na+ channels?
Action potential is generated
What are the 5 phases of an action potential?
- RMP
- Depolarising stimulus
- Upstroke
- Repolarisation
- After-hyperpolarisation
What is the Nernst Equation?
E = (RT/zF).ln.(X2/X1)
E = Eqm potential R = gas constant T = Temperature in Kelvin z = charge on ion (-1 for Cl-, +2 for Ca2+) F = Faraday’s number - charge per mol of ion ln = natural logarithm (log to base e) X2 = intracellular ion concentration X1 = extracellular ion concentration
What can the Nernst Equation be simplified to?
E = (-61/z).log (Xinside/Xoutside) (mV)
What are the typical concentrations of K+?
150 mM inside and 5 mM outside
What are the typical concentrations of Na+?
10 mM inside and 150 mM outside
Why does the Goldman-Hodgkin-Katz equation describe membrane potential (Em) more accurately than the Nernst equation?
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+).
GHK takes into account permeability.
What produces the initial change in membrane potential that determines what happens next i.e. initiate or prevent AP?
Graded potentials
In what type of cells do APs occurs?
Neurones and muscle cells but also some endocrine tissues.
What are APs also known as in neurones?
Nerve impulses and allow the transmission of information reliably and quickly over long distances
What does permeability of a membrane depend on?
Conformational state of ion channels
Opened by membrane depolarisation
Inactivated by sustained depolarisation
Closed by membrane hyperpolarisation/repolarisation
Outline phase 3 - upstroke of an AP.
Starts at threshold potential
^Na+ because VGSCs open quickly [Na+ enters the cell down electrochemical gradient]
^PK as the VGKCs start to open slowly [K+ leaves the cell down electrochemical gradient]. Less than Na+ entering.
Membrane potential moves toward the Na+ equilibrium potential
Outline phase 4 - repolarisation of an AP.
^Na+ because the VGSCs close - Na+ entry stops
^K+ as more VGKCs open & remain open. K+ leaves the cell down its electrochemical gradient. Membrane potential moves toward the K+ equilibrium potential.
What happens after hyperpolarisation?
At rest voltage-gated K+ channels are still open. K+ continues to leave the cell down the electrochemical gradient. Membrane potential moves closer to the K+ equilibrium - some voltage-gated K+ channels then close. Membrane potential returns to the resting potential
