Lecture 2: Membrane potentials and action potentials Flashcards
How do you measure membrane potential?
By placing a reference electrode outside the cell (zero-volt level) and another electrode is placed inside the cell - it measures a voltage difference that is negative compared with the outside.
What are ion channels?
permeable pores in the membrane that open and close depending on transmembrane voltage, presence of activating ligands or mechanical forces
How is a membrane potential generated?
due to diffusion through a selectively permeable membrane
When is electrochemical equilibrium achieved?
when electrical force prevents further diffusion across the membrane
What is the equilibrium potential?
the potential at which electrochemical equilibrium has been reached - potential that prevents diffusion of ion down its concentration gradient.
How can the equilibrium potential be calculated?
Using the Nernst equation
Which ions contribute to the real membrane potential?
K+, Na+ and Cl-
What is the size of each ion’s contribution to the real membrane potential proportional to?
How permeable the membrane is to the ion
What does the GHK equation describe?
The resting membrane potential
What is mean by depolarisation?
membrane potential increases from negative towards 0 and becomes positive
What is meant by repolarisation?
membrane potential decreases towards resting potential
What is meant by overshoot?
membrane potential becomes more positive
What is meant by hyperpolarisation?
membrane potential decreases beyond resting potential
What are the most important ions for the resting potential of neurons?
Na+ and K+
What does P mean in the GHK equation?
Permeability or channel open probability
What does it mean if P=0?
channel is 100% closed
What does it mean if P=1?
channel is 100% open
What does it mean if P = 0.5?
channel is open 50% of the time
What are graded potentials?
Change in membrane potential in response to stimulation
What are the 2 key features of graded potentials?
- the size depends on the strength of the stimulus
- they dissipate with distance from the stimulus as charge ‘leaks’ from the axon as the impulse propagates
Where do graded potentials occur?
at synapses and in sensory receptors
What do graded potentials contribute to?
initiating or preventing action potentials
In what cells do action potentials occur?
in excitable cells (mainly neurons and muscle cells but also in some endocrine tissues)
What roles do action potentials have?
- cell-to-cell communication and can be used to activate intracellular processes
What does permeability depend on?
conformational state of ion channels
How is an ion channel opened?
by membrane depolarisation
How is an ion channel inactivated?
by sustained depolarisation
How is an ion channel closed?
by membrane hyperpolarisation/repolarisation
What are the 5 phases of the action potential?
- resting membrane potential
- depolarising stimulus
- upstroke
- repolarisation
- after-hyperpolarisation
What happens in phase 1 of an action potential?
RESTING MEMBRANE POTENTIAL:
- P(K) > P(Na) therefore potassium moves out of the cell and very little sodium comes in,
- nearer equilibrium potential for K+
What happens in phase 2 of an action potential?
DEPOLARISING STIMULUS:
- stimulus depolarises membrane potential,
- moves it in positive direction towards threshold
What happens in phase 3 of an action potential?
UPSTROKE:
- starts at threshold potential
- increased P(Na) because VGSCs open quickly
- increased P(K) as VGKCs open slowly
- less K+ leaving than Na+ entering
- membrane potential moves toward Na+ equilibrium potential
What happens in phase 4 of an action potential?
REPOLARISATION:
- decreased P(Na) as VGSCs close –> Na+ entry stops
- increased P(K) as more VGKCs open and remain open –> K+ leaves cell down electrochemical gradient
- membrane potential moves towards K+ equilibrium potential
What is the absolute refractory period?
Is the period of time during which a second action potential ABSOLUTELY cannot be initiated, no matter how large the applied stimulus is - the activation and inactivation gates are closed
What happens in phase 5 of an action potential?
AFTER-HYPERPOLARISATION:
- as membrane potential moves closer to K+ equilibrium some VGKCs close
- returns to resting potential
What is the relative refractory period?
stronger than normal stimulus required to trigger an action potential - inactivation gate is open
When is an action potential triggered?
Once the threshold potential is reached
What are action potentials described as?
‘All-or-nothing’ events
When does the positive feedback behaviour of the depolarisation cycle stop?
When the voltage-gated Na+ channels inactivate (i.e. closed and voltage-insensitive)
How is the electrochemical equilibrium restored following the action potential?
by K+ and Na+ ions moving through non voltage-gated ion channels and some ions are exchanged through pumps (like Na+K+ATPase)
Where are the voltage-gated channels mostly located?
At the Nodes of Ranvier formed by gaps between the myelin sheath surrounding the axon
What factors influence conduction velocity?
- axon diameter (Larger diameter = faster AP)
- myelination (more myelin = faster AP)
What does passive propagation result from?
Passive (graded) propagation results from a local change in ionic conductance (e.g. synaptic or sensory that produces a local current) that spreads along a stretch of membrane becoming exponentially smaller
What is the process called when APs propagate along an axon using nodes of Ranvier?
saltatory conduction
