membrane potentials and action potentials Flashcards
definition of Flux
the number of molecules that cross a unit area per unit of time (number of particles)
what are 2 properties of ions
they are charged molecules
opposites attract and like repel
3 electrical properties of excitable cells
- voltage (potential difference) = generated by ions to produce a charge gradient
- current = movement of ions due to a potential difference
- resistance = barrier that prevents the movement of ions
V = I x R
STEPS to measure membrane potential
1) place reference electrode outside of the cell = zero volt level
2) another electrode is placed inside the cell - measures a voltage that is negative compared with the outside (ie. reference)
properties of ion channels
- lipid (hydrophobic) cell membrane barrier to ion movement and separates ionic environments
- ion channels can open or close
what 3 things cause ion channels to open or close
1) transmembrane voltage
2) activating ligands
3) mechanical forces
when will movement across the membrane occur
when the concentration of the ion is different on one sided of the membrane and will stop when equilibrium is reached
when is the electrochemical equilibrium reached
when the conc gradient exactly balances the electrical gradient
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
what is the Nersnt equation (to calculate equilibrium potential)
E = RT/zF ln X2/X1 X2 = intracellular ion conc X1 = extracellular ion conc
what are the most important ions in determining resting potential of neurones
Na+ and K+
what is typical resting membrane potential
-70mV and each ions contribution to membrane potential is proportional to how permeable the membrane is to the ion at any time
GHK equation
Em (mV) = -61 log Pk[k]i + Pna[Na]i + Pcl[Cl]o/Pk[K]o + Pna[Na}o + Pcl[Cl}i
[x] represents conc and the subscript i or o indicates inside or outside the cell
P is permeability or channel open probability (0 = 100% closed, 1 = 100% open, 0.5 = open 50% of the time)
what is depolarisation
membrane potential becomes positive towards 0mV
what is repolarisation
membrane potential decreases towards resting potential (becomes more negative)
what is overshoot
membrane potential becomes more positive
what is hyperpolarisation
membrane potential decreases beyond resting potential
what is a graded potential
change in membrane potential in response to external stimulation or NTs - and the change in membrane potential is graded in response to the type or strength of stimulation
what do graded potentials do
produce the initial change in membrane potential that determines whether an action potential is initiated or prevented
what happens to graded potentials over time
graded potentials decay along the axon as small amounts of charge leaks from the axon and so the size of that potential change decreases along its length
what are ion channels opened by
membrane depolarisation
what are ion channels inactivated by
sustained depolarisation
what are ion channels closed by
hyperpolarisation/repolarisation
what are the 5 phases of the action potential
phases 1-5
1) resting membrane potential
2) depolarising stimulus
3) upstroke
4) repolarisation
5) after-hyperpolarisation
what is phase 1
resting membrane potential
permeability for Pk > Pna
membrane potential is nearer equilibrium potential for K+ (-90) than that for Na+ (+72)
what is phase 2
depolarising stimulus
- produces graded potential
- the stimulus depolarises the membrane potential
- moves in the positive direction towards threshold
what is phase 3
upstroke
- starts at threshold potential
- increased permeability for Na because VGSCs open quickly
- increased permeability for K because VGKCs open slowly
K+ leaves the cell less than Na+ entering
membrane potential moves toward Na+ equilibrium potential
what is phase 4
repolarisation
- decreased permeability for Na as VGSCs close - entry stops
- increased permeability for K as channels are still open so K leaves cell down EC gradient
- membrane potential moves towards K+ Eq potential
at the start of repolarisation…
- absolute refractory period
Na channel activation gate is open
inactivation gate is closed
new action potential cannot be triggered even with strong stimulus
what happens at the peak of an AP
just after all Na+ channels are open they undergo a conformational change called inactivation
what is inactivation and what does it do
impedes ion flow through Na+ channel > cannot be reopened
- only when cells repolarise completely - inactivation is removed from Na+ channel > closes and can be reopened
what happens later in repolarisation
- absolute refractory period continues
- activation and inactivation gates closed
- until repolarisation - inactivation is removed
- cell channels are primed for reopening
(during this time - cell cannot be stimulated)
what happens in phase 5
after hyperpolarisation
- at rest, VGKCs still open
K+ continues to leave down EC gradient
- membrane potential moves closer to K+ equilibrium - some VGKCs close
- membrane potential returns to resting potential
- some Na+ channels open = relative refractory period (some Na+ channels have recovered from inactivation - gate is open)
stronger than normal stimulus is required to trigger AP
what 2 things determine decay at the site of depolarisation
- internal resistance of axon
- membrane insulation
alters propagation distance and velocity
how does diameter affect decay
- resistance of neurone is determined by diameter
the larger the diameter = smaller resistance = slower decay
smaller diameter = greater resistance = faster decay
what does myelination do to decay
if there is insulation - graded potential decays further along axon due to insulation being provided
what 2 factors affect conduction velocity
axon diameter
myelination
how does axon diameter affect conduction velocity
as diameter increases = conduction velocity increases
conduction velocity is proportional to the square root of axon diameter
how does myelination affect conduction velocity
as myelination increases so does conduction velocity
what are 3 factors that influence movement of ions across a membrane
charge of ions
conc of ions on both sides
voltage across membrane
