Neuronal Excitability (Action Potentials) Flashcards
how does a nerve cell respond to a stimulus?
neurone is activated- its membrane potential will depolarise from RMP
what is a graded depolarisation?
graded depolarisations of the membrane:
the level of depolarisation will be proportional to the strength of stimulation applied
what happens if the membrane is continuously depolarised?
if the membrane is sufficiently depolarised to a certain critical level of membrane potential, it will suddenly generate an all-or-none event known as an ‘action potential’
what factors determine the movement of ions during RMP, depolarisation and AP?
concentration differences of ions between intra-
& extra-cellular compartments of the cell are the
source of energy for movements of ion in nerves at
RMP
define efflux and influx
the movement of ions in the cells is usually called flux:
influx: inward movement into the cells
efflux: the outward movement
what factors effect the movement of ions (flux) across the membrane?
chemical gradient
electrical force
how does chemical gradient effect movement of K+ and Na+ across the PM?
there is an unequal ion distribution so ions will flow down their concentration gradient
K+ concentration is higher inside driven to leave cell through ion channels (efflux)
Na+ concentration is higher outside- driven to enter cell through ion channels (influx)
how does chemical gradient effect movement of K+ and Na+ across the PM?
ions are charged (Na+/K+) thus are attracted by voltage inside cell (Em)
At negative Em, drive for K+ and Na+ to move into the cell (influx)
what does driving an ion across the membrane electrically require?
- the membrane possesses channels permeable to that ion to provide conductance
- there is an electrical potential difference across the membrane
define equilibrium potential (Em)
The voltage of the membrane potential necessary to perfectly oppose the net movement of an ion down its concentration gradient - i.e. the Ion is in equilibrium (no net flux)
how is an equilibrium potential achieved?
Different ions have different concentrations on either side of the membrane and the Chemical force will be different for each ion. Therefore, the membrane potential that must be achieved to equalise the two forces will be different. So, each ion has its own equilibrium potential.
what happens to equilibrium potential if the membrane is permeable to only one ion?
where in the body does this happen?
the resting membrane potential will be equal to the equilibrium potential for that ion.
true for skeletal muscle cells and glial cells within the nervous system- their membranes are permeable to K+ only and so their membrane potential is equal to the equilibrium potential for K+
what is the Nernst equation?
can be used to calculate the membrane potential at equilibrium for each of the ions in question
why is the resting membrane potential closer to EK than ENa?
At rest the amount of Na+ entering is the same as K+
leaving, but because the permeability to K+ is much greater the resting membrane potential is much closer to EK (equilibrium potential of K+)
define ionic driving force and when is it present?
the net force resulting from chemical (conc. gradient) and electrical influences (attract to opposite charge)
driving Force is present whenever Em is different from equilibrium potential for the ion (i.e. Em - Eion ≠. 0- this equation is how you calculate driving force for an ion)
what happens to K+ and Na+ when Em is -65mV?
Em is approx. = -65mV
- If Em ≠ EK then the influences on K+ movement are unequal
- At -65 mV the chemical influence (efflux) is greater than the electrical influence (influx) so the ionic driving forces causes a net movement K+ out of the cell (efflux)
- Em ≠ ENa at -65 mV so both chemical and electrical influences result in an ionic driving forces which causes Na+ to move into the cell (influx)
at -65mV, what is the permeability of the membrane to ions?
- there are more K+ leak ion channels than Na+ leak channels (as Em is -65 which is close to Ek rather than ENa)
- at rest the permeability of K+ is around 40 times that of the permeability of Na+ (PK = 40 x PNa)
- as there are more non-gated K+ channels, it has more influence in setting membrane potential: membrane potential (Em) controlled (mostly) by K+ movement).
at -65mV, what is the driving force of the membrane to ions?
Na+ influx: due to both chemical gradient and electrical force
Na+: Large Driving force ( -65mv – 62mv = -127mv)
K+ efflux: Chemical gradient (causing efflux) is greater than electrical force (causing influx)
K+: Small Driving force (-65mv - -80mv = 15mv)
As the Em is at rest (voltage is constant) so Na+ influx must equal K+ efflux - achieved by: (look at table)
what is the Em in neurones?
K+ efflux (trying to bring Em to -80 mV - EK)
Na+ influx (trying to bring Em to +62 mV - ENa)
So, Em must rest between EK and ENa
what is the Goldman equation?
- equation is a modification of the Nernst Equation
- allows you to calculate Em whilst taking into account relative permeabilities of ions in question
- important modification because ion channels can modulate their permeabilities to ions
briefly explain the process of how membrane potential changes throughout a neuronal excitation
- A neurone (with a resting membrane potential: -65mV) is stimulated at one end causing its membrane potential to become less negative.
- Membrane potential has to get to a voltage level less negative than the threshold value to generate an action potential.
- Then there is a steep rise in membrane potential during depolarisation where the voltage increases up to +40mv.
- The voltage then steeply goes down (repolarisation).
- For a short time the membrane potential becomes more negative than resting potential (hyperpolarisation) before coming back to resting membrane potential.
what is conductance (g) and what is it used for?
It is difficult to measure permeability so conductance is used as a measure instead. Conductance represents the activity of the ion channels.
Conductance (g) is directly proportional to how many channels are opening in the membrane (permeability does not have such a simple relationship)
Membrane acts as an electrical resistor (R)
Conductance, g = 1 / R (i.e. greater the resistance, the lower the conductance)
Each ion has its own conductance (gK; gNa)
Change in g for one ion results in a change in Em- this is due to changes in amounts of charge (ions) inside the cell
how does voltage gated ion channel opening explain change in Em during an AP?
- at resting Em (-65 mV) Na+ influx = K+ efflux via leak channels
- voltage gated ion channels are closed at the resting membrane potential; Depolarisation = open; Repolarisation = close
- the more depolarisation the more ion VG ion channels open
Initial Stimulus (depolarisation) causes the opening of voltage gated ion channels
what happens during depolarisation of the membrane?
- an initial stimulus opens the voltage-gated Na+ ion channels (i.e. increases (gNa)) which results in Na+ influx (in addition to that via ionic driving force) causing depolarisation.
- if this becomes more positive than the threshold potential an action potential will be generated.
- as you get further depolarisation you get the opening of more vg sodium ion channels so an even higher gNa.
- Em rapidly approaches ENa - -doesn’t reach it due to leak k+ channels in membrane, it would only reach it if no other ion was influencing membrane potential.