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).