Equilibrium, Resting Membrane, and Action Potentials Flashcards
what is RMP?
the potential difference that exists across membranes in the period between action potentials
how are RMPs established?
by diffusion potentials
what are diffusion potentials?
- defined as the potential difference generated across a membrane WHEN A CHARGED ION DIFFUSES DOWN CONC. GRADIENT (K+ increases inside)
- can be positive or negative, depending on charge of ion
- set by K+ leak and channels and maintained by Na+/K+ ATPase pump
- K+ leaks out of cell and carries + charge -> inside of cell is (-) -> cation Na wants to rush into cell
what ions contribute most to RMP?
- ions with high permeabilities
- K+, Cl-
- K+ separated from immobile anions inside cell
normal RMP range?
usually -70 to -90 mV
Na+/K+ ATPase
- helps to maintain K+ concentration gradient across the membrane
- helps to set K+ diffusion potential
why is RMP always close to K equilibrium potential?
RMP is negative because there are more Na ions outside cell than K ions inside cell (membrane more permeable to K+ than Na+)
K equilibrium potential=
-85 to -90 mV
K+ is in equilibrium when…
cell is 85-90 mV lower than EC environment
K+ equilibrium potential is point at which…
movement of K+ INTO cell because of negative electrical potential is balanced by diffusion of K+ OUT of cell due to concentration gradient
at K+ equilibrium…
electrical and diffusion forces are equal and opposite
driving force=
difference between the measured membrane potential and the ion’s calculated equilibrium potential
when driving force is negative…
- ion will enter cell if it is a cation and leave cell if it is an anion
- membrane potential is too negative- try to bring it towards equilibrium potential
Action Potentials (AP)
- transmit information in nervous systems and all muscles
- occurs in EXCITABLE cells- rapid depolarization followed by depolarization
depolarization=
membrane potential LESS negative
hyperpolarization=
membrane potential MORE negative; harder to get new AP when in this state
inward current=
Na+, flow of positive charge into cell (must occur for AP)
outward current=
K+, flow of positive charge out of cell; helps reset Na+K+ pump working
threshold potential
membrane potential at which AP is inevitable
overshoot
portion of AP where membrane potential is positive
undershoot
portion of AP where membrane potential is more negative than RMP
refractory period
period during which another AP can’t be generated
1st event of AP
- RMP is -70 mV, K+ conductance is high, Na+ conductance low
- K channels fully open -> K leaks out -> diffusion potential
2nd event of AP
- upstroke of AP- membrane depolarization to threshold (-60 mV)
- voltage-gated Na+ channel
- rapid opening of activation gates in Na channel, Na flows into cell -> depolarization
3rd event of AP
- REPOLARIZATION: inactivation (terminate upstroke) gates on Na+ channels close and K+ channels open (allowing depolarization of membrane)
4th event of AP
UNDERSHOOT (hyperpolarization): K+ conductance higher than at rest- slowly return to RMP
Refractory periods=
excitable cells unable to produce normal APs
absolute refractory period (ARP)
- majority of APs
- overlaps with most of the AP
- no stimulus can occur to cause another AP
- inactivation gates on Na channels remain closed (once closed) until cell repolarized
relative refractory period (RRP)
- from end of ARP until through most of hyperpolarization
- AP occurs with greater than normal depolarization
- membrane more negative, it takes more stimulus to reach threshold
refractory period timeline
- A: at RMP- activation gate closed, inactivation gate open
- B: activated by signal- voltage gated and ligand gated
- C: inactivated state- time and return to RMP required to reset; refractory to subsequent stimulus (inactivation gate closes)
characteristics of AP?
- size and shape for a given cell type
- propagation: APs propogate -> depolarization spreads to adjacent areas on membrane
- all-or-none response: depolarization either brings to threshold or doesn’t
steps in propagation of APs
- APs start close to the cell body of a neurons, spread down the axon via local currents
1. initial area of axon depolarization to threshold, AP fires, cell interior positive
2. positive charges inside cell flow towards negative charges in adjacent areas of cell- those areas then depolarized to threshold
3. at this point, the initial area has repolarized
what is conduction velocity of APs?
speed at which an AP travels along a nerve or muscle fiber
what is a time constant?
- how quickly a membrane depolarized in response to inward Na+ current
- membrane resistance and capacitance affect the time constant
what is resistance?
- high membrane resistance -> current doesn’t readily flow across membrane -> force current to spread internally -> path of least resistance
- INCREASE time constant
what is capacitance?
-ability of a membrane to store charge
- high capacitance -> time constant INCREASE- current must discharge membrane before depolarization can occur
what is length constant?
- how far depolarization current will spread along a nerve
- membrane resistance and internal resistance affect length constant
longer length constant means…
current spreads farther
internal resistance is inversely related to…
- how easily current spreads within cytoplasm
- high resistance, current won’t spread as far
current travels farthest when diameter…
large, membrane resistance high (force current to flow along interior), internal resistance is lowing o
in order to increase conduction velocity, we need to…
- increase size of the nerve fiber- by increasing diameter of a fiber, internal resistance DECREASES
- however, there are anatomical limits to nerve size - myeline the nerve fiber-insulation of nerves with lipid will increase membrane resistance and decrease capacitance
- where nerve insulated and membrane resistance increases, force current to move along interior
- also increases constant time -> at breaks in myelin sheath, membrane depolarize faster
nodes of Ranvier?
breaks in myelin sheath every 1-2 mm
- membrane resistance low, current flow and AP occurs
Saltatory conduction
APs jump from node to node
