Lecture 11, The Action Potential Flashcards
Resting Membrane Potential
all cells have a resting membrane potential
- the intracellular fluid is more negatively charged than the extracellular fluid
- remember, solutes want to flow DOWN their gradient gradient to reach diffusion equilibrium
- negatively charged ions are attracted to positively charged ions, and vice versa
- result: negatively charged ion want to diffuse OUT of the cell, while positively charged ions will want to move INTO the cell
ions are not able to pass the membrane via simple diffusion
- instead, they are only able to pass across the membrane with the help of transporter proteins
- only a few types of cells are able to change their membrane potential (muscle cells and nerve cells)
> the highly selectivity and tight regulation of membrane transport allows neurons to maintain their resting potential
-> we do not want the membrane to be at equilibrium rather maintained homeostatic state
Neuron Membrane Resting Potential
neuron resting potential = -70 mV (-40 to -90 range)
- there is a net negative charge inside the cell
- there extracellular fluid is approximately neutral in charge
- the difference between the intracellular and extracellular fluid is about -70 mV
◦ net negative electrochemical gradient across the membrane
neuron action potential = depolarization to +30 mV
- ion channels in the membrane open, allowing for the influx of positive ions
- the intracellular fluid becomes more positive, causing for depolarization of the membrane
◦ results in a net positive electrochemical gradient across the membrane
Electrochemical Gradient
the difference in ion concentration across a membrane that also results in a difference in charge (+/-) across the membrane
Distribution of Ions Across the Membrane
Na+, Cl- and K+ are present in the highest concentrations, however other molecules also influence the intracellular and extracellular electrochemical gradients
- Mg2+, Ca2+, H+, HCO-, HPO42-, amino acids, proteins, etc.
Na+/K+ Pump
for every ATP molecule hydrolyzed, 3 Na+ are pumped out of the cell, and 2 K+ and brought into the cell
1. the concentration gradient across the membrane is established by the Na+/K+ pump
A. chemical-aspect = high [Na+] outside of the cell,
high [K+] inside the cell
- K+ moves DOWN its concentration gradient out of the cell via ion channels (ion “leak” channels)
A. causes the intracellular fluid to become more
negative compared to the extracellular fluid - a stable resting negative potential is achieved
A. balance between the Na+/K+ pump activity and f
flux through ion channels
B. maintained electrochemical gradient -> difference in charge (+/-) AND difference in ion concentration across the plasma membrane
Ion Leak Channels (K+)
some ion channels allow for some ions to “leak” through and cross the membrane
* selective to specific ions
* remember, movement through ion channels is a type of passive transport
◦ movement is DOWN the concentration gradient
◦ no ATP is used in this process
* ion leak channels are constitutively open (not gated)
K+ movement across the membrane is in a balance between:
- concentration potential - K+ wants to move DOWN its concentration gradient OUT of the cell
- electrical potential - K+ wants to stay IN the cell, as it is attracted to the negatively charged fluid
equilibrium potential: the membrane potential when the concentration and electrical potential of an ion (K+) are equal and opposite in magnitude
*together, activity of the Na+/K+ pump and selective K+ ion leak channels are the “key players” that generate the electrochemical gradient in cells
Membrane Potential - resting membrane potential (depolarization, hyperpolarization, repolarization)
resting membrane potential: the electric potential difference across the plasma membrane of an unstimulated cell (i.e., a cell at “rest”)
- depends on the [ion] on either side of the membrane, as well as the membrane’s permeability to those ions
- depolarization: a change to a less negative membrane potential
- hyperpolarization: a change to a more negative membrane potential (resting potential that goes below -70 mV)
- repolarization: a return to the resting potential following depolarization
Permeability
the ability of a substance or barrier to allow for molecules to pass through it
Equilibrium Potential
the membrane potential when the concentration and electrical potential of an ion are equal and opposite in magnitude
- when the flux of the ion across the membrane is in equilibrium
Current
when charged particles flow in a net direction
- example: at the NMJ, voltage-gated ion channels open and Ca2+ ions flood into the synaptic end bulb of the motor neuron
the membrane potential depends on:
the membrane potential depends on:
- the concentration gradient on either side of the membrane (of Na+ and K+)
◦ the larger the concentration gradient, the greater
the membrane potential
- the membrane’s permeability to Na+ and K+
◦ at rest, the membrane is more permeable to K+
than Na+
‣ more K+ ion leak channels than Na+ ion leaks
channels
even at rest, the movement of ion across the membrane is dynamic
- continuous influx and efflux of ions across the membrane, but maintained membrane potential
- dynamic constancy (not static) - that a given variable may fluctuate a little in the short term, but stable and predictable in the long term
Polarization
signals passed between neurons come in the form of action potentials
- action potentials are rapid depolarizations of the membrane, that occur in an “all-or-none” fashion, and quickly repolarizes
- the membrane is “polarized” → the electrical environment (charge) inside the cell is different from that outside the cell
- overshoot : a reversal in the membrane potential’s polarity
◦ transition from a net negative potential to a net
positive potential
- depolarization and overshoot = membrane potential becomes less negative
- repolarization and hyperpolarization = membrane potential becomes more negative
Graded Potentials
graded potentials: a change in the membrane potential that only occurs in one confine sport in the membrane
- “graded” -> the magnitude of depolarization may vary
- neighbouring segments of the membrane are initially still at resting potential
graded potential can be either depolarizing or hyperpolarizing
- the local current created can spread to adjacent areas of the membrane
- as the depolarization/hyperpolarization spreads out to adjacent areas, it gets weaker and eventually disappears
- decremental: the flow of charge decreases as the distance from the site of origin of the graded potential increases
- very different from action potentials, which are propogated down the plasma membrane
What are 3 Factors of Graded Potentials?
- can be either depolarizing or hyperpolarizing (depending on the stimulus) -> the two kinds of graded potentials
- can vary in size (depending on the stimulus) -> depending on the stimulus
- are are conducted decrementally (decay of a graded potential with distance)
Action Potentials
action potential: a rapid depolarizations of the membrane, that occurs in an “all-or-none” fashion, and quickly repolarizes
- occurs in plasma membranes that contain voltage-gated ion channels
◦ aka excitable membranes
- generally very rapid (1-4 msec in duration)
- provides long-distance transmission through the nervous system
depolarization of the membrane occurs via the opening of Na+ ion channels
- rapid influx of Na+
◦ the membrane potential becomes less negative
- K+ ion channels also open, but at a slower rate
◦ K+ leaves the cell, halting membrane
depolarization
Action Potential Step (1-2)
- resting membrane potential
A. Na+/K+ pump and ion leak channels allow for the
membrane potential to be maintained at -70mV
B. no external stimulus - Na+ ion channels open
A. an external stimulus causes Na+ ion channels to open, and Na+ floods into the cell
B. the external stimulus may be a signal from
another cell, a chemical, a hormone, a physical
perturbation, etc.
C. enough of Na+ enters the cell, the membrane will
reach the threshold potential
threshold potential (-55 mV): the critical membrane potential that initiates the depolarization phase of an action potential
* if ion channels close before -55 mV is reached, the membrane will repolarize without initiating an action potential
-> “all-or-none” response
Subthreshold Potentials
subthreshold potentials are when the membrane is depolarized but does not reach the threshold potential
- the membrane repolarizes, and does not initiate an action potential
a stimulus that is not sufficient in initiating an action potential is called a sub threshold stimulus
- if the stimulus IS able to initiate an action potential, it is called a threshold stimulus
Action Potential Steps (3-4)
- the threshold potential has been reached, initiating the action potential
◦ reaching the threshold potential opens voltage-
gated Na+ channels, causing rapid depolarization
of the next segment of the membrane
◦ action potential propation: the local current
produced by an action potential that triggers a
new action potential at a site farther down the
membrane
◦ depolarization overshoots 0 mV and the
membrane potential becomes positive - Na+ ion channels close and K+ channels open
◦ Na+ ion channels close, halting the influx of Na+
into the cell
◦ K+ ion channels open and allow for the efflux of
K+ into the extracellular space
◦ this halts membrane depolarization
- K+ ion channels are triggered by the same stimuli as Na+ ion channels. they just have a delayed response
Action Potentials
5. the plasma membrane repolarizes
◦ Na+ channels have closed and K + channels
remain open, allowing for repolarization of the
membrane
◦ Na + influx has stopped, but K + efflux continues
- the plasma membrane is hyperpolarized
◦ the plasma membrane becomes slightly
hyperpolarized, because K+ ion channels take a
little more time to close
◦ this phase is known as afterhyperpolarization - K+ ion channels finally close*
◦ Na+ /K+ pumps activate to restore Na+ and K+
concentration gradients
◦ Na+ is pumped out of the cell and K+ is pumped
into the cell
◦ resting membrane potential is restored
the majority close, but some remain open as K+ ion leak channels
