NS - Membrane Potential: RMP, Graded and some AP Flashcards
True or False: Only excitable cells have transmembrane potential
False
True or False: Neurons are the only excitable cells in the body
False
True or False: The transmembrane potential is created by ion pumps (such as the Na/K-ATPase)
False
True or False: Ions move across plasma membranes through ion channels according to their concentration gradient
False
True or False: When a membrane is at a permeable ion’s equilibrium potential, the ion no longer moves
False
Match the following to their correct type of transport:
- Co-transporters
- Pumps
- Ion channels
- Co-transporters
- active secondary transport - Pumps
- active primary transport - Ion channels
- passive
What is co-transport/secondary transport?
one particle is moving against its gradient with the help of another molecule moving with its gradient
- does not directly use ATP
Ions only move according to their concentration gradients: True or False?
FALSE! Ions don’t actually move according to JUST their concentration gradients
The movement of the ion differs depending on whether it has a charge or not
Ions, since they have a charge, move across membranes according to their ELECTROCHEMICAL GRADIENT!
What is an electrochemical gradient?
Charged molecules will experience repulsion and attraction according to their sign
And this is known as the ELECTROCHEMICAL GRADIENT/ELECTROMOTIVE FORCES
How do uncharged molecules move?
Uncharged molecules move passively according to a DIFFUSIVE FORCE
- a force that pushes molecules DOWN THEIR CONCENTRATION GRADIENT
How do charged molecules move?
Charged molecules such as ions are also affected by the diffusive force created by uncharged molecules moving down their concentration gradient
HOWEVER, they also experience ELECTROMOTIVE FORCES
Initially, it may look like an ion moves down its concentration gradient, but its movement relies more strongly on its electrochemical gradient (repulsion or attraction) to its surroundings
What is a diffusive force?
A force that pushes UNCHARGED molecules down their concentration gradient
What kind of cells have a transmembrane potential?
Transmembrane Potential (Vm)
ALL CELLS HAVE A TRANSMEMBRANE POTENTIAL
What are two types of excitable cells?
Neurons and myocytes (muscle cells)
Describe excitable cells and their transmembrane potentials at rest or when excited (Briefly)
Excitable cells can alter their Vm to send and/or receive signals
- involves the movement of particles down their electrochemical/concentration gradient to change the transmembrane potential
When an excitable is EXCITED, its Vm will be less negative than at rest
- exciting a cell turns the membrane potential down
What is a potential?
Potentials are signals that are sent/received when excitable cells alter their membrane potential
- very quickly
- rapid time scale
- measured in millivolts and take place in milliseconds
What are the two types of potentials?
Graded potentials
Action potentials
What is a graded potential?
- changes in membrane potential that vary in size rather than being all-or-none
- temporary fluctuation in membrane potential
- associated with dendrites
ex: dendrite in sensory neuron
What is an action potential?
- transient = lasting only for a short time
- associated with axons
- abrupt change that is very rapid (spike)
What is the difference between a graded potential and an action potential?
Graded potentials come in varying sizes and shapes, and their size varies relative to the number of ion channels that are open
- typically associated with dendrites and cell bodies (somas)
- not able to propagate because they get weaker as they move further from the site of initial induction
Action potentials are all the same size and they function in an all-or-none manner where they either happen because the threshold amount was reached or they do not occur at all
- typically associated with axons
- are able to propagate down axons
How do potentials occur? What allows Vm signals to occur?
Vm signals occur by PROTEINS that allow IONS TO RAPIDLY CROSS THE PLASMA MEMBRANE
Which of the following types of transporter proteins are used in generating potentials?
- Ion channel (passive transport)
- Permease (co-transporter)
- ATP-powered pump (primary transport)
- ion channels
- fastest transporters
- involved with signals because they are the FASTEST
ONLY ION CHANNELS ALLOW ARE USED IN GENERATING POTENTIALS
What does the term “resting membrane potential” mean?
- Any changes in membrane potential are relative/deviating from the RMP
What is depolarization?
Any potential changes that go from rest to becoming less negative
Moving away from the negative value
(more negative to less negative/more positive)
What is repolarization?
Potential changes that bring the membrane potential back to resting potential
Restoring resting potential
(less negative back to more negative)
What is hyperpolarization?
Overshooting the restoration of the resting potential
- when the membrane potential temporarily becomes more negative than what you started with
All potentials (resting, graded, and action) are created by the movement of ions through _________________________________
All potentials (resting, graded, and action) are created by the movement of ions through DIVERSE ION CHANNELS
What are the two types of ion channels involved in generating potentials?
- Leak channels (ungated)
- Gated channels
How are ion channels categorized?
Ion channels are categorized by gating and ion selectivity
Gating = what makes them open and shut
Ion selectivity = when they are open, what can come through?
What are leak ion channels?
Gating = not gated
- create a pathway for ions to get through
- always open
- passive transport
- always providing a place for ions to leak from one place to another
- pretty good selectivity
How is resting membrane potential maintained?
RMP is created by leak channels, which are always open
RMP arises from the unequal permeability of ions due to different leak channel abundance
(Abundance of Na+ leak channels to K+ leak channels is roughly ~1:20)
- Unequal permeability to ions (# of channels)
- Unequal distribution of ions (Na+/K+)
What are gated channels?
Gating = not automatically open
- something must happen to it in order for the channels to open
- require GATING FACTORS to open and close the channels
- three types of gated channels (ligand-gating, voltage-gating, mechanical-gating)
What are the three types of gated ion channels?
- Ligand-gated channel
- Voltage-gated channel
- Mechanical-gated channel
What are ligand-gated channels?
- requires a ligand to bind to the channel to open it
- molecules that bind to receptors to open channels
Ex: muscle excitation occurs when acetylcholine released from the motor neuron binds to the channel and the neurotransmitters are released
What are voltage-gated channels?
- closed at resting potential, but if you excite the cell, the channels will open and allow ions to pass through
- will not pass ions at rest (closed at resting potential)
- if a potential is generated to the correct level (threshold) then these VGC will open
Ex: Sodium voltage-gated channels and potassium voltage-gated channels
What are mechanical-gated channels?
- requires mechanical pressure to make the channels open
Why are ion channels the only channel that can generate/pass ions in potentials?
Only ion channels are FAST ENOUGH to produce potentials/pass ions
What is the distribution of ions outside and inside the plasma membrane?
K+ in abundance INSIDE (ICF)
Na+ in abundance OUTSIDE (ECF)
The ratio of Na+ to K+ leak channels in the plasma membrane is ~1:20
What happens when K+ moves out of the cell down its concentration gradient? In an extreme example where K+ was the only permeable ion
At the starting state, there would be no electrical force because the charges would be balanced on both sides of the membrane, but if there was a greater abundance of K+ in the cell, the diffusive force would push K+ out of the cell
When selective K+ channels open and K+ diffuses out, a negative charge inside the cell is generated because the (+) ions have moved out and there now exists an ELECTRICAL FORCE!!
There is now a negative membrane potential
When would K+ reach equilibrium potential? In an extreme example where K+ was the only permeable ion
If K+ was the only permeable ion, net K+ movement would cease when its two opposing forces (diffusive and electrical) are balanced or equal and opposite
- when the forces on an ion are balanced, it moves inward and outward at the same rate
- ions are still moving but there is no net movement of ions
- still moving and still going through channels, but movement is equal
What is equilibrium potential?
When there is no net movement in Vm because the ion’s electrical and diffusive forces are equal and opposite (balanced)
EQUILIBRIUM !!
- becomes so negative or positive to the point where it just gets steady because the electrical force is equal to the diffusive force
At equilibrium potential, there is ________________ movement of ions because there is _______________ force on the ion
At equilibrium potential, there is NO NET movement of ions because there is NO NET force on the ion
What is resting potential for a cell?
-70mV
What is equilibrium potential for K+ in a myofiber?
-90mV
ions are always trying to get the membrane potential value back to its equilibrium potential
- K+ desire to move the membrane potential to -90mV
If we were at -70mV, what direction would K+ want to move to achieve equilibrium potential?
K+ would want to move out of the cell to make the inside membrane potential more NEGATIVE - bring it down to -90mV
When do ions want to move towards their equilibrium potential?
At any value of Vm other than the equilibrium potential for an ion, there will be a net force on that ion (a small or large electrochemical gradient) to makes the ion move so that the membrane potential becomes closer to its equilibrium potential
*Will the equilibrium potential for sodium be positive or negative based on its concentration gradient?
There is a smaller gradient for Na+, but since there is a large abundance of Na+ outside of the cell, they have a tendency to move INTO the cell down their concentration gradient
As they carry a positive charge, they bring an excess positive charge into the cell as well
When sodium moves inwards, there is an electrical gradient that is created moving outwards… Eventually sodium reaches an equilibrium potential
But now that there are more Na+ inside the cell, the membrane potential has increased to become more POSITIVE
How would you describe the net force (electrochemical gradient) experienced by K+ and Na+ at RMP?
At RMP (-70mV)
K+ wants to make the membrane potential MORE NEGATIVE, so there is going to be a small electrochemical gradient moving K+ ions OUTWARDS (less net force because K+ is closer to its equilibrium potential)
Na+ wants to make the membrane potential more POSITIVE, so there is going to be a large electrochemical gradient moving Na+ ions INWARDS (big net force because it is very far from its equilibrium potential)
Will the equilibrium potential for K+ be positive or negative?
For K+ in most all cells, equilibrium potential is LARGE AND NEGATIVE
-90mV
Why isn’t resting potential equal to the equilibrium potential for K+?
When the neuron is at rest, K+ is the MOST permeable, but NOT the ONLY permeable ion
There are still other Na+ and Cl- ions moving, though there are fewer Na+ leaker channels in comparison to the many K+ leak channels
K+ wants to drive the membrane potential to be -90mV, ,but the other ions are simultaneously trying to get the membrane potential to their equilibrium potential, which explains why RMP is not equal to K+ equilibrium potential
What is the relative permeability for K+/Na+/Cl- at rest?
Relative permeability of K+ = 1.00 (1)
Relative permeability of Cl-/K+ = 0.45 (1:2)
Relative permeability of Na+/K+ = 0.05 (1:20)
True or False: Each ion’s contribution to the membrane potential is directly proportional to its permeability
True
True or False: Each permeable ion moves the membrane towards its own equilibrium potential
True
What is RMP for a sensory neuron? A myofiber?
Sensory neuron = -70mV
Myofiber = -90mV
A sensory neuron has a Vm of -70mV and a myofiber has a Vm of -90mV, what predictions might you make about the relative proportion different leak channels present in the two cell types?
In the myofiber, there must be a greater proportion of K+ leak channels to Na+ leak channels OR less Na+ leak channels
ratio = 1:100 instead of 1:20
which is what allows the membrane potential to be more negative
What prediction can you make, based on the equilibrium values, about what would happen if the sensory neuron opened gated channels that were permeable to sodium?
Eion Na+ = +70mV
Eion K+ = -90mV
Sensory neuron RMP = -70mV
If more gated channels opened and Na+ was more permeable,
There would be an increase in Vm as the Na+ would try to move towards its equilibrium potential
The Vm would become LESS NEGATIVE !!
What is the equilibrium potential for Na+ in a myofiber?
+70mV
What maintains ion concentration differences?
Ion concentration differences are critical for the membrane potential and they are maintained by ACTIVE TRANSPORT (PUMPS)
Ex: Na+/K+ pump
Differentiate the roles of passive ion channels and active transport in membrane potential
Ion channels are what allow for potentials to be generated because they are the fastest transporters for ions
Active transport does not do membrane potentials because they are not fast enough, instead they maintain ion concentration differences
What maintains Na+ and K+ gradients at the plasma membrane/cell?
Maintained by the primary active transport pump (Na-K-ATPase pump)
Maintains a higher abundance of Na+ outside of the cell and a higher abundance of K+ inside of the cell
What maintains Cl- gradients at the plasma membrane/cell?
Maintained by secondary active transport (co-transport with K+)
Equilibrium potential of Cl- = -65mV
What is responsible for maintaining resting membrane potential?
The transmembrane potential is indirectly maintained by ion pumps (Na/K-ATPase pump)
Leak channels create resting membrane potential as ions leak in and out
What drives potential signals in an excitable membrane? (Graded vs action potentials)
Driven by the opening and closing of gated ion channels
- gated ion channels create temporary fluctuations from RMP
Graded potentials = dendrites and cell bodies
- come in different grades, shapes, sizes, etc.
Action potentials = axons (require the right voltage gated channels to propagate)
- bigger in amplitude compared to graded potentials (more than 20mV deviation from RMP)
What are the two types of graded potentials?
- (Post)synaptic potentials (PSPs)
- Receptor/generator potentials
What determines the size of a graded potential?
Graded potentials have variable sizes that are affected by the number of ion channels that open
- graded potentials have different amplitudes (different amounts of change from resting potential)
- can be bigger or smaller depending on what caused the potential to happen
- produce membrane potential changes from RMP for as long as the gated channels are open
Recall: Membrane potential is determined by permeability = more channels open means more permeability… A BIGGER GRADED POTENTIAL !!
During excitation, which ions are permeable?
Excitation = Depolarization
- only Na+ and K+ ions are permeable
During inhibition, which ions are permeable?
Inhibition = Hyperpolarization
- only Cl- or K+ ions are permeable
Explain: A synaptic potential can be inhibitory or excitatory.
During excitation: Na+ and K+ ions are permeable
- excitation is associated with DEPOLARIZATION
- making the membrane potential more positive/less negative
- deviating from the RMP
During inhibition: K+ and Cl- ions are permeable
- inhibition is associated with REPOLARIZATION/HYPERPOLARIZATION
- making the membrane potential less positive/more negative
- restoring the RMP
What happens to membrane potential changes as they spread away from the site where they are first induced?
As membrane potential changes spread away from the site where they are first induced, they get WEAKER the further they spread
What is transmembrane current?
the flow of ions across the membrane
*What is electrotonic current?
the flow/spreading out of membrane potentials = getting weaker as they get further from the origin
What type of potential is used for transmitting information?
Action potentials are specialized membrane potential signals that can rapidly transmit long distances through an axon membrane (associated with axons)
Axons are too long to rely on graded potentials for transmitting information
- usually small and do not last long
- cannot spread out very far because they get weaker and weaker as they move away
What is the threshold value of an action potential?
Action potentials are ALL OR NONE
- only generated if there is enough excitation to hit the threshold amount
- if the threshold value isn’t reached then the action potential will not happen at all
List the four key features of action potentials
- All or none (threshold)
- Propagation
- Stereotyped dimensions
- All or none (refractory)
Describe the all or none key feature of action potentials
The threshold value must be reached in order for the action potential to occur
If the threshold value isn’t reached (~15-20mV above the RMP) then no action potential will happen
Describe the propagation of an action potential
Action potentials are able to propagate down the axons because they are an all-or-none type of potential
Graded potentials get weaker as they move further from the site of initial induction, so they cannot propagate
Describe the all or none refractory feature of action potentials
The refractory period describes a period after an action potential in which another action potential cannot be generated
- this is like a grace period for the cell
What are the two types of voltage gated ion channels that generate action potentials?
- Voltage gated sodium channels (VGNC)
- Voltage gated potassium channels (VGKC)
What are voltage-gated channels?
When you depolarize to about the threshold level, these voltage gated channels that didn’t allow ion channels to get through before will open and allow ions to go through
*When do VGNCs open? When do VGKCs open?
VGNC open during depolarization = aid depolarization so that Na+ ions can flow into the cell and make the membrane potential more positive
VGKC open during the downhill of the depolarization curve because K+ ions open and K+ ions can flow INWARDS to balance out of the electrical gradient
What are the two distinct gates on VGNCs?
- Activation gates
- Inactivation gates
What are the two types of fates in VGNCs
Activation gate
Inactivation gate
What are activation gates?
Activation gates are found on one side of voltage-gated sodium channels as a response to depolarization
More depolarization = more activation gates open
- open quickly and Na+ floods in quickly since more gates are open
- after a certain amount of time, activation gates close
What are inactivation gates?
Inactivation gates are found on on side of the voltage-gated sodium channels
- they start OPEN then CLOSE
- control the movement of sodium ions after depolarization
How do activation gates work?
At rest, activation gates are closed
Crossing the “threshold” amount for an axon causes a few VGNCs to open
A single open channel produces extra depolarization that causes nearby VGNCs to open
The channels eventually inactivate, closing themselves
If either the activation gates or inactivation gates are closed, no sodium goes through. Why?
Activation gates = stop the flow of Na+, which in turn causes the membrane potential to stop leaving!!
Compare and contrast VGKCs to VGNCs
VGKC = have only one gate (an activate gate)
VGKC = very slow at opening/closing
VGKC = only occur after a delay
The gate responds to the membrane crossing threshold, but only AFTER A DELAY (refractory period)
The opening and closing of both types of VG channels determines….
The SHAPE and BRIEF DURATION of an AP reflects the opening and closing of both types of VG channels
The permeability of the membrane to both types of ions rises markedly during AP, just at slightly different times
What type of VG channel is open/closed at each stage:
- RMP - graded potential
- Threshold/depolarization
- Repolarization
- AHP
- AHP
Positive or Negative Feedback:
“Every VG Na+ channel that
opens leads to more
depolarization, increasing
the likelihood that more VG
channels will open”
Explain this passage
Because the Na+ channels opening leads to MORE DEPOLARIZATION, the action potential is accelerated or driven to completion
- as more Na+ channels open, it causes even MORE to open
- builds upon it
- depolarization (Na+ channels) is an example of positive feedback
Negative feedback:
“Every VG K+ channel that
opens leads to repolarization,
increasing the likelihood that
more VG channel activation
gates will close once again.”
Explain this passage
Negative feedback:
When the original stimulus is shut off by the end of the stimulus
- restores the initial conditions
- brings back to the original condition
- repolarization (K+ channels) is an example of negative feedback
Describe the behaviour of voltage-gated potassium channels
- They open very slowly after the suprathreshold depolarization
- VERY SLOWLY after repolarization
- gate responds to membrane potential crossing threshold, but only after a delay
