Term
| What are the stages in an action potential? During what periods can another action potential NOT be fired? |
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Definition
Depolarizing phase (Na+ channels open) --> Threshold (signal becomes self-propagating) --> Peak --> hyperpolarization phase --> repolarization…
A second action potential could not be fired from threshold--->hyperpolarization. A second action potential may be fired during the hyperpolarization phase but an increased voltage would be required. (stronger stimulus) |
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Term
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Definition
| A neurotransmitter or something that will bind to a protein and cause a confirmational change (cause it to open if it's a channel). |
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Term
| The cell interior has what charge relative to the exterior of the cell? |
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Definition
| The interior has a negative charge relative to the exterior of the cell. |
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Term
| What are the different modes of channel activation? |
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Definition
- Voltage activated
- Stretch activated
- Extracellular activation (by ligand binding)
- Intracellular activation (by ligand binding) |
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Term
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Definition
| Process of random motion of molecules. Movement from one location to another as a result of random thermal movement. |
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Term
| What is able to cross the phospholipid bilayer? |
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Definition
| Charged molecules are unable to cross the bilayer. |
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Term
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Definition
Jx = Px ([X]_o - [X]_i)
That is, that the flux of an ion (Jx) depends on the permeability coefficient of X (Px) multiplied by the difference in the external and internal concentrations (Xo-Xi)
Permeability can be increased by increasing the number of channels that are open to that substance. |
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Term
| What is Ohms law? What is it in terms of conductance? |
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Definition
V = IR --> Current * Resistance = Voltage
I = GV where G = conductance.
Unit of conductance is a siemen. |
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Term
| If the conductance of a membrane increases by 2x, what can we expect of the current? |
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Definition
| We can expect the current to increase by 2x as well. |
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Term
| What are the reasons that ions flow across a membrane? |
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Definition
- Concentration difference
- Difference in Charge |
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Term
| If you make the interior of a cell more positive (Say, up to +50mV) than what will positive and negative ions want to do? |
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Definition
| If you have the interior of the cell at +50 mV, then positive ions will want to flow out of the cell but negative ions will want to move inwards. |
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Term
| If you make the interior of a cell more negative than regular (-50mV) what happens to the flow of ions? |
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Definition
| At negative values, positive ions will want to move into the cell and negative ions will want to flow out. |
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Term
| What other name is the equilibrium potential called? |
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Definition
| It is called the Nernst Potential (for the equation) but it is also called the reversal potential because past this potential the ion will flow against its concentration gradient. |
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Term
| What is the Nernst Equation? |
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Definition
| The nernst equation describes the equilibrium potential for any ion species. It is the electrical potential necessary to balance an ionic concentration gradient across a membrane so that the net flux is zero. E_x=−RT/zFln([X_i ]/[X_o ] ) Ex = equilib potential for ion X R = absolute gas constant T = absolute temperature F = the faraday constant [X]o = extracellular concentration of ion [x]I = intracellular concentration of ion Z = valence of ion. Simplified version for 20*C Ex = 50mV/zlog10([Xi]/[Xo]) Z = valence of ion [xo] extracellular concentration of ion [xi] intracellular concentration of ion |
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Term
| What does it mean for sodium that it's Nernst potential is +67? |
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Definition
| It means that sodium would want to flow into the cell unless the cell reached +67mV relative to the outside of the cell. At that point sodium would want to flow outwards. |
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Term
| What is the approximate Nernst Potential for sodium, potassium and calcium? |
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Definition
Sodium - +67
Potassium - -93
Calcium - +123 |
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Term
| What would happen to Ek (Equilibrium Potential for Potassium) if we increased or decreased the external concentration of KCl? What would happen to the membrane potential? |
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Definition
Ek will become more positive.
By decreasing the concentration ratio, you decrease the driving force.
Nothing would happen to the membrane potential. The resting membrane potential depends primarily on the permability for the sodium Ion. |
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Term
| How do you calculate the driving force? |
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Definition
Driving force = equilibrium potential - membrane voltage.
Example: for potassium, when the membrane potential is -64.
Driving Force = -94-(-64) = -30 mV … |
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Term
| What are the techniques for electrophysiological measurement? |
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Definition
Current Clamp
- Measurement of cell voltage while controlling applied current.
Voltage Clamp
- Measurement of cell currents while controlling cell voltage |
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Term
| In voltage clamping, what is the parameter that you are able to observe? |
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Definition
| The current, or conductance, of the cell. |
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Term
| In current clamping, what is the parameter that you are able to observe? |
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Definition
| You can observe a response in Vm (membrane potential?) |
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Term
| What is the rate at which voltage change occurs dependant on? |
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Definition
| It depends on the capacitance of the cell. Capacitance depends on the size of the cell. |
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Term
| What are the different patch clamp configurations? |
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Definition
1) Cell attached patch -- without breaking the surface of the cell, but making a seal around one (or more) channels, you are able to observe opening/closing of single (or few) channels
2) Whole-cell patch -- Cell membrane is broken open for direct access to cytoplasm. Enables recording of all currents on cell membrane.
3) Inside-out excised patch -- the outside portion of the ion channel is found within the pipette and is bathed with the pipette contents. A seal is formed on the cytoplasmic side.
4) Outside-out excised patch -- the outside portion of the ion channel is outside the pipette and the channel is no longer attached to the cell. Allows for manipulation of the solution treatment. |
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Term
| What is the absolute and relative refractory periods? |
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Definition
Periods during an action potential.
Absolute refractory period: A second response is not possible regardless of duration/strength of stimulus.
Relative refractory period: Second response can be elicited, but strength/duration of stimulus must be increased. |
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Term
| What are the ways that the speed of transmission can be increased? |
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Definition
Through myolin sheathing (skipping to nodes of Ranvier)
Making the axons bigger (increase cross sectional area) |
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Term
| What does the amplitude of an action potential depend on? |
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Definition
| The external concentration of sodium. |
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Term
| The resting membrane potential depends strongly on the concentration of what ion? And is it the concentration inside or outside the cell? |
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Definition
| The resting membrane potential depends strongly on the internal potassium concentrations. |
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Term
| What is the difference between an inward current and outward current? How would they look on a graph with membrane current on the Y axis? |
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Definition
Inward current = Negative deflection on membrane current graph. Caused by the inwards movement of sodium ions.
Outward current = positive deflection on membrane current graph. Caused by the outwards flow of potassium ions. Also caused by negative ions flowing into the cell. |
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Term
| Tetrodotoxin is used to block what channels? |
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Definition
| Tetrodotoxin is used to block Na+ channels. |
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Term
| Tetraethylammonium is used to block which channels? |
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Definition
| Tetraethylammonium blocks K+ channels. |
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Term
| What is saxitoxin, where do you find it and what is it used for? |
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Definition
| Saxitoxin is also known as red tide, which refers to a bloom of a certain type of bacteria that creates this compound. Saxitoxin blocks Na+ currents |
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Term
| What chemicals can be used to block Na+ currents? |
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Definition
| Saxitoxin and tetrodotoxin |
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Term
| What chemicals can be used to block K+ channels? |
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Definition
|
|
Term
| How can the voltage dependence of a current be determined? |
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Definition
| The voltage dependence of a current can be determined by measuring the amplitude of that current at a series of different voltages imposed on the cell using voltage clamping. |
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Term
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Definition
| A measure of the permeability of the cell. That is, it's a measure of the number of cells that are open. |
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|
Term
| What determines the amplitude of total cell current at each voltage? |
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Definition
1) The number of channels that are open
2) The current passing thru each of those channels |
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Term
| How can we determine the number of channels that are open? |
|
Definition
The opening of voltage-dependent channels depends on the voltage.
If we knew the total number of channels present, the voltage, the probability that that channel would be open at that voltage (or the open probability, Po) |
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Term
| What determines the amount of current passing through each open channel? |
|
Definition
Ohms Law.
The current passing thru each channel depends on the conductance (g) of that channel and the driving force for that ion (diff between membrane voltage / equilibrium potential) |
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Term
| Channels, pores and carriers are all constructed from what? |
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Definition
| Integral membrane proteins |
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Term
| What is responsible for the maintenance in the differences between the concentrations of key ions inside / outside mammalian cells? |
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Definition
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Term
| What happens at the level of individual channels (Na and K) during a voltage step? (Say, from -80mV to -20mV) |
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Definition
Na: Single Sodium channels open, but close near immediately during the depolarization phase. This is because sodium has an inactivation gate (with chain) that closes with time.
K: Potassium maintains the open-state of their channels for the entire step. IE the channels do not close once they have been opened until repolarization occurs. |
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Term
| What is different about the gating structures for sodium channels and potassium channels? |
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Definition
| Sodium channels have a separate inactivation gate that potassium channels do not have. Activation/inactivation are both membrane voltage dependent. |
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Term
| By which mechanism does the inactivation gate for sodium act on the membrane channel? |
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Definition
By the "ball and chain" method. The sequence of activation/inactivation is as follows:
1) Activation gate is closed, inactive gate is open.
2) Activation gate opens and current flows
3) Inactivation gate closes
4) Inactivation gate opens and activation gate will close when the cells becomes repolarized. |
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Term
| Which step of the sodium-channel open/closing process is time dependent? |
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Definition
The recovery from inactivation is a time dependent process.
If a second voltage step is applied, that step evokes less current than the first unless sufficient time has been interval has been allowed for full recovery. |
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Term
| What is the sodium, recovery from inactivation period, responsible for? |
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Definition
| Na+ channel inactivation (a time dependent process) is responsible for the absolute refractory period of the action potential because channels are inactivated and cannot be opened. |
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Term
| The opening of Na+ channels and the opening of K+ channels are what types of feedback cycles? Explain? |
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Definition
Na+ channels opening = positive feedback cycle.
The opening of Na+ channels causing depolarizing (bringing cell interior to more + mV) which opens more Na+ channels. Results in extremely fast depolarization.
K+ channels open more slowly but as they repolarize the cell they cause the sodium channels to turn off and return to cell back to membrane potential +. |
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Term
| At rest, is the membrane permeability higher for sodium or for potassium? |
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Definition
| @ rest, the permeability to potassium is much higher than that for sodium. This is due to leak channels. |
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Term
| What contributes to the repolarization of a cell? |
|
Definition
The closing of the inactivation gates on Sodium channels.
The opening of potassium channels (in addition to their leak channels) |
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Term
| What makes an ion channel voltage dependent? Explain for sodium (as an example) |
|
Definition
An sodium ion channel is made up of four different sub units, put together. Each sub unit is made up of 6-trans membrane regions.
One of those transmembrane regions will have a large number of charged amino acids in it. This region serves as the voltage sensor.
These regions are programmed to shift in response to voltage changes. (regions are drawn to the interior of the cell by its relative negative polarity. When it becomes more positive (depolarization) those regions begin to move towards the extracellular surface, thereby changing the protein shape) |
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Term
| How can an ion channel be selective for a specfic ion |
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Definition
| An ion channel is made up of four different sub units, put together. Each sub unit is made up of 6-trans membrane regions. In addition to the trans-membrane region that serves as the voltage center (#4) there is also a "P" region which forms the "pore" section for that sub unit. This region is hydrophobic, but unlike the other trans-membrane proteins, this section is too small to cross the entire membrane. The actual amino acids that form the pore region will determine what type of ion can flow through |
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Term
| What are the two, key structural elements of a membrane channel? |
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Definition
1) The S4 segment, which has positive charged residues every third amino acid and which is thought to be involved in voltage sensing
2) The P region, which dips into the membrane but does not cross it and is thought to be responsible for the ion selectivity of a channel. |
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Term
| What is different between the overall structures for sodium, calcium and potassium channels? |
|
Definition
Sodium channels:
Made up of 4 sub units put together. Each sub unit contains 6-transmembrane regions. (S4 = voltage sensor, P = pore lining)
+ Sodium channels have an inactivation gate
(total of 24 transmembrane segments)
Composed of: Alpha + Beta1 + Beta2 subunits.
Calcium channels:
Made up of 1 sub units put together (alpha1). Each sub unit contains 6-transmembrane regions.
Composed of: Gamma + Alpha 1 + Alpha 2 + Beta + Delta
Potassium channels:
Requires only one sub-unit to form a channel, with 6-transmembrane segments. However, often is seen with 4 sub units.
Composed of: 4 Alpha sub units + 1 Beta sub unit. |
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Term
| Why do all of the voltage-gated cation channels have similar structural components? |
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Definition
| They are thought to have evolved from a common ancestor billions of years ago. |
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Term
| From which channel type (or protein type) have modern day cation channels evolved from? |
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Definition
| It is thought that all VG channels have evolved from early potassium transporters. |
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Term
| In what order is it thought that the voltage gated cation channels diverged in evolution? Starting from the oldest to the most recent. |
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Definition
It is thought that all VG channels have evolved from early potassium transporters. Therefore 1 = potassium.
Calcium then broke off from this and formed a channel. Therefore, Ca = 2.
Sodium diverged from the evolution of the calcium channels (which explains why they're more similar than Na - K). Therefore, Na = 3. |
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Term
| Which channel family is the most diverse? Why? |
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Definition
Potassium channels have the largest and most diverse family.
Why? Potassium channels came first in evolution and so there has been more time for them to evolve into different family members than any other. |
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Term
| How did the 6-membrane segment K+ channel alpha-sub unit evolve into the 24 transmembrane protein found in Na+ and Ca+ channels? |
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Definition
Probably through a duplication event.
IE: 1 --> 2 --> 4 --> 8 --> 16 --> 24… |
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Term
| Where are sodium channels most concentration in a neuronal cell? |
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Definition
They are most concentrated at the axon hillock + initial segment of a nerve cell. This is the feature that makes this region the most likely location for the triggering of an action potential.
*IE if channels are very close together then their effect with summate + there will be a positive feedback loop that increases the depolarization.
A second place on the neuronal cell where the sodium channels are most dense is at the Nodes of Ranvier. |
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Term
| How does hypocalcemia (IE low calcium levels) effect the firing of action potentials? |
|
Definition
The extracellular concentrations of calcium are very important in the opening of voltage-gated Sodium channels.
At high concentrations of calcium, (5mM) the Po (probability of open) for sodium shifts towards positive values. (-45~mV for 5mM calcium)
At low concentrations of calcium (0.5mM) the Po for sodium shifts in the negative direction (-57mV for 0.5mM).
THIS MEANS THAT A LOW CONCENTRATION OF CALCIUM CAN LEAD TO A HIGHER NUMBER OF ACTION POTENTIALS BEING FIRED (hyperexcitability) WHEN THERE NEED BE NONE. |
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Term
| What is charge screening? |
|
Definition
| Occurs when positive / negative charges are attracted to each other on opposite sides of a membrane channel. It makes the channel itself think that it is more negative than it really is. So, if a lot of this binding is occuring than it will be more difficult for channels to recognize and detect |
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Term
| What happens if you have very low concentrations of calcium in the extracellular fluid? |
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Definition
You will experience hyperexcitability.
That is that sodium channels will begin being opened at much more negative values (closer to resting membrane potential). |
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Term
| What can be used to block Sodium channels? |
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Definition
Local anesthetics block sodium channels. Local anesthetics are typically derived from cocaine. For these the block is use dependent -- this means that they are more likely to block open Na+ channels.
Tetradotoxin, a toxin from puffer fish, can also block Na+ channels. |
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Term
| What are local anesthetics typically derived from? List the most commonly used ones. |
|
Definition
Cocaine
- Tetracaine
- Procaine
- Lidocaine |
|
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Term
| Do local anesthetics block open or clocked Na+ channels? |
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Definition
| Local anesthetics that are derived from cocaine (like lidocaine, tetracaine, procaine) are more likely to be lock the open-confirmation of the sodium channel. |
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Term
| How does calcium differ in function from the other types of channels (sodium and potassium?), offer some examples. |
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Definition
The influx of calcium causes not only a change in membrane potential, but also acts as a signal to activate processes in the cell.
As the cytoplasmic concentration of calcium increases, calcium sensitive switch proteins are triggered and cause changes in:
Contraction, secretion, channel gating, metabolic enzymes, signalling enzymes, gene expression. |
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Term
| Which ion acts as a trigger for most kinds of intracellular communication? |
|
Definition
Calcium.
Why? Ca2+ influx causes the secretion of neurotransmitters (Calcium binds to the surface of vesicles to be secreted). This fusing helps the vesicles to bind to the membrane and eject their contents into extracellular space. |
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Term
| How can calcium channels be divided? What is the differences between them? |
|
Definition
Calcium channels can be separated based on the voltage required to open those channels.
1) T-type ("transient") channel.
• LOW THRESHOLD
• Rapidly acting/inactivating currents. Opens once (or few times) then then closes.
• Single channels are opening, so "tiny" current. (ind. Conductance for each channel is low)
2) L-type (long lived)
• Requires a positive threshold (-50mV --> +10mV by example)
• Much longer lived currents.
• Individual channel conductance is large relative to T-type channels.
• Channels continue to open (and close) throughout the voltage change step (contributing, therefore, to the long-lived current) |
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Term
| L-type Ca+ channels are blocked by what? Is the effect of this substance to only block the channel? |
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Definition
L-type Ca+ channels are blocked by dihydropyridines which is a family of drugs.
They can be either inhibitors (blocks the channel) or activators (causes the channel to remain open or to open more often). |
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Term
| What are all the types of calcium channels, what are they blocked (or activated) by? |
|
Definition
L-type Ca+ channels --> blocked/activated by dihydropyridines (which can act as antagonists or agonists)
T-type Ca+ channels -->
N-type Ca+ Channels (found in neurons) --> blocked by w-conotoxin
P/Q-type Ca+ channels (found in purkinje cells) --> blocked by w-agatoxin |
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Term
| What are the subfamilies from calcium channel encoded genes? |
|
Definition
1) L-type family - high voltage activated
2) T-type family - low voltage activated
3) Channel types that are most important for synaptic release |
|
|
Term
| How does calcium play a role in burst firing of action potentials? |
|
Definition
T-type Ca+ channels are activated at thresholds below the sodium channels. Therefore, they contribute to the depolarization effect.
T-type channels generate a slow Ca+ wave in neuronal somata, which increases the probability of achieving an action potential (with the contribution of other channels as the cell depolarizes) |
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Term
| which Ca+ channels are most important in neurotransmission? What do these have that the others do not? What is the significance of these additional structures? |
|
Definition
The N and P/Q types are most important.
They have two key localization sequences underlying synpatic transmission.
1) Synprint site (synaptic protein interaction site)
2) Mint/CASK binding sites
Synprint site --> can interact with proteins involved in exocytosis
Mint/CASK binding sites --> |
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|
Term
| What are the roles of K+ channels? |
|
Definition
- Play role in setting the resting membrane potential (make interior more negative)
- Repolarization phase of action potential
- Resists depolarization of the cell towards AP threshold
- Slows the rate of firing of action potentials |
|
|
Term
| What are the major types of current made by K+ channels? |
|
Definition
1) Delayed outward rectifiers -- The activation of the current is delayed
2) Transient outward rectifiers (A-type currents) --> refers to currents that are low threshold and inactivating K+ currents. Active at thresholds below resting membrane potential.
3) Ca2+-activated K+ currents --> BK (large conductance) and SK (small conductance) channels. Sensitive only to Ca+
4) Inward rectifiers --> Have only 2 transmembrane spanning regions, so do not possess the S4 voltage sensor region. (S5 - P - S6 regions remain). More likely to have a current at negative voltages. **There is inward ratification, however, ONLY IN THE PRESENCE OF Mg+ in the cytosol. Cytosolic Mg+ occludes the channel pore and prevents exit of K+, but does not prevent the entrance (even if it's blocked on the cyt. Side). |
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Term
| What does "rectifier" refer to? |
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Definition
| "rectifier" refers to a current that flows better in one direction than another. An outward rectifier, for example, carriers current preferentially in an outward direction. |
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Term
| Delayed outward rectifiers and transient outward rectifiers are what types of ion channels? |
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Definition
| They are voltage-gated K+ channels. |
|
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Term
| Which channel type can regulate the rate at which a cell reaches the threshold for an action potential? |
|
Definition
T-type K+ channels and T-type Ca+ channels.
They activate below threshold and therefore can regulate the rate at which a cell reaches the threshold for an action potential. |
|
|
Term
| What are K_v type channels? |
|
Definition
They are a type of outward rectifying potassium channel that has two modes of inactivation similar to Na+ inactivation channels.
They are inactivated by the "ball and chain" model which is modified somewhat by whether or not there are beta subunits attached to the channel.
N-type inactivation = amino terminus of the protein chain is a "ball" which can swing into the channel and block the pore.
2nd type -- beta subunits provide the ball and chain for Kv-channel alpha sub units that themselves lack this capacity at their N-termini. (different beta sub units give different inactivation properties) |
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Term
| What channel does the Kv7 family belong to? What are the currents for this channel called and why? |
|
Definition
Kv7 is a family of K+ channels.
The neuronal versions were originally identified through their sensitivity to muscarinic activation and were therefore named M-type K+ currents. |
|
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Term
| What are some of the features of the K+ channels in the Kv7 family? |
|
Definition
Members of the Kv7 family mediate currents that are low-threshold, non-inactivating and are VERY SLOW.
- Can contribute to the repolarization of cardiac action potentials because the action potential in cardiac muscle is very slow as well. |
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|
Term
| Activation of M-type currents leads to what? What channel is M-type currents associated with? What family? |
|
Definition
M-type currents are K+ channels from the Kv7 family.
Activation of M-type currents tends to inhibit firing over a relatively long time scale. M-current blocks repetitive firing. |
|
|
Term
| What are the two main families of calcium activated potassium channels? |
|
Definition
(BK) Channels = big conductance channels which are sensitive to both voltage and calcium.
- Increase their open probability in response to membrane depolarization + increasing concentration of intracellular calcium ions.
(SK) channels = small conductance channels which are sensitive to only Calcium. |
|
|
Term
| What is spike frequency adaptation? |
|
Definition
Spike frequency adaptation occurs when the there is multiple action potentials being fired, which can lead to a progressively larger internal [Ca+] and therefore a progressively larger Ca+ activated K+ current, which can turn off cell firing.
This current (the K+ current due to internal calcium concentrations) may be referred to as the after-hyperpolarization current. |
|
|
Term
| Inwardly rectifying K+ channels. Describe them. |
|
Definition
Have only 2 transmembrane spanning regions, so do not possess the S4 voltage sensor region. (S5 - P - S6 regions remain).
More likely to have a current at negative voltages.
Selective for K+, so tend to hyperpolarize a cell and make it less likely to fire.
Not gated channels, but outward flow of K+ is partially blocked by internal Mg++ at potentials above Ek. **However, there is some outward flow since the cell voltage is never below Ek (VERY negative, -95mV) so the function of these channels is to let K+ ions flow out of the cell.
Contribute to the resting membrane potential since they're not gated.
Modulation of these channels will cause changes in the resting membrane potential and therefore change the excitability of the cell.
Example: Katp channel: regulates the excitability of the cells that release insulin from the pancreas |
|
|
Term
| Katp channel does what? What kind of channel is it? |
|
Definition
Katp channel is an inwardly rectifying K+ channel that regulates the excitability of the cells that release insulin from the pancreas.
Are closed by increases in the ATP/ADP ratio. |
|
|
Term
| What molecule enhances insulin release? How does it do this? Is there any real world application to this? |
|
Definition
Sulfonlyureas is a drug that is used to enhance insulin release by blocking Katp channels through the action of a sulfonylurea receptor.
This drug is therefore used by diabetics. |
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