Term
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Definition
electric current is proportional to voltage and inversely proportional to resistance (in a circuit)
I = GV G = 1/R I = V/R
I=current, G=conductance, R=resistance, V=voltage |
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Term
| Define equilibrium potential |
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Definition
| membrane potential where the net flow through any open channel is 0. The chemical and electrical forces are balanced. |
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Term
| Give the Nersnt equation at 37°C |
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Definition
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Term
| What does the Nersnt equation show? |
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Definition
| gives a formula that relates the numerical values of the concentration gradient to the electrical gradient that balances it |
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Term
| Give the intracellular and extracellular concentrations of K+ in mammalian skeletal muscle and the equilibrium potential |
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Definition
intracellular: 155mM
extracellular: 4mM
equilibrium potential: -98mV |
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Term
| Give the intracellular and extracellular concentrations of Na+ in mammalian skeletal muscle and the equilibrium potential |
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Definition
intracellular: 12mM
extracellular: 145mM
equilibrium potential: +67mV |
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Term
| Give the intracellular and extracellular concentrations of Cl- in mammalian skeletal muscle and the equilibrium potential |
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Definition
intracellular: 4.2mM
extracellular: 123mM
equilibrium potential: -90mV |
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Term
| Give the intracellular and extracellular concentrations of Ca2+ in mammalian skeletal muscle and the equilibrium potential |
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Definition
intracellular: 0.0001mM
extracellular: 1.5mM
equilibrium potential: +110mV |
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Term
| Give the Nernst equation for any temperature |
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Definition
| Ex = (RT/zF)ln([X]o/[X]i) |
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Term
| How does the Nernst equation change for Na+? |
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Definition
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Term
| How does the Nernst equation change for K+? |
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Definition
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Term
| How does the Nernst equation change for Ca2+? |
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Definition
| Ca2+ has divalency, therefore a '2' is before Faraday's constant (F) |
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Term
| How does the Nernst equation change for Cl-? |
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Definition
| Cl- has an inverted ratio as it is negatively charged. |
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Term
| When is the GHZ equation used? |
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Definition
| to calculate composite membrane potential by incorporating different ions |
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Term
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Definition
- selective
- conduct small ionic currents along the electrochemical gradient
- open/close in milliseconds |
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Term
| Describe transporters/exchangers |
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Definition
- integral membrane proteins
- move along electrochemical gradient
- binding/releasing ion
- do not normally conduct electrical current |
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Term
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Definition
- against electrochemical gradient
- takes up ATP
- Examples: Na+/K+ ATPase, PMCA (plasma membrane Ca2+ ATPase). |
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Term
| Calculate the membrane potential if the membrane is mainly permeable to K+ ions but also has small permeability to Na+ (PK = 0.935; PNa = 0.065) |
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Definition
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Term
If the cell is permeable only to K+ what is the membrane potential?
2. External K+ is changed to 10 mM. What is the new membrane potential?
3. External K+ is changed to 20 mM. What is the new membrane potential?
4. Suppose the cell is now only permeable to Na+, what is the membrane potential?
5. External Na+ is changed to 70 mM. What is the membrane potential?
6. External Na+ is changed to 40 mM. What is the membrane potential?
7. If the cell is only permeable to Cl-, what is the membrane potential?
7. External Cl- is reduced to 70 mM. What is the membrane potential?
8 External Cl- is increased to 200 mM. What is the membrane potential? |
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Definition
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Term
| Describe the structure of voltage-gated ion channels |
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Definition
- Voltage-gated ion channels have an ion conducting pore and a gate(s) controlling the pore.
- The pore has a region called a selectivity filter that allows permeable ions to go through much easier than the other ions.
- The gate is coupled to a voltage sensor: a moveable portion of the protein molecule that is sensitive to changes in voltage across the membrane. |
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Term
| How does movement of the voltage sensor control the ion channel gate? |
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Definition
- when the gate occludes the channel pore, the channel is closed.
- the gate opens due to a conformational change in the structure of the
channel protein brought about by the voltage sensor. When the gate opens, the ion channel conducts ionic current. |
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Term
| When do most voltage-gated channels open? |
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Definition
| Most voltage-gated channels are closed at resting membrane potential, but will open upon depolarisation. |
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Term
| Describe the structure of voltage-gated sodium channels |
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Definition
| Sodium channels consist of a large α-subunit that associates with other proteins, such as β-subunits. An α-subunit forms the core of the channel and is functional on its own. When a cell expresses the α-subunit protein, it is able to form channels that conduct Na+ in a voltage-gated way, even if β-subunits or other known modulating proteins are not expressed. |
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Term
| Describe the structure of the α-subunit |
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Definition
| The α-subunit has four repeat domains, labelled I through IV, each containing six membrane-spanning segments, labelled S1 through S6. The highly conserved S4 segment acts as the channel's voltage sensor. |
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Term
| What are voltage-dependent Ca2+ channels formed by? |
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Definition
| Voltage-dependent Ca2+ channels are formed as a complex of several different subunits: α1, α2δ, β1-4, and γ. The α1 subunit forms the ion-conducting pore while the associated subunits have several functions including modulation of gating (Dolphin, A.C. (2003)). |
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Term
| How are K+ channels selective to K+ when Na+, still a monovalent ion, is even smaller in size than K+ so should pass through easily? |
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Definition
| The Na+ channel just has a smaller pore, so it’s easy to understand why Na+ channels are selective for Na+ and don’t allow K+ through – it just doesn’t fit (too fat). |
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Term
| What gates are involved in Na+ channels? |
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Definition
1. activation gate
2. inactivation gate ('h' gate) |
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Term
| Describe the activation gate |
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Definition
| The activation gate– ‘m ’gate – opens rapidly upon depolarisation and allows Na+ to flow through, into the cell. |
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Term
| Describe the inactivation gate |
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Definition
The inactivation gate– ‘h’ gate –also responds to depolarisation and plugs the channel pore therefore preventing ion flow (after a delay of around 5ms).
The ‘h’ gate remains in place for the duration of the refractory period. For this short time, further Na+ entry is not possible. |
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Term
| Why is the resting potential membrane 0? |
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Definition
| biological membranes have ion channels, transporters and pumps that together create asymmetric distribution of ions across the plasma membrane. |
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