Membrane Potential Flashcards
Composition of Cell membrane
bilayers of phospholipids
polar heads on outside, lipophilic fatty tails inside
separates cellular contents from ECF
contains:
Integral Proteins: Transverse the membrane
Peripheral proteins: sit on intra or extracellular surface.
Protein channels: Traverse the structure and determine ionic permeability and resultant electrical potential across membrane.
Resting Membrane potential value for
- SKM
- SMM
- Neurons
Skeletal M: -90mV
SMM: -50 to - 60mV
neurons: -70 mV
Cardiac M: -80 to -90mV
Cardiac pacemaker = -60mV
What Maintains RMP
- Na K atpase Pumps
- Move 2 K into cell and 3Na out of cell, against concentration gradient
- Active process, requiring ATP
- Net negative intracellular (3Na+ lost for 2K+ gained) - Potassium leakage channels - passive
- K+ is moving from intracellular to extracellular, down a conc gradient therefore passive process
- Loosing positive charge to ECF therefore makes cell more negative
RMP is when these two processes are at equilibrium
- NaKatpase can be seen as an electrical -in of K
- K leakage channels can be seen as a chemical - out of K
- When these two electrochemical gradients are at equilibrium, this is RMP.
NERNST EQUATION
Equation that relates the numerical values of the Conc gradient to the electrical gradient that balances it.
Used to calculate the potential difference that any ion would produce if the membrane was fully permeable to it. Therefore it will give a value for the voltage that must exist across a membrane in order to balance a chemical gradient that exists for the ion in question
E = (RT/zf) x lx ([ion]o/[ion]i)
- E = equilibrium potential for a specific ion
- R = universal gas constant (8.314Jdeg-1mol-1)
- T = absolute temperature (degrees kelvin= 273+ degrees centigrade)
- z = ionic valency (number of valence bonds a given atom can form with one or more other atoms. 1 for K, 1 for Na, -1 for Cl
- F= Faraday’s Constant (96500 coulombs/mol)
- ln = log to base e
- o = ionic concentration outside the cell
- i = ionic concentration inside the cell.
Simplified
E= 58log10 ([ion]o/[ion]i) mV
Describe the Nernst Potential for potassium and sodium in a cell in its resting state
- K
- At rest, excitable membranes such as those found in cardiac myocytes, are nearly fully permeable to K, therefore equilibrium potential for K is comparable to the cells RMP (approx -90mV) - Na
E = 58log10 ([150]/[15])
= 58 x log10(10)
= 58 x 1
K role in RMP
K is the main driver of the membrane potential.
At rest, excitable membranes are nearly fully permeable to K, therefore the equilibrium potential for K is similar to RMP.
K transverses the cell membrane via
- Na/K/ATPase pumps: 2 K in to cell for 3 Na out. (Net neg charge)
- Potassium leakage channels - K out of cell, down conc gradient (net neg charge)
Na Role in RMP
At normal RMP of -70 to -80mV, both the electrical and chemical gradient of Na tent to push it into the cell, despite the relative impermeability of the membrane to Na.
Some Na enters cell, and if this is not actively removed, the membrane potential would gradually deminish and move more positive.
Na is moved out of the cell using Na/K Atpase pumps
- 2 K into cell, 3NA out of cell, net negative
Ca Role in RMP
Ca is used as a physiological trigger, therefore intracellular concentrations are kept low
In Muscle cells, Ca is sequestered by SR, only released during excitation contraction coupling.
Chemical and electrical gradients both tend to push this ion across the membrane into cells.
Ca is removed from cells via:
Na/K ATPase pump
3 Na out of cell, 2K into cell.
Na binds to a subunit, ATP binds -> ADP, energy used to extrude 3 Na and move 3 K into cell for each unit of ATP
Regulation:
1) Increased intracellular Na
2) Thyroid hormones - Increase number and activity of pumps
3) Aldosterone = increase number of pumps
4) Dopamine = inhibits pumps in kidneys = natriresis
5) Insulin = increase activity
Draw the following points of a graph of an action potential
1. Rising phase
2. Peak
3. Falling phase
4. Positive phase
5. Negative after potential
6. Positive after potential
7. RMP
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What is the threshold potential
The level to which the membrane potential must be depolarised to initiate an action potential.
The size of an action potential in a single nerve fibre is unaffected by the stimulus intensity above the threshold. It is all or none.
