Lecture 5: Membrane Potentials COPY Flashcards
Why do you want to “excite” a cell?
To allow cells communicate to other cells or its own interior
What is resting membrane potential primarily due to?
The permiability of the plasma membrane to potassium ions
Potassium Leaky Channels
What is the accepted resting membrane potential for cardiac and skeletal muscle cells?
-80 to -90 mV
What is the accepted resting membrane potential for smooth muscle?
-60 mV
What is the accepted resting membrane potential for Neurons?
-60 to -70 mV
What are the differences between the following?
Voltage Gated
Ligand Gated
Signal Gated
Voltage Gated: Channel is opened in response to change in membrane potential
Ligand Gated: Channel is opened in response to a specific extracellular NT (e.g. acetylcholine)
Signal Gated: Opens in resposne to a specific intracellular molecule
In a Na+/K+ ATPase pump, what goes in and out of the cell?
3 Na+ Out and 2 K+ In
Requires Energy
Which ion are membranes most permeable to?
Potassium
Potassium Leak Channels are open all the time
What two forces act on ions to help develop membrane potential, and what determines those forces?
Diffusion/Chemical Gradients: Concentration of a given ion in vs out of the cell
Electrostatic Forces: Like repels like and opposites attract
(Negative ions are less likely to cross over to the side of the membrane with more negative ions, but more likely to cross over to the side with more positive ions)
What is equilibrium potential for a given ion?
The membrane potential when electrical and chemical forces on that ion are equal, and no further movement of that ion occurs.
Not the same as resting membrane potential
What is the equilibrium potential for Sodium and Potassium?
Sodium: 66 mV
Postassium: -91 mV
What is the equilibrium potential for Calcium?
+123 mV
What is the equilibrium potential for Chloride?
-66.4 mV
What is the Nernst Equation?
Eion= (61.5/x)(log [out]/[in])
Where [out] is the extracellular concentration of a given ion
Where [in] is the intracellular concentration of a given ion
In terms of the Nernst Equation, if the concentration of an ion is greater inside the cell, will the log come out as positive or negative?
Negative
Log [x]out/[x]in
The denominator is larger, requiring the log to be negative.
What is the equation to determine the “driving force” of an ion?
[Resting membrane potential] - [Equalibrium constant of a given ion] = Driving Force
If a driving force is positive, what is the direction of movement for an ion?
Out of the cell
Example: Potassium
If a driving force is negative, what is the direction of movement for an ion?
Into the cell
Example: Sodium
Other than the potassium leak channels, what other forces contribute to the resting membrane potential?
Sodium Diffusion
Na+/K+ ATPase Pump
What is the resting membrane potential for Sodium?
+61.5 mV
In terms of potassium movement, what is happening when a neuron is at its resting membrane potential?
Potassium movement out of the cell is roughly equal to potsassium movement into the cell
Is it easier to depolarize a cell with a higher or lower resting membrane potential?
Higher
The higher the resting membrane potential, the closer it is to threshold, and the less is required to depolarize it.
What is a graded potential?
A stimulus that does not reach the threshold to become an action potential
What is the function of voltage gated sodium channels?
Depolarization phase of an action potential
Do voltage gated sodium channels open quickly or slowly?
Quickly
Voltage Gated Sodium Channels
What gate is open upon activation?
What gate blocks out sodium after activation?
Activation Gate
Inactivation Gate
What state are voltage gated sodium channels in during repolarization?
Their activation gates are open, but their inactivation gates are closed
What is the function of a voltage gated potassium channel?
Repolarization phase of an action potential
(positive potassium leaves the cell)
Do voltage gated potassium channels open quickly or slowly?
Open slowly
What is a main contributor to causing the hyperpolarization period?
Voltage gated potassium channels stay open a bit too long.
What makes up the absolute refractory period?
Depolarization (Overshoot)
Repolarization
Why can you not cause sodium channels to open during the absolute refractory period?
Because they are either already open, or the inactivation gates are closed, and have not yet reset.
What phase makes up the relative refractory period?
Hyperpolarization
What two things are happening relating to voltage gated ion channels to make it harder to generate an action potential during the relative refractory period?
Voltage gated Potassium channels are still open, or too much potassium has left the cell.
Not all voltage gated sodium channels are reset and ready to open.
What type of cell has an unusual repolarization graph?
What is a consequence of this fact?
Cardiac Ventricle cells - their repolarization curve is very broad, and they have a long absolute refractory period.
What do you see in patients with intermittantly low blood potassium?
Hypokalemia
- Paralysis due to a larger gap between resting membrane potential and threshold.
- Repolarization occurs more quickly
What happens with patients with higher levels of potassium in their blood?
Hyperkalemia
- Unable to compensate like regular people
- Prolonged action potential
- Absoulte Refractory Periods lengthened
