Chapter 5: Membrane Potentials and Action Potentials Flashcards
This is the potential difference between the inside and outside of the cell
diffusion potential
In normal mammalian nerve fiber, what is the potential difference inside the fiber membrane?
-94 millivolts (as potassium goes out)
+61 millivolts (as sodium goes in)
This equation describes the relationship of diffusion potential to the ion concentration difference across a membrane
Nernst equation
This is the diffusion potential across a membrane that exactly opposes the net diffusion of a particular ion through the membrane
Nernst potential
Give the Nernst equation
Electromotive Force = ±(61/z) (Concentration inside/Concentration outside)
where z is the electrical charge of the ion (e.g., 1+ for K+)
What is the Nernst potential if the concentration of the K+ ions inside is 10 times that on the outside?
- 61 mV
* since log of 10 is 1
What is the equation used to calculate the diffusion potential when the membrane is permeable to several different ions?
Goldman equation or
Goldmann-Hodgkin-Katz equation
What are the most important ions involved in the development of membrane potentials in nerve and muscle fibers, as well as in the neuronal cells?
Sodium, Potassium, Chloride
Resting Membrane Potential of Neurons
-60 to -70 mV
Resting Membrane Potential of Skeletal Muscle
-85 to -95 mV
Resting Membrane Potential of Smooth muscle
-50 to -70 mV
Resting Membrane Potential of Cardiac Muscle
-80 to -90 mV
Resting Membrane Potential of Hair cells (cochlea)
-15 to -40 mV
Resting Membrane Potential of Astrocytes
-80 to -90 mV
Resting Membrane Potential of Erythrocytes
-8 to -12 mV
Resting Membrane Potential of Photoreceptors
-40 (dark) to -70 (light) mV
This is primarily responsible for the signal transmission in neurons
Rapid changes in sodium and potassium ion permeability
What is the sign of the electrochemical driving force if the predicted movement of cations is out of the cell?
positive
Cations are predicted to move (?outside/inside?) the cell if the electrochemical driving force of cations is negative
inside
This pertains to the difference between the membrane potential and equilibrium potential of the ion
electrochemical driving force
Vdf = Vm – Veq
A positive value indicates outward flux of the ion, and a negative value indicates inward flux of
the ion. A typical equilibrium potential for sodium (calculated using the Nernst equation) is +62 millivolts, so the electrochemical driving force for sodium is −65 − 62 = −127 millivolts. This means that a 127-millivolt force attempts to drive sodium into the cell. The equilibrium potential is about −86 millivolts for potassium and about −70 millivolts for chloride; hence, the electrochemical driving forces for these two ions are +21 and +5 millivolts, respectively (and both ions tend to be driven out of the cell).
This is the membrane potential where the net flow through any open channels is 0.
equilibrium potential or reversal potential
This is known as the reversal potential
equilibrium potential
**When Vm = Veq, there is no net movement of the ion into or out of the cell. Also, the direction of ion flux through the membrane reverses as Vm becomes greater than or less than Veq; hence, the equilibrium potential (Veq) is also called the reversal potential.
When measuring the membrane potential, which electrode is place in the extracellular fluid?
indifferent electrode
This is the voltage change area at the cell membrane that causes the potential to decrease abruptly to -70 mV as the recording electrode passes through.
electrical dipole layer
The resting membrane potential of large nerve fibers when they are not transmitting nerve signals is about:
−70 millivolts
What are the large concentration gradients for sodium and potassium across the resting nerve membrane caused by the Na+-K+ pump?
Na+ (outside):142mEq/L
Na+ (inside):14mEq/L
K+ (outside): 4mEq/L
K+ (inside):140mEq/L
The ratios of these two respective ions from the inside to the outside are as follows:
Na inside /Na outside = 0.1
K inside /K outside = 35.0
These are protein channels in the nerve membrane through which potassium ions can leak, even in a resting cell
tandem pore domain, potassium channel, or potassium [K+] “leak” channel
How much degree of negativity (in mV) is added by the Na-K pump on the inside of the cell membrane?
-4 mV
Most of the membrane potential of the cell is due to the diffusion potentials of which ion?
Potassium
**the diffusion potentials alone caused by potassium and sodium diffusion would give a membrane potential of about −86 millivolts, with almost all of this being determined by potassium diffusion. An additional −4 millivolts is then contributed to the membrane potential by the continuously acting electrogenic Na+-K+ pump, and there is a contribution of chloride ions.
These are rapid changes in the membrane potential that spread rapidly along the nerve fiber membrane
action potential
What stage of the action potential pertains to the resting membrane potential before the action potential begins?
Resting stage
- polarized membrane with -70mV
This is the process by which the normal polarized state of −70 millivolts is immediately neutralized by the inflowing, positively charged sodium ions, with the potential rising rapidly in the positive direction
depolarization
This is the reestablishment of the normal negative resting membrane potential by the rapid diffusion of potassium ions to the exterior of the cells as the potassium channels open to a greater degree than normal.
repolarization
In a normal resting membrane (-70mV) what is which gate of the voltage-gated sodium channel is closed?
activation gate
Around what voltage from the resting membrane potential does the voltage-gated sodium channels undergo a sudden conformational change in its activation gate?
-55 mV (activation gate flips all the way to open position)
True or False:
The conformational change that flips the activation gate to the open state is a slower process than the conformational change that closes the inactivation gate.
False:
the conformational change that flips the inactivation gate to the closed state is a slower process than the conformational change that opens the activation gate.
This is the method used to measure the flow of ions through different voltage-gated protein channels
Voltage Clamp method
This toxin blocks sodium channels when applied to the outside of the membrane
tetrodotoxin
This ion blocks the potassium channels when applied to the interior of the membrane
tetraethylammonium
What causes the potassium channels to close at the end of action potential?
the return of the membrane potential to a negative state
What causes the membrane potential to be positive at the action potential onset?
More sodium ions flow to the interior of the fiber than potassium ions to the exterior
These are responsible for the negative charge inside the fiber when there is a net deficit of positively charged potassium ions and other positive ions.
Impermeant negative ions
These protein channels are known as the slow channels
voltage-gated calcium ion channels
These channels play a key role in initiating action potential
voltage-gated sodium ion channels
These channels provide more sustained depolarization
voltage-gated calcium ion channels
In some types of smooth muscle, the fast sodium channels are hardly present. In these cells, the action potentials are caused almost entirely by the activation of which channels.
slow calcium channels
What is expected to change as for the membrane potential of cells in cases of hypocalcemia?
a small increase of the membrane potential from its normal, very negative level
**When there is a deficit of calcium ions, the sodium channels become activated (opened) by a small increase of the membrane potential from its normal, very negative level. Therefore, the nerve fiber becomes highly excitable, sometimes discharging repetitively without provocation, rather than remaining in the resting state
This condition can occur in the muscles if the calcium ion concentrations in the ECF falls 50% below normal
Muscle tetany
**due to the spontaneous discharge of peripheral nerves
Which ions appear to bind to the exterior surfaces of the sodium channel protein, altering the electrical state of the sodium channel protein, thus altering the voltage level required to open the sodium gate.
calcium ion
What type of feedback cycle opens the sodium-channels (?positive/negative?)
positive-feedback cycle
This is the level of stimulation that causes the explosive development of action potential (about 15-30 mV of sudden rise in membrane potential)
threshold for stimulation
Once an action potential has been elicited at any point on the membrane of a normal fiber, the depolarization process travels over the entire membrane if conditions are right, but it does not travel at all if conditions are not right. This principle is called the:
all-or-nothing principle
For continued propagation of an impulse to occur, the ratio of action potential to threshold for excitation must at all times be greater than 1. This “greater than 1” requirement is called the ___________ for propagation
safety factor
Which cellular protein pump is strongly stimulated when excess sodium ions accumulate inside the cell membranes?
sodium-potassium pump
The plateau of action potential that occurs in heart muscle fibers occurs for as long as ______ second
0.2 to 0.3 second
What are the 2 types of channels that contribute to the depolarization process in the heart muscle?
(1) the usual voltage-activated sodium channels, called fast channels; and (2) voltage-activated calcium-sodium
channels (L-type calcium channels), which are slow to open and therefore are called slow channels.
This is a drug (an alkaloid neurotoxin found in Liliaceae family) that activates sodium ion channels causing repetitive discharge of excitable tissues
veratridine
**DRUG HAS BEEN TESTED IN MYASTHENIA GRAVIS TO INCREASE MUSCLE RESPONSE TO GIVEN MOTOR NEURON STIMULATION, AND
USED EXPERIMENTALLY TO ALTER SODIUM CHANNEL KINETICS
What is the resting membrane potential in the rhythmical control center of the heart?
-60 to -70 mV
**which is not enough voltage to keep the sodium and calcium channels closed; hence, rhythmical or spontaneous depolarization
This is the state in action potential wherein increased outflow of potassium ions leaves considerably more negativity inside the fiber drawing the membrane potential nearer to the potassium Nernst potential
hyperpolarization
This is the central core of a myelinated fiber
axon
This is a viscid intracellular fluid of an axon.
axoplasm
This is the lipid substance that make up the multiple layers of Schwann cell membrane surrounding the peripheral nerve axon
sphingomyelin
This substance is an excellent electrical insulator that decreases ion flow through the membrane about 5000-fold
sphingomyelin
This is the electrical current flow through the surrounding extracellular fluid outside the myelin sheath, as well as through the axoplasm inside the axon from node to node, exciting successive nodes one after another.
saltatory conduction
Range of velocity of action potential conduction in nerve fibers
0.25 m/sec in small unmyelinated fibers to as much as 100 m/sec—more than the length of a football field in 1 second—in large myelinated fibers.
When electricity is applied with positive and negative electrodes, the excitable membrane becomes stimulated at the _______ electrode.
negative
**negative current from the electrode decreases the voltage on the outside of the membrane to a negative value nearer to the voltage of the negative potential inside the fiber. This effect decreases the electrical voltage across the membrane and allows the sodium channels to open, resulting in an action potential
These are local potential changes that fail to elicit an action potential
acute subthreshold potentials
What is the cause of refractory periods after action potential is initiated?
inactivation of sodium channels (closing of inactivation gates)
The period during which a second action potential cannot be elicited, even with a strong stimulus, is called the:
absolute refractory period (1/2500 second in a large myelinated fiber)
Membrane-stabilizing factors can decrease excitability. Which ions are said to be stabilizers?
Calcium ions
**For example, a high extracellular fluid calcium ion concentration decreases membrane permeability to sodium ions and simultaneously reduces excitability.
This is known as the ratio of action potential strength to excitability threshold
safety factor
Procaine and tetracaine affect which gate of voltage-gated sodium channels?
activation gate
**Most of these agents act directly on the activation gates of the sodium channels, making it much more difficult for these gates to open and thereby reducing membrane excitability.
