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Neurons (M A & P)
Terms in this set (48)
Ions are unequally distributed across the plasma membrane of all cells. This ion distribution creates an electrical potential difference across the membrane. What is the name given to this potential difference?
Resting membrane potential (RMP)
The resting membrane potential is the baseline potential that can be recorded across the plasma membrane of an excitable cell prior to excitation.
Sodium and potassium ions can diffuse across the plasma membranes of all cells because of the presence of what type of channel?
Leak channels for Na+ and K+ are ubiquitous, and they allow for the diffusion of these ions across plasma membranes.
On average, the resting membrane potential is -70 mV. What does the sign and magnitude of this value tell you?
The inside surface of the plasma membrane is much more negatively charged than the outside surface.
The inside surface of the plasma membrane accumulates more negative charge because of the presence of Na+ and K+ gradients and the selective permeability of the membrane to Na+ and K+.
The plasma membrane is much more permeable to K+ than to Na+. Why?
There are many more K+ leak channels than Na+ leak channels in the plasma membrane.
More leak channels translates into more leakiness. Thus the outward flux of K+ is greater than the inward flux of Na+.
The resting membrane potential depends on two factors that influence the magnitude and direction of Na+ and K+ diffusion across the plasma membrane. Identify these two factors.
The presence of concentration gradients and leak channels
The concentration gradient and the large number of K+ leak channels allow for rather robust K+ diffusion out of a cell. In contrast, the concentration gradient and the relatively few Na+ leak channels allow for much less Na+ diffusion into a cell.
What prevents the Na+ and K+ gradients from dissipating?
Also known as the Na+-K+ pump, or simply the pump, this transporter moves three Na+ out of the cell and two K+ into the cell for every ATP it hydrolyzes. This pumping action prevents the Na+ and K+ gradients from running down as these ions passively move through leak channels.
The membranes of neurons at rest are very permeable to _____ but only slightly permeable to _____.
more K+ moves out of the cell than Na+ moves into the cell, helping to establish a negative resting membrane potential.
During depolarization, which gradient(s) move(s) Na+ into the cell?
Both chemical and electrical gradient
a positive ion is driven into the cell because the inside of the cell is negative compared to the outside of the cell, and Na+ is driven into the cell because the concentration of Na+ is greater outside the cell.
What is the value for the resting membrane potential for most neurons?
the resting membrane potential for neurons depends on the distribution of both Na+ and K+ across the cell membrane. The potential is closer to the equilibrium potential of K+ because the cell is more permeable to K+.
The Na+-K+ pump actively transports both sodium and potassium ions across the membrane to compensate for their constant leakage. In which direction is each ion pumped?
Na+ is pumped out of the cell and K+ is pumped into the cell.
Na+ is pumped out of the cell against its electrochemical gradient and K+ is pumped into the cell against its concentration gradient.
The concentrations of which two ions are highest outside the cell.
Na+ and Cl-
both Na+ and Cl- are in higher concentrations outside the cell.
In a neuron, sodium and potassium concentrations are maintained by the sodium-potassium exchange pump such that __________.
the sodium concentration is higher outside the cell than inside the cell and the potassium concentration is higher inside the cell than outside the cell.
Because the sodium-potassium exchange pump moves sodium and potassium ions in opposite directions, the pump generates concentration gradients for these ions that are opposite in direction. The opposite direction of these concentration gradients explains why the equilibrium potentials for those ions are opposite in sign (that is, -90 mV for potassium and +66 mV for sodium).
The sodium-potassium exchange pump transports potassium and sodium ions in which direction(s)?
Sodium ions are transported out of the cell. Potassium ions are transported into the cell.
The energy of ATP is used to actively transport potassium and sodium ions against their electrochemical gradients. Potassium and sodium ions diffuse in the opposite direction through channels.
Leak channels allow the movement of potassium and sodium ions by what type of membrane transport?
Ions move through leak channels because of chemical and electrical gradients.
The electrochemical gradient for potassium ions when the transmembrane potential is at the resting potential (-70 mV) is caused by what?
a chemical gradient going out of the cell and an electrical gradient going into the cell
The higher concentration of potassium inside the cell than outside the cell results in an outward chemical gradient. However, the electrical gradient is in the opposite direction (inward) because, at the resting potential, the inside of the cell is more negative, which is attractive to the positively charged potassium ions.
What is the electrochemical gradient of an ion?
the sum of the electrical and chemical gradients for that ion
Together, these two gradients determine the net movement of a particular ion across the plasma membrane.
In a typical neuron, what is the equilibrium potential for potassium?
The potassium equilibrium potential is the transmembrane potential at which the chemical and electrical gradients would be equal in magnitude, but opposite in direction. In neurons, potassium tends to exit the cell because of the greater concentration of potassium ions inside the cell than outside the cell (that is, the concentration gradient for potassium). Therefore, the equilibrium potential for potassium must be negative, because it must oppose the exit of potassium ions. The specific value of the potassium equilibrium potential depends on the size of the potassium chemical gradient.
The electrochemical gradient for sodium ions in a neuron when the transmembrane potential is at the resting potential is caused by what?
chemical and electrical gradients both going into the cell
The higher concentration of sodium outside the cell than inside the cell, creates an inward chemical gradient. In addition, the electrical gradient for sodium is also inward becuase, at the resting potential, the inside of the cell is relatively more negative than the outside, which is attractive to the positively charged sodium ions.
Compared to the electrical gradient for sodium at rest, the electrical gradient for potassium at rest is __________.
in the same direction and of the same magnitude.
The electrical gradients for both potassium and sodium are inward because these positively charged ions are both attracted to the negatively charged interior of the cell. Because sodium and potassium each carry a single positive charge, the transmembrane potential affects them the same. The electrical gradient is entirely independent of the chemical gradient or the absolute concentrations of the ions.
In a typical neuron, what is the equilibrium potential for sodium?
The sodium equilibrium potential is the transmembrane potential at which the chemical and electrical gradients would be equal in magnitude, but opposite in direction. In a typical neuron, sodium tends to enter the cell because of the large concentration of sodium ions outside the cell relative to the concentration of sodium ions inside the cell (that is, the concentration gradient for sodium). Therefore, the equilibrium potential for sodium must be positive, because it must oppose the entry of sodium ions. The specific value of the sodium equilibrium potential depends on the size of the sodium chemical gradient.
At rest, why is the transmembrane potential of a neuron (-70 mV) closer to the potassium equilibrium potential (-90 mV) than it is to the sodium equilibrium potential (+66 mV)?
The membrane is much more permeable to potassium ions than to sodium ions.
The greater number of potassium leak channels allows potassium ions to more easily cross the membrane than do sodium ions.
Where do most action potentials originate?
The first part of the axon is known as the initial segment. The initial segment is adjacent to the tapered end of the cell body, known as the axon hillock.
What opens first in response to a threshold stimulus?
Voltage-gated Na+ channels
The activation gates of voltage-gated Na+ channels open, and Na+ diffuses into the cytoplasm.
What characterizes depolarization, the first phase of the action potential?
The membrane potential changes from a negative value to a positive value.
The plasma membrane, which was polarized to a negative value at the RMP, depolarizes to a positive value.
What characterizes repolarization, the second phase of the action potential?
Once the membrane depolarizes to a peak value of +30 mV, it repolarizes to its negative resting value of -70 mV.
The plasma membrane was depolarized to a positive value at the peak of the first phase of the action potential. Thus, it must repolarize back to a negative value.
What event triggers the generation of an action potential?
The membrane potential must depolarize from the resting voltage of -70 mV to a threshold value of -55 mV.
This is the minimum value required to open enough voltage-gated Na+ channels so that depolarization is irreversible.
What is the first change to occur in response to a threshold stimulus?
Voltage-gated Na+ channels change shape, and their activation gates open.
The activation gates of voltage-gated Na+ channels open very rapidly in response to threshold stimuli. The activation gates of voltage-gated K+ channels are comparatively slow to open.
Where in the neuron is an action potential initially generated?
this region (first part of the axon) receives local signals (graded potentials) from the soma and dendrites and has a high concentration of voltage-gated Na+ channels.
The depolarization phase of an action potential results from the opening of which channels?
voltage-gated Na+ channels
when the voltage-gated Na+ channels open, Na+ rushes into the cell causing depolarization.
The repolarization phase of an action potential results from __________.
the opening of voltage-gated K+ channels
as the voltage-gated K+ channels open, K+ rushes out of the cell, causing the membrane potential to become more negative on the inside, thus repolarizing the cell.
Hyperpolarization results from __________.
slow closing of voltage-gated K+ channels
the slow closing of the voltage-gated K+ channels means that more K+ is leaving the cell, making it more negative inside.
What is the magnitude (amplitude) of an action potential?
the membrane goes from -70 mV to +30 mV. Thus, during the action potential, the inside of the cell becomes more positive than the outside of the cell.
In what part of the neuron does the action potential typically initiate?
The initial segment has the lowest threshold and, therefore, is the place where most action potentials are initiated.
During an action potential of a neuron, what directly causes the different channels to open and close?
the transmembrane potential (voltage)
Changes in transmembrane potential directly cause voltage-gated channel proteins to change shape and allow the flow of ions across the cell membrane. The tiny electrical current associated with a single channel opening can actually be measured with sensitive recording techniques.
What is the typical duration of a nerve action potential?
From initiation to completion, the action potential is rarely over a few milliseconds. This is an extremely brief period of time-about as long as it takes for a handgun bullet to travel one meter.
Around what transmembrane potential does threshold commonly occur?
At approximately -60 mV, an action potential is initiated. A transmembrane potential -60 mV corresponds to a depolarization of 10-15 mV away from the resting membrane potential.
What ion is responsible for the depolarization of the neuron during an action potential?
The influx of sodium ions causes the rapid depolarization during the action potential. The influx of sodium ions through open channels is favored by two factors. (1) The sodium concentration inside the neuron is only about 10% of the sodium concentration outside the neuron. (2) Most of the time, the interior of the cell is electrically negative, which is attractive for the positively charged sodium ions.
What type of membrane transport causes the depolarization phase of the action potential in neurons?
Ions move through channels according to their electrochemical gradient from one side of the membrane to the other. This movement is known as channel-mediated diffusion. The transport rate for channels, unlike that for the carrier proteins involved in facilitated diffusion, does not saturate when the electrochemical gradient for the diffusing ion is increased.
During an action potential, after the membrane potential reaches +30 mV, which event(s) primarily affect(s) the membrane potential?
Voltage-gated sodium channels begin to inactivate (close) and voltage-gated potassium channels begin to open.
The repolarization phase of the action potential involves decreasing sodium influx via inactivation of sodium channels and increasing potassium efflux (exit) via opening potassium channels. Both of these processes begin near the peak of the action potential.
What ion causes repolarization of the neuron during an action potential?
The exit of potassium from the cell causes the cell to become more negative, repolarizing the membrane. The exit of potassium ions through open channels is caused by the large concentration of potassium ions inside the neuron compared to the concentration of potassium ions outside the neuron (the chemical gradient for potassium). Even though the transmembrane potential during most of the repolarization phase is negative, this small electrical gradient (tending to pull potassium ions inward) is not enough to change the overall outward direction of the potassium electrochemical gradient.
What causes repolarization of the membrane potential during the action potential of a neuron?
potassium efflux (leaving the cell)
Positively charged potassium ions flowing out of the cell makes the transmembrane potential more negative, repolarizing the membrane towards the resting potential.
What is primarily responsible for the brief hyperpolarization near the end of the action potential?
voltage-gated potassium channels taking some time to close in response to the negative membrane potential
Although both types of voltage-gated channels open and close in response to changes in membrane voltage, the voltage-gated potassium channels open and close much more slowly than the voltage-gated sodium channels. This slowness means that voltage-gated potassium channels do not immediately close after the resting potential (-70 mV) is reached. Instead, they drag the membrane potential briefly towards the potassium equilibrium potential (-90 mV).
Which is the correct statement regarding action potentials?
Action potentials depend on voltage-gated channels.
Which of the following accurately describes the all-or-none principle?
All stimuli that bring the membrane to threshold generate identical action potentials.
Choose the correct pairing of events that occur in the formation of an action potential to the corresponding changes in the transmembrane potential.
Inactivation of sodium channels; +30 mV
How is an action potential propagated along an axon?
An influx of sodium ions from the current action potential depolarizes the adjacent area.
the influx of sodium ions depolarizes adjacent areas, causing the membrane to reach threshold and cause an action potential. Thus, the action potential is regenerated at each new area.
Why does the action potential only move away from the cell body?
The areas that have had the action potential are refractory to a new action potential.
sodium channels are inactivated in the area that just had the action potential.
The velocity of the action potential is fastest in which of the following axons?
small myelinated axon
the myelination acts as insulation and the action potential is generated only at the nodes of Ranvier. Propagation along myelinated axons is known as saltatory conduction.
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