Neurons Mastering AP quiz Flashcards
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)
Sodium and potassium ions can diffuse across the plasma membranes of all cells because of the presence of what type of channel?
Leak channels
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 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.
The resting membrane potential depends on two factors that influence the magnitude and direction of Na+ and K+ diffusion across the plasma membrane. What are these two factors.
The presence of concentration gradients and leak channels
What prevents the Na+ and K+ gradients from dissipating?
Na+-K+ ATPase
he membranes of neurons at rest are very permeable to _____ but only slightly permeable to _____. What does this help do inside the cell?
K+, Cl- <> 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 the electrical and chemical gradients
What is the value for the resting membrane potential for most neurons?
-70mV
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.
The concentrations of which two ions are highest outside the cell.
Na+ and Cl–
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.
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.
Leak channels allow the movement of potassium and sodium ions by what type of membrane transport? What is this membrane transport mechanism called and how is it worked??
channel-mediated diffusion <> Leak channel via the electrochemical gradient
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
What is the electrochemical gradient of an ion?
the sum of the electrical and chemical gradients for that ion
In a typical neuron, what is the equilibrium potential for potassium? Why is it this number?
-90mV <> 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
Compared to the electrical gradient for sodium at rest, the electrical gradient for potassium at rest is __________. Why is this?
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? Why this value?
+66 mV <> 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.
Where do most action potentials originate?
Initial segment
What is the initial segment? Where is it? What is it also called?
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
What characterizes depolarization, the first phase of the action potential?
The membrane potential changes from a negative value 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.
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.
What is the first change to occur in response to a threshold stimulus?
Voltage-gated Na+ channels change shape, and their activation gates open.
How does the speed of the activation gates of Na+ and K+ compare?
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.
The depolarization phase of an action potential results from the opening of which channels?
voltage-gated Na+ channels
The repolarization phase of an action potential results from __________.
the opening of voltage-gated K+ channels
Hyperpolarization results from __________.
slow closing of voltage-gated K+ channels
How does hyperpolarization make the cell more polar?
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? What is the range?
100mV <> -70mV to 30mV
During an action potential of a neuron, what directly causes the different channels to open and close?
the transmembrane potential (voltage)
What is the typical duration of a nerve action potential?
2ms
Around what transmembrane potential does threshold commonly occur? What happens at this voltage?
-60mV <> an action potential is initiated
What ion is responsible for the depolarization of the neuron during an action potential? What does it do to make this depolarisation possible?
Na+ <> The influx of sodium ions causes the rapid depolarization during the action potential.
What factors favour the influx of sodium ions through open channel?
(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? What type of movement is this?
facilitated diffusion <> Channel-mediated diffusion
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.
What causes repolarization of the membrane potential during the action potential of a neuron? Why does this cause repolarisation?
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
Label the diagram
What do action potential depend on?
Voltage gated channels
Describe the all-or-none principle
All stimuli that bring the membrane to threshold generate identical action potentials.
Which one of these occurs correspondingly? Potassium channels close; –60 mV Depolarization to threshold; +10 mV Resting potential; +30 mV Inactivation of sodium channels; +30 mV
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.
Why does the action potential only move away from the cell body? Why is this?
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.
Where are action potentials regenerated as they propagate along an unmyelinated axon?
at every segment of the axon
The movement of what ion is responsible for the local currents that depolarize other regions of the axon to threshold? How does it work?
Na+ <> Sodium ions enter the cell during the beginning of an action potential. Not only does this (further) depolarize the membrane where those channels are located, but it also sets up local currents that depolarize nearby membrane segments. In the case of myelinated axons, these local currents depolarize the next node, 1-2 mm away.
In an unmyelinated axon, why doesn’t the action potential suddenly “double back” and start propagating in the opposite direction? What causes this behaviour?
The previous axonal segment is refractory. <> A propagating action potential always leaves a trail of refractory membrane in its wake. The trailing membrane takes some time to recover from the action potential it just experienced, largely because the membrane’s voltage-gated sodium channels are inactivated. By the time this membrane segment is ready to (re)generate another action potential, the first propagating action potential is long gone.
Label the steps
