When we measure the voltage across the membrane of a neuron, we are measuring the _______.
membrane potentialExplanationThe term 'membrane potential' refers to the voltage across the membrane. The 'resting potential' is the membrane potential when the neuron is not activated. The Nernst potential is not related to the membrane but depends solely on ion concentrations inside and outside the neuron. Finally, 'voltage potential' is not a scientific term - all electrical potentials are voltages!
Ions move across the membrane through channels and can thus cause changes in voltage. Each ion has its own characteristic _______ based on the inside and outside concentrations, which is an example of a(n) _______.
Nernst potential; equilibriumExplanationIf a single ion is allowed to move until it reaches an equilibrium, this occurs at that ion's Nernst potential; this Nernst potential will change depending on the concentrations of the ion inside and outside of the cell.
Normally, for neurons there are many ions that move across the membrane, and all of these ions' movements balance at the ______, which is an example of a(n) ______.
resting potential; steady stateExplanationFor a neuron at rest, many ions are flowing in different directions. At some point the net movement of all ions balances out and the voltage is stable, this is the resting potential of the neuron (calculated by the GHK equation). This is a steady state and not an equilibrium because each ion individually is not at equilibrium (even though the net movement of all ions cancels out).
Which of the following would lead to the membrane of a neuron having a higher resistance? (Indicate all that apply.)
Decrease the number of ion channels 正解Decrease the individual conductance of ion channels 正解ExplanationIf there are fewer channels in the membrane, then it is harder for ions to pass through the membrane and so the membrane resistance is higher. Conductance refers to how easily ions pass through something like a channel, and so conductance is the inverse of resistance. So, decreasing the conductance of individual ion channels would increase the resistance of the membrane.
In the video, we referred to one method to measure resistance, such as membrane resistance in ohms/square mm. This measurement refers to one patch of membrane, and these measurements can be useful for comparing general membrane properties across cells. However, the size of a cell will affect its electrical properties, so we need other measures of resistance. The most common method is the 'total membrane resistance' or 'total axial resistance,' which takes into account the size of the cell. Usually, when we discuss membrane or axial resistance, it will be most helpful to think in terms of 'total resistance.'Given the above explanation, what would happen if we increase the diameter of an axon, but keep the same density of ion channels in the membrane? (Indicate all that apply.)
Membrane resistance decreasesAxial (internal) resistance decreasesExplanationIf we make the axon larger in diameter, we increase the cross-sectional area of the axon, and in so doing, decrease resistance of current (ions) moving through the internal solution. This is how "giant" axons in invertebrates work. Here's the slightly tricky part: if we increase the diameter, we also increase the circumference of the axon, which increases the surface area. Since the density of channels per unit area remains the same, there will be more channels in the membrane per unit length and thus the membrane resistance will decrease as well.
Which of the following would lead to a decrease in membrane capacitance of an axon? (Indicate all that apply.)
Decrease the diameter of the axonIncrease the thickness of the membraneExplanationCapacitance refers to the ability to store charge. If the membrane is thicker, then the positive and negative charges (ions) are more separated across the membrane; this then results in a reduced ability to accumulate charge and so the capacitance decreases. Decreasing the diameter of the axon would decrease the overall surface area. With a lower surface area there is less space over which charge can build up and so the membrane capacitance again decreases.
Consider an ion for which there are only passive leakage channels. If the membrane potential is farther from the ion's Nernst potential, the magnitude of the current associated with that ion will be...
greaterExplanationIf there are only passive leakage channels, then making the membrane potential farther from the ion's Nernst potential increases the driving force, which increases the current.
As we've considered in previous lessons, many biological factors can influence membrane and axial resistance. Given what you know, changing the number of ion channels in a membrane will have a more direct effect on the
membrane resistanceExplanationA larger number of ion channels means that more current can leak out of the membrane, and so the membrane resistance is lower. Conversely, a smaller number of channels means that less current (fewer ions) can pass through the membrane, and so the membrane resistance is higher. Axial resistance affects current flow through the cytoplasm and is not directly affected by the number of ion channels.
What happens to the length constant if we decrease the membrane resistance?
Length constant decreasesExplanationAgain, we can use the expression for the length constant to understand this question. Axial resistance is in the denominator of the equation, and so a decrease in axial resistance will also lead to an increase in the length constant.
A researcher is investigating the properties of motor axons in mice, and finds that voltage changes are able to travel far down the axon without much attenuation. Which of the following explanations could lead to this result? (select all that apply)
The axon membrane has a small number of ion channels.The axon diameter is very large.ExplanationWe can think about this question in terms of resistance. If voltage changes can travel farther down the axon (a larger length constant), then it is likely that the membrane resistance is high and the axial resistance is low. So, if there are a small number of ion channels, fewer charges can leak out of the axon (higher membrane resistance) and allow the voltage change to travel farther. Also, if the axon diameter is large then the opposition to the flow of ions is smaller (lower axial resistance) and so the voltage change can travel farther.
The length of an axon is 10 mm. At one end of the axon, a voltage change of 150 mV results in a change of about 1 mV at the opposite end. Given this information, what is the approximate length constant of the neuron in mm?
2ExplanationThe length constant is the length over which a voltage change decreases to about 37% of its initial value. So, we can set up the equation that 150*(.37^x) = 1, where x is the number of length constants the signal traveled. We find that x is approximately 5, so the length constant is 10 mm/5 = 2 mm.
A researcher is again investigating the properties of a motor axon in a mouse, and finds that voltage changes are very fast for this neuron. Which of the following explanations could lead to this result? (Indicate all that apply.)
The axon membrane has a large number of ion channels.The axon membrane is very thick.ExplanationWe can think about this question in terms of the time constant t = RC, where this axon has a low t because the membrane voltage changes quickly. So, if the axon has a large number of ion channels, the membrane resistance is low and the time constant is smaller (faster). Also, if the axon is thicker, the membrane capacitance is smaller and so again the time constant is smaller.
If the membrane length constant increases, then the time constant must also increase.
FalseExplanationAlthough both the length and time constants are affected by the membrane resistance, this isn't the only factor that can change the length constant. It may be the case that the length constant increases due to a decrease in the axial resistance.