The Nerve Cell › Action Potentials

Resting Membrane Potential

Notes

Resting Membrane Potential

Sections



Summary

Overview

  • Ions flow along their electrochemical gradient (combination of concentration gradient and electric potential)
  • Neurons have open channels ("leak" channels) that allow potassium and sodium ions to travel across the membrane

Definitions

Voltage

  • Measure of the potential energy between two points that arises from separating electrical charges

Voltmeter

  • Device that measures the potential difference between two points
  • Measures the membrane potential of a neuron as around -70mV (though some variability exists) which means the inside is slightly more negative than the outside

CREATION OF RESTING POTENTIAL BY POTASSIUM ONLY

Here, we address the creation of the resting potential by potassium, only.

Stage 1

  • We show a cell within an enclosed environment and specify the higher concentration of potassium within the cell.
  • The membrane potential is zero at the beginning.
  • Next, we introduce a potassium leak channel, which allows potassium to pass through based on its concentration gradient.

Stage 2

  • The efflux of potassium ions out of the cell makes the inside of the cell negatively charged.
  • There is a large concentration gradient driving potassium ions out of the cell.
  • A weak electric force attract potassium ions (which are positively charged) back inside the cell.
  • However, the force from the concentration gradient overpowers the electric force, so potassium ions continue to leave the cell though slower than before.

Stage 3

  • There is a greater negative charge (inside the cell) than before and a greater positive charge (outside the cell).
  • The concentration gradient still favors an efflux of potassium ions, but is weaker than before (because of the higher concentration of potassium ions in the extracellular space than before).
  • The electric force is stronger than before, because there is a greater negative charge within the cell.
  • So now both forces are equivalent, so the cell has reached its equilibrium potential and there is no net movement of potassium ions.
  • For potassium, the membrane equilibrium potential is about -90 mV.

CREATION OF RESTING POTENTIAL WITH BOTH SODIUM AND POTASSIUM

Here, we address the creation of the resting potential by both sodium and potassium, which approximates what actually happens with neurons.

Stage 1

  • There is a high sodium concentration outside the cell and high potassium concentration inside the cell.
  • We show the sodium and potassium leak channels.
  • There is sodium influx into the cell and potassium efflux out of the cell (based on their concentration gradients).
    • Pay attention that our potassium arrow is larger than the sodium arrow because there are more potassium "leak" channels than sodium "leak" channels, not as a reflection of the volume of individual ions that pass through the channel types.

Stage 2

  • The concentration gradients still promote the influx of sodium into the cell and the efflux of potassium out of the cell.
  • There is still a slight negative charge within the cell and the slight positive charge outside the cell.
  • We show an electrical force that enhances the movement of sodium (a positively charged ion) into the cell and opposes the movement of potassium (also a positively charged ion) out of the cell.
  • This force slows down the flow of potassium ions and speeds up the flow of sodium ions.

Stage 3

  • Now we show that there is a greater negative charge (inside the cell) than before and a greater positive charge (outside the cell).
  • Ultimately the ion movement of sodium influx into the cell is balanced by the potassium efflux (make both arrows of equal size).
    • The resting state of the neuron refers to the steady state that occurs when there is no longer any net charge movement across the membrane (each positive charge that leaves the cell as a potassium ion is replaced by a positive charge entering the cell as a sodium ion).
  • The resting potential is measured at about -70mV.
  • The sodium/potassium pump we learned about elsewhere transports sodium out of the cell and potassium back into the cell to continually "recharge" their concentration gradients.

Full-Length Text

  • Here we will learn about the resting membrane potential of a neuron and how it is created.
  • First, make a table so we can learn some key concepts about resting potential.
  • Denote that ions flow along their electrochemical gradient, a combination of their concentration gradient and electric potential.
  • Denote that neurons have open channels in their membranes that allow potassium and sodium ions to "leak" across the membrane.
  • Denote that voltage (or potential difference) is a measure of the potential energy between two points that arises from separating electrical charges.

First, let's look at the resting membrane potential of a typical neuron.

  • Draw a neuron.
  • Next to the neuron, draw a simple voltmeter, which measures the potential difference between two points.
    • Again, voltage is a measure of the energy difference between two points.
  • Draw a wire with a microelectrode from the voltmeter to the inside of the cell: intracellular.
  • Draw another wire from the voltmeter to outside the cell: extracellular fluid.
  • Write that the voltmeter reads -70mV, which indicates that the intracellular fluid is slightly more negative relative to the extracellular fluid.
    • The resting potential of -70mV is only an example of a typical neuron but variability exists between neuron types.

Now let's explore how this resting potential arises.

To understand this, let's first look at a simplified situation in which an example cell is permeable only to potassium.

  • Draw a cell within an enclosed environment.
  • Stage 1: Show 8 representative potassium ions within the cell and zero within the extracellular space.
  • Specify the higher concentration of potassium within the cell.
  • Establish that the membrane potential is zero at the beginning.
  • Next, introduce a potassium leak channel, which allows potassium to pass through based on its concentration gradient.
  • Stage 2: Now show 2 potassium ions outside the cell and the remaining 6 still inside the cell.
  • Write that the efflux of potassium ions out of the cell makes the inside of the cell negatively charged.
  • Show that there is a large concentration gradient driving potassium ions out of the cell.
  • Now show a weak electric force attract potassium ions (which are positively charged) back inside the cell.
  • However, indicate that the force from the concentration gradient overpowers the electric force, so potassium ions continue to leave the cell though slower than before.
  • Stage 3: Draw 3 total potassium ions outside the cell and the remaining 5 still inside the cell.
  • Indicate that there is a greater negative charge (inside the cell) than before and a greater positive charge (outside the cell).
  • Show that the concentration gradient still favors an efflux of potassium ions, but is weaker than before (because of the higher concentration of potassium ions in the extracellular space than before).
  • Show that the electric force is stronger than before, because there is a greater negative charge within the cell.
  • Indicate that now both forces are equivalent, so the cell has reached its equilibrium potential and there is no net movement of potassium ions.
  • Indicate that for potassium, the membrane equilibrium potential is about -90mV (though there is some textual variation on the exact value).

Now let's look at what happens with both sodium and potassium in a single cell, which approximates what actually happens with neurons.

  • Again, draw a cell in an enclosed environment.
  • Stage 1: Draw 8 representative sodium ions in the extracellular fluid and 8 representative potassium ions inside the cell.
  • Indicate the high sodium concentration outside the cell and high potassium concentration inside the cell.
  • Draw the sodium and potassium leak channels.
  • Show that there is sodium influx into the cell and potassium efflux out of the cell (based on their concentration gradients).
  • Pay attention that our potassium arrow is larger than the sodium arrow because there are more potassium "leak" channels than sodium "leak" channels, not as a reflection of the volume of individual ions that pass through the channel types.
  • Stage 2: Now, show 1 sodium ions within the cell (7 still outside the cell).
  • Draw 2 potassium ions outside the cell (6 still inside the cell).
  • Indicate that the concentration gradients still promote the influx of sodium into the cell and the efflux of potassium out of the cell.
  • Indicate the slight negative charge within the cell and the slight positive charge outside the cell.
  • Show an electrical force that enhances the movement of sodium (a positively charged ion) into the cell and opposes the movement of potassium (also a positively charged ion) out of the cell.
  • This force slows down the flow of potassium ions and speeds up the flow of sodium ions.
  • Stage 3: Now, show 3 sodium ions within the cell (5 still outside the cell).
  • Draw 3 potassium ions outside the cell (5 still inside the cell).
  • Indicate that there is a greater negative charge (inside the cell) than before and a greater positive charge (outside the cell).
  • Indicate that ultimately the ion movement of sodium influx into the cell is balanced by the potassium efflux (make both arrows of equal size).
  • Indicate that the resting state of the neuron refers to the steady state that occurs when there is no longer any net charge movement across the membrane (each positive charge that leaves the cell as a potassium ion is replaced by a positive charge entering the cell as a sodium ion).
  • Write that this resting potential is measured at about -70mV.
  • Also write that the sodium/potassium pump we learned about elsewhere transports sodium out of the cell and potassium back into the cell to continually "recharge" their concentration gradients.

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