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Chapter 12: Problem 6

Explain as precisely as you can but in no more than 100 words the ionic basis of an action potential and how it is passed along an axon.

Short Answer

Expert verified
Action potentials are generated by ion exchanges through voltage-gated channels: depolarization (Na鈦� in), followed by repolarization (K鈦� out), which propagates the signal along the axon.

Step by step solution

01

Resting Potential

The axon is at a resting potential, typically around -70 mV, maintained by the sodium-potassium pump, which moves 3 Na鈦� ions out and 2 K鈦� ions in, creating a concentration gradient.
02

Depolarization Initiation

Once the neuron receives a stimulus, voltage-gated sodium channels open, allowing Na鈦� to rush into the axon, causing depolarization as the inside becomes more positive.
03

Action Potential Peak

The influx of Na鈦� continues until the inside of the axon reaches around +40 mV. At this peak, sodium channels close, and voltage-gated potassium channels open.
04

Repolarization

K鈦� ions move out of the axon, returning the inside to a negative potential. This process is called repolarization, restoring the overall charge difference across the membrane.
05

Hyperpolarization and Return to Resting State

An overshoot may occur (hyperpolarization), but the sodium-potassium pump restores the resting potential. Meanwhile, the action potential travels down the axon.
06

Propagation of Action Potential

The action potential opens nearby voltage-gated sodium channels, creating a domino effect that propagates the signal along the axon to the axon terminals.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Resting Potential
The resting potential is essential for nerve cells to function correctly. At rest, a neuron's inside has a negative charge compared to the outside. This difference creates a resting potential of approximately -70 mV. It's maintained by the sodium-potassium pump, an essential protein that actively moves ions across the cell membrane.
  • The sodium-potassium pump moves 3 sodium ions (Na鈦�) out for every 2 potassium ions (K鈦�) it brings in.
  • This action creates a concentration gradient, leading to a negatively charged interior relative to the outside.
The resting potential allows neurons to quickly respond to stimuli, setting up the conditions necessary for an action potential.
Sodium-Potassium Pump
The sodium-potassium pump is a critical component in maintaining the neuron's resting potential. It's an active transport mechanism, meaning it uses energy (ATP) to function.
  • By constantly exchanging 3 Na鈦� ions outward and 2 K鈦� ions inward, the pump maintains the ion concentration gradients.
  • This helps keep the inside of the neuron negatively charged relative to the outside.
Without the pump, neurons would not be able to recover their resting state after an action potential and would ultimately cease to function. This process is continuous and essential for neuron excitability.
Voltage-Gated Sodium Channels
Voltage-gated sodium channels are crucial in starting and propagating an action potential.
  • These specialized proteins open in response to a change in membrane potential, typically when a neuron receives a large enough stimulus.
  • Once open, they allow Na鈦� ions to flood into the neuron, which changes the membrane potential.
This sudden influx results in depolarization, a vital first step in the action potential process. Once the inside of the cell reaches around +40 mV, these channels quickly close, which prevents further sodium influx and sets the stage for the next phase.
Depolarization
Depolarization is the process of reversing the charge difference across a neuron's membrane, triggered by opening voltage-gated sodium channels.
  • When these channels open, Na鈦� ions rush into the cell, making the inside more positive.
  • This shift from a negative to a positive interior is critical for generating an action potential.
Depolarization moves along the axon as each segment reaches the threshold, opening neighboring Na鈦� channels. This process creates a wave of electrical activity that travels down the neuron's length, enabling the communication of signals between nerve cells.
Repolarization
Repolarization follows the peak of the action potential, where the neuron's interior returns to a negative state.
  • Once the sodium channels close, voltage-gated potassium channels open, allowing K鈦� ions to exit the cell.
  • This outward movement of K鈦� restores the negative internal environment.
Repolarization is crucial for resetting the membrane potential after depolarization, allowing the neuron to fire another action potential. Occasionally, the membrane may become overly negative, a situation called hyperpolarization, which is corrected by the sodium-potassium pump returning the cell to resting potential.

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Most popular questions from this chapter

The resting membrane potential of a typical animal cell is about \(-70 \mathrm{mV}\), and the thickness of a lipid bilayer is about \(4.5 \mathrm{nm} .\) What is the strength of the electric field across the membrane in \(\mathrm{V} / \mathrm{cm} ?\) What do you suppose would happen if you applied this field strength to two metal electrodes separated by a \(1-\mathrm{cm}\) air gap?

Which of the following statements are correct? Explain your answers. A. The plasma membrane is highly impermeable to all charged molecules. B. Channels have specific binding pockets for the solute molecules they allow to pass. C. Transporters allow solutes to cross a membrane at much faster rates than do channels. D. Certain \(\mathrm{H}^{+}\) pumps are fueled by light energy. E. The plasma membrane of many animal cells contains open \(\mathrm{K}^{+}\) channels, yet the \(\mathrm{K}^{+}\) concentration in the cytosol is much higher than outside the cell. F. A symport would function as an antiport if its orientation in the membrane were reversed (i.e., if the portion of the molecule normally exposed to the cytosol faced the outside of the cell instead). G. The membrane potential of an axon temporarily becomes more negative when an action potential excites it.

One thousand \(\mathrm{Ca}^{2+}\) channels open in the plasma membrane of a cell that is \(1000 \mu \mathrm{m}^{3}\) in size and has a cytosolic \(\mathrm{Ca}^{2+}\) concentration of \(100 \mathrm{nM}\). For how long would the channels need to stay open in order for the cytosolic \(\mathrm{Ca}^{2+}\) concentration to rise to \(5 \mu \mathrm{M} ?\) There is virtually unlimited \(\mathrm{Ca}^{2+}\) available in the outside medium (the extracellular \(\mathrm{Ca}^{2+}\) concentration in which most animal cells live is a few millimolar), and each channel passes \(10^{6} \mathrm{Ca}^{2+}\) ions per second.

We will see in Chapter 15 that endosomes, which are membrane-enclosed intracellular organelles, need an acidic lumen in order to function. Acidification is achieved by an \(\mathrm{H}^{+}\) pump in the endosomal membrane, which also contains \(\mathrm{Cl}^{-}\) channels. If the channels do not function properly (e.g., because of a mutation in the genes encoding the channel proteins), acidification is also impaired. A. Can you explain how CI' channels might help acidification? B. According to your explanation, would the Cl-channels be absolutely required to lower the pH inside the endosome?

Acetylcholine-gated cation channels do not discriminate between \(\mathrm{Na}^{+}, \mathrm{K}^{+},\) and \(\mathrm{Ca}^{2+}\) ions, allowing all to pass through them freely. So why is it that when acetylcholine binds to this protein in the plasma membrane of muscle cells, the channel opens and there is a large net influx of primarily \(\mathrm{Na}^{+}\) ions?
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