Lesson Explainer: The Nerve Impulse Biology

In this explainer, we will learn how to explain how a resting potential is maintained and describe the electrical and chemical changes that occur during an action potential.

The human body contains over seven trillion nerves. Each signal these nerves send can travel at rapid speeds of up to 120 metres (almost 400 feet) every second! This amazing evolutionary development allows us to think quickly and even act without thinking, to respond to our environment and aid our survival.

A neuron is a specialized cell found within the nervous system. Neurons’ function is to transmit information in the form of an electrical signal: a nerve impulse.

A nerve impulse is initiated by a stimulus, that is, a change in the internal or external environment. This stimulus triggers a receptor to send a nerve impulse to our central nervous system (CNS). The CNS, consisting of the brain and spinal cord, processes the information. Nerve impulses are then transmitted from the CNS to different organs that allow us to react to the stimulus appropriately. For example, a stimulus of touching a hot object will cause a series of nerve impulses to contract muscles in your arm to pull your hand away.

Definition: Neuron

A neuron is a specialized cell that transmits nerve impulses.

Let’s look at the structure of a neuron. Neurons come in many shapes and sizes; however, most of them have a similar basic structure. Figure 1 shows an example of a neuron.

The nerve impulse first starts at the dendrites, then arrives at the cell body, which contains the nucleus of the neuron. The red arrows in Figure 1 show the path that the nerve impulse will take from the cell body and along the threadlike part of the neuron called an axon. Some neurons, like the one in Figure 1, have an insulating layer surrounding the axon called a myelin sheath. There are small gaps in the myelin sheath, called nodes of Ranvier, that play an important role in increasing the speed of a nerve impulse.

Key Term: Axon

An axon is the long threadlike part of a neuron along which nerve impulses are conducted.

To initiate and propagate a nerve impulse, a neuron must be excitable. What makes neurons electrically excitable?

The cytoplasm of neurons and the extracellular space are different fluids with different chemical compositions. As a consequence, they do not contain the same amounts of charged ions. There is normally an excess of positive charges in the extracellular space as we will see later in this explainer. This creates an electric tension, or potential, between both sides of the membrane, with the positive ions outside attracted by the negatively charged cytoplasm. In physics, this kind of electric force is called a voltage. The membrane is said to be polarized because of this difference of potential. Potentially, if there was a hole or a channel in the membrane, the positive ions would move freely inside until their concentration and charges equilibrate on both sides of the membrane.

The difference between the voltage inside the neuron’s cytoplasm and the extracellular space is called the membrane potential.

Key Term: Membrane Potential

The membrane potential, or potential difference, is the difference in electrical potential between the interior and exterior of a neuron.

When a neuron is not transmitting a nerve impulse, it is said to be at rest, and the membrane has its resting potential. The mechanism by which the resting potential is maintained is summarized in Figure 2.

Key Term: Resting Potential

The resting potential is the potential difference across the membrane of a neuron at rest (around 70 mV).

The resting potential is maintained through active transport by proteins embedded in the neuron membrane called sodium–potassium pumps. The sodium–potassium pump moves positively charged sodium (Na+) and potassium (K+) ions across the membrane using ATP energy. It requires energy, as sodium and potassium are being transported against their concentration gradients from an area of low concentration to an area of high concentration. For every three sodium ions pumped out of the neuron, two potassium ions are pumped in. This makes the voltage in the extracellular space more positive than the neuron’s cytoplasm. It also increases the concentration of potassium ions inside the neuron. In fact, the concentration of sodium ions is 10x–15x higher outside the neuron than inside, and the concentration of potassium is 30x higher inside the cell than outside!

The constant activity of sodium–potassium pumps plays a vital role in keeping neurons excitable. Ouabain, a plant-derived poison, has been used for several thousand years by West African tribes to make poisonous arrows. Ouabain is a potent blocker of the sodium–potassium pump as it attacks the nervous system, and one poisonous arrow is enough to rapidly kill any hunted animal, even an elephant.

Key Term: Sodium–Potassium Pump

The sodium–potassium pump maintains the resting potential of the axon membrane by transporting three sodium ions out and two potassium ions into the neuron.

The activity of the pump creates an imbalanced distribution of Na+ and K+ across the membrane, with a higher concentration of K+ inside the neuron than outside and a higher concentration of Na+ outside than inside. At rest, the membrane allows a minimal flow of these ions and remains 40x more permeable to K+ than to Na+. K+ passively diffuses through pores called “leak” channels specific to these ions, moving down their concentration gradient from an area of high to low K+ concentration in the extracellular space.

The “leak” channels are always open, so the membrane is permeable to K+ and the flow of Na+ remains forty times smaller. This net flow of ions ultimately lowers the membrane potential, as the outside of the cell becomes more positively charged.

Key Term: “Leak” Channels

“Leak” channels, or potassium ion channels, are always open making the neuron membrane permeable to potassium ions.

There are also negatively charged ions, such as chloride, and negatively charged proteins in a higher concentration inside the neuron. With the action of the sodium–potassium pump and “leak” channels, this contributes to making the extracellular space outside the neuron more positively charged than the cytoplasm inside the neuron. The membrane is polarized, achieving a resting potential of around 70 mV.

Example 1: Describing the Status of Ion Channels in Maintenance of the Resting Potential

When the resting potential is being maintained, are potassium ion channels (leak channels) open or closed?

Answer

When the neuron is at rest, the extracellular space is more positively charged than the neuron’s cytoplasm. The membrane is polarized, and the membrane potential is around 70 mV.

The resting potential is maintained primarily through active transport by proteins embedded in the neuron membrane called sodium–potassium pumps. The sodium–potassium pump moves positively charged sodium (Na+) and potassium (K+) ions across the membrane using ATP. It requires energy, as Na+ and K+ are being transported against their concentration gradients from an area of low concentration to an area of high concentration. For every 3Na+ ions that are pumped out of the neuron, 2K+ ions are pumped in. This makes the voltage in the extracellular space more positive than the neuron cytoplasm. It also increases the concentration of K+ inside the neuron.

Due to the higher K+ concentration inside the neuron, K+ will also “leak” across the neuron membrane out of the cytoplasm into the extracellular space. It passively diffuses through pores called “leak” channels specific to K+, moving down its concentration gradient from an area of high to low K+ concentration. “Leak” channels are always open, so the membrane is permeable to K+. This lowers the membrane potential, as the outside of the cell is becoming more positively charged, achieving the resting potential of 70 mV.

Therefore, when resting potential is maintained, the potassium ion channels (leak channels) are open.

When the neuron is not at rest, it is conducting a nerve impulse called an action potential.

Action potentials are electrical signals that transmit information by the movement of charged ions across the membrane of a neuron as the action potential passes along it. This temporarily changes the potential difference at the particular point on the neuron where ions are moving.

The main stages of an action potential are

  1. depolarization,
  2. repolarization,
  3. Hyperpolarization,
  4. a brief refractory period during which another action potential cannot be generated.

The movement of ions in depolarization and repolarization is summarized in Figure 3.

Key Term: Action Potential

An action potential is the transient change in the potential difference across the neuron membrane when stimulated (approximately +40 mV).

Let’s look at depolarization first.

Depolarization is when the membrane potential at one point on the neuron reverses from negative to positive. This is initially caused by the activation of chemical receptors at synapses located at the dendrites of a neuron. The activation of these receptors triggers the opening of voltage-gated Na+ channels that were previously shut, making the membrane more permeable to Na+.

Na+ diffuses into the neuron cytoplasm as it is less concentrated there than in the extracellular space due to the action of the sodium–potassium pump. The increased concentration of Na+ makes the neuron cytoplasm less negatively charged as you can see in Figure 4. The increased positivity of the membrane potential causes more voltage-gated Na+ channels to open. This means that Na+ diffuses into the neuron at a faster rate, which continues until the membrane potential reaches a value of around +40 mV.

Key Term: Depolarization

Depolarization is a change in the membrane potential at one point in a neuron from negative to positive.

Key Term: Voltage-Gated Ion Channels

Voltage-gated ion channels are those that open and close in response to changes in the membrane potential of the cell and, as a result, enable a flow of ions across a membrane.

When the membrane potential has reached +40 mV, the voltage-gated Na+ channels close, and voltage-gated K+ channels open. Na+ can no longer enter the neuron. K+ is more concentrated in the neuron cytoplasm than in the extracellular space due to the action of the sodium–potassium pump, so K+ can now diffuse out. This lowers the membrane potential, and the neuron cytoplasm again becomes less positively charged than the extracellular space. This is called repolarization, as you can see in Figure 5.

Key Term: Repolarization

Repolarization is a change in the membrane potential at one point in a neuron from positive back to negative.

So much K+ diffuses out of the neuron when the voltage-gated K+ channels open that the membrane potential temporarily becomes even more negative than its resting potential. This is called hyperpolarization.

Hyperpolarization causes the voltage-gated K+ channels to close, and the sodium–potassium pump resets the membrane to its resting potential. You can see this occurring in the final stage of Figure 3. This period of time is called the refractory period, during which no more action potentials can be generated as the voltage-gated Na+ channels remain closed. Refractory periods last a very short time, usually between 0.001 and 0.003 seconds!

Key Term: Hyperpolarization

Hyperpolarization is a change in the membrane potential at one point in a neuron to more negative than its original resting potential.

Key Term: Refractory Period

The refractory period is a brief period immediately following an action potential during which a neuron is unresponsive to further stimulation and therefore cannot generate another action potential.

Example 2: Stating the Sequence of Stages in an Action Potential

The diagram provided shows the stages of an action potential, with each stage assigned a number. State the correct sequence of numbers.

Answer

An action potential is a change in the electrical potential of the neuron membrane as the nerve impulse passes along the neuron. Its main stages are depolarization, repolarization, hyperpolarization, and a brief refractory period.

Depolarization is when the electrical charge at one point on the neuron membrane reverses from negative to positive. This is caused by energy from a stimulus triggering the opening of voltage-gated Na+ channels so Na+ diffuses into the neuron cytoplasm. The increased concentration of Na+ makes the neuron cytoplasm less negatively charged, which causes more voltage-gated Na+ channels to open. Na+ diffuses into the neuron at a faster rate until the membrane potential reaches around +40 mV.

The voltage-gated Na+ channels now close so Na+ can no longer enter the neuron. Voltage-gated K+ channels open so K+ can diffuse out of the neuron cytoplasm. This lowers the membrane potential, and the neuron cytoplasm again becomes less positively charged than the extracellular space. This is called repolarization.

So much K+ diffuses out of the neuron that the membrane potential becomes even more negative than its resting potential. This is called hyperpolarization, and it causes the voltage-gated K+ channels to close. The sodium–potassium pump resets the membrane to its resting potential in a period of time called the refractory period. During the refractory period, no more action potentials can be generated as the voltage-gated Na+ channels remain closed.

Therefore, the correct sequence of events in an action potential is 4, 2, 6, 1, 5, 3.

Let’s look at the graph in Figure 6 showing how the membrane potential changes during an action potential.

  1. In stage 1, the resting potential is being maintained at stage 1, with the sodium–potassium pump and “leak” channels keeping the membrane potential at around 70 mV.
  2. In stage 2, stimulus has caused voltage-gated Na+ channels to open at stage 2, depolarizing the membrane to +40 mV.
  3. The voltage-gated Na+ channels close at +40 mV and voltage-gated K+ channels open. Stage 3 shows repolarization of the membrane, as K+ diffuses out of the axon.
  4. Stage 4 shows hyperpolarization of the membrane, overshooting its resting potential. The sodium–potassium pump works to reset the resting potential in the refractory period.
  5. The resting potential has been reset in stage 5, returning the membrane potential to 70 mV.

Example 3: Describing the Events of an Action Potential

The graph provided shows how the potential difference across an axon membrane changes during the course of an action potential. What is happening during stage 2?

Answer

The resting potential is being maintained at stage 1, with the sodium–potassium pump keeping the membrane potential at around 70 mV. A stimulus has caused voltage-gated Na+ channels to open at stage 2, depolarizing the membrane to +40 mV. The voltage-gated Na+ channels close at +40 mV and voltage-gated K+ channels open. Stage 3 shows repolarization of the membrane, as K+ diffuses out of the axon. Stage 4 shows hyperpolarization of the membrane, overshooting the resting potential. Following this refractory period, the resting potential is reset in stage 5, returning the membrane potential to 70 mV.

Therefore, at stage 2, a stimulus has triggered the opening of voltage-gated sodium ion channels, and sodium ions depolarize the membrane.

Example 4: Describing the Events of an Action Potential

The graph provided shows how the potential difference across an axon membrane changes during the course of an action potential. What is happening during stage 3?

Answer

The resting potential is being maintained at stage 1, with the sodium–potassium pump keeping the membrane potential at around 70 mV. A stimulus has caused voltage-gated Na+ channels to open at stage 2, depolarizing the membrane to +40 mV. The voltage-gated Na+ channels close at +40 mV and voltage-gated K+ channels open. Stage 3 shows repolarization of the membrane, as K+ diffuses out of the axon. Stage 4 shows hyperpolarization of the membrane, overshooting the resting potential. Following this refractory period, the resting potential is reset in stage 5, returning the membrane potential to 70 mV.

Therefore, at stage 3, voltage-gated potassium ion channels open, and potassium ions diffuse out of the axon.

An action potential is then propagated from one end of the neuron’s axon to the other, in one direction only. This propagation is referred to as a wave of depolarization.

This is because as one section of the axon’s membrane depolarizes, positively charged Na+ moves into the axon cytoplasm, as you can see in the green section of stage 1 in Figure 7.

Voltage-gated sodium channels next to the initial site of depolarization get activated so that sodium diffuses along the axon to depolarize the next section as you can see in stage 2 in Figure 8. This triggers voltage-gated Na+ channels in this next section to open, and the membrane at this point becomes fully depolarized.

The wave of depolarization can only travel in one direction, as the section behind the depolarized section in stage 3 is repolarizing, as you can see in Figure 9. The voltage-gated K+ channels have opened and K+ diffuses out of the axon, making it more negative than the extracellular space, and the membrane hyperpolarizes. During this refractory period, the voltage-gated Na+ channels remain shut, so no Na+ can move into the axon and the Na+ in the wave of depolarization cannot diffuse backward.

The strength of a stimulus determines whether an action potential will be generated. If the stimulus passes a threshold value, it will always trigger an action potential. If the stimulus does not pass this value, no action potential will be generated. Therefore, action potentials are called all-or-nothing responses.

Though the action potential will always be the same size, if a stimulus is stronger, the frequency of action potentials will be higher and so more will be generated per unit time.

Key Term: The All-or-Nothing Principle

The all-or-nothing principle states that if a stimulus is large enough to pass a threshold value, an action potential of the same size will always be generated. If the stimulus is not large enough to pass this value, no action potential will be generated.

Three factors affect the speed of transmission of an action potential.

At higher temperatures, ions diffuse faster as they have more kinetic energy. This increases the speed of the action potential. At temperatures above 40C, however, proteins such as the sodium–potassium pump start to denature, which causes transmission rate to drop.

The diameter of the axon also affects the speed of an action potential. The larger the diameter, the faster the transmission, as the diffusing ions encounter less resistance. This is like if lots of people were trying to walk along a wide corridor, it would be much easier than the same number of people walking along a narrow one!

Whether or not an axon is myelinated also affects the speed of transmission. Myelinated axons conduct nerve impulses faster than nonmyelinated axons. The speed of propagation of a nonmyelinated axon is around 12 metres per second, whereas propagation along a myelinated axon can reach up to 140 metres per second!

The voltage-gated ion channels are only found in the nodes of Ranvier in myelinated axons, so depolarization can only occur at these points. This means that the action potential “jumps” from one node to the next as represented by the pink arrows in Figure 10. This process is called saltatory conduction, from the Latin word meaning “leap,” and it speeds up the transmission as less time is taken in opening and closing ion channels.

Comparatively, lots of ion channels are opening and closing in the nonmyelinated axon in Figure 10, so the speed of propagation of the action potential is much slower.

Key Term: Saltatory Conduction

Saltatory conduction describes how action potentials propagate along a myelinated axon by “jumping” from one node of Ranvier to the next, increasing the speed of conduction compared to nonmyelinated axons.

Let’s recap some of the key points we have covered in this explainer.

Key Points

  • The movement of sodium and potassium ions across a neuron’s membrane determines the membrane potential.
  • The resting potential of a neuron’s membrane is maintained by the sodium–potassium pump and “leak” channels.
  • An action potential transmits electrical information along a neuron and consists of depolarization, repolarization, hyperpolarization, and a refractory period.
  • The speed of transmission of an action potential is affected by temperature, axon diameter, and myelination of the neuron.
  • The all-or-nothing principle states that if a stimulus passes the threshold value, an action potential will always be generated.

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