Lesson Explanation: The Nervous Impulse (2023)

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 more than seven trillion nerves. Any signal sent by these nerves can change at speeds of up to120 Metros(fast400 pies) every second! This amazing evolutionary development allows us to think quickly and even act without thinking to respond to our environment and support our survival.

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

A nerve impulse is triggered by a stimulus, ie a change in the internal or external environment. This stimulus activates a receptor to send a nerve impulse to our central nervous system (CNS). The CNS, consisting of the brain and spinal cord, processes information. Nerve impulses are then relayed from the CNS to various organs that allow us to respond appropriately to the stimulus. For example, the stimulus of touching a hot object will cause a series of nerve impulses to contract the arm muscles to withdraw the hand.

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.

Lesson Explanation: The Nervous Impulse (1)

The nerve impulse first starts in the dendrites and then reaches the cell body, which contains the nucleus of the neuron. The red arrows in Figure 1 show the route the nerve impulse will take from the cell body and along the filamentous part of the neuron, the axon. Some neurons, like the one in Figure 1, have an insulating layer surrounding the axon called the myelin sheath. There are small spaces in the myelin sheath called Ranvier nodes 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 travel.

In order to initiate and transmit 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. Consequently, they do not contain the same number of charged ions. Normally there is an excess of positive charges in the extracellular space, as we will see later in this explanation. This creates an electrical voltage or potential between the two sides of the membrane, attracting positive ions from the outside to the negatively charged cytoplasm. In physics, this type of electrical force is called voltage. Because of this potential difference, the membrane is said to be polarized. If there were a hole or channel in the membrane, positive ions would potentially move freely inside until their concentration and charge on either side of the membrane were balanced.

The difference between the voltage in the cytoplasm of the neuron and in 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 inside and outside of a neuron.

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

Lesson Explanation: The Nervous Impulse (2)

Key term: resting potential

The resting potential is the potential difference across the membrane of a resting neuron (approxmV).

The resting potential is maintained by active transport by proteins embedded in the neuron's membrane called sodium-potassium pumps. The sodium-potassium pump moves positively charged sodium () and potassium () ions across the membrane using ATP energy. It requires energy because sodium and potassium are 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 out. This makes the voltage in the extracellular space more positive than in the cytoplasm of the neuron. It also increases the concentration of potassium ions within the neuron. In fact, the concentration of sodium ions outside the neuron is 10 to 15 times higher than inside, and the concentration of potassium inside the cell is 30 times higher than outside.

The constant activity of the sodium and potassium pumps plays a crucial role in keeping neurons excitable. Uabain, a plant-derived poison, has been used by West African tribes to make poison darts for several thousand years. Ouabain is a powerful sodium-potassium pump blocker as it attacks the nervous system, and one poison dart is enough to quickly kill any animal being hunted, 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 pump activity creates an unbalanced distribution to and across the membrane, with a higher concentration inside the neuron than outside and a higher concentration outside than inside. At rest, the membrane allows minimal flux of these ions and remains 40 times more permeable to than to.it diffuses passively through pores called “leakage” channels, specific to these ions, and moves along their concentration gradient from a high to low concentration region in the extracellular space.

The "leak" channels are always open, so the membrane is permeable and the flow of dirt is forty times less. This net ion flux ultimately lowers the membrane potential as the outside of the cell becomes more positively charged.

Key Term: "Leakage" Channels

"Leak" channels, or potassium ion channels, are always open, allowing the neuron's membrane to leak potassium ions.

There are also negatively charged ions like chloride and negatively charged proteins in higher concentrations within the neuron. Through the action of the sodium-potassium pump and the "leak" channels, this contributes to the extracellular space outside the neuron becoming more positively charged than the cytoplasm inside the neuron. The membrane is polarized and reaches a resting potential of approxmV.

Example 1: Description of the state of ion channels when maintaining the resting potential

Are the potassium ion channels (leakage channels) open or closed when the resting potential is held?

responder

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 roundmV.

The resting potential is maintained primarily by active transport by proteins embedded in the neuron's membrane called sodium-potassium pumps. The sodium-potassium pump moves positively charged sodium () and potassium () ions across the membrane with the help of ATP. It requires energy because they are transported against their concentration gradient from an area of ​​low concentration to an area of ​​high concentration. For every ion pumped out of the neuron, ions are pumped out. This makes the voltage in the extracellular space more positive than in the cytoplasm of the neuron. It also increases concentration within the neuron.

Lesson Explanation: The Nervous Impulse (3)

It also “leaks” through the neuron membrane from the cytoplasm into the extracellular space due to the higher concentration inside the neuron. It diffuses passively through pores specifically named for "leakage channels".,Downward movement of its concentration gradient from an area of ​​high to low concentration. The "leakage" channels are always open, so the membrane is permeable.This lowers the membrane potential as the outside of the cell becomes more positively charged and reaches the resting potential ofmV.

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

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

Action potentials are electrical signals that carry information through the movement of charged ions across a neuron's membrane as the action potential penetrates them. This temporarily changes the potential difference at the point in the neuron where the ions are moving.

The main stages of an action potential are

  1. Depolarisation,
  2. Repolarisation,
  3. Hyperpolarisation,
  4. a short refractory period during which no further action potential can be generated.

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

Key term: action potential

An action potential is the transient change in potential difference across the neuron's membrane when stimulated (approxmV).

Lesson Explanation: The Nervous Impulse (4)

Let's look at depolarization first.

Lesson Explanation: The Nervous Impulse (5)

Depolarization is when the membrane potential at a point on the neuron changes from negative to positive. This is initially caused by activation of chemical receptors at synapses located on the dendrites of a neuron. Activation of these receptors triggers the opening of previously closed voltage-gated channels, making the membrane more permeable.

it diffuses into the cytoplasm of the neuron, since it is less concentrated there than in the extracellular space due to the action of the sodium-potassium pump. Increasing the concentration makes the neuron's cytoplasm less negatively charged, as seen in Figure 4. An increase in membrane potential positivity causes more voltage-gated channels to open. This means that it diffuses faster into the neuron, which continues until the membrane potential reaches a value of aboutmV.

Key term: depolarization

Depolarization is a change in membrane potential at one point on 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 cell's membrane potential, allowing ions to flow across the membrane as a result.

When the membrane potential is reachedmV,close voltage-gated channels and open voltage-gated channels. it can no longer enter the neuron. it is more concentrated in the neuron's cytoplasm than in the extracellular space due to the action of the sodium-potassium pump, so it can now diffuse. As a result, the membrane potential drops and the cytoplasm of the neuron is again less positively charged than the extracellular space. This is called repolarization as seen in Figure 5.

Key term: repolarization

Repolarization is a positive to negative change in membrane potential at one point on a neuron.

When voltage-gated channels open, so much is diffused out of the neuron that the membrane potential temporarily becomes even more negative than its resting potential. This is called hyperpolarization.

Hyperpolarization causes voltage-gated channels to close, and the sodium-potassium pump restores the membrane to its resting potential. You can see this in the final stages in Figure 3. This period is called the refractory period, during which action potentials can no longer be generated because the voltage-gated channels remain closed. Refractory periods last a very short time, usually between 0.001 and0.003 seconds!

Key term: hyperpolarization

Hyperpolarization is a change in membrane potential at a point in a neuron to a more negative value 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 unable to generate another action potential.

Example 2: Describe the sequence of steps in an action potential

The provided diagram shows the stages of an action potential and each stage is assigned a number. Enter the correct sequence of numbers.

Lesson Explanation: The Nervous Impulse (7)

responder

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

Depolarization is when the electrical charge at a point on the neuron's membrane reverses from negative to positive. This is caused by the energy of a stimulus that triggers the opening of voltage-gated channels, allowing it to diffuse into the neuron's cytoplasm. Increasing the concentration of makes the neuron's cytoplasm less negatively charged, causing more voltage-gated channels to open. diffuses faster into the neuron until it approximately reaches the membrane potentialmV.

The voltage-gated channels are now closed, preventing them from entering the neuron. Voltage-gated channels open to diffuse out of the neuron's cytoplasm. As a result, the membrane potential drops and the cytoplasm of the neuron is again less positively charged than the extracellular space. This is called repolarization.

So much is diffused 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 channels to close. The sodium-potassium pump returns the membrane to its resting potential over a period called the refractory period. No further action potentials can be generated during the refractory period because the voltage-gated channels remain closed.

Hence the correct sequence of events in an action potential4,2,6,1,5,3.

Let's look at the diagram in Figure 6, which shows how the membrane potential changes during an action potential.

Lesson Explanation: The Nervous Impulse (8)

  1. In Stage 1, the resting potential is maintained in Stage 1, with the sodium-potassium pump and "leak" channels maintaining the membrane potential around themmV.
  2. In stage 2, the stimulus has caused voltage-gated channels in stage 2 to open and depolarize the membranemV.
  3. Voltage controlled channels close atmVand open the voltage-gated channels. Stage 3 shows the repolarization of the membrane as it diffuses out of the axon.
  4. Stage 4 shows hyperpolarization of the membrane, exceeding its resting potential. The sodium-potassium pump works to restore resting potential during the refractory period.
  5. The resting potential was restored in stage 5, restoring the membrane potentialmV.

Example 3: Describing the events of an action potential

The graph provided shows how the potential difference across the membrane of an axon changes over the course of an action potential. What happens in phase 2?

Lesson Explanation: The Nervous Impulse (9)

responder

The resting potential is maintained at level 1 with the sodium-potassium pump maintaining the membrane potential all aroundmV.A stimulus has caused voltage-gated channels to open in stage 2, depolarizing the membranemV.Voltage controlled channels close atmVand open the voltage-gated channels. Stage 3 shows the repolarization of the membrane as it diffuses out of the axon. Stage 4 shows a hyperpolarization of the membrane that exceeds the resting potential. After this refractory period, the resting potential is restored in step 5, whereby the membrane potential is again reachedmV.

Therefore, in 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 the membrane of an axon changes over the course of an action potential. What happens in phase 3?

Lesson Explanation: The Nervous Impulse (10)

responder

The resting potential is maintained at level 1 with the sodium-potassium pump maintaining the membrane potential all aroundmV.A stimulus has caused voltage-gated channels to open in stage 2, depolarizing the membranemV.Voltage controlled channels close atmVand open the voltage-gated channels. Stage 3 shows the repolarization of the membrane as it diffuses out of the axon. Stage 4 shows a hyperpolarization of the membrane that exceeds the resting potential. After this refractory period, the resting potential is restored in step 5, whereby the membrane potential is again reachedmV.

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

An action potential then propagates from one end of the neuron's axon to the other, and only in one direction. This propagation is called a depolarization wave.

Lesson Explanation: The Nervous Impulse (11)

This is because when a section of the axon membrane depolarizes, the positive charge moves into the axon's cytoplasm, as seen in the green section of Stage 1 in Figure 7.

Lesson Explanation: The Nervous Impulse (12)

Voltage-gated sodium channels adjacent to the initial site of depolarization are activated, allowing sodium to diffuse down the length of the axon to depolarize the next segment, as seen in stage 2 in Figure 8. This triggers the opening of voltage-gated channels in the next segment, and the membrane at this point is completely depolarized.

Lesson Explanation: The Nervous Impulse (13)

The depolarization wave can only propagate in one direction because the section behind the depolarized section repolarizes in Stage 3, as you can see in Figure 9. The voltage-gated channels have opened and are diffusing out of the axon, making it more negative. than the extracellular space, and the membrane becomes hyperpolarized. During this refractory period, the voltage-gated channels remain closed, preventing them from moving into the axon and preventing the depolarization wave from back-diffusing.

The strength of a stimulus determines whether an action potential is generated. If the stimulus exceeds a threshold value, it always triggers an action potential. If the stimulus does not exceed this value, no action potential is generated. Therefore, action potentials are referred to as all-or-none responses.

Although the action potential is always the same magnitude, when a stimulus is stronger, the frequency of action potentials is higher, and therefore more is generated per unit time.

Key concept: The all-or-nothing principle

The all-or-none principle states that a stimulus large enough to exceed a threshold will always produce an action potential of the same magnitude. If the stimulus is not large enough to exceed this value, no action potential will be generated.

Three factors influence the transmission rate of an action potential.

At higher temperatures, the ions diffuse faster because they have more kinetic energy. This increases the speed of the action potential. At higher temperatures,However, proteins such as the sodium-potassium pump begin to denature, causing the rate of transmission to decrease.

The diameter of the axon also affects the speed of an action potential. The larger the diameter, the faster the transfer because the diffusing ions encounter less resistance. That's like having a lot of people trying to walk down a wide corridor, it would be a lot easier than the same number of people walking down a narrow one!

Lesson Explanation: The Nervous Impulse (14)

Whether an axon is myelinated or not also affects transmission rate. Myelinated axons conduct nerve impulses faster than unmyelinated axons. The propagation speed of a myelinated axon is approx12 meters per second,while spreading along a myelinated axon can reach up to140 meters per second!

Voltage-gated ion channels are only found at Ranvier nodes 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 shown by the pink arrows in Figure 10. This process is called saltatoric conduction, from the Latin word for "leap," and it at least speeds up transmission. It is taken in by opening and closing ion channels.

In comparison, many ion channels in the unmyelinated axon of Figure 10 open and close, so the propagation speed of the action potential is much slower.

Key term: Saltatory conduction

Saltatoric conduction describes how action potentials propagate along a myelinated axon by "hopping" from one Ranvier node to the next, increasing conduction velocity compared to non-myelinated axons.

Let's recap some of the key points we've covered in this explainer.

Important points

  • The movement of sodium and potassium ions across a neuron's membrane determines the membrane potential.
  • The resting membrane potential of a neuron is maintained by the sodium-potassium pump and "leak" channels.
  • An action potential carries electrical information along a neuron and consists of depolarization, repolarization, hyperpolarization, and a refractory period.
  • The transmission rate of an action potential is affected by the temperature, the diameter of the axon, and the myelination of the neuron.
  • The all-or-nothing principle states that an action potential is always generated when a stimulus exceeds the threshold value.
Top Articles
Latest Posts
Article information

Author: Tuan Roob DDS

Last Updated: 02/26/2023

Views: 5909

Rating: 4.1 / 5 (62 voted)

Reviews: 85% of readers found this page helpful

Author information

Name: Tuan Roob DDS

Birthday: 1999-11-20

Address: Suite 592 642 Pfannerstill Island, South Keila, LA 74970-3076

Phone: +9617721773649

Job: Marketing Producer

Hobby: Skydiving, Flag Football, Knitting, Running, Lego building, Hunting, Juggling

Introduction: My name is Tuan Roob DDS, I am a friendly, good, energetic, faithful, fantastic, gentle, enchanting person who loves writing and wants to share my knowledge and understanding with you.