In and how depolarization is much like ‘the

In this video lesson, you’ll learn about neurons, which are specialized cells in the nervous system. Check out how far neurons can send signals throughout the body and how depolarization is much like ‘the wave’ at a football game.

Neuron Anatomy

Neurons are specialized cells of the nervous system that transmit signals throughout the body. You may already know that neurons can do many different things from sensing external and internal stimuli, to processing information and also directing muscle actions.

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But do you know how far a neuron can send a signal or how neurons actually work at the cellular level? In this lesson, we’ll take a look at neurons and how they work.

The two types of neuron extensions
Dendrites and axons

The most unique and important structures of a neuron are the long extensions that extend out from the cell body. There are two types of extensions: dendrites and axons. Dendrites are extensions of neurons that receive signals and conduct them toward the cell body.

Axons are extensions of neurons that conduct signals away from the cell body to other cells. Both axons and dendrites can extend far away from the cell body, and some human axons can reach lengths of over 3 feet, but that’s nothing compared to giraffes, which have axons as long as 15 feet!

Membrane Polarization

Okay, so neurons have dendrites and axons that can extend far from the cell body and relay signals to and from other cells, but how do they actually relay these signals? For this, let’s take a closer look at a section of a neuron’s dendrite.When the dendrite is not transmitting a signal it is said to be in its resting state. In this state, the inside of the cell has a net negative charge, and the outside of the cell has a net positive charge.

The membrane is said to be polarized because negative and positive charges exist on opposite sides. The polarized state of the membrane is actively maintained by the neuron through the use of sodium-potassium pumps. These sodium-potassium pumps pump three positively charged sodium ions out of the cell for every two positively charged potassium ions it pumps into the cell. Each cycle of the pump increases the polarization a little more. In addition, potassium ions leak back across the membrane and out of the cell by diffusion which, again, creates more negative charge inside the cell and more positive charge outside the cell.

Membrane Depolarization

When a neuron receives a signal, sodium channels in the membrane are opened and allow a localized influx of positive sodium ions into the cell, which causes depolarization, or a reduction of the difference in charge across the membrane. The localized depolarization also triggers nearby sodium channels to open up and depolarize the membrane nearby, which then causes more sodium channels to open up further away and depolarize the membrane there, and so a chain reaction is started.

Like the wave in a football stadium, depolarization occurs in a wave across the membrane.
Depolarization example

Depolarization occurs in a wave across the membrane, starting at the dendrite that received the signal, moving toward the cell body, across the cell body, and then away from the cell down the axon. You can think of depolarization as being like people standing up to do the wave in a football stadium. When the wave reaches people who are seated, it triggers them to stand up, which then triggers the people next to them to stand up, and so on, continuing the wave through the stands. And much like the people in the stands who sit back down, the membrane repolarizes by closing the sodium channels and firing up the sodium-potassium pumps to re-establish the difference in charge across the membrane, and the neuron is ready to pass along another signal.

Synapses

So what happens when the signal reaches the end of the axon? How does the neuron pass the signal along to another cell? At the ends of axons, the axon usually splits into several smaller branches.

Each of these branches terminates at another cell, at a junction called a synapse, which is the site where an axon terminates at a target cell. At the synapse, there is a small gap between the terminal end of the axon and the target cell. When the depolarizing signal reaches the synapse, it triggers the release of signaling molecules called neurotransmitters, which are the signaling molecules used at the synapse to pass a signal from a neuron to its target cell. These neurotransmitters diffuse across the very short gap from the axon to the surface of the target cell. The membrane of the target cell at the synapse has lots of receptors that the neurotransmitter can specifically bind to, and these receptors are coupled to ion channel proteins that are controlled by the receptor.

An excitatory signaling neuron will allow sodium ions to enter the cell and cause depolarization.

How synapses work

Typically, when the neurotransmitter binds to the receptor, the ion channel is opened and a specific type of ion is allowed into the target cell. If the signaling neuron is an excitatory neuron, the ion channel will allow sodium ions to enter the cell and cause depolarization at the target cell. However, if the signaling neuron is an inhibitory neuron, a different ion channel will be opened that will allow a different type of ion, like negatively charged chloride ions, into the cell that will increase polarization of the target cell and decrease the chances of depolarization even if the cell receives an excitatory signal at the same time.

Lesson Summary

So, to review, neurons are specialized cells of the nervous system that transmit signals throughout the body. Neurons have long extensions that extend out from the cell body called dendrites and axons. Dendrites are extensions of neurons that receive signals and conduct them toward the cell body. Axons are extensions of neurons that conduct signals away from the cell body to other cells.When a neuron is in its resting state, the membrane is said to be polarized because negative and positive charges exist on opposite sides. When a neuron receives a signal, sodium channels in the membrane are opened and allow a localized influx of positive sodium ions inside the cell, which causes depolarization, or a reduction of the difference in charge across the membrane.

The localized depolarization also triggers nearby sodium channels to open up and depolarize the membrane nearby, which then causes more sodium channels to open up further away and depolarize the membrane there, and so a chain reaction is started. Depolarization occurs in a wave across the membrane, starting at the dendrite that received the signal, moving toward the cell body, moving across the cell body, and then away from the cell down the axon.Axons terminate at junctions with their target cells called synapses. At the synapse, there is a small gap between the terminal end of the axon and the target cell. When the depolarizing signal reaches the synapse, it triggers the release of signaling molecules called neurotransmitters. These neurotransmitters diffuse across the very short gap from the axon to the surface of the target cell and bind to receptors that control ion channels, causing the ion channels to open. If the signaling neuron is excitatory, the ion channel will allow sodium ions to enter the cell and cause depolarization at the target cell.

However, if the signaling neuron is inhibitory, a different ion channel will be opened that will allow inhibitory ions, like negatively charged chloride ions, into the cell. This will increase polarization of the target cell and decrease the chances of depolarization even if the cell receives an excitatory signal at the same time.

The Neuron’s Extensions

Dendrites and axons are responsible for the polarization and depolarization of a neuron cell.
Graphic of a neuron
Dendrites Axons
Receive signals and conduct them towards the cell body Receive signals and conduct them away from the cell body
When a membrane receives a signal from a dendrite, channels are opened and the cell becomes depolarized. When the signal pass through the cell and reaches an axon’s synapse, it can either cause depolarization or polarization of the cell.

Learning Outcomes

Complete this lesson on neurons and strengthen your ability to:

  • Discuss the two components of the neuron responsible for polarization and depolarization
  • Sum up the polarization and depolarization processes
  • Indicate what happens at the synapse
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