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Vocabulary: neuron, cell body, dendrite, axon, membrane potential, resting potential, threshold, action potential
Hey there. This is Eric Simon, and welcome to another MP3 Tutor session for tough terms. In today’s tutorial, I’ll talk you through the various terms that are used to describe neurons and how they communicate with each other.
First, let’s talk about the structure of neurons. Neurons are the fundamental units of the nervous system, so they are specialized for transmitting and receiving signals. Repeating a theme we see throughout biology, the shape of neurons reflects their function. Unlike most other animal cells, neurons are not round. Instead, they have many thin extensions that trail out from a central cell body. These extensions fulfill the functions of neurons by receiving and sending signals.
The cell body of a neuron contains the nucleus and most of the organelles. The extensions that emanate from the cell body come in two kinds: dendrites and axons. Dendrites are usually short and branched; they look like a bushy shrub with many branches.
Dendrites receive signals from other neurons. So, you can think of dendrites as the “ears” of a neuron. Axons, on the other hand, are usually long and branched only at the ends. They look like a broom. Axons send signals to other cells. So, you can think of an axon as the “mouth” of a neuron, since it talks to other neurons. Sometimes neurons talk to targets that are really far away, so axons can be very long. For example, you have neurons where the cell body resides in your spinal cord, but the axons go all the way down to the muscles in your toes!
To recap, neurons have a central cell body with two types of extensions for communicating with other cells. The dendrites listen to other neurons, while the axon talks to other cells. Picture a neuron as a cell with many ears and one mouth.
Are you with me? So far, we’ve focused only on what neurons look like: their structure.
Now let’s think about what neurons do: their function. Neurons convey signals throughout the nervous system as well as to target cells outside of it. Let’s think a minute about a baseball player stepping up to the plate to bat. It takes less than a half a second for a ball to leave the pitcher’s hand and travel to home plate. In that time, a batter must ascertain how fast the ball is approaching and where the ball will cross the plate so that he can swing the bat to the correct location to hit the ball. Half a second is not much time to think about all this and send appropriate signals to the muscles. How do neurons communicate so quickly?
The answer is that they use electricity to talk to each other. Electricity is the movement of electrons, which travel at the speed of light. Due to certain physical constraints, electrical signals do not move at the speed of light in the nervous system, but they still move very quickly.
So how do neurons use electricity to talk to each other? The key to understanding this very important point is this: the movement of ions, or charged particles, across a neuron’s cell membrane is responsible for electrical communication in the nervous system. That is, charged ions moving back and forth produce electrical differences.
To understand this, let’s first consider an analogy: a battery. In a battery, electrical charges are separated by a barrier. This separation of electrical charges represents potential energy that can be tapped into at any time. When the two ends of a battery are connected by a wire, the electricity flows, releasing the stored energy to perform some work.
Neurons are like batteries. They separate electrical charges, creating potential energy that can be tapped into at any time. The plasma membrane of a neuron is the barrier that separates ions. That is, there is a greater concentration of specific ions, and therefore more electrical charge, on one side of the membrane than on the other side. This difference in electrical charge across the plasma membrane is the membrane potential.
The membrane potential is the potential energy that exists across the plasma membrane due to the separation of charge. As a neuron does its job, the membrane potential changes. For the remainder of this tutorial, we’ll discuss three different elements of membrane potentials: resting potentials, action potentials, and thresholds.
When neurons are not receiving or sending any signals, they are said to be “at rest.” In this situation, proteins in the plasma membrane move ions around, creating a difference in electrical charge across the plasma membrane. This membrane potential at rest is the resting potential. The resting potential is the difference in electrical charge across the plasma membrane while the neuron is at rest. This is similar to the potential energy that is stored in a battery when it is not being used.
When a neuron conveys a signal to a target cell, the resting potential is temporarily reversed and then restored. This rapid change in membrane potential is the action potential. You can think of this as connecting a lightbulb to our battery for a split second, and then disconnecting it again.
What causes an action potential to be generated? Remember that a neuron has many dendrite “ears” that are listening for signals from other neurons. If the neuron “hears” a loud enough signal, in the form of action potentials from other neurons, an action potential will be triggered. In order to trigger an action potential, the signals from other neurons must be sufficiently “loud.” This usually happens when incoming signals arrive at the dendrite around the same time. You can think of the volume of a solo singer compared to the volume of a choir; hearing many signals at once produces a louder signal, whether in your ear or in a neuron.
The minimum change in the signal heard by the dendrites that is needed to trigger an action potential is called the threshold. The threshold is like the energy necessary to push the battery’s power switch. If the threshold is reached, a switch is turned on and the electricity flows, producing a signal. Below the threshold, the switch remains off, and no signal flows.
Did you follow that? Let’s review.
• Neurons have a cell body with many dendrite “ears” that listen for signals, and a single axon “mouth” that can send a signal.
• Neurons communicate using electricity by separating ions across the plasma membrane.
• The difference in electrical charge across the plasma membrane is the membrane potential.
• The resting potential is the membrane potential for a neuron that is not receiving or sending any signals.
• If the neuron receives strong enough signals via its dendrites, the membrane potential can move from the resting potential to the threshold.
• When the threshold is reached, a rapid reversal of electrical charge occurs, creating an action potential.
• The action potential is the signal that is transmitted down the axon to its target.
I hope this tutorial helped you learn the terms that describe neuron structure and function. If you need more help, you can try listening to this again, or consult your textbook or lecture notes. Good luck with your studying, and remember: dendrites listen and axons talk!