Nerve Cells and Nerve
Impulses
Jessica Harwood
Douglas Wilkin, Ph.D.
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AUTHORS
Jessica Harwood
Douglas Wilkin, Ph.D.
EDITOR
Douglas Wilkin, Ph.D.
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Printed: April 14, 2015
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Corliss Karasov
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C HAPTER
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Chapter 1. Nerve Cells and Nerve Impulses
1
Nerve Cells and Nerve
Impulses
Define neuron and synapse.
Describe neuron structure.
Explain the role of the dendrites and axon.
Distinguish sensory neurons from motor neurons.
Summarize what happens when a nerve impulse reaches the end of an axon.
What do nerve cells look like?
Note that like most other cells, these nerve cells have a nucleus. They also have other organelles. However, the long,
threadlike extensions of the nerve cells are unique. This is where the nerve impulses are transmitted.
Neurons and Nerve Impulses
The nervous system is made up of nerves. A nerve is a bundle of nerve cells. A nerve cell that carries messages
is called a neuron ( Figure 1.1). The messages carried by neurons are called nerve impulses. Nerve impulses can
travel very quickly because they are electrical impulses.
Think about flipping on a light switch when you enter a room. When you flip the switch, the electricity flows to the
light through wires inside the walls. The electricity may have to travel many meters to reach the light, but the light
still comes on as soon as you flip the switch. Nerve impulses travel just as fast through the network of nerves inside
the body.
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FIGURE 1.1
The axons of many neurons, like the one
shown here, are covered with a fatty layer
called myelin sheath. The sheath covers
the axon, like the plastic covering on an
electrical wire, and allows nerve impulses
to travel faster along the axon. The node
of Ranvier, shown in this diagram, is any
gap in the myelin sheath; it allows faster
transmission of a signal.
What Does a Neuron Look Like?
A neuron has a special shape that lets it pass signals from one cell to another. A neuron has three main parts ( Figure
1.1):
1. The cell body.
2. Many dendrites.
3. One axon.
The cell body contains the nucleus and other organelles. Dendrites and axons connect to the cell body, similar to
rays coming off of the sun. Dendrites receive nerve impulses from other cells. Axons pass the nerve impulses on to
other cells. A single neuron may have thousands of dendrites, so it can communicate with thousands of other cells
but only one axon. The axon is covered with a myelin sheath, a fatty layer that insulates the axon and allows the
electrical signal to travel much more quickly. The node of Ranvier is any gap within the myelin sheath exposing
the axon, and it allows even faster transmission of a signal.
Types of Neurons
Neurons are usually classified based on the role they play in the body. Two main types of neurons are sensory
neurons and motor neurons.
• Sensory neurons carry nerve impulses from sense organs and internal organs to the central nervous system.
• Motor neurons carry nerve impulses from the central nervous system to organs, glands, and muscles—the
opposite direction.
Both types of neurons work together. Sensory neurons carry information about the environment found inside or
outside of the body to the central nervous system. The central nervous system uses the information to send messages
through motor neurons to tell the body how to respond to the information.
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Chapter 1. Nerve Cells and Nerve Impulses
The Synapse
The place where the axon of one neuron meets the dendrite of another is called a synapse. Synapses are also found
between neurons and other types of cells, such as muscle cells. The axon of the sending neuron does not actually
touch the dendrite of the receiving neuron. There is a tiny gap between them, the synaptic cleft ( Figure 1.2).
FIGURE 1.2
This diagram shows a synapse between
neurons. When a nerve impulse arrives at
the end of the axon, neurotransmitters are
released and travel to the dendrite of another neuron, carrying the nerve impulse
from one neuron to the next.
The following steps describe what happens when a nerve impulse reaches the end of an axon.
1.
2.
3.
4.
When a nerve impulse reaches the end of an axon, the axon releases chemicals called neurotransmitters.
Neurotransmitters travel across the synapse between the axon and the dendrite of the next neuron.
Neurotransmitters bind to the membrane of the dendrite.
The binding allows the nerve impulse to travel through the receiving neuron.
Did you ever watch a relay race? After the first runner races, he or she passes the baton to the next runner, who
takes over. Neurons are a little like relay runners. Instead of a baton, they pass neurotransmitters to the next neuron.
Examples of neurotransmitters are chemicals such as serotonin, dopamine, and adrenaline.
You can watch an animation of nerve impulses and neurotransmitters at http://www.mind.ilstu.edu/curriculum/neu
rons_intro/neurons_intro.php .
Some people have low levels of the neurotransmitter called serotonin in their brain. Scientists think that this is one
cause of depression. Medications called antidepressants help bring serotonin levels back to normal. For many people
with depression, antidepressants control the symptoms of their depression and help them lead happy, productive lives.
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Summary
• Neurons, or nerve cells that carry nerve impulses, are made up of the cell body, the axon, and several dendrites.
• Signals move across the synapse, the place where the axon of one neuron meets the dendrite of another, using
chemicals called neurotransmitters.
Explore More
Use the resource below to answer the questions that follow.
• Neuroscience For Kids at http://faculty.washington.edu/chudler/cells.html
1.
2.
3.
4.
5.
What are the three types of neurons?
What neurons are most abundant in the central nervous system?
What is the function of sensory neurons?
What is the function of motor neurons?
What is the role of interneurons?
Review
1.
2.
3.
4.
Describe a neuron and identify its three main parts.
Distinguish between dendrites and the axon.
Distinguish between sensory and motor neurons.
Explain how one neuron transmits a nerve impulse to another neuron.
References
1. Zachary Wilson, modifying image by Mariana Ruiz Villarreal. The axons of many neurons are covered in a
myelin sheath to allow for faster signal transmission . CC BY-NC 3.0 (original image in public domain)
2. Laura Guerin. At a synapse, neurotransmitters are released to carry a nerve impulse from one neuron to the
next . CC BY-NC 3.0
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