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31.1 The Neuron
Lesson Objectives
Identify the functions of the nervous system.
Describe the function of neurons.
Describe how a nerve impulse is transmitted.
Lesson Summary
Functions of the Nervous System The nervous system collects information
about the body’s internal and external environment, processes that information, and
responds to it.
The peripheral nervous system consists of nerves and supporting cells. It collects
information about the body’s internal and external environment.
The central nervous system consists of the brain and spinal cord. It processes
information and creates a response that is delivered through the peripheral nervous
system.
Neurons Nervous system impulses are transmitted by cells called neurons. The three types
of neurons are sensory, motor, and interneurons. All neurons have certain features:
The cell body contains the nucleus and much of the cytoplasm.
Dendrites receive impulses from other neurons and carry impulses to the cell body.
The axon is the long fiber that carries impulses away from the cell body. In some
neurons, the axon is surrounded by an insulating membrane called the myelin sheath.
The Nerve Impulse Nerve impulses are similar to the flow of an electric current through
a wire.
Neurons have a charge, or electric potential, across their membranes. When resting, the
inside of a neuron has a negative charge compared to the outside. This difference is called
the resting potential.
When a neuron is stimulated, the inside of its membrane temporarily becomes more
positive than the outside. This reversal of charges is called an action potential, or nerve
impulse. The nerve impulse moves along the axon.
Not all stimuli are capable of starting an impulse. The minimum level of a stimulus that is
required to start an impulse in a neuron is called its threshold.
At the end of the axon, impulses can be transmitted to the next neuron. The point at which
a neuron transfers an impulse to another cell is called a synapse. When an impulse
arrives at the synapse, neurotransmitters, chemicals that transmit an impulse across a
synapse to another cell, are released from the axon.
Functions of the Nervous System
1. What are the main functions of the nervous system?
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2. Name the two parts of the nervous system and explain what each part does
Neurons
3. THINK VISUALLY Draw and label a diagram of a neuron. Be sure to include
the following features in your drawing: axon, axon terminals, cell body, dendrites,
myelin sheath, nodes, and nucleus.
For Questions 4–8, complete each statement by writing the correct word or words.
4. Neurons that carry impulses from the eyes to the spinal cord and brain are called
5. Motor neurons carry impulses from the brain and spinal cord to
and
6. The neuron’s cell body has short, branched extensions called
receive impulses from other neurons.
7. In most animals,
in bundles called nerves.
and
which
of different neurons are clustered
8. The insulating membrane that surrounds a single axon in some neurons is called the
.
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The Nerve Impulse
9. Describe the role of sodium ions in propagating a nerve impulse.
For Questions 10–11, use the Visual Analogy comparing falling dominoes to a moving
impulse.
10. VISUAL ANALOGY Dominoes require a push to
begin falling. What “push” starts a nerve impulse?
11. What is the threshold of a neuron, and how is it
similar to a row of falling dominoes?
12. What are neurotransmitters, and how do they function?
Apply the Big idea
13. How is the structure of a neuron suited to its function?
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