Lesson Overview
31.1 The Neuron
Lesson Overview
The Neuron
Functions of the Nervous System
What are the 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.
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The Neuron
Functions of the Nervous System
The functions of the nervous system are accomplished by the peripheral nervous system and the central nervous system.
The peripheral nervous system, which consists of nerves and supporting cells, collects information about the body’s external and internal environment.
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The Neuron
Functions of the Nervous System
The central nervous system, which consists of the brain and spinal cord, processes that information and creates a response that is delivered to the appropriate part of the body through the peripheral nervous system.
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The Neuron
Neurons
What is the function of neurons?
Nervous system impulses are transmitted by cells called neurons.
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The Neuron
Neurons
The messages carried by the nervous system are electrical signals called impulses.
Nervous system impulses are transmitted by cells called neurons.
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The Neuron
Types of Neurons
Neurons can be classified into three types according to the direction in which an impulse travels.
Sensory neurons carry impulses from the sense organs, such as the eyes and ears, to the spinal cord and brain.
Motor neurons carry impulses from the brain and the spinal cord to muscles and glands.
Interneurons process information from sensory neurons and then send commands to other interneurons or motor neurons.
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The Neuron
Structure of Neurons
**Draw and Label the neuron as we go through the next 7 slides
The largest part of a typical neuron is its cell body, which contains the nucleus and much of the cytoplasm.
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The Neuron
Structure of Neurons
Spreading out from the cell body are short, branched extensions called dendrites.
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The Neuron
Structure of Neurons
Dendrites receive impulses from other neurons and carry impulses to the cell body.
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The Neuron
Structure of Neurons
The long fiber that carries impulses away from the cell body is the axon.
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The Neuron
Structure of Neurons
An axon ends in a series of small swellings called axon terminals.
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The Neuron
Structure of Neurons
In some neurons, the axon is surrounded by an insulating membrane known as the myelin sheath.
The myelin sheath that surrounds a single, long axon has many gaps, called nodes, where the axon membrane is exposed.
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The Neuron
Structure of Neurons
As an impulse moves along the axon, it jumps from one node to the next. This arrangement causes an impulse to travel faster than it would through an axon without a myelin sheath.
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The Neuron
The Nerve Impulse
How does a nerve impulse begin?
An impulse begins when a neuron is stimulated by another neuron or by the environment.
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The Neuron
The Resting Neuron
Neurons have a charge, or electrical potential, across their cell membranes.
The inside of a neuron has a voltage of –70 millivolts (mV) compared to the outside. This difference is known as the resting potential.
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The Neuron
The Resting Neuron
Active transport proteins pump sodium ions (Na+) out of the cell and potassium ions (K+) into it.
Ungated potassium channel proteins make it easier for K+ ions than for Na+ ions to diffuse back across the membrane.
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The Neuron
The Resting Neuron
Because there is a higher concentration of K+ ions inside the cell as a result of active transport, there is a net movement of positively charged K+ ions out of the cell.
As a result, the inside becomes negatively charged compared to the outside, producing the resting potential.
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The Neuron
The Moving Impulse
A neuron remains in its resting state until it receives a stimulus large enough to start a nerve impulse.
Once it begins, the impulse travels quickly down the axon away from the cell body toward the axon terminals.
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The Neuron
The Moving Impulse
The neuron cell membrane contains thousands of “gated” ion channels.
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The Neuron
The Moving Impulse
At the leading edge of an impulse, gated sodium channels open, allowing positively charged Na+ ions to flow into the cell. The inside of the membrane temporarily becomes more positive than the outside, reversing the resting potential.
This reversal of charges, from more negatively charged to more positively charged, is called a nerve impulse, or an action potential.
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The Neuron
The Moving Impulse
Once the impulse passes, sodium gates close and gated potassium channels open, allowing K+ ions to flow out.
This restores the resting potential so that the neuron is once again negatively charged on the inside. (Like falling dominoes)
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The Neuron
Threshold
Not all stimuli are capable of starting an impulse. The minimum level of a stimulus that is required to cause an impulse in a neuron is called its threshold.
Any stimulus that is weaker than the threshold will not produce an impulse.
The brain determines if a stimulus, like touch or pain, is strong or weak from the frequency of action potentials.
A weak stimulus might produce three or four action potentials per second, while a strong one might result in as many as 100 per second.
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The Neuron
The Synapse
At the end of the neuron, the impulse reaches an axon terminal, which may pass the impulse along to another cell. A motor neuron, for example, may pass impulses to a muscle cell, causing the muscle cell to contract.
The point at which a neuron transfers an impulse to another cell is called a synapse.
A space, called the synaptic cleft, separates the axon terminal from the adjacent cell.
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The Neuron
The Synapse
The axon terminal at a synapse contains tiny vesicles filled with neurotransmitters. Neurotransmitters are chemicals that transmit an impulse across a synapse to another cell.
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The Neuron