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Biology
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35-2 The Nervous System
Photo Credit: AP/Wide World Photos.
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35-2 The Nervous System
What are the functions of the nervous system?
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35-2 The Nervous System
The nervous system controls and coordinates functions throughout the body and responds to internal and external stimuli.
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Neurons
Neurons
The messages carried by the nervous system are electrical signals called impulses.
The cells that transmit these impulses are called neurons.
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Neurons
Neurons are classified according to the direction in which an impulse travels.
Sensory neurons carry impulses from the sense organs to the spinal cord and brain.
Motor neurons carry impulses from the brain and spinal cord to muscles and glands.
Interneurons connect sensory and motor neurons and carry impulses between them.
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Neurons
Structures of a Neuron
Nucleus
Dendrites
Axon terminals
Cell body
Myelin sheath
The nervous system controls and coordinates functions throughout the body. The basic units of the nervous system are neurons.
Axon
Nodes
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Neurons
The largest part of a typical neuron is the cell body.
It contains the nucleus and much of the cytoplasm.
Cell body
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Neurons
Dendrites extend from the cell body and carry impulses from the environment toward the cell body.
Dendrites
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Neurons
The axon is the long fiber that carries impulses away from the cell body.
Axon terminals
Axon
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Neurons
The axon ends in axon terminals.
Axon terminals
Axon
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Neurons
The axon is sometimes surrounded by an insulating membrane called the myelin sheath.
There are gaps in the myelin sheath, called nodes, where the membrane is exposed.
Impulses jump from one node to the next.
Myelin sheath
Nodes
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The Nerve Impulse
The Nerve Impulse
The Resting Neuron 
When resting, the outside of the neuron has a net positive charge.
The inside of the neuron has a net negative charge.
The cell membrane is electrically charged because there is a difference in electrical charge between its outer and inner surfaces.
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The Nerve Impulse
The sodium-potassium pump in the nerve cell membrane pumps sodium (Na+) ions out of the cell and potassium (K+) ions into the cell by means of active transport.
As a result, the inside of the cell contains more K+ ions and fewer Na+ ions than the outside.
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The Nerve Impulse
Sodium-Potassium Pump
The sodium-potassium pump in the neuron cell membrane uses the energy of ATP to pump sodium out of the cell and, at the same time, to pump potassium in. This ongoing process maintains resting potential.
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The Nerve Impulse
More K+ ions leak across the membrane than Na+ ions. This produces a negative charge on the inside and a positive charge on the outside.
The electrical charge across the cell membrane of a neuron at rest is known as the resting potential.
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The Nerve Impulse
How is a nerve impulse transmitted?
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The Nerve Impulse
The Moving Impulse
An impulse begins when a neuron is stimulated by another neuron or by the environment.
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The Nerve Impulse
At the leading edge of the impulse, gates in the sodium channels open allowing positively charged Na+ ions to flow inside the cell membrane.
An impulse begins when a neuron is stimulated by another neuron. At the leading edge of an action potential, gates in the sodium channels open, allowing Na+ ions to flow into the cell. This flow of ions causes the action potential to move.
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The Nerve Impulse
The inside of the membrane temporarily becomes more positive than the outside, reversing the resting potential.
An impulse begins when a neuron is stimulated by another neuron. At the leading edge of an action potential, gates in the sodium channels open, allowing Na+ ions to flow into the cell. This flow of ions causes the action potential to move.
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The Nerve Impulse
This reversal of charges is called a nerve impulse, or an action potential.
An impulse begins when a neuron is stimulated by another neuron. At the leading edge of an action potential, gates in the sodium channels open, allowing Na+ ions to flow into the cell. This flow of ions causes the action potential to move.
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The Nerve Impulse
As the action potential passes, gates in the potassium channels open, allowing K+ ions to flow out restoring the negative potential inside the axon.
At the trailing edge of an action potential, gates in the potassium channels open, allowing positive ions to flow out and restoring the resting potential of the neuron.
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The Nerve Impulse
The impulse continues to move along the axon.
An impulse at any point of the membrane causes an impulse at the next point along the membrane.
At the trailing edge of an action potential, gates in the potassium channels open, allowing positive ions to flow out and restoring the resting potential of the neuron.
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The Nerve Impulse
Threshold 
A stimulus must be of adequate strength to cause a neuron to transmit an impulse.
The minimum level of a stimulus that is required to activate a neuron is called the threshold.
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The Nerve Impulse
A stimulus that is stronger than the threshold produces an impulse.
A stimulus that is weaker than the threshold produces no impulse.
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The Synapse
The Synapse
At the end of the neuron, the impulse reaches an axon terminal. Usually the neuron makes contact with another cell at this site.
The neuron may pass the impulse along to the second cell.
The location at which a neuron can transfer an impulse to another cell is called a synapse.
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The Synapse
A Synapse
When an impulse reaches the end of the axon of one neuron, neurotransmitters are released into the synaptic cleft. The neurotransmitters bind to receptors on the membrane of an adjacent dendrite.
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The Synapse
The synaptic cleft separates the axon terminal from the dendrites of the adjacent cell.
Synaptic cleft
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The Synapse
Terminals contain vesicles filled with neurotransmitters.
Neurotransmitters are chemicals used by a neuron to transmit an impulse across a synapse to another cell.
Vesicle
Neurotransmitter
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The Synapse
As an impulse reaches a terminal, vesicles send neurotransmitters into the synaptic cleft.
These diffuse across the cleft and attach to membrane receptors on the next cell.
Receptor
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The Synapse
Sodium ions then rush across the membrane, stimulating the next cell.
If the stimulation exceeds the cell’s threshold, a new impulse begins.
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The Synapse
Moments after binding to receptors, neurotransmitters are released from the cell surface.
The neurotransmitters may then be broken down by enzymes, or taken up and recycled by the axon terminal.
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35-2
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35-2
Neurons that carry impulses from the brain and spinal cord to the muscles are
interneurons.
sensory neurons.
resting neurons.
motor neurons.
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35-2
The part of the neuron that carries impulses toward the cell body is the
axon.
myelin sheath.
dendrite.
nodes.
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35-2
The minimum level of a stimulus that is required to activate a neuron is called its
action potential.
resting potential.
threshold.
synapse.
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35-2
Chemicals that are used by a neuron to transmit impulses are called
neurotransmitters.
synapses.
axons.
inhibitors.
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35-2
An action potential begins when
sodium ions flow into the neuron.
potassium ions flow into the neuron.
sodium and potassium ions flow into the neuron.
sodium and potassium ions flow out of the neuron.
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39. END OF SECTION