Transcript 35-2 The Nervous System

35-2 The Nervous System
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35-2 The Nervous System
35-2 The Nervous System
What are the functions of the nervous
system?
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35-2 The Nervous System
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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35-2 The Nervous System
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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35-2 The Nervous System
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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35-2 The Nervous System
Neurons
Structures of a Neuron
Nucleus
Dendrites
Axon
terminals
Cell body
Myelin sheath
(Schwann cells)
Nodes of
Ranvier
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Axon
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35-2 The Nervous System
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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35-2 The Nervous System
Neurons
The axon is the long fiber that carries impulses away
from the cell body.
Axon
terminals
Axon
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35-2 The Nervous System
Neurons
The axon ends in axon terminals.
Axon
terminals
Axon
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35-2 The Nervous System
Neurons
The axon is sometimes surrounded by an insulating
membrane called the myelin sheath.
There are gaps in the myelin sheath, called Nodes of
Ranvier, where the membrane is exposed.
Impulses jump from one node to the next.
Myelin sheath
Nodes of
Ranvier
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35-2 The Nervous System
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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35-2 The Nervous System
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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35-2 The Nervous System
The Nerve Impulse
Sodium-Potassium Pump
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35-2 The Nervous System
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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35-2 The Nervous System
The Nerve Impulse
How is a nerve impulse transmitted?
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35-2 The Nervous System
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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35-2 The Nervous System
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.
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35-2 The Nervous System
The Nerve Impulse
The inside of the membrane temporarily becomes
more positive than the outside, reversing the resting
potential.
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35-2 The Nervous System
The Nerve Impulse
This reversal of charges is called a nerve impulse, or
an action potential.
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35-2 The Nervous System
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.
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35-2 The Nervous System
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.
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35-2 The Nervous System
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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35-2 The Nervous System
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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35-2 The Nervous System
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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35-2 The Nervous System
The Synapse
A Synapse
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35-2 The Nervous System
The Synapse
The synaptic cleft
separates the axon
terminal from the
dendrites of the
adjacent cell.
Synaptic cleft
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35-2 The Nervous System
Terminals contain
vesicles filled with
neurotransmitters.
The Synapse
Vesicle
Neurotransmitters
are chemicals used by
a neuron to transmit
an impulse across a
synapse to another
cell.
Neurotransmitter
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35-2 The Nervous System
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.
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The Synapse
Receptor
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35-2 The Nervous System
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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35-2 The Nervous System
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 The Nervous System
Neural transmitters and neural inhibitors
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35-2 The Nervous System
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35-2
Neurons that carry impulses from the brain and
spinal cord to the muscles are
a. interneurons.
b. sensory neurons.
c. resting neurons.
d. motor neurons.
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35-2
The part of the neuron that carries impulses
toward the cell body is the
a. axon.
b. myelin sheath.
c. dendrite.
d. nodes.
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35-2
The minimum level of a stimulus that is required
to activate a neuron is called its
a. action potential.
b. resting potential.
c. threshold.
d. synapse.
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35-2
Chemicals that are used by a neuron to transmit
impulses are called
a. neurotransmitters.
b. synapses.
c. axons.
d. inhibitors.
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35-2
An action potential begins when
a. sodium ions flow into the neuron.
b. potassium ions flow into the neuron.
c. sodium and potassium ions flow into the
neuron.
d. sodium and potassium ions flow out of the
neuron.
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END OF SECTION