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Nervous System:
Neurons
Biology 105
Lecture 8
Chapter 7
Copyright © 2009 Pearson Education, Inc.
Outline
I. Nervous system overview and function
II. Central and peripheral nervous systems
III. Nervous system cells
IV. Myelinated neurons
V. Neuronal signal transmission
VI. Neuronal synapse
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Nervous System
Includes nervous tissue and sensory organs
Nervous system functions to conduct
messages throughout the body:
Sense the environment – receives information
from both outside and inside the body.
Process the information it receives.
Respond to information – sends out orders.
When a neuron is stimulated, an electrical
signal quickly travels through the neuron,
triggering events.
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Two Parts of the Nervous System
1. Central Nervous System (CNS)
Brain and spinal cord
2. Peripheral Nervous System (PNS)
Nervous tissue outside brain and spine
Sensory organs
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Central Nervous System
Peripheral
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Nervous System Cells
Two types of cells found in nervous tissue:
Neurons – the cells that are responsible
for transmitting messages.
Glial cells – cells that support the
neurons.
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Glial Cells
Microglia – immune system cells, engulf
bacteria and cellular debris
Astrocytes – provide nutrients to neurons,
maintain balance of the extracellular
environment
Oligodendrocytes
and Schwann cells –
form myelin sheaths
Figure 4.6
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Parts of a Neuron
Cell body – the main body of the cell.
Contains the nucleus.
Dendrites – many projections from the cell
body that carry messages to the cell body.
Axons – one large projection that carries
messages away from the cell body.
Copyright © 2009 Pearson Education, Inc. Figure 7.2
The cell body
integrates input
from other neurons.
Dendrites receive
information from
other neurons or
from the environment.
The cell body controls
the cell’s metabolic
activities.
An axon conducts the
nerve impulse away
from the cell body.
Axon endings release
chemicals called
neurotransmitters that
affect the activity of
nearby neurons or an
effector (muscle or gland).
Receiving portion of
neuron
Sending portion of neuron
Cell
body
Axon
endings
Nucleus
Neuron!
DIRECTION OF NEURONAL SIGNAL
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Neurons of the Peripheral Nervous System
Neurons in the PNS are either carrying
messages to or from the CNS.
Afferent = sensory neurons = neurons carrying
messages TO the CNS.
Arriving in the CNS.
Efferent = motor neurons = neurons carrying
messages FROM the CNS.
Exiting from the CNS.
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Interneurons in the Central Nervous System
Located between sensory and motor neurons
within the CNS.
Interneurons integrate and interpret sensory
signals.
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Sensory Neurons
The afferent or sensory neuron cell bodies are
located in dorsal root ganglion.
Motor Neurons
The efferent or motor neuron cell bodies are
located in the gray matter of the spinal cord.
Their axons leave the CNS and go to the
skeletal muscles.
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Neurons of the Nervous System
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Q: The cell bodies of these neurons are located in the
dorsal root ganglion:
1. Motor
2. Sensory
Q: These glial cells provide nutrients to neurons:
1. Microglia
2. Astrocytes
3. Oligodendrocytes
4. Schwann cells
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Q: These are projections of the neuron cell body that
carry messages to the cell body:
1. Axons
2. Dendrites
Q: Which of the following type of neuron would alert
the brain that you had touched a hot object?
1. Efferent neuron
2. Afferent neuron
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Q: What type of neuron is the arrow pointing to?
1. Sensory
2. Motor
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Myelinated Neurons
Some glial cells form a substance called myelin
that is rich in lipid and contains proteins.
These glial cells wrap themselves around
neuronal axons.
The neurons that have axons covered with these
glial cells are called myelinated neurons.
Main function: myelinated neurons are able to
carry messages faster than non-myelinated
neurons.
Myelin is an excellent electrical insulator!
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Myelin Sheath
Figure 7.3b
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Myelin Sheath
Figure 7.3c
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Two Types of Cells Myelinate Neurons
Schwann cells are found in the PNS.
Oligodendrocytes are found in the CNS.
Schwann cells and oligodendrocytes both wrap
around neuronal axons.
Myelin sheaths from Schwann cells also help
regenerate injured PNS neuronal axons.
Nodes of Ranvier are spaces on the axon
between the glial cells.
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Myelinated Neurons
Figure 7.3a
(a)
Cell
body
Dendrites
Myelin sheath
Node of
Ranvier
Nucleus
Schwann cell
In saltatory conduction, the
nerve impulses jump from one
node of Ranvier to the next.
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Multiple Sclerosis (MS)
Caused by the destruction of the myelin sheath
that surrounds axons found in the CNS.
Can result in paralysis and loss of sensation,
including loss of vision.
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Nerve
A nerve contains many neuronal axons
bundled together.
These bundles contain:
Axons
Blood vessels
Connective tissue
Figure 8.9d
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Q: An ion is an atom that has gained or lost a(n):
1. Neutron
2. Proton
3. Electron
Q: How can an ion pass through a membrane?
1. Simple diffusion
2. Facilitated diffusion
3. Active transport
4. Both 2 and 3
5. All of the above
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The Neuronal Impulse is an Electrochemical
Signal
A neuronal impulse, or action potential,
involves sodium ions (Na+) and potassium ions
(K+) that cross the cell membrane through ion
channels.
Each ion channel is designed to allow only
certain ions to pass through.
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Action Potential
Figure 7.4
Extracellular
fluid
Neuron plasma
membrane
Cytoplasm
Sodium-potassium pump
The sodium-potassium pump uses cellular energy (ATP) to
pump sodium ions out of the
cell and potassium ions into the cell
Continually open ion channels “Gated” ion channels Sodium-potassium pump
Ion channels
Ion channels can be open continuously or opened and closed by a molecular gate
Cross section
Axon membrane
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The difference in charge between the inside
and outside of the neuron is the membrane
potential.
Membrane Potential
A neuron that is not conducting a message is
said to be “resting”.
When a neuron is resting, there is more
sodium (Na+) outside the neuron cell and more
potassium (K+) inside the cell.
The inside of the cell has a negative charge
compared to the outside the cell.
Resting Membrane Potential
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Resting Membrane Potential
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The Neuronal Impulse
Figure 7.5 (1 of 4)
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Sodium-Potassium Pump
To maintain this resting membrane potential the
neuron pumps Na+ out of the cell and K+ into
the cell.
The transport proteins (= Na+/K+ pumps) move
3 Na+ ions out for every 2 K+ ions into the cell.
This is Active Transport and requires ATP!
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Action Potential
Action Potential – an electrochemical signal
conducted along an axon.
An action potential is a wave of depolarization
followed by repolarization.
Depolarization is caused by sodium ions
entering the axon.
Repolarization is caused by potassium ions
leaving the axon.
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Steps of an Action Potential
1. The axon is depolarized when voltage-gated
sodium ion channels open and Na+ comes
rushing in.
This causes the inside of the neuron to
become positively charged.
Figure 7.5 (2 of 4)
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Steps of an Action Potential
2. The axon is repolarized when voltage-gated
potassium ion channels open and allow K+ to
leave the axon.
This returns the membrane potential to
negative on the inside of the neuron.
3. The action potential travels down the axon.
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Action Potential
Figure 7.5 (3 of 4)
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Action Potential
After the action potential, the sodium-
potassium pump restores the original
conditions by pumping sodium (Na+) out of
the cell and potassium (K+) back into the cell.
Figure 7.5 (4 of 4)
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The Neuronal Impulse
Figure 7.6
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Action Potentials
An action potential is an all-or-nothing
response:
If there is not enough stimulation, the ion
channels will not open and there will not be an
action potential.
The level of the action potential is always the
same.
An action potential always moves in the same
direction down the axon.
The sodium channels are inactivated for awhile
after the action potential passes = refractory
period.
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Q: When a neuron is resting, sodium ions have a
greater concentration:
1. Inside the neuron cell
2. Outside the neuron cell
3. Concentration is the same
Q: When a neuron is depolarizing, which ions come
into the neuron?
1. Calcium (Ca2+)
2. Sodium (Na+)
3. Potassium (K+)
4. Chlorine (Cl-)
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Neuronal Synapse
How are messages passed from one neuron
to the next, or from a neuron to a muscle?
The junction between two neurons or
between a neuron and a muscle is called a
synapse.
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Components of the Synapse
1. Presynaptic neuron is the transmitting
neuron.
It has synaptic vesicles that contain
neurotransmitters.
2. Postsynaptic neuron is the receiving
neuron or the muscle.
3. And the gap in between them = synaptic
cleft.
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Synaptic Transmission
Figure 7.8 (1 of 3)
Nucleus
Impulse
Synaptic
knob
Axon
Dendrites
Cell
body
Synaptic
cleft
Synaptic
vesicle
Impulse
Membrane of
postsynaptic neuron
Step 1: The impulse reaches
the axon ending of the
presynaptic membrane.
Step 2: Synaptic
vesicles release
neurotransmitter
into the synaptic
cleft.
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Synaptic Transmission
Figure 7.8 (2 of 3)
Neurotransmitter
Receptor (of sodium ion
channel) on postsynaptic membrane
Step 3: Neurotransmitter
diffuses across synaptic cleft.
Synaptic
vesicle
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Synaptic Transmission
Figure 7.8 (3 of 3)
Step 5: Sodium ion channels open.
Step 4: Neurotransmitter molecules bind to receptors on the postsynaptic neuron.
Step 6: Sodium ions enter the postsynaptic neuron, causing depolarization and possible action potential.
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1. The action potential gets to the end of the presynaptic axon.
2. The action potential triggers Ca2+ to enter the presynaptic axon terminal.
3. The Ca2+ triggers synaptic vesicles located at the axon terminal to merge with the neural membrane.
Transmission Across Synaptic Cleft
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4. The synaptic vesicles release the neurotransmitters into the synaptic cleft.
5. These neurotransmitters travel across the synaptic cleft to the postsynaptic neuron (or the muscle).
6. Neurotransmitter binds to receptors on the postsynaptic neuron (or muscle).
Transmission Across Synaptic Cleft
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Transmission Across Synaptic Cleft
7. These receptors are ligand-gated sodium
ion channels which allow Na+ to enter the
postsynaptic neuron (or muscle) and
triggers an action potential in the
postsynaptic neuron (or muscle
contraction).
8. Once the neurotransmitters are released,
they need to be destroyed or contained
quickly or they will continue to stimulate
the nerve.
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Neurotransmitters
Acetylcholine
Acts in both the PNS and the CNS as a
neurotransmitter.
Causes voluntary muscles to contract.
Acetylcholinesterase: an enzyme that breaks
down excess acetylcholine in the synaptic cleft.
Myasthenia gravis is an autoimmune disease
that attacks the acetylcholine receptors,
resulting in reduced muscle strength.
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Important Concepts
Read Chapter 7
What are the functions of nervous system?
What are the two types of cells in nervous
tissue? (Neuroglial cells and neurons!)
What are the three types of neuroglial cells and
what are their functions?
What are the two main divisions of the nervous
system (CNS, PNS) and where are they each
found?
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Important Concepts
What are the parts and functions of a neuron?
What are the three types of neurons (sensory,
interneuron, and motor neuron) and their
functions, and where are they located?
Where are the cell bodies located for motor and
sensory neuronal cells?
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What are Schwann cells and oligodendrocytes,
and what are their functions?
Where are Schwann cells and oligodendrocytes
found?
What is the cause and what are the effects of
multiple sclerosis?
What are the parts of a nerve?
Important Concepts
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How do ions pass through membranes?
What is the function of the sodium-potassium
pump?
What are the steps of signal transmission
through the nervous system, starting with the
resting stage of one neuron and ending with
the next neuron or muscle being stimulated?
Important Concepts
Copyright © 2009 Pearson Education, Inc.
Which ions enter and exit the neuron during the
depolarization and repolarization steps of an
action potential?
What is the relative charge of the inside versus
the outside of the neuron during these events,
and what is the correct order of these events?
What are the components of a synapse?
What is the function of neurotransmitters? How
do they work and where do they work?
Important Concepts
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What is acetylcholine, where is it found, what
effect does it have, and how is acetylcholine
removed from the synaptic cleft?
What is the cause and effect of Myasthenia
gravis?
Important Concepts
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Definitions
Afferent neuron, efferent neuron, dendrite, axon, sensory neuron, interneuron, motor neuron, myelin, myelin sheath, myelinated neuron, Schwann cell, oligodendrocytes, nodes of Ranvier, nerve, ion, ion channel, ligand-gated ion channel, voltage-gated ion channel, action potential, repolarization, depolarization, membrane potential, resting potential, sodium-potassium pump, refractory period, synapse, synaptic cleft, synaptic vesicle, neurotransmitter, acetylcholinesterase, presynaptic neuron, postsynaptic neuron, stimulate, inhibit