Download Nervous System

Nervous System
FUNCTION: Senses, processes, interprets, and
determines the response to stimuli from the
environment
• Central Nervous System (CNS) - made of the brain
and spinal chord
• Peripheral Nervous System (PNS) - all nerve cells
outside of the CNS
Divisions of the Nervous System
(see pages 388-89 for more detail on each)
Sensory division receives
Information (senses
Motor division - relays
information to organs/glands
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involuntary
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“Fight or flight”
“rest & digest”
voluntary
Cells of the Nervous System
• Neurons - cells that conduct
nerve impulses (action
potentials) to communicate
with organs and glands
• Neuroglia (glial cells) support, protect and nourish
neurons (do not send nerve
impulses
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Structure of a Neuron
• Axon - transmits nerve impulses to
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•
•
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communicate with other cells and
organs
Dendrites - receive signals from
other neurons
Myelin sheath - fatty coating on
axon that speeds up action potential
Nodes of Ranvier - gaps in the
myelin sheath where the axon is
exposed
Cell body - part of neuron from
which dendrites arise (also contains
nucleus of cell)
Axon terminals - end of axon/part
that releases neurotransmitters to
communicate with other neurons
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Functional Regions of the Neuron
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Receptive zone
-Receives
input from other
neurons
Conducting zone
-generates action
potential
(nerve impulse)
Secretory zone
-Releases
neurotransmitters
Functional Classification of
Neurons
Sensory (afferent) neurons
• Receive information from the
environment (senses)
Motor (efferent) neurons
• Send signals to
muscles/glands/organs to carry out
response
Interneurons
• Relay signals between sensory
and motor neurons
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Nerves vs. Neurons
Nerves are bundles
of neurons
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Synapse - area where two or more
neurons communicate with each other
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Action Potentials
• A brief reversal in charge
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across a membrane
Happens in the axon
membrane at Nodes of
Ranvier (Saltatory
conduction)
Voltage gated ion
channels for Na+ and K+
open and close in
response to changes in
membrane potential
(charge)
Action potential
is initiated by the
axon hillock
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Stages of an Action Potential
1) More Na+ outside cell/more
2)
3)
4)
5)
K+ inside
Na+ enters the axon (charge
becomes more positive depolarization)
K+ leaves axon (charge
becomes more negative)
Too much K+ has left
(hyperpolarization - more
negative than resting)
Na/K pump restores original
conditions
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View action potential stages in action
Neurotransmitters
• Chemical messengers that cross the
synapse allowing one neuron to
communicate with another
• Can be excitatory (cause postsynaptic neuron to depolarize
(become more positive))
• Can be inhibitory (cause post
synaptic membrane to hyperpolarize
(become more negative))
Neurotransmitters (cont.)
• Stored in vesicles in axon terminals
• Ca2+ rushes into the terminal in response to arriving
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action potentials
Ca2+ causes vesicles to release neurotransmitters into
synaptic cleft (example)
Neurotransmitters bind to their receptors on the postsynaptic membrane
Neurotransmitters are broken down by enzymes in the
synaptic cleft or are taken back up by the pre-synaptic
neuron via transporter proteins
Neurotransmitters (cont)
Examples:
• GABA (inhibitory)
• Glutamate (excitatory)
• Dopamine, serotonin, norepinephrine
(excitatory or inhibitory depending on the
nature of the synapse)
• Over 50 identified
IPSP vs. EPSP
• Inhibitory post-synaptic potentials
(IPSP) decrease the likelihood of
the post-synaptic neuron sending
an action potential (hyperpolarizes
post-synaptic neuron: lets Cl- in or
lets K+ out)
• Excitatory post-synaptic potentials
(EPSP) increase the likelihood of
the post-synaptic neuron sending
an action potential (depolarizes
post-synaptic neuron: lets Na+ in)
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Summation
• Additive effect of the inputs of
all pre-synaptic neurons
• If there are more excitatory
than inhibitory signals, then
depolarization may occur and
an action potential may be sent
by the post-synaptic neuron
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Long-term Potentiation (LTP)
• LTP is the long-lasting strengthening of synapses
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between two neurons
Post-synaptic neurons become more sensitive to
neurotransmitters coming from the pre-synaptic
neuron(s) by:
1) making more receptors
2) increased sensitivity of existing receptors
Involved in learning and memory formation
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BEFORE:
Glutamate (excitatory
neurotransmitter) stimulates
NMDA receptors (in green) at a
high frequency
AFTER:
Because of the frequency of
stimulation, there is an increase
in the number and sensitivity
of receptors on the
post-synaptic neuron (increasing
the strength of the synapse)
What causes this to happen?