Nervous System
AP Biology
2007-2008
Essential Knowledge:
Animals have nervous systems that
detect external and internal signals,
transmit and integrate information and
produce responses.
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Nervous system cells
 Neuron
signal

direction
a nerve cell
dendrites
cell body
 Structure fits function
many entry points
for signal
 one path out
 transmits signal

axon
signal direction
myelin sheath
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dendrite
 cell body  axon
synaptic terminal
synapse
Myelin sheath
 Axon coated with Schwann cells
signal
direction
Insulation material (lipid)
 speeds up signal

 saltatory conduction

150 m/sec vs. 5 m/sec
(330 mph vs. 11 mph)
myelin sheath
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action potential
saltatory
conduction
Na+
myelin
axon
+
+
+
+
+
–
–
Na+
Multiple Sclerosis
 immune system (T cells)
attack myelin sheath
 loss of signal
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Neuron Functional Differences
Integrates and coordinates
info from afferent, sends out
response to efferent
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Information Processing
 Describe the basic pathway of
information flow through neurons that
cause you to turn your head when
someone calls your name (big picture).
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Neuron at Resting Potential
 Opposite charges on opposite sides of
cell membrane

membrane is polarized
 negative inside; positive outside
 charge gradient (-70mv)
 stored energy (like a battery)
+ + + + + + + + + + + + + + +
– – – – – – – – – – – – – –
– – – – – – – – – – – – – –
+ + + + + + + + + + + + + + +
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What makes it polarized?
 Cells live in a sea of charged ions

anions (negative)
 more concentrated within the cell
 Cl-, charged amino acids (aa-)

cations (positive)
 Na+ more concentrated in the extracellular fluid
Na+
Na+
K+
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aa-
K+
Na+
aaCl-
Na+
ClK+
Na+
aa-
Na+
K+
aa-
K+
Na+
ClCl-
Na+
aa-
Na+
Na+
Na+
Claa- Cl-
–
K+
+
channel
leaks K+
How does a nerve impulse travel?
 Stimulus: nerve is stimulated

reaches threshold potential
 open Na+ channels in cell membrane
 Na+ ions diffuse into cell

charges reverse at that point on neuron
 positive inside; negative outside
The 1st
domino
goes
down!
 cell becomes depolarized
– + + + + + + + + + + + + + +
+ – – – – – – – – – – – – – –
Na+
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+ – – – – – – – – – – – – – –
– + + + + + + + + + + + + + +
Depolarization
 Wave: nerve impulse travels down neuron



The rest
of the
dominoes
fall!
Gate
change in charge opens
+ –
next Na+ gates down the line
 “voltage-gated” channels
channe
l
closed
Na+ continues to diffuse down neuron
“wave” moves down neuron = action potential
+
+
channe
l open
– – – + + + + + + + + + + + +
+ + + – – – – – – – – – – – –
Na+
+ + + – – – – – – – – – – – –
– – – + + + + + + + + + + + +
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wave 
Voltage-gated channels
 Ion channels open & close in response to
changes in charge across membrane
Structure
& function!
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Repolarization
 Re-set: 2nd wave travels down neuron

K+ channels open
 K+ channels open up more slowly than Na+ channels


K+ ions diffuse out of cell
charges reverse back at that point
 negative inside; positive outside
Set
dominoes
back up
quickly!
K+
+ – – – – + + + + + + + + + +
– + + + + – – – – – – – – – –
Na+
– + + + + – – – – – – – – – –
+ – – – – + + + + + + + + + +
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wave 
How does a nerve impulse travel?
 wave of opening ion channels moves down
neuron
 flow of K+ out of cell stops activation of Na+
channels in wrong direction
 Action Potential Propagation
Ready
for
next time!
K+
+ + + – – – – + + + + + + + +
– – – + + + + – – – – – – – –
Na+
– – – + + + + – – – – – – – –
+ + + – – – – + + + + + + + +
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wave 
How does the nerve re-set itself?
 Sodium-Potassium pump

active transport protein in membrane
 requires ATP
3 Na+ pumped out
+
 2 K pumped in
 re-sets charge
across
membrane

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That’s a lot
of ATP !
Feed me some
sugar quick!
ATP
Action potential graph
1. Resting potential
2. Stimulus reaches
40 mV
4
30 mV
Membrane potential
threshold potential
3. Depolarization
Na+ channels open;
K+ channels closed
4. Na+ channels close;
K+ channels open
5. Repolarization
reset charge gradient
6. Undershoot
+ channels close slowly
K
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20 mV
10 mV Depolarization
Na+ flows in
0 mV
–10 mV
3
–20 mV
Repolarization
K+ flows out
5
–30 mV
–40 mV
–50 mV
Threshold
–60 mV
2
–70 mV
–80 mV
1
Resting potential
Hyperpolarization
(undershoot)
6 Resting
All or nothing response
 Once first one is opened, the rest open
in succession
a “wave” action travels along neuron
 have to re-set channels so neuron can
react again

How is a nerve impulse
similar to playing with
dominoes?
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What happens at the end of the axon?
Impulse has to jump the synapse!



junction between neurons
has to jump quickly from one cell
to next
Synaptic Signaling
How does
the wave
jump the gap?
Synapse
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The Synapse
axon terminal

action potential
synaptic vesicles
synapse



Ca++
receptor protein

neurotransmitter
acetylcholine (ACh)
 ion-gated channels open
muscle cell (fiber)

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We switched…
from an electrical signal
to a chemical signal
Action potential
depolarizes membrane
Opens Ca++ channels
Neurotransmitter vesicles
fuse with membrane
Release neurotransmitter
to synapse  diffusion
Neurotransmitter binds
with protein receptor
Neurotransmitter
degraded or reabsorbed
Neurotransmitters
 Acetylcholine

transmit signal to skeletal muscle
 Epinephrine (adrenaline) & norepinephrine

fight-or-flight response
 Dopamine



affects sleep, mood, attention & learning
lack of dopamine in brain associated with
Parkinson’s disease
excessive dopamine linked to schizophrenia
 Serotonin

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affects sleep, mood, attention & learning
Weak point of nervous system
 Any substance that affects
neurotransmitters or mimics them affects
nerve function
 Ex: Gases, drugs, poisons

Acetylcholinesterase inhibitors =
neurotoxins!
 Ex: snake venom, insecticides, cone snail
toxins
Snake toxin blocking
acetylcholinesterase
active site
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Vertebrate Brains
 Evolutionary trends towards
“Cephalization”

Central region for integrating and
coordinating information.
 Different regions have different functions:
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How are they similar?
How are they different?
More mass, more
neurons, more
connections….
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Ponder this…
Any Questions??
Action Potential Brain U
Action Potential Propagation
Synaptic Signaling
cone snail toxins
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2007-2008
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