Download Ch 34 Action Potential and Neurons

CH 34 Neurons and Action
Potential
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Neurons
dendrites
cell body
 Structure fits function
many entry points
 one path out
 transmits signal

axon
signal direction
synapse
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dendrite
 cell body  axon
Cells are surrounded by charged ions
 Anions [more within the cell]
 Cl-, charged amino acids (aa-)
 Cations [more in the extracellular fluid]
 K+, Na+
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+
voltage!
 membrane is polarized
 negative charge inside; positive outside
 stored energy (battery)
 nerve impulses are called action
potentials
+ + + + + + + + + + + + + + +
– – – – – – – – – – – – – –
– – – – – – – – – – – – – –
+ + + + + + + + + + + + + + +
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Action potential steps
1. Resting potential [-70mv]
2. Threshold potential [-55mv]
3. Depolarization [30mv]
K+
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channels close slowly
40 mV
4
30 mV
Membrane potential
Na+ channels open;
K+ channels closed
4. Na+ channels close;
K+ channels open
5. Repolarization
reset charge gradient
6. Hyperpolarization
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
Action Potential
1. Stimulus
 Na+ channels in membrane open  Na+ diffuses in
 Axon reaches threshold potential
2. Charges reverse at that point = depolarized
 positive inside; negative outside
– + + + + + + + + + + + + + +
+ – – – – – – – – – – – – – –
Na+
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+ – – – – – – – – – – – – – –
– + + + + + + + + + + + + + +
3. Wave travels down neuron


change in charge opens next Na+ gates
Na+ ions continue to diffuse into cell
– – – + + + + + + + + + + + +
+ + + – – – – – – – – – – – –
Na+
+ + + – – – – – – – – – – – –
– – – + + + + + + + + + + + +
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wave 
4. 2nd wave travels down neuron



K+ channels open  K+ diffuses out
charges reverse [negative inside; positive outside]
Combined waves are called an action potential
K+
+ – – – – + + + + + + + + + +
– + + + + – – – – – – – – – –
Na+
– + + + + – – – – – – – – – –
+ – – – – + + + + + + + + + +
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wave 
5. Re-set
 Na+ needs to move back out
 K+ needs to move back in
 both move against concentration gradients need a pump!!
K+
+
Na
Na+
+
Na+ +
K
K Na+
+
Na+
Na+
K+
K
Na+
+Na
+
Na
Na
+ + + + + + + + + + – – – – +
– – +– – – – – – – – + + + + –
Na+
Na
K+
K+
+
+
K
K++ Na
+
+
+
+
Na
K
K
Na K
Na+
Na+
K+
– – – – – – – – – – + + + + –
+ + + + + + + + + + – – – – +
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Na+
+
 Na+ / K+ pump

active transport protein in membrane
 requires ATP
3 Na+ pumped out
+
 2 K pumped in

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ATP
Neuron is ready to fire again
Na+
Na+
Na+
K+
aa-
aaNa+
Na+
Na+
K+
Na+
Na+
K+
Na+
aa-
K+
Na+
Na+
Na+
Na+
K+
aaNa+
Na+
Na+
K+
Na+
Na+
Na+
K+
aa-
aa- K+
K+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
resting potential
+ + + + + + + + + + + + + + +
– – – – – – – – – – – – – – –
– – – – – – – – – – – – – – –
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+ + + + + + + + + + + + + + +
Myelin sheath
 Axon insulated
signal
direction

speeds signal
 signal hops from node to node
 saltatory conduction
myelin sheath
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What happens at the end of the axon?
Impulse has to jump the synapse!

junction between neurons
Synapse
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Chemical synapse
1.
2.
3.
4.
5.
6.
action potential reaches synapse
Ca++ channels open
neurotransmitters released into synapse
neurotransmitters bind with protein receptor
Ion channels open
neurotransmitter broken down or reabsorbed
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Examples of Neurotransmitters
 Acetylcholine  signals skeletal muscle
 Epinephrine (adrenaline)  fight-or-flight
response
 Dopamine  affects sleep, mood, learning



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widespread in brain
Too little  Parkinson’s disease
Too much  schizophrenia
The Trouble with Neurotransmitters
 Weak point
 substances that affect neurotransmitters
affect nerve function
 Gases like carbon monoxide
 Mood altering drugs
 Stimulants  caffeine

amphetamines, caffeine, nicotine
 poisons
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Acetylcholinesterase
 Enzyme which breaks
down acetylcholine n

inhibitors = neurotoxins
 snake venom, insecticides
neurotoxin
in green
active site
in red
acetylcholinesterase
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snake toxin blocking
acetylcholinesterase active site
Reflex Arc
 nerve pathway involving a sensory nerve and

a motor nerve with a synapse between
five (5) parts of a reflex:
1.
2.
3.
4.
5.
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Receptor - sense organ in skin, muscle, organ
Sensory neuron - carries impulse TO CNS
Interneuron - carries impulse IN CNS
Motor neuron - carries impulse AWAY from CNS
Effector - structure that responds (ex: muscle, gland)