AP Biology 2007-2008

Nervous System

AP Biology

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

dendrites

cell body

axon

synaptic terminal

Neuron a nerve cell

Structure fits function many entry points

for signal one path out transmits signalsignal direction

signaldirection

dendrite cell body axon synapse

myelin sheath

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Myelin sheath

signaldirection

Axon coated with Schwann cells 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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myelin

axon

Na+

Na+

++ + + + –

–

action potential

saltatoryconduction

Multiple Sclerosis immune system (T cells)

attack myelin sheath loss of signal

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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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+ Na+ Na+ Na+ Na+ Na+Na+ Na+ K+ Na+ Na+

Cl-

K+ Cl- Cl- Cl-K+

aa-K+ Cl- Cl-

aa- aa-aa-

aa- aa-K+

K+channel leaks K+

channel leaks K+ +

–

Salty Banana!

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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 cell becomes depolarized

– + + + + + + ++ + + + + + +

– + + + + + + ++ + + + + + +

+ – – – – – – –– – – – – – –

+ – – – – – – –– – – – – – –Na+

The 1stdomino

goesdown!

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Gate

+ –

+

+

channel

closed

channel open

Depolarization Wave: nerve impulse travels down neuron

change in charge opens next Na+ gates down the line “voltage-gated” channels

Na+ continues to diffuse down neuron “wave” moves down neuron = action potential

– – + + + + + +– + + + + + +

– – + + + + + +– + + + + + +

+ + – – – – – –+ – – – – – –

+ + – – – – – –+ – – – – – –Na+

wave

The restof the

dominoes fall!

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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

+ – – + + + + +– – + + + + +

+ – – + + + + +– – + + + + +

– + + – – – – –+ + – – – – –

– + + – – – – –+ + – – – – –Na+

K+

wave

Setdominoesback upquickly!

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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

+ + – – + + + ++ – – + + + +

+ + – – + + + ++ – – + + + +

– – + + – – – –– + + – – – –

– – + + – – – –– + + – – – –Na+

wave

K+Readyfor

next time!

Animation

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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

ATP

That’s a lot of ATP !

Feed me somesugar quick!

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1. Resting potential

2. Stimulus reaches threshold potential

3. Depolarization Na+ channels open; K+ channels closed

4. Na+ channels close; K+ channels open

5. Repolarizationreset charge gradient

6. UndershootK+ channels close slowly

Action potential graph

–70 mV

–60 mV

–80 mV

–50 mV

–40 mV

–30 mV

–20 mV

–10 mV

0 mV

10 mV DepolarizationNa+ flows in

20 mV

30 mV

40 mV

RepolarizationK+ flows out

ThresholdHyperpolarization(undershoot)

Resting potential Resting1

2

3

4

5

6

Mem

bra

ne

po

ten

tial

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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 againHow is a nerve impulse similar to playing with

dominoes?

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Synapse

Impulse has to jump the synapse! junction between neurons has to jump quickly from one cell

to next

What happens at the end of the axon?

How does the wave

jump the gap?

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axon terminal

synaptic vesicles

muscle cell (fiber)

neurotransmitteracetylcholine (ACh)receptor protein

Ca++

synapse

action potential

The Synapse Action potential

depolarizes membrane Opens Ca++ channels Neurotransmitter vesicles

fuse with membrane Release neurotransmitter

to synapse diffusion Neurotransmitter binds

with protein receptor ion-gated channels open

Neurotransmitter degraded or reabsorbedWe switched…

from an electrical signalto a chemical signal

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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 affects sleep, mood, attention & learning

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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

Snake toxin blockingacetylcholinesterase

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….

AP Biology 2007-2008

Ponder this…Any Questions??