AP Bio – 3/19/13

• The Nervous System, Chp.48• Body Systems Test Thursday

• (Chp.40, 43 (Immune), 45 (Endocrine), & 48 (Nervous)

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• Animals have nervous systems that detect external and internal signals, transmit and integrate information, and produce responses.

The Nervous System

What trends do you notice?

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Noteworthy Trends In Development

• Increase in ganglia• Increase in sensory reception• Increase in cephalization

– Cephalization is the concentration of nervous tissue in the anterior region of the organism.

Human Nervous System

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Neuron = nerve cells

• The neuron is the basic structure of the nervous system that reflects function.

• Neuron structure allows for the detection, generation, transmission, and integration of signal information.

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Neuron Anatomy

• A typical neuron has a cell body, axon and dendrites.

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Myelin Sheath Axon coated with Schwann cells

insulates axon speeds signal

signal hops from node to node saltatory conduction

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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

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

dendrites

cell body

axon

synaptic terminal

Structure fits function many entry points for

signal one path out transmits signal

signal direction

signaldirection

synapse

myelin sheath

dendrite cell body axon

Evolutionary Adaptations of Axon Structure

• The speed of an action potential increases with the axon’s diameter

• In vertebrates, axons are insulated by a myelin sheath, which causes an action potential’s speed to increase

Axon Myelin sheath

Schwanncell

Nodes ofRanvier

Node of Ranvier

Layers of myelin

Axon

SchwanncellNucleus ofSchwann cell

0.1 m

Cell body

Schwann cell

Depolarized region(node of Ranvier)

Myelinsheath

Axon

Saltatory Conduction• Saltatory conduction. Notice that the conduction

along a myelinated axon can occur quickly as large spaces can be skipped and impulse propagation occurs only at the nodes of Ranvier.

Describe a Resting Potential:

• Membranes of neurons are polarized by the establishment of electrical potentials across the membranes

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• What is the charge inside the neuron at rest?

Source of Charge Differences:

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Action Potential

• Action potentials propagate impulses along neurons. – In response to a stimulus, Na+ and K+ gated

channels sequentially open and cause the membrane to become locally depolarized.

–Na+/K+ pumps, powered by ATP, work to maintain membrane potential.

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Figure 48.11a

Actionpotential

Threshold

Resting potential

Time

Mem

bran

e po

tenti

al(m

V)

50

100

50

0

1

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1. Resting potential2. Stimulus reaches threshold

potential3. 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 mV10 mV Depolarization

Na+ flows in

20 mV30 mV

40 mV

RepolarizationK+ flows out

ThresholdHyperpolarization(undershoot)

Resting potential Resting1

2

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Mem

bran

e po

tenti

al

OUTSIDE OF CELL

INSIDE OF CELLInactivation loop

Sodiumchannel

Potassiumchannel

Threshold

Resting potentialTime

Mem

bran

e po

tenti

al(m

V)

50

100

50

0

Na

K

Key

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1

Resting state

At resting potential1. Most voltage-gated

sodium (Na+) channels are closed; most of the voltage-gated potassium (K+) channels are also closed

OUTSIDE OF CELL

INSIDE OF CELL

Inactivation loop

Sodiumchannel

Potassiumchannel

Threshold

Resting potential

Time

Mem

bran

e po

tenti

al(m

V)

50

100

50

0

2

1

1

2

Resting state

Depolarization

When an action potential is generated

2.Voltage-gated Na+ channels open first and Na+ flows into the cell

OUTSIDE OF CELL

INSIDE OF CELLInactivation loop

Sodiumchannel

Potassiumchannel

Actionpotential

Time

Mem

bran

e po

tenti

al(m

V)

50

100

50

0

Na

K

Key

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3

1

2

3

Resting state

Depolarization

Rising phase of the action potential

3. During the rising phase, the threshold is crossed, and the membrane potential increases to and past zero

OUTSIDE OF CELL

INSIDE OF CELLInactivation loop

Sodiumchannel

Potassiumchannel

Actionpotential

Threshold

Resting potentialTime

Mem

bran

e po

tenti

al(m

V)

50

100

50

0

Na

K

Key

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1

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1

2

3

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Resting state

Depolarization

Rising phase of the action potentialFalling phase of the action potential

4. During the falling phase, voltage-gated Na+ channels become inactivated; voltage-gated K+ channels open, and K+ flows out of the cell

5. During the undershoot, membrane permeability to K+ is at first higher than at rest, then voltage-gated K+ channels close and resting potential is restored

***Action potentials travel in only one direction: toward the synaptic terminals

OUTSIDE OF CELL

INSIDE OF CELLInactivation loop

Sodiumchannel

Potassiumchannel

Actionpotential

Threshold

Resting potentialTime

Mem

bran

e po

tenti

al(m

V)

50

100

50

0

Na

K

Key

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1

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1

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5 1

Resting state Undershoot

Depolarization

Rising phase of the action potentialFalling phase of the action potential

Sequence the following in order of occurrence

• Depolarization• Resting state• Repolarization• Hyperpolarization

Sequenced in order of occurrence

• Resting state• Depolarization• Hyperpolarization• Repolarization• Resting state

• Resting state• Depolarization• Hyperpolarization• Repolarization• Resting state

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?

?Time

Mem

bran

e po

tenti

al(m

V)50

100

50

0

1

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a. the resting membrane potential to drop to 0 mV.b. the inside of the neuron to become more negative

relative to the outside.c. the inside of the neuron to become positively

charged relative to the outside.d. sodium to diffuse out of the cell and potassium to

diffuse into the cell.

Adding a poison that specifically disables the Na+/K+ pumps to a culture of neurons will cause

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

Name three specific adaptions of the neuron

membrane that allow it to specialize in conduction

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What happens when the impulse reaches the end of the axon?

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Synapses• Transmission of information between neurons

occurs across synapses.• A chemical synapse is a junction between two nerve

cells consisting of a narrow gap across which impulses pass by means of a neurotransmitter

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Cell To Cell Communication Events1. Action potential depolarized the membrane

of synaptic terminal, this triggers an influx of Ca2+.

2. That causes synaptic vesicles to fuse with the membrane of the pre-synaptic neuron.

3. Vesicles release neurotransmitter molecules into the synaptic cleft.

4. Neurotransmitters bind to the receptors of ion channels embedded in the postsynaptic membrane.

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Note the structural features that allow the cell to cell communication to occur in the synaptic region:

• Calcium gated channels in the synaptic knob• Sodium channels in the post-synaptic membrane• Fluidity of the lipid bi-layer allows for exocytosis of

the neurotransmitter

Exocytosis

• Neurotransmitter release is a form of exocytosis.

• In exocytosis, internal vesicles fuse with the plasma membrane to secrete macromolecules out of the cell.

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Neuron Transmitter Binds With A Receptor On The Postsynaptic Membrane

• The neurotransmitter will then be released from the postsynaptic membrane and degraded.

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Response

• Transmission of information along neurons and synapses results in a response.

• The response can be stimulatory or inhibitory.

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• Acetylcholine – transmit signal to skeletal muscle

• Epinephrine (adrenaline) & norepinephrine– fight-or-flight response

• Dopamine– widespread in brain– affects sleep, mood, attention & learning– lack of dopamine in brain associated with Parkinson’s

disease– excessive dopamine linked to schizophrenia

• Serotonin– widespread in brain– affects sleep, mood, attention & learning

***There are more than 100 neurotransmitters1 neurotransmitter may have more than a dozen different receptors

Neurotransmitters

• Weak point of nervous system– any substance that affects neurotransmitters or

mimics them affects nerve function • gases: nitrous oxide, carbon monoxide• mood altering drugs:

– stimulants» amphetamines, caffeine, nicotine

– depressants» quaaludes, barbiturates

• hallucinogenic drugs: LSD, peyote• SSRIs: Prozac, Zoloft, Paxil• poisons

Injecting ethylene glycol tetraacetic acid (EGTA), a chelating agent that prevents calcium ions from moving across membranes, to a synaptic region would likely

a. increase the release of neurotransmitters by the presynaptic neuron.

b. decrease the release of neurotransmitters by the presynaptic neuron.

c. result in neurotransmitters being released, but could not bind to its receptors on the post synaptic neuron.

d. result in the lack of calcium ions keeping the ligand-gated ion channels open on the post synaptic neurons.

• The contraction of a muscle is a typical response generated by the nervous system.

• Muscle contraction demonstrates the interdependence of the nervous and muscle systems.

Ex - Nervous and muscular

Motor cortex(control ofskeletal muscles)

Frontal lobe

Prefrontal cortex(decision making,planning)

Broca’s area(forming speech)

Temporal lobe

Auditory cortex (hearing)

Wernicke’s area(comprehending language)

Somatosensory cortex(sense of touch)

Parietal lobe

Sensory associationcortex (integration ofsensory information)

Visual associationcortex (combiningimages and objectrecognition)

Occipital lobe

CerebellumVisual cortex(processing visualstimuli and patternrecognition)