Chapter 20 - Coordination in Mammals (Part 3)

7/30/2019 Chapter 20 - Coordination in Mammals (Part 3)

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CHAPTER 20

COORDINATION

20.1 Nervous System


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Figure 28.4

Resting state: voltage gated Na+

and K+ channels closed; resting

potential is maintained.

1

2

3

4

A stimulus opens some Na+

channels; if threshold is reached,

action potential is triggered.

Additional Na+ channels open,

K+ channels are closed; interior of

cell becomes more positive.

5 The K+ channels close

relatively slowly, causing

a brief undershoot.

Na+ channels close and

inactivate. K+ channels

open, and K+ rushes

out; interior of cell more

negative than outside.

Neuron

interior

Actionpotential

Thresholdpotential

Resting potential

1

2

3

4

5

Na+

Na+

Na+

Na+

1 Return to resting state.

1

Neuron

interior

K+

K+

PREVIOUS LECTURE

Objective:

(c) Explain the generation of action potential, transmission and characteristic of nerveim ulse alon an axon.


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OBJECTIVES

(d)Describe the structure of synapse and

explain the mechanism of synaptic

transmission across synapses.

(e)Compare the transmission of impulse at the

synapse and along the axon.

(f) Explain the mechanism of action of drugs on

the nervous system.


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OVERVIEW:

Two connecting neurons


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OVERVIEW:

Two connecting neurons and

axon ending


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OVERVIEW:

Synapse transmission

Transfers the

electrical nerveimpulse

By chemical neuron-transmitters

From one neuron tothe next
http://c/Documents%20and%20Settings/kmpk/Desktop/coordination/D:/10.Coordination/Animation/chemical%20synapse.swf


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SYNAPSE

An area of functional

contact between one

neuron and another fortransferring information


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Dendrites Synaptic knobs

Myelinsheath

Receivingcell body

Axon

Synapticknobs


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Found between the

terminal end of the axon of

one neuron and the

dendrites or cell body ofanother neuron

Objective:

(c) Explain the generation of action potential, transmission and characteristic of nerveimpulse along an axon.

SYNAPSE


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Electrical synapse

Pre and postsynaptic membrane are very close together

No synaptic cleft

Allow electrical current to flow directly from one neuron to

another

Types

Objective:



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Chemical synapse

Most common

Pre and postsynaptic membrane are separated by

synaptic cleft Involve the release of chemical neurotransmitter into the

synaptic cleft

Objective:



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Synaptic cleft A gap between the presynaptic neuron and the

postsynaptic neuron

About 20nm width

Synaptic knob Enlargement of the terminal end of the axon

Contains: Synaptic vesicles Mitochondria

Structure

Contains neurotransmitter- acetylcholine or

norepinephrine

Objective:



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Presynaptic membrane

The membrane nearest to the

synaptic cleft

Postsynaptic membrane

The thickened membrane of

the dendrite

Objective:



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Ligand-gated channels

Found on the postsynaptic

membrane

Have receptors for the

neurotransmitter

substance

Allow the movement of

ions into the postsynaptic

neurons

Objective:



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Mechanism of Synaptic

Transmission

Nerve impulses arrive at the synaptic knob Depolarizes the presynaptic membrane

Permeability of the membrane to Ca

2+

ions isincreased

Enter the knob

Causes the synaptic vesicles to fuse with thepresynaptic membrane Rupture and discharging the neurotransmitter into the

synaptic cleftObjective:

(c) Explain the generation of action potential, transmission and characteristic of nerve impulse alongan axon.


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New vesicles filled with neurotransmitter substancesare reformed in the knob

The neurotransmitter substance diffuses across thesynaptic cleft

Attaches to a specific receptor site on the postsynaptic

membrane

Causing the opening of the Na+ channels

Objective:



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The Na+ ions enter the postsynaptic neurons

Depolarization of the postsynaptic membrane

occurs This new potential generated is known as excitatory

postsynaptic potential (EPSP)

Temporary depolarization of postsynaptic membranepotential

caused by the flow of positively charged ions into thepostsynaptic cell

Objective:



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Single EPSP does notsufficiently depolarize

the membrane to

generate an action

potential.

The summation of these

three EPSPs generatesan action potential.

Objective:



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Followed by the leaving of the K+ ions down their

respective concentration gradient

Repolarization

Objective:



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After the action potential is produced, theneurotransmitter is removed

2 methods of neurotransmitter removal: Direct uptake of neurotransmitter Diffuse by active transport back into the synaptic knob

Degraded by enzymes

Acetylcholinesterase degrades acetylcholine

Monoamine oxidase degrades norepinephrine

Objective:



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SUMMARY

A synapse is the junction

between 2 neurones.

There is a very narrow gapbetween neurones called the

synaptic cleft.

An action potential cannotcross the synaptic cleft, so

nerve impulses are carried by

chemicals called

neurotransmitters.


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SUMMARY

Chemical transmitter released

from presynaptic neurone

diffuses across synaptic cleft &

fits into receptors on

postsynaptic membrane

cause postsynaptic neurone

to depolarise & set up action

potential.

C i f I l T i i


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Comparison of Impulse Transmission:

Across the Synapse and Along the Axon

Synapse Axon

Impulse is chemically transmitted Impulse is electrically transmitted

Involves the neurotransmitter

substances

No neurotransmitter substances

are involved

Impulse transmission is slowerbecause the neurotransmitter

need to diffuse across the synapticcleft

Impulse transmission is very fast

Involves the diffusion of Ca2+ ions

into the synaptic knob to activate

the vesicles

Ca2+ ions are not involved

The diffusion of Na+ across the

membrane is needed

The diffusion of Na+ across the

membrane is needed

Objective: (e) Compare the transmission of impulse at the synapse and along the axon.


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Drugs and Impulse

Transmission

DRUGS

Any chemical substances that alters the

physiological state of a living organism

Lead to addiction if abused

Give harmful effect

Most drugs interfere with the impulse

transmission: Changing, releasing, reabsorbing or hindering synthesis

Objective:

(f) Explain the mechanism of action of drugs on the nervous system.


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The Mechanism of Drugs(cocaine)

Action at Synapse

Affects the brains limbic system (pleasure center)

Release neurotransmitter dopamine into the synaptic cleft

Bind to the receptors on the postsynaptic membrane

Stimulates the feeling of euphoria

Objective:



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Dopamine is quickly

transported back to the

presynaptic knob by

transport molecules

Objective:



Action at Synapse


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In the presence ofcocaine:

Cocaine will binds to

the transport

molecules

Blocks the reuptake of

dopamine

Stays in the

synaptic cleft

Objective:



Action at Synapse


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Depolarization occursrepeatedly

Cause continuous

impulse

transmission

Leads to intense

pleasure, increased

energy and feeling ofpower

Objective:



Action at Synapse


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In response, the neuron will reduce the number of dopaminereceptors in the postsynaptic membranes

Less receptors will bind with dopamine

Thus, more and more drug is needed for the addict to

experience the pleasurable effects

Objective:



Action at Synapse


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Addiction build,

Cocaine addicts find that their pleasure centers cant

function at all without the stimulation of drugs

Objective:



Action at Synapse

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Chapter 20 - Coordination in Mammals (Part 3)

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