Download C The Electrochemical Impulse

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Background
 Messages are sent throughout the nervous system by The Electrochemical Impulse
electrochemical impulses
 Electrical impulses transmitted by movement of ions
 Hence the “electro” and the “chemical”
Section 9.2
Section
92
Page 418
Recall: a long, long time ago...
Recall: a long, long time ago...
 Passive transport: Movement of a substance along its  Ion channels are not constantly open:
concentration gradient
 Voltage‐gated – Open when a change in voltage across the membrane is detected
 Charged entities cannot pass freely through the cell membrane
 Ligand‐gated – Open when something binds to the channel
 Can move by passive transport through ion channels
Nerve membrane potential
Nerve membrane potential
 Electrical potential – A difference in charge
 When a nerve is excited, the potential becomes +40 mV
 Reversal of potential (+110 mV)
 Called an action potential
Resting potential
 The normal “resting” potential of a cell membrane is ‐70 mV
 The membrane has more negative charges inside than it does outside
 The charge difference between the outer and inner surfaces is 70 mV
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Outline
The resting potential
1.
The action potential
2.
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
Generation
Th ll
The all‐or‐nothing response
thi
Propagation along an axon
Synaptic transmission
Establishing the resting potential
At rest,  The concentration of sodium ions (Na+) outside the membrane is higher than the concentration of potassium ions (K+) inside.
 Uneven
Uneven concentrations cause the inside to be more negative
 Resting potential 3.
Diseases and disorders (if time)
is actually an accumulation of a lesser amount of positive charges
Maintaining the resting potential
 The cell membrane is “leaky”: Some ions diffuse across
 Resting potential is maintained by the sodium‐
The Action Potential
potassium pump (active transport – antiport)
 3 Na+ out, 2 K+ in
Generating the action potential
Generating the action potential
 Na+ ions can also diffuse in through ion channels
During excitation,
 A stimulus disrupts the resting potential of the cell
 The Na + channels are opened
 Na + rushes in through voltage‐gated ion channels
 Recall that channels can be voltage‐gated  they can be stimulated to open by disrupting membrane potential
 The inner surface of the membrane becomes positive
relative to the outer  The process of Na + rushing in is called depolarization.
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Repolarization
 Once the membrane is depolarized, Na + gates close
 K+ channels open, allowing K+ to exit the cell
 The original polarity of the membrane is restored to ‐70 mV
 The membrane is said to be repolarized
The membrane is said to be repolarized
Then...
 The Na +/ K+ pump restores the balance of ions
Refractory period
The all‐or‐nothing response
 The membrane must be repolarized before it can  What can stimulate initial depolarization of the generate another nerve impulse
membrane?
 Pressure
 Refractory period: The time required for repolarization
 1 to 10 ms
1 to 10 ms
 Changes in pH
 Electrical shock
Electrical shock
 The intensity of the stimulus must reach a threshold level in order to stimulate an action potential.
The all‐or‐nothing response
The all‐or‐nothing response
 Any stimulus below the threshold will not elicit a response.
 Additionally, increasing the stimulus intensity beyond the threshold will not change the degree of response
the threshold will not change the degree of response.
What is the stimulus threshold?
What happens when the stimulus is beyond the threshold?
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 All action potentials travel with the same speed and intensity.
 Differences in stimulus intensity are detected by changes in the number and frequency of action potentials.
The all‐or‐nothing response: Neurons either fire maximally, or they do not fire at all.
More intense stimulus = More + More frequent action potentials
Propagation of the action potential
Propagation along a neuron
 Nerve impulses are transmitted:
a. along the length of neurons
b. from neuron to neuron (synaptic transmission)
 The action potential h
l
travels down axons, like a wave
Propagation along a neuron
Propagation along a neuron
Animation
http://highered.mcgraw‐
hill.com/olc/dl/120107/bio_d.swf
How?
 The positive charges inside the cytoplasm are attracted to the negatively‐charged areas adjacent to them
 Movement of the positive charges depolarizes the area "downstream"
 Upstream area is still experiencing a refractory period, so will be unaffected
 Na + channels in this area now open, causing sodium to rush in
 The cycle repeats itself in order to propagate the action potential
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Synaptic transmission
Result
 Neurons are separated by a gap (the synapse)
 Each successive region becomes depolarized, passing the impulse along the length of the axon
 One direction only
 The wave of depolarization is followed by a wave of  Nerve impulses must somehow get across the gap from the axon of one neuron to the dendrites of another
repolarization
Synaptic transmission
Synaptic transmission
 Synaptic transmission occurs due to neurotransmitters
 Chemicals contained within vesicles, at the ends of axons
 Neurotransmitters diffuse across the synapse and bind to the dendrites post‐synaptic neuron
 Ligand‐gated Na+ channels
 The action potential reaches p
the end of the axon, which stimulates the release of neurotransmitters
Excitatory effect
 Binding depolarizes the membrane of the dendrites
 Na+ channels open, and the action potential is re‐
intiated
Inhibitory effect
 Opens K + channels in the membrane, so K + diffuses out
 The inner surface becomes even more negative in relation to the outer
 the membrane is hyperpolarized
 Binding can either:
g
 Stimulate another action potential (excitatory effect); or
 Make another action potential less likely (inhibitory effect)
Animation
Featuring the excitatory effect of neurotransmitters: http://highered.mcgraw‐hill.com/olc/dl/120107/anim0015.swf
Acetylcholine and cholinesterase
Two neurotransmitters:
 Acetylcholine (ACh) acts has an excitatory effect on most
neurons
 Causes depolarization
 Cholinesterase is then released by the postsynaptic neuron
 It destroys Ach so that the sodium channels close, and the cell can be repolarized
 makes it harder to depolarize the membrane in order to initiate an action potential
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Summation
Summation
 Often the axons of many neurons will synapse  The effect on the postsynaptic neuron will be determined with the dendrites of another.
 Some of the presynaptic neurons may release excitatory neurotransmitters, while some may release inhibitory ones.
Diseases and disorders by the sum of all the effects of the neurotransmitters released.
 This principle is called summation.
Diseases and disorders Depression
Parkinson’s disease
 Linked to decreased production of the neurotransmitter dopamine.
 Dopamine is the messenger between p
g
parts of the brain that control smooth muscle movement.
Diseases and disorders Addiction
 Dopamine is associated with feelings of pleasure.
 Many
Many drugs mimic, or stimulate drugs mimic or stimulate
the release of, dopamine.
 Massive stimulation of dopamine receptors gives the “high”
 Linked to imbalances in neurotransmitters serotonin, norepinephrine, and dopamine.
 The nature of the relationship is currently unclear.
 Anti
Anti‐depressant
depressant drugs act on pre
drugs act on pre‐ and post
and post‐synaptic
synaptic neurons to alter rate of neurotransmitter breakdown.
 Overstimulation of dopamine receptors on postsynaptic neurons causes the neuron to decrease the number of dopamine receptors.
 This is responsible for drug tolerance.
 Progressively increasing amounts of drug are required to achieve the same effect.
 Body also decreases the amount of dopamine it makes.  This is responsible for withdrawal once drug use is discontinued.
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Homework
 Pg. 426 #3‐10
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