Download Nerve Impulses ppt

HOW MESSAGES ARE SENT
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A message travelling down a neuron
It comes from:
 Another neuron
 A sensory receptor
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Also called an ACTION POTENTIAL
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Sodium (Na+)
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Potassium (K+)
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Since both are positive…. Its all relative
 Positive = more positive ions
 Negative = fewer positive ions
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Not sending an impulse
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Neuron impermeable to Na+
 Na+ collects outside
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Potassium moves freely (permeable)
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Outside is positive
 All Na+ and some K+
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Inside is negative
 Some K+
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Polarized
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Na+ channels (gates) open
 Neurons now permeable to Na+
 Na+ diffuse into cell
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Polarity reversed
 Inside positive
 Outside negative
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Depolarized
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K+ moving out of the cell (closes Na+ gates)
Depolarization occurs in a small area
 Affects adjacent gates
▪ Creates “wave” of electricity
▪ Travels length of axon

Na+ gates are only open for a fraction of a
second
 Na+ gets trapped inside the cell
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Recovery period
 Few thousandths of a second
 Neuron cannot be stimulated again

Neuron must ne returned to “resting
potential”
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Sodium potassium pumps returns membrane
to rest
 Na+ moves out
 K+ moves in
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Repolarized
 Outside now +
 Inside now -
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Myelinated neuron
Faster transmission (100 m/s)
 2m/s (unmyelinated)
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Jumps from one node of Ranvier to next
Uses less energy
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Minimum strength stimulus required for
action potential to occur
 Different for each neuron
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Impulses are all alike once threshold reached
 Strength only changes with number sent