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Published byCaroline Barton Modified over 9 years ago
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HOW MESSAGES ARE SENT
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It is a message travelling down a neuron The message comes from: Another neuron or A sensory receptor A nerve impulse is also called an ACTION POTENTIAL.
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Neurons have a rich supply of positive and negative ions both inside and outside the cell. Sodium (Na+) ions and Potassium (K+) ions play a key role the creation of a nerve impulse.
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Since both are positive…. It’s all relative. ▪ Positive = more positive ions ▪ Negative = fewer positive ions
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The highly concentrated potassium ions inside the nerve cells have a tendency to diffuse outside the nerve cells. Similarly, the highly concentrated sodium ions outside the nerve cell have a tendency to diffuse into the nerve cell.
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Not sending an impulse The neuron is impermeable to Na+ meaning Na+ remains outside Potassium (K+) moves freely (permeable) in and out of cell.
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Outside is positive All Na+ and some K+ Inside is negative Some K+ Polarized more positive outside cell then inside
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Only when polarization can occur will Na+ be able to enter. Na+ channels (gates) open, allowed Na+ to diffuse into cell… for a fraction of a second! Charges begin to switch sides… This happens very quickly
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Polarity reverses: Inside positive Outside negative “Depolarized”
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K+ moving out of the cell (closes Na+ gates), trapping Na+ inside cell. Depolarization (reversal of charges) occurs in a small area ▪ Triggers a “wave” of electricity ▪ Travels length of axon
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Recovery period Few thousandths of a second Neuron cannot be stimulated again Neuron must be returned to “resting potential”
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Sodium potassium pumps returns membrane to rest Na+ moves out K+ moves in Repolarized Outside now + Inside now -
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Myelinated neuron Faster transmission (100 m/s) 2m/s (unmyelinated) 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
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Great animation of the creation of a nerve impulse: Action Potential Demo (Harvard University) Action Potential Demo
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