action potential the all-or-none principle. all-or-none a threshold must be overcome (between -60 mv...
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Action PotentialThe All-or-none Principle
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All-or-None
A threshold must be overcome (between -60 mV and -55 mV)
Will open voltage-regulated sodium channels
If less than -62 mV will produce a graded depolarization
The action potential is independent of the relative strength of the depolarizing stimulus
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Generation of Action Potential
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Generation of Action Potential
Depolarization to Threshold
Activation of Sodium Channels & Rapid Depolarization (this is positive feedback)
Inactivation of Sodium Channels & the Activation of Potassium Channels
Return to Normal Permeability
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Action Potential Animations
http://outreach.mcb.harvard.edu/animations/actionpotential_short.swf
http://brainu.org/files/movies/action_potential_cartoon.swf
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Propagation of Action Potentials
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Continuous Propagation
An action potential is propagated (moved) along an unmyelinated axon
Action potential moves at the same speed along the entire length of the axon
Moves at about 1 meter/second
Similar to dominoes falling
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Saltatory Propagation
Myelinated axons allows the action potential to travel faster
Action potential travels only along the nodes
Node to node – skips the internodes
Faster and uses less energy
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General Properties
Electrical – the Presynaptic & Postsynaptic membranes are locked together at gap junctions, start action potential more quickly
Located int CNS & PNS but very rareFew areas of the brain, the eye & ciliary ganglia of the PNS
Chemical – Presynaptic & Postsynaptic membranes do not touch, most abundant type
Several types of neurotransmitters
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Cholinergic Synapses
Release Acetylcholine (Ach)
Released at neuromuscular junctions
At many synapses in CNS
At all neuron-to-neuron synapses in PNS
At all neuromuscular and neuroglandular junctions within the parasympathetic division of the ANS
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Other Neurotransmitters
Norepinephrine – in brain and ANS, has an excitatory, depolarizing effect on the postsynaptic membrane
Dreaming, learning, emotions, sleeping, controls body temp.
Dopamine – a CNS neurotransmitter, may be inhibitory or excitatory
inhibitory – imp. role in our precise control of movements
excitatory – cocaine inhibits the removal of dopamine from synapses in areas of the brain resulting in a “high”
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Other Neurotransmitters
Serotonin – inadequate production can have widespread effects on a person’s attention & emotional state & may be responsible for many cases of severe chronic depression
Thought to be involved with regulation of sleep and wake patterns
Gamma aminobutyric (GABA) – inhibitory, in the CNS appears to reduce anxiety
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Information Processing by Individual Neurons
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Postsynaptic Potentials
Excitatory - a graded depolarization caused by the arrival of a neurotransmitter, affects only the area immediately surrounding the synapse
Inhibitory – a graded hyperpolarization of the postsynaptic membrane, a larger than usual stimuli would be needed to bring the membrane potential to threshold
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Postsynaptic Potential
Summation – individual excitatory postsynaptic potentials combine which integrates the effects of all graded potentials
Facilitation – a neuron whose transmembrane potential shifts closer to threshold
the larger the degree of facilitation, the smaller is the additional stimulus needed to trigger an action potential