Neurobiology 2 - Action Potential (cont) Since speed of...

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Action Potential (con’t) Since speed of response to a stimulus is obviously of some advantage, it is not surprising that animals have evolved at least two ways to speed up the conduction of the AP : since electrical resistance is inversely related to the diameter of the wire (1/r 2 ), larger neurons will conduct at a faster rate the usual neuron , with a diameter of approx. 10 microns , conducts at about 1 meter per sec some invertebrate neurons (e.g., squid giant axon) are 1 mm (1000 microns) in diameter--conduct at a velocity of 20 meters per sec the major problem is that such large neurons take up too much room in the nervous system insulate the neuron so that the impulse moves through the cytoplasm of the neuron at the speed of electricity = the speed of light = 3 x 10 8 meters per sec!! the trouble with that is that the resistance of the cytoplasm reduces the voltage as it conducts through the cell (it doesn't generate any new APs because the cell membrane is covered and therefore can't generate an AP) thus, although the speed is increased, the AP would be decremental , which means that it could become so small at the end of a long neuron that it wouldn't stimulate the release of neurotransmitter at the synapse (see below), and the 'message' would be lost the Vertebrates have evolved a simple solution to this problem: the myelin sheath is composed of cells ( Schwann cells ) that are wrapped around the neuron, with gaps between the cells called nodes of Ranvier (about 1 mm apart) at the nodes of Ranvier, the neuronal membrane
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is uninsulated, so a normal AP can be generated thus, the myelin sheath allows rapid, decremental conduction of the AP between the nodes, but the impulse regenerates an all-or-none AP at each node 0 termed saltatory conduction -- can be 100 meters per sec 1 the evolution of the myelin sheath has allowed the vertebrates to have small, rapidly conducting neurons. Literally gives more room in the central nervous system for more wires. Synaptic transmission
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Neurobiology 2 - Action Potential (cont) Since speed of...

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