BME_552_2007_lecture2_Berger_neurosci_part3

BME_552_2007_lecture2_Berger_neurosci_part3 - BME 552...

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Synaptic Transmission: Interneuron Communication • original “circulatory system” model for the nervous system: neurons as nodes in a continuous “nerve net” • mechanisms underlying action potential generation (review) • role of Na + • role of K + problem : how to continue active propagation, or regeneration, of the action potential across the synapse BME 552 Lecture 2
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Synaptic Transmission 1: Electrical Coupling fundamental concept : • hypothesis: transmission between neurons results from electrotonic coupling through the extra-cellular space • same mechanisms as propagation of the action potential: “jumping the intercellular membranes” • however, if neurons are not continuous, i.e., if there is a substantial physical space between them, then electrical coupling between neurons is not favored example : • assume for neurons 1 and 2 a resistance of the terminal membranes of 1000 Mohms • assume an input resistance for neuron 2 of 100 Mohm • assume resistance of extracellular space of 1 Mohm • when action potential reaches the terminal, low resistance of synaptic cleft will shunt majority of electrical current away from postsynaptic membrane and into extracellular space BME 552 Lecture 2
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fundamental concept : results in an attenuation of potential from neuron 1 to neuron 2 by 10 -4 thus for every 100mV in neuron 1, only 100μV would be generated in neuron 2 • electrical synapses have structural characteristics that overcome this shunting effect of the extracellular space • smaller cleft (20 Å) • low resistance channels between pre- and postsynaptic membranes: gap junctions BME 552 Lecture 2
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channels have the effect of decreasing the resistance between pre- and postsynaptic neuron membranes and thus decreasing the shunting of the presynaptic current to the extracellular space BME 552 Lecture 2
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example of gap junction coupling of multiple neurons revealed through “dye coupling”: injection of a low molecular weight dye capable of diffusing through the gap junction channels the dye used here (“Lucifer yellow”) has the property that it becomes fluorescent upon exposure to ultraviolet light; note that in this case gap junction coupling occurs at the level of the dendrites BME 552 Lecture 2
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increasing evidence that electrical synapses are widespread in the mammalian brain • dye coupling in hippocampus; at soma level • dye coupling in neocortex / striatum at dendritic level • notion that electrical synapses are particularly important for synchronization of activity among populations of neurons; potential importance of gap junctions in epilepsy • evidence that gap junction strength varies as a function of intracellular pH, and thus, as a function of past history of activity • evidence that gap junction strength is regulated by multiple neurotransmitters, e.g., dopamine in striatum evidence that some species have combinations of chemical and electrical synapses: ciliary ganglion of the chick • alternating 400 Å and 20 Å synaptic clefts
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BME_552_2007_lecture2_Berger_neurosci_part3 - BME 552...

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