Synaptic transmission in the CNS

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Synaptic transmission in the CNS

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University of Southern California
Bisc 307l - General Physiology
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BISC 307L 2nd Edition Lecture 9 Current Lecture Synaptic transmission in the CNS Entire surface of the neuron(purple) is covered with synapses (blue). A few thousand synaptic inputs in one nerve cell of the CNS. - Each input is quite weak. Not like NMJ. They each only have a slight effect on postsynatpic potential – the overall effect depends on integration(summation) of the large number of synapses over short period of time. -Can be both excitatory and inhibitory – there are more inhibitory inputs than excitatory. -Different nerve transmitters released by different synapses. Postsynaptic neurons create and put different receptors in its membrane. If you look at postsynaptic membranes directly opposite the input/transmitter release sites, you will find high concentrations of receptor that are specific for the transmitter. So you make many receptors and send them to the right places, concentrating them in high density. -Each transmitter can have multiple receptors. -Synapses can have ionotropic or metabotropic effects. Integration of Excitation and Inhibition Left –lightning bolt represents the stimulus that stimulates the axon. There is a typical excitatory synapse in the upper left that releases glutamate (the most abundant excitatory transmitter in the CNS). Glutamate binds to 2 types of receptors – in this case, it binds to AMPA-type GluR. The binding opens the channel which is specific for monovalent cations, of which the two most abundant are Na and K. Given that this has a normal negative resting potential when the synapse is active, the fact that Na is so far out of equilibrium creates a large driving force for inward sodium current, and that inward sodium current far exceeds the outward K current, because the outward K current is driven by a smaller driving force because the membrane potential is close to Ek. Big inward sodium current + small outward potassium current = net inward current that is depolarizing. When it flows out across the membrane capacitance, it depolarizes the cell. Shown on graph on the bottom: resting potential, then stimulation, then depolarization, decays back as transmitter gets inactivated or diffuses away. This is an excitatory postsynaptic potential because it brings the cell closer to threshold for an AP. -Think of threshold in terms of space/location around the neuron - it matter that this inward current exceeding outward current should happen? If the distribution of ion channels were even, it wouldn’t matter. But it’s not. Typically, where the axon comes off the cell body, called the first hemi-node or axon hillock, is the trigger zone. That’s the important place to depolarize – it will reach threshold first because there is a high density of VG Na channels so it

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