L09-2010 - BioNB222 Cornell University Spring 2010 Ronald...

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BioNB222 Spring 2010 Cornell University Ronald Harris-Warrick Lecture 09. Synaptic Inhibition and Neuronal Integration Abstract: Electrical synapses are another form of communication between neurons. At these gap junctions, current flows directly through pores that link the pre- and post-synaptic neurons, encouraging synchronization of neuron firing. Chemical inhibitory synapses try to block the post-synaptic neuron from firing, by activating receptors whose channels have a reversal potential below the threshold for firing action potentials. The decision made by the post-synaptic neuron to spike or not to spike reflects its integration of many synaptic inputs, both excitatory and inhibitory. IPSPs can shunt EPSPs by applying current of opposite sign or direction, thus canceling out the depolarization evoked by the EPSP. A single synapse’s effect on the post-synaptic neuron depends on many geometrical and electrical factors affecting the passive flow of current to the spike initiation zone; in many neurons, voltage-dependent channels are found in dendrites, and help “boost” the synaptic current or even fire action potentials in the dendrites. Learning Objectives 1. To understand the mechanisms of electrical synaptic transmission. 2. To understand how inhibitory synapses reduce neuronal excitability. 3. To understand the dynamic and integrative interactions of EPSPs and IPSPs, and how IPSPs can shunt a simultaneous EPSP. 4. To understand how the principles of passive electrical flow apply to synaptic currents and their ability to influence the post-synaptic neuron’s firing. Reading Assignment Purves, et al., Chapter 5, especially pp. 112-118. properties. Lecture Outline A. Electrical synapses Another class of synaptic interactions occurs in the nervous system, in addition to chemical transmission: electrical synapses (also called electrotonic coupling). Whereas in all chemical synapses the pre- and post-synaptic neurons are separated by the space of the synaptic cleft, in electrical synapses the pre- and post-synaptic cells are connected by actual pores which connect the intracellular cytoplasm of one cell to the other. These channels, called connexons, are large enough to let ions and even small fluorescent molecules to pass from one cell to another. In electron micrographs, the membranes of the pre- and post-synaptic neurons appear to fuse due to their tight apposition at the gap junction. To show that two neurons are
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BioNB222 Spring 2010 Cornell University Ronald Harris-Warrick electrically coupled, one can inject current (either depolarizing or hyperpolarizing) into one cell and see a smaller version of that current immediately (without a synaptic delay) in the other cell. One can also inject fluorescent molecules into one cell and see them diffuse to nearby coupled neurons. Many electrical synapses are bidirectional: current flows equally well in both directions. Other electrical synapses are "rectifying", where current flows preferentially in one direction. There are several advantages of electrical synapses over chemical synapses:
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L09-2010 - BioNB222 Cornell University Spring 2010 Ronald...

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