Introduction to Neurobiology - Lecture Notes 09 - Synaptic Inhibition & Neuronal Integration

Introduction to Neurobiology - Lecture Notes 09 - Synaptic Inhibition & Neuronal Integration

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BioNB222 Spring 2008 Cornell University Ronald Harris-Warrick 1 Lecture 9. Synaptic Inhibition and Neuronal Integration Reading Assignment Purves, et al., Chapter 5, especially pp. 112-118. Summary: 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. Whether this occurs depends on whether the synapses are near one another so that they influence current flow through each other’s channels. 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. 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 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 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 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
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2 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 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. Many electrical synapses are bidirectional: current flows equally well in both directions. Other electrical synapses are "rectifying", where current flows preferentially in one direction.
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This note was uploaded on 02/13/2008 for the course BIO 222 taught by Professor Hopkins during the Spring '08 term at Cornell University (Engineering School).

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Introduction to Neurobiology - Lecture Notes 09 - Synaptic Inhibition & Neuronal Integration

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