L08-2010 - BioNB222 Cornell University Spring 2010 Ronald...

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BioNB222 Spring 2010 Cornell University Ronald Harris-Warrick Lecture 08. Ionic Mechanisms of Synaptic Excitation Abstract: Neurons communicate with one another primarily at synapses via the release of neurotransmitters like acetylcholine and glutamate. In this lecture, we focus on the post-synaptic side, where the neurotransmitter binds to its receptor and evokes a change in the post-synaptic neuron’s excitability. Excitatory synapses are those that tend to make the post-synaptic neuron become more excitable and fire action potentials. As an example of how this works, we discuss the action of acetylcholine at the vertebrate neuromuscular junction. The concept of the reversal potential is introduced and calculated for this synapse, which allows a quantitative determination that the synapse is excitatory because its reversal potential is more depolarized than the threshold for spike initiation. Learning Objectives 1. To understand the basic mechanisms of chemical synaptic transmission 2. To understand how Ohm’s Law can be used to calculate the current flowing through the transmitter receptors at an excitatory synapse. 3. To understand the concept of the reversal potential of a synaptic event and how it can be used to determine whether a fast synapse is excitatory or inhibitory Reading Assignment Purves et al., Chapter 5, especially pp. 107-112. Lecture Outline A. Introduction to chemical synaptic transmission Synapses are the sites of interaction between neurons in the nervous system. For most neurons, they are the sole source of excitation and inhibition, and thus they play essential roles in shaping activity. The great majority of synapses are chemical in nature: the electrical signal in the pre-synaptic cell is transduced to a chemical signal, the neurotransmitter, which is released into the
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BioNB222 Spring 2010 Cornell University Ronald Harris-Warrick synaptic cleft, the space separating the two cells. The transmitter diffuses across this extracellular space and binds to a specialized protein, the receptor, on the post- synaptic membrane. This binding interaction leads, directly or indirectly, to changes in the flow of ions across the membrane, usually by opening or closing transmitter- sensitive ion channels. This electrical signal depolarizes or hyperpolarizes the post-synaptic cell, leading to changes in its activity. We will discuss electrical synapses (which bypass the release of neurotransmitter) in the next lecture. B. Types of synapses There are three major types of chemical synapses in the brain: 1. Excitatory : Results in an attempt to depolarize the cell above threshold for the generation of an action potential. Typically, this is due to a net inward flow of positive current (i.e., Na + or Ca 2+ flowing into the cell) to depolarize it. 2. Inhibitory
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This note was uploaded on 05/25/2010 for the course BIONB 2220 taught by Professor Hopkins,c.d. during the Spring '10 term at Cornell.

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L08-2010 - BioNB222 Cornell University Spring 2010 Ronald...

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