New Chapt 8-synap trans 1(1)

New Chapt 8-synap trans 1(1) - CHAPTER 8 INTRODUCTION TO...

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82 CHAPTER 8 INTRODUCTION TO CHEMICAL SYNAPTIC TRANSMISSION Now that you know that the nerve cell uses the action potential to convey signals along its axon, we now want to turn to consider how signals are passed between neurons. That is, we want to look at what happens when the action potential reaches the axon terminals, i.e. at how information is conveyed from one neuron to the next. I talked briefly about this process earlier in the introductory lecture and told you that the term applied to it is synaptic transmission and the area of contact between two neurons where the transmission takes place is called a synapse (from the Greek meaning "point of contact"). General features of synaptic transmission In order to make some general points about synaptic transmission, let's consider in a little more detail the synapse we considered in the first lecture, namely the synapse between a muscle spindle afferent (i.e. sensory) neuron and a motor neuron to the same muscle where the spindle is located. Let us consider the following experiment. We impale the motor neuron in the spinal cord with a microelectrode. With a second microelectrode we then impale the axon terminal of a single muscle spindle afferent (in actuality this is an impossible experiment because the small size of these afferent endings means that they are easily damaged by the microelectrode; nonetheless, this "gedanken" (thought) experiment will serve to illustrate the points that need to be made). Imagine that we stretch the muscle in such a way that a single action potential is generated in the spindle afferent neuron. This action potential would be conducted into the spinal cord over the axon of this neuron and a short time later we would record this action potential arriving at the afferent terminal (see Fig. 1). Less than a millisecond after we observed the action potential in the afferent terminal, we would observe a small depolarizing local potential in our motor neuron. This small local potential is called a synaptic potential or more commonly a postsynaptic potential . (The potential is described as "postsynaptic" because it occurs in the postsynaptic as opposed to the presynaptic neuron. In this case the sensory afferent is the presynaptic neuron, because it is on the sending end of the transmission, and the motor neuron is the postsynaptic neuron, because it is on the receiving end of the transmission). Moreover, because this synaptic potential is depolarizing, and brings the membrane potential of the motor neuron closer to threshold, it is most commonly called an excitatory postsynaptic potential and is abbreviated as epsp. Fig. 1.
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83 Note that if the epsp remains below threshold, the information being sent to the motor neuron by the sensory afferent is lost. You may be slightly perplexed by this, for you may remember that in our example of the stretch reflex we considered in an early lecture, excitation of the sensory afferent by muscle stretch generated an action potential in the motor neuron, which
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This note was uploaded on 09/19/2011 for the course BIO 365R taught by Professor Draper during the Spring '08 term at University of Texas.

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New Chapt 8-synap trans 1(1) - CHAPTER 8 INTRODUCTION TO...

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