Lec 5 - Synaptic Transmission

Lec 5 - Synaptic Transmission - Synaptic Transmission What...

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1 Synaptic Transmission What happens when the action potential gets to the end of the sensory neuron and motor neuron in the patellar reflex and encounters the synaptic terminal ? Both of these synapses are chemical synapses , in which the presynaptic cell releases a chemical neurotransmitter substance Bernard Katz Summary of Synaptic Transmission We will consider first the synapse at the output end: the neuromuscular junction Then, we will look at neuron-to-neuron synapses in the central nervous system
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2 Release of acetylcholine from the synaptic terminal of the motor neuron At vertebrate neuromuscular junctions, the neurotransmitter substance is acetylcholine (ACh) : Synthesized in the motor neuron terminal by the enzyme choline acetyltransferase (ChAT) What links the presynaptic action potential to the release of ACh? Influx of calcium ions from the ECF triggers release Ca 2+ is a relatively minor component of the ECF, present at approximately 1 mM. When the action potential invades the synaptic terminal, the resulting depolarization opens voltage-gated calcium channels (functionally similar to those of Paramecium) Intracellular concentration of free Ca 2+ is about 10 -7 M. From the Nernst equation, E Ca is +116 mV (remember that Z = +2 for calcium). So, both concentration and electrical gradients drive Ca 2+ into the terminal when voltage-gated calcium channels open.
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3 Summary of presynaptic events Neurotransmitter molecules are stored in synaptic vesicles inside the synaptic terminal
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4 Neurotransmitter molecules are stored in synaptic vesicles inside the synaptic terminal Synaptic Vesicle Exocytosis Synaptic vesicles are membrane-bound structures. To release their neurotransmitter cargo to the extracellular space at the synaptic cleft, the vesicle membrane must fuse with the plasma membrane. This is a specialized instance of exocytosis .
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5 Postsynaptic response to ACh How does ACh affect the permeability of the postsynaptic muscle cell? Answer: ACh-gated ion channels Like the voltage-gated channels that underlie the action potential, neurotransmitter-gated ion channels are controlled by gates. These gates are not sensitive to voltage, however, and instead are controlled by binding of specific neurotransmitter substance to specific binding sites that are part of the channel protein. ACh-gated channels are an example of a large family of ion channels, called ligand-gated ion channels. As we will see, ligand-gated ion channels of various kinds are important in chemical neurotransmission. ACh-gated ion channels at the neuromuscular junction The ACh-gated channel is a non- specific cation channel , with approximately equal permeability to sodium and potassium.
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6 How can a channel that lets both sodium and potassium through cause depolarization of the postsynaptic cell? Consider what happens to p
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Lec 5 - Synaptic Transmission - Synaptic Transmission What...

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