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Lecture 29 Neurotransmitters and Vesicle Fusion

Lecture 29 Neurotransmitters and Vesicle Fusion - BME 418...

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BME 418, Quantitative Cell Biology Alan J. Hunt Lecture #29: Neurotransmitter release and vesicle fusion. A chemical (left) and electrical (right) synapse. When an action potential reaches the end of an axon there must be a mechanism for propagating the signal in and adjacent neuron or target cell. This occurs at the synapse: At and electrical synapse, depolarizating currents can pass directly through gap junctions as previously discussed. Some examples of electrical synapse’s are in the retina between photoreceptors and on amicrine and bipolar cells, and some have recently been found in the CNS. In the more common chemical synapse, a series of events passes a signal from the pre to post-synaptic cells: - AP depolarizes the synaptic bouton. - Calcium influx - Vesicle fusion and NT release - Diffusion across synaptic cleft - Bind on post-synaptic receptors - Open or close channels, or induce second messenger cascades. - Possibly launch AP in post-synaptic nerve. A chemical synapse has more flexibility then an electrical synapse, as it is capable of producing graded changes in the postsynaptic cell, including depolarization, hyperpolarization, and activation of signal tranduction cascades leading to long term changes. This allows complicated logic and processing, for example a hypothetical and gate .
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BME 418, Quantitative Cell Biology Alan J. Hunt From: Textbook of Physiology, Patton et al. In an excitatory synapse like the neuromuscular junction stimulation of the presynaptic nerve results in depolarization of the postsynaptic cell: - Several millisecond delay. - Depolarization larger near synapse. - This is called the end-plate potential.
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BME 418, Quantitative Cell Biology Alan J. Hunt - Blocked by curare. From: Textbook of Physiology, Patton et al. Ion substitution experiments like those discussed earlier demonstrated that the end-plate potential is the result of opening a relatively non-selective post-synaptic cation channel : - As expected, reversal potential is around zero since sodium and potassium move in opposite directions down their electrochemical gradients. - Channel is rectified. Closes more quickly when membrane is depolarized. Although it makes intuitive sense that sodium and potassium currents will tend to cancel each other out, is it really sensible to expect the potential to go to zero? This can be answered analytically:
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BME 418, Quantitative Cell Biology Alan J. Hunt ( 29 ( 29 ( 29 ( 29 K m K Na m Na K m K Na m Na E E g E E g E E g E E g - + - = - = - = + = EPSP K Na K Na EPSP I I I I I I
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