Week 1, Lecture 5

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Computer Science and Engineering
CSE 528
Rajesh Rao

What is a Synapse? - When the spike (action potential) reaches the end of an axon, we meet the synapse - A synapse is a “connection” or junction between two neurons o Electrical synapses use gap junctions  Very similar to the connections in your technology: they allow for activity to be propagated from neuron A to B  These are essentially ionic channels spanning the membranes of both neurons A and B  If you have excitation on one side, and a higher concentration of Na is on that side, it can flow through the gap junctions  These are typically found when you need neurons to synchronise, because they’re very fast  The other case would be an escape reflex, for example that of the crayfish o Chemical synapses use neurotransmitters  Bags of neurotransmitters, known as vesicles, fuse with the membrane of a cell when a spike comes in, which causes the release into the synaptic cleft (the gap between the two neurons) of the neurotransmitters  When the neurotransmitters bind to the channels, they’re going to open them.  Suppose that they allow Na to enter – this is going to cause a change in the membrane potential. This is both electrical and chemical  Why did evolution go to the trouble of constructing something so complicated as a chemical synapse when an electrical synapse works?  A possible answer: a synapse allows you to change the number of inputs allowed into Neuron B by changing the number of ionic channels or the density or ionic channels  If you want to decrease the amount of possible excitation, you could remove some of the ionic channels  This is why chemical synapses have been suggested as the basis of learning and memory Q1 True or False: a chemical synapse is used for smooth coordination of several neurons, whereas the electrical synapse is used for applications in which a customized amount of current transmission is desired between neurons upon spiking, such as in building memories. - An overhead picture of a neuron can look like an overhead shot of a city at night o A cortical neuron can have up to 10000 synapses - Synapses can be Excitatory or Inhibitory o Increase or decrease postsynaptic membrane potential  The neuron passing the information along is the pre-synaptic neuron, the one receiving it is the post-synaptic neuron o An excitatory synapse increases postsynaptic membrane potential (tends to excite the post-synaptic neuron)  Input spike  Neurotransmitter release (e.g., Glutamate, which is an excitatory neurotransmitter)  Binds to ion channel receptors after + release into synaptic cleft  Ion channels open  Na influx (if they are sodium selective channels)  Depolarization due to EPSP (excitatory postsynaptic potential) o An inhibitory synapse decreases postsynaptic membrane potential (tends to supress the post-synaptic neuron)  Very similar, just a different neurotransmitter and different channel
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