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How Neurons Work

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University of Toronto St. George
Martin Ralph

Lecture Three The question we will focus on today is: What is the system for the mechanistic change of our information? The neuroscience will not be on the test (that much). The sections on LTP and LTD are really important however and we need to understand more about these two than was mentioned in the textbook. So refer to slide notes and lecture notes to understand these two processes the way the professor wants. What is the mechanism that allows for learning and memory to take place? Anatomy of the neuron should give us more information about the way we learn and memorize. In general, higher organisms (with a very developed central brain) tend to encode information not based on just the frequency of signals. It is less so in more simple systems where there are fewer neurons. The opportunity to do what the mammalian brain does not exist in any other specie (or computer, to a degree). The systems we use to learn, to encode, to do everything, relies more so on code than frequency of signals. The process of neural firing We must first understand that a single neuron passes an all or nothing response. A working neuron has a potential difference across the membrane that is maintained down the axon. It uses this difference within the axon (which involves the separation of charges) and by changing the permeability of the membrane (usually this is done in a coordinated fashion), the neuron has the capacity to do work (learn, encode, etc). This is the mechanism of the propagation of signals. But what is the mechanism for receiving and responding to input? The axon is a tube with the inside separated from outside. It has holes in it (channels) that can be controlled (this allows ions to cross, which allow changes in membrane potential, which allows for changes down the axon to occur and an action potential to be fired). The separation of charge. This separation really only means that different types of ions are concentrated on one side of the membrane and one on the other side of the membrane. This function is mostly performed by Sodium and Potassium ions. By separating charges you create diffusion potential in both directions. The membrane potential is based on the fact that you have equal positive charges on the outside and inside of this membrane but on both sides you have a particular type of ion that wants to diffuse across. There is a tendency or probability that potassium, given the opportunity, will cross form the inside to the outside and there is a probability that sodium, which is concentrated on the outside, will push its way in. Its the electrostatic charge that holds those ions back (that we measure as the potential to do work, the potential difference across the membrane). In the case of the neuron in general, the pressure, the probability that these positive charges will move out is positive because of the higher concentration and the fact that they can move more easily across the membrane (more easily than the sodium ions can move back in). They are carrying and providing most of that potential difference. It takes more electrostatic charge to get potassium in than to keep sodium out (when everything is at rest). This leaves us with a tendency to be negative inside versus the outside. We tend to see a negative internal cytoplasm and a positive outside. This can change. Propagation of a signal down a neuron changes the permeability of the membrane to one or the other of these ions. The membrane separates charges, the charges have to be kept in place by electrostatic charges which produce the potential to do work, which is to move ions. The formula (on the slide) represents the difference in concentrations of each ion on the inside and outside and what it takes to keep those ions where they are. The voltage (overall) that one measures is the function of the difference inside and outside (mainly potassium). Understand what is going on in this equation. How the membrane potential is produced. How does it transmit a signal? The difference between the neuron and its membrane and just a plain wire is that when you put a voltage at one end of a wire you can detect it almost instantly at another end. But it takes a while for a neuron to charge up and thats because the membrane is a capacitor. It doesnt allow all charges to go across. It then takes a while to switch of the voltage. When the voltage reaches a certain threshold something else happens in the membrane. The permeability of the membrane changes so that different ions can move through with different probabilities. In the beginning of an action potential we see that it starts of with a positive feedback effect. You make the inside of the cell more positive and then at some point the sodium channels open up because of that and more sodium can potentially move in. This creates an even more positive charge. This causes the rising charge for an action potential. Very quickly after that the sodium channels are deactivated and potassium channels open up, bringing the membrane potential back down. There is then a refractory period. It is there to allow the channels to reset so that they may be open again. Why does the action potential tend to move in one direction once it is propagated? It can be that an action potential goes both ways but usually, once it is initiated it tends to move in one direction and not spread out. As the action potential brings in positive charge to the axon, making the potential more positive inside and opening sodium channels so that sodium comes in and raises this potential even further. As this AP moves, this positive charge can be distributed along the membrane. It is moved because the next region of the membrane has become more positive. The region we leave is becoming more negative because the sodium channels are being deactivated and also because the potassium channels are being activated to bring the membrane potential down. The AP wont reverse since there are deactivated sodium and highly negative region behind it. The positive charge brought in by sodium ions opens up the potassium channels. As they open up it increases potassium influx. Driven by sodium, the AP moves through the axon. Cant have action potentials right on top of the other. We have to wait for the channels to reset before a second AP can be propagated down the axon. The myelin sheath directs the flow of the charge down the axon instead allowing the charge to contribute to the capacitants. So basically, the AP does not charge the membrane (the capacitant). You have positive nodes between two Schwann cells (these are wrapped around the axon). Sodium can come in and potassium can come out along the axon except the channels are concentrated (in one area) and the movement of charge and change of potential happens in these nodes. As the node depolarizes the inside of the axon becomes positive and that positivity spreads down the axon instead of to the membrane. This will influence sodium channels in the next node. The previous node has potassium channels which open up and repolarise that part of the axon and just like before the sodium channels are deactivated. This bounces down the axon. This form of firing has an increased speed at which the signal moves through the whole cell. Hence the myelin sheath is like an insulator (it prevents the membrane from absorbing the charge, thus propagating a signal faster). Why is the myelin sheath important for healthy brain function? There are some cells that require very fast transmission. Any cell that is involved in cortical activity, where information has to be sent and received from a variety of difference places over distances through the brain requires fast transmission in order to keep information coordinated. If it the signal firing is slow, information will be delayed and reaction to events or situations will be slow. This process (explained above) is what gets information from one place to another. We need to remember that the function of one cell is simply to take a piece of information, something that contributes to all the information, and take it to another place in an all or nothing fashion. There is a way to control if a cell is to do this at any particular time. Cells arent always active or there would be confusion in the brain. Controlling the way a cell moves information One way to regulate/control this process (so that the neuron does not act just like a cable) is to actually integrate multiple signals. There are two ways of integrating signals. o (1) One is through spatial summation: this is when the signals coming in and being detected by the dendrites, will each contribute to a very small potential difference at these points/synapses. They are not enough to trigger an action potential alone. Spatial summation means that whatever happens in two places at the membrane can add up. In fact it multiplies. So you change the potential in one place and at the other and then those will summate to produce a large signal (by the time they get to the beginning of the axon). So what arrives at the axon hilla is really a summary of all the different inputs that have been
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