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Biology and Neurology of Neurons

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Martin Ralph

Lecture Two Today we will consider the biology and neurology of neurons. There are tons of neurons in the brain. They are the basic components that define the limits of our brain. If we knew nothing about how a neuron works we would make different assumptions than what we make today (in regards to how the brain works). The smallest neurons in the brain are in the hypothalamus. Hypothalamic neurons tend to be simple. Cortical ones are usually complex The neuron has a dendrite field that picks up information and interpretative field that integrates the information and sends it down. Neurons in the brain can have fast dendrite fields. The shape of the neuron tells us what its function is. Neurons are able to integrate massive amounts of signals into one package. Simple neurons cannot integrate very much because they have a more focused function. Hypothalamic neurons for example tend to be concentrated in nuclei and the nuclei have very specific functions (maintaining of the organism, hormonal levels, ionic levels, etc). So they are much targeted in their function. Learning and plasticity is not something that needs to be maintained on a simple level but something that requires structural change of sorts. How neurons transmit information There are four parts and anatomy tells us quite a bit about the functioning. We have a place were information can enter the cell through various sources (sensory, other neural information). The cell is built this way in order to integrate the vast amount of information that comes in. It will pass an electrical pulse down the axon. This is an all or nothing process. It may diverge into multiple targeted areas. These signals are brought through the dendrite membrane into the cell body where the signals simply add up. There is a signal generation point in the hilla of the neuron that initiates an action potential. That is propagated to the targets. How is this done? Do neurons have cable properties? Consider a wire, just an electric wire; it is a cable of some sort. It carries information. If that information is organized it carries electrons down the wire. One basic property is that if you apply a direct voltage at one end you will detect it at another. However there will be a loss of signal the longer the wire is (the longer the wire, the smaller the voltage at the other end). It is a direct current. Because things are moving there is a loss of energy all the way down the wire at the other end. You will lose the amplitude of the signal. Neurons do not work this way but they are cables.They take a signal and send it along the axon. We can measure the difference in an experiment by putting the voltage by one end of a neuron and measure the result at the other end. A neuron doesnt look like a wire in this case. The axon itself is a tube. That tube is made up of a cell membrane. Cell membranes are lipid bi- layers. They naturally form these bi-layers by keeping water and therefore other soluble substances outside the bi-layer. This produces a biological capacitor. Built within the structure (it is not bonded, it is electrically bonded together, weakly) but with lots of components it becomes very strong and can create spheres of these lipid bi-layers. My-cells. There are structural components that help support the actual tubular structure of this axon. Built into the axon are pores, they are the active component within the axon itself and are critical for propagating a signal down the axon. There is ion selectivity in these channels (pores). The main thing that makes the axon work is the fact that this structure can be used to set up a reservoir of energy that can do work. It does it by producing a difference in the electrical potential between the inside and the outside of this tube. The difference in potential is based upon the different concentrations of ions (ionize proteins) of outside versus inside the cell. That potential can be used by the cell to do work. In transmitting the signal it is working. It stores energy by producing this potential difference between inside and outside and uses it by discharging that potential when needed. What we need to know is what this difference really
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