BISC 230 UT Readings

BISC 230 UT Readings - , First,, system,...

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Despite the enormous complexity of the brain, it is possible to obtain an   understanding of its function by paying attention to two major details: First, the ways in which individual neurons, the components of the nervous   system, are wired together to generate behavior. Second, the biophysical, biochemical, and electrophysiological properties of   the individual neurons.  A good place to begin is with the components of the nervous system and how the  electrical properties of the neurons endow nerve cells with the ability to process and   transmit information. 1.1  Introduction to the Action Potential Figure 1.1 Hover over the  colored circles (light  stimulus) to activate. Important insights into the nature of electrical signals used by nerve cells were  obtained more than 50 years ago. Electrodes were placed on the surface of an optic  nerve of an invertebrate eye. (By placing electrodes on the surface of a nerve, it is  possible to obtain an indication of the changes in membrane potential that are  occurring between the outside and inside of the nerve cell.) Then 1-sec duration  flashes of light of varied intensities were presented to the eye; first dim light, then  brighter lights. Very dim lights produced no changes in the activity, but brighter lights  produced small repetitive spike-like events. These spike-like events are called action  potentials, nerve impulses, or sometimes simply spikes. Action potentials are the  basic events the nerve cells use to transmit information from one place to another. 1.2  Features of Action Potentials The recordings in the figure above illustrate three very important features of nerve  action potentials.  First , the nerve action potential has a short duration (about 1  msec).  Second , nerve action potentials are elicited in an all-or-nothing fashion.  Third nerve cells code the intensity of information by the frequency of action potentials.  When the intensity of the stimulus is increased, the size of the action potential does  not become larger. Rather, the frequency or the number of action potentials  increases. In general, the greater the intensity of a stimulus, (whether it be a light 
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stimulus to a photoreceptor, a mechanical stimulus to the skin, or a stretch to a  muscle receptor) the greater the number of action potentials elicited. Similarly, for the  motor system, the greater the number of action potentials in a motor neuron, the  greater the intensity of the contraction of a muscle that is innervated by that motor  neuron. 
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