ࡱ> 02/ bjbj,, ( NN RTTTTTT;@TTiRRVT*Ͷw: F 0 {{** k:    Simulation: the WAVEnerve message transmission The purpose of this simulation is to visualize the phenomena that occurs as a nerve message travels from the dendrites of a neuron to the axon and synapse with the next nerve or with an effector. Recall that the cell surface membrane allows diffusion of molecules and ions down a concentration gradient from high concentration to low concentration of that ion/ molecule. It is also true that ions will move to equalize electrical differences so that the positives equal the negatives. Recall also that cell surface membranes are able to perform active transport of ions and molecules against the gradient. This process costs the cell ATP. What you may not know is that the cell surface membrane has a resting state (normal) that is negative inside and positive outside. In order to maintain this state the cell surface membrane constantly pumps sodium ions (Na+) out of the cell by active transport. The sodium ions are constantly trying to get in by diffusion. The cell surface membrane is not very permeable to sodium ions so that at least helps the cell maintain itself. The negative charge inside the cell is maintained by there being more negative ions especially chloride ions (Cl-) inside than out. There is another ion that is involved in the sensitivity of cell surface membranes. That ion is potassium (K+). In nerve cells potassium is generally kept inside the cell by active transport. Set up your simulation stage according to the diagram. Na Na Na Na Na Na Na Na K K K K K K K K synapse/ Dendrite K K K K K K K K neuromuscular junction Na Na Na Na Na Na Na Na Add the imitation voltmeter and set the dial at 70 millivolts. Note: By movement of positive ions the voltage across the membrane will change. The chloride ions do not move to make this change occur---only sodium and potassium. Since the positives are moving, the relative negativity changes. Script: A stimulus whacks the dendrite changing the permeability of the cell surface membrane momentarily so that sodium floods down both the concentration and the electronegative gradient. (Move sodium into the cell and change the voltmeter dial to +35) Potassium moves out through the membrane along the concentration gradient. (Move potassium out and move the voltmeter downward) Sodium is pumped out of the cell. At the same time potassium is pumped back into the cell simulating the sodium potassium pump. (Move sodium out of the cell and potassium into the cell and continue to move the voltmeter down to 75 and back to 70 millivolts) Now that you have the basic mechanism at the dendrite end make the wave. As soon as the sodium floods into the cell the membrane next to the affected area is stimulated and the action continues on down the cell until the synapse or junction is reached. Once the wave reaches the sequestered neurotransmitter, it is kicked out of the cell into the space (the junction or synapse). The neurotransmitter diffuses across the space and tries to change the permeability of the next membrane too. In essence, three waves are occurring, one after another, in the neuron. The first wave moves sodium into the cell, the second wave moves potassium out of the cell, and the third wave exchanges one sodium for one potassium, returning that part of the cell to the resting state. 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