1 3 1 Physics and Decrement BIO 361T Fall 2014 This activity will focus on the circuit properties resistance conductance capacitance and cable properties length constant time constant of the neuron membrane The cell membrane conducts electricity and changes in permeability affect its ability to do so efficiently 1 Define each of the following terms in as few words as possible a voltage V Difference in charge Defines the electrical gradient across a membrane b current I Flow of charge c resistance R Impedance to flow of charge d capacitance C Storage of charge 2 Ohm s Law a According to Ohm s Law Im Vm Rm Define each variable and explain why Im is negatively proportional to Rm mathematically and conceptually Im current across the membrane Vm voltage across the membrane Rm resistance of the membrane Mathematically because R is in the denominator as it increases I will decrease Conceptually as the membrane impedes the flow of charge less current will flow across it b Rearrange Ohm s Law to illustrate that increasing the resistance of the membrane will increase the voltage across the membrane V m I mR m c How can the cell accomplish this By decreasing the number of leak channels conductance 3 Length constant The following changes will increase Explain why both mathematically and conceptually a Decrease resistance of intracellular fluids mathematically because Ri is in the denominator as it decreases will increase conceptually if the inside of the neuron impedes flow of charge less the charge can go farther b Decrease number of leak channels in the membrane mathematically fewer channels makes the membrane more resistant to ion flow across it Rm Because Rm is in the numerator as it increases so will conceptually if less current can leak out the membrane more will flow through the neuron so it can go farther without decaying 2 4 Time constant The following changes will increase Explain why both mathematically and conceptually a Decrease number of leak channels in the membrane mathematically fewer channels makes the membrane more resistant to ion flow across it Rm Because Rm is in the numerator as it increases so will conceptually if less current can leak out the membrane more will flow through the neuron so it can go for longer without decaying b Increase length of the membrane mathematically more membrane means more Cm which will increase conceptually more space to charge up will take longer both to increase in voltage and decrease 5 Preview for the next lesson The neuron that controls the leg of a crayfish has a diameter of 30 40 m and can send signals at 8 m sec The giant neuron that controls the escape reflex of a squid has a diameter of 50 1 000 m and can send signals at 30 m sec Explain this difference in signal speed Increasing neuron diameter decreases Ri which increases so an electrical signal can travel farther without decaying Rm also decreases but a factor slower surface area vs volume does not change appreciably as the increase in Cm with surface area cancels out the decrease in Rm
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