2 1 Neurons Electrochemical Gradients BIO 361T Fall 2014 1 Neurons vary widely in their cellular anatomy Specialized structures are reflective of specialized functions Match each specialized structure left to its function right I Large number of dendrites iv i Neural pathway with high resolution II Single dendrite i ii Source of input far from target tissue III Extremely long axon ii iii Neuron controls a set of target cells IV Axon branching with multiple axon iv Neuron integrates input from many sources terminals iii 2 mRNA transcripts are produced only in the nucleus located in the soma of the neuron Many neural proteins are also produced in the soma but some are produced in the axon terminal Which of the following statements explains this phenomenon a Transcription requires access to the DNA genome which is in the nucleus b Translation requires ribosomes which are not restricted to the soma c There is a nucleus like structure in the axon terminal d Protein packaging occurs in the endoplasmic reticulum which is spread throughout the neuron 3 Label the effect of each of the following as either an increase or decrease in Vm increase cations entering the cell decrease cations exiting the cell decrease anions entering the cell increase anions exiting the cell 4 If extracellular K 5 mmol L and intracellular K 150 mmol L calculate the Keq Keq 61 mV 1 log 5 150 61 log 0 033 90 mV 5 Use this Keq for K to predict which way K will move into or out of the cell at each of the following voltages List what gradient the ion is following electrical chemical or both 120 mV in electrical 40 mV out chemical 0 mV out chemical 50 mV out both 6 Use the Nernst equation to give a mathematical explanation for each of the following statements For a cation that is more concentrated inside the cell Keq will always be negative Log of number less than 1 is negative For a cation that is more concentrated outside the cell Keq will always be positive Log of number greater than 1 is positive Rewrite the two statements but for anions Change cation to anion and switch negative and positive 1 2 7 The equilibrium potential Keq for an ion at given concentrations is also called its reversal potential Explain why Ion flow will switch directions at this voltage where the balance between the chemical and electrical gradients also switches direction 8 The Nernst equation is used to calculate the membrane potential necessary to counteract a specific chemical gradient given particle charge and concentration on either side of the membrane We do not recalculate Keq as ions move according to their electrochemical gradients This would result in constantly recalculating Keq every time a few ions moved Why is our original calculation sufficient Because the chemical gradients concentrations are basically unchanged We are looking only right around the membrane The ions that are moving are a very small proportion of the ions present in the system 9 Use the GHK equation to calculate Vm using the concentrations in 4 if the membrane is impermeable to Na and Cl Same as Nernst because P for Na and Cl will be 0 Relative permeability for K is 1 What do you notice about Vm Same as Keq for K What would Vm be equivalent to if the membrane were impermeable to K and Cl Keq for Na 10 The membrane potential of a typical neuron at rest is 70 mV Look at the GHK equation and explain why this value is much closer to the Keq of K than Na Relatively more permeable to K than Na at rest so that chemical gradient contributes more Keq for Na is 61 mV 11 The Na K ATPase pump counters the effects of the passive Na and K channels in the neuron membrane Without this active process Vm would eventually become 0 mV Explain why it would not happen immediately as well as why it would happen eventually K would go out until Vm 90 mV but Na is also continually going in opposing this change Thus as long as there are strong chemical gradients for both Vm never reaches Keq for either ion neither ion ever reaches equilibrium because their movements oppose each other Na will constantly leak in and K will constantly leak out resulting in a resting potential of about 70 mV However eventually chemical gradients will be depleted and original Nernst calculations no longer apply because now there are different out and in values As the in and out concentrations for each ion approach each other Vm goes to 0 mV because log 1 0 Would also stop losing net 1 due to 3 Na out for every 2 K in but pump does not contribute much to the difference in charge just a few mV This is the mechanism of the neurotoxin ouabain which selectively binds this ATPase and inhibits it
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