Neuron Physiology: Ouabain, Action Potentials, and Ion Conductances, Assignments of Introduction to Sociology

Download Neuron Physiology: Ouabain, Action Potentials, and Ion Conductances and more Assignments Introduction to Sociology in PDF only on Docsity! BIPN 140 2006 Problem Set II Question 1a The toxin ouabain has been used for hunting purposes in Africa. The source of ouabain is the ouabaio tree (Strophanthus gratus). At sufficient dosage, it completely inhibits the sodium/potassium pump (ATPase). What happens if you apply ouabain to a neuron? Explain. The Na+/K+ ATPase pumps Na+ ions out of the cell and K+ ions into the cell against their gradients. After application of ouabain the resting potential slowly disappears because the ion concentration gradient required for the membrane voltage dissipates as the ions diffuse down their respective concentration gradients. Question 1b Would you expect the loss of membrane potential to be faster or slower if an action potential were triggered in the neuron once the ATPase was inhibited? Present your logic. Faster. The AP brings Na+ ions into the cell which contribute to the dissipation of the concentration gradient. Question 2a Draw the corresponding activation curve (conductance as a function of membrane voltage) for the conductance shown in the I-V-plot below. -50 -40 -30 -20 -10 0 10 20 30 40 50 -100 -50 0 50 100 150 Vm [mV] I [ pA ] Question 2b Draw the corresponding activation curves for the Na+ and K+ conductance given in the I- V-plot below. Rising, overshoot, falling, and undershoot. Depolarization of cell triggers voltage gated Na+ channels to open. Inward current of Na+ influx. Positive feedback. Overshoot occurs. Then, there is an outward current of K+ ions. Cell is hyperpolarized. Refractory periods occur. Question 11 You have a neuron with a resting potential of -55 mV, EK of –70 mV, and ENa of 50 mV. You decide to voltage clamp the axon. Now, draw the membrane current over time. What will the responses look like? Explain why. a. Cell depolarized to -10 mV. There will be an initial negative (inward) current due to rapidly initiating sodium influx, followed by a prolonged positive (outward) current due to slowly initiating potassium efflux. b. Cell depolarized to +40 mV. There will be an initial negative current due to rapidly initiating sodium influx, followed by a prolonged positive current due to slowly initiating potassium efflux. The negative current will be smaller (reduced driving force for Na+ since closer to equilibrium potential) and the positive current will be greater than in (a) because of increased driving force for potassium. c. Cell hyperpolarized to -100 mV. There will be no voltage-activated sodium or potassium current; the channels remain closed. Because –100 mV is negative to EK, potassium ions will move into the axon (inward current) through voltage independent, “leak” potassium conductances to restore the membrane potential. Question 14 What is the term “n” in the following equation? gK=gK(max)*n4. What two variables affect n? What is the assumption underlying raising n to the 4th power? Variable n is the probability that the “n-gate” of the potassium channel is open, and varies between 0 and 1, depending on voltage and time. The power of 4 reflects the assumption that there are 4 independent gates in a potassium channel and that each of the gates has to be open to allow K+ flux. Hodgkin and Huxley found that the power of 4 best describes their experimental data. Question 15 The sodium channel has two variables that describe its gating behavior. Explain. Variable m describes the activation kinetics for the Na+ channel. Variable h represents inactivation of that channel. Both variables are time and voltage dependent. H&H found that three independent activation gates and one inactivation gate fits best their experimental data from the squid giant axon. Question 16 Why is the squid giant axon so big in diameter? Speculate which behavior could be mediated by this axon. The squid giant axon is the very large (approximately 0.5 to 1 mm in diameter) axon that controls part of the Atlantic squid's (Loligo pealei) water jet propulsion system. Squid use this system primarily for making brief but very fast movements through the water usually when escaping predators. Between the tentacles of a squid is a siphon through which water can be rapidly expelled by the fast contractions of the body wall muscles of the animal. This contraction is initiated by action potentials in the giant axon. Action potentials travel faster in a larger axon than a smaller one, and squid have evolved the giant axon to improve the speed of their escape response. This has obvious adaptive advantage when escaping from predators. [from Wikipedia]