cellular physiology exam
Cellular Physiology Test 1. The membrane potential of a red cell is -14.3mV. The concentrations of K +, Na + and Cl- on both sides of the membrane are as follows: • [K +] o = 4.5mM; [K +] i = 140mM • [Na +] o = 145mM; [Na +] i = 11mM • [Cl-] o = 116mM; [Cl-] i = 80mM Initially, the membrane is permeable only to Na + ions and K +. a. What is the potential of the cell at rest? (2 points) b. What is the reversal potential of Cl- for this cell? (2 points) c. Will the membrane become more positive, more negative, or will no change occur if it suddenly becomes permeable to Cl-? Explain your answer (2 points) 2. A colleague tells you about an interesting mutation in the voltage-gated Na + channel inactivation gate. You notice that the mutation involves several amino acids associated with the pore region of the canal. Upon reflection, you realize that these changes are likely to make the inactivation gates "voltage-dependent". Besides, you think the inactivation gates will now be activated (in other words, opened) by hyperpolarization. (4 points) (a) (2 marks) draw a graph showing the predicted conductance for this gate according to membrane potential (labeled carefully). (b) (2 marks) Assuming that the mutant inactivation gate is completely deactivated (closed) at the resting potential, what would be your prediction of the effects of this mutation on the action potential (relative to an action potential of a "normal" neuron). Use a diagram if you prefer. Explain your reasoning. 3. You successfully record action potentials of an unknown neuron in an octopus. In order to determine the ionic basis of these action potentials, you consider your knowledge of "classical" action potentials. You decide to investigate how the profile of the action potentials will be modified according to a gradual decrease in the concentrations of intracellular K + ions ([K +] in) and surprisingly, you do not see any change in the profile of the action potentials. A colleague (who is not a physiologist) does not understand your reaction. Explain in detail the reasons for your surprise based on your knowledge of classic action potentials. (5 points) 4. The figure shows an action potential in a cell of a recently-discovered species of fish as well as the concentrations of ions in intracellular and extracellular fluids. What conclusion can be drawn from the changes in membrane potential? Explain your reasoning. (4 points) (note: there is no error in the graph!) [Na+]E = 120mM ; [Na+]i = 12mM [K+]E = 5mM; [K+]i= 125mM [Cl-]E = 125mM; [Cl-]i = 10mM 5. Draw the profile of the action potentials according to the different scenarios in the given spaces. Do not forget to label the axes, indicate the membrane potentials (at rest, at the plateaus, etc.). Use the action potential you drew in Scenario A as a reference point to indicate changes in the other scenarios. (2 points each; 6 points) Scenario A Typical action potential [K+]o = 4.5mM ; [K+]i = 140mM [Na+]o = 145mM ; [Na+]i = 11mM Threshold = -55mV Senario B [K+]o = 7mM ; [K+]i = 140mM [Na+]o = 145mM ; [Na+]i = 11mM Threshold = -55mV Senario C [K+]o = 4.5mM ; [K+]i = 140mM [Na+]o = 145mM ; [Na+]i = 11mM Threshold = -55mV Pump inhibition 3Na+/2K+ 6. Some human neurological diseases cause loss of myelination. What effect does this loss have on the length constant of these axons? Describe an experimental manipulation (not involving myelination) that would compensate for this effect on the length constant? Explain your reasoning. (4 points) 7. You record the membrane potential in the muscle of a grasshopper to determine the neuromuscular junction. You can generate excitatory postsynaptic potentials (PPSE) in the muscle by electrically stimulating a presynaptic motor neuron called Mn1. To determine the ionic basis of PPSEs, you systematically monitor the membrane potential of the muscle and find that the PPSEs induced by Mn1 stimulation 'reverse' at a membrane potential of +20 mV (i.e., the post synaptic potential becomes hyperpolarizing following stimulation of the Mn1 neuron when the potential imposed on the muscle is greater than +20mV). Suggest an ionic mechanism for the PPS evoked by Mn1. Then, describe an experiment that would allow you to test your suggestion. Explain your reasoning. (4 points) 8. The chromatophores of fish scales balance the transport of pigment granules between the plus and minus ends of the microtubules in order to regulate their color. You have discovered a mutant zebrafish unable to regulate its color normally and is almost always pale. Based on your knowledge of transport along microtubules, list 4 possibilities that could explain this observation. (8 points) 9. You determine the "stepping behavior" of a recently discovered myosin isoform. You find that the steps involve a random combination of forward and backward steps. The backward steps have an average length of 36 nm. However, the forward steps are either 36 nm or 72 nm. It is important to note that all three lengths occur with equal probability. (a) Using a diagram, note the expected distribution (i.e. histogram) of molecular motors given the behavior of the steps (stepping). Be sure to label your diagram correctly and consider all of the information provided. (4 points) (b) State a hypothesis that might explain the observed progression behavior. Explain your reasoning. (4 points) 10. You have studied the honey bee's olfactory system, but you are increasingly frustrated with the results of the experiments. Sometimes you observe a strong behavioral response to a brief exposure to a particular scent, and other times, no response to that same scent. After months of research, you identify a signaling network in an olfactory receptor cell that involves three molecules (A, B, and C); the concentration of C varies cyclically with time, as shown in the figure below. As you do not know of any oscillatory input in this network, the temporal variation of the concentration of C must therefore be produced by the interactions between A, B and C. To your great satisfaction, you find that C inhibits a cationic channel dependent on the cAMP which is abundantly expressed by this recipient cell. (a) Using a diagram (with specific labels), propose a set of interactions between A, B and C that are compatible with the observations. Next, describe two properties of these interactions that may influence the changes in the concentration (oscillation) and / or the magnitude of the concentration of C. (4 marks) (b) Now explain why “C-concentrations” might be the cause of your varying behavioral responses. Be specific. (4 points) (c) With this new knowledge of the influence of C on odor transduction, you discover a drug that blocks the effects of C on the "cAMP-dependent cation channel". You apply the medication and find that, as expected, the behavioral responses are constant with very little variation (i.e., the C levels do not vary). Explain the observation. (4 points) 11. As you determine the neuromuscular junction of the claws muscle of a crayfish, you attempt to record the postsynaptic potential (PPS) in the muscle while electrically and individually stimulating different motor neurons. Unfortunately, the movement caused by the contraction makes it very difficult to get reliable recordings. (a) Based on your knowledge of the excitation-contraction coupling in skeletal muscles, suggest two experimental manipulations that would allow you to measure PPSs in the muscle in the absence of contraction (in other words, when the contractions are blocked). Explain your reasoning. (4 points) (b) In another experiment, you focus your efforts on studying the contractile properties of this muscle. (You are no longer interested in recording PPS in isolation). You observe that the rate of relaxation of these muscles initially decreases and increases when the muscle is repeatedly stimulated (see the figure of muscle tension as a function of time). You determine that this observation is a property of the muscle, not of the synapse, because other studies have not shown evidence for a role of synaptic plasticity in this response. Suggest a mechanism that could explain the dynamics of the contractions of these muscles. (4 points) 12. A microscopic study revealed the presence of 2 new amino acid-based substances (therefore peptides) in the terminal of the nerves which control the adrenal tissue of rainbow trout (the equivalent of the adrenal gland in frogs, reptiles, birds and mammals). To do this, the researchers used two antibodies that were previously produced to detect the neurotransmitters called VIP and PACAP in mice. In addition, multiple studies in mice and rats have determined these neurotransmitters and have shown that VIP and PACAP interact with the same receptor, but with different affinities (PACAP has 10X more receptor affinity than VIP). While the authors of the study drew the conclusion that the substances detected in the nerves of the trout was indeed VIP and PACAP, they did indeed opened the criticism that the conclusion was premature due to the likelihood that the antibodies used have formed nonspecific bonds with proteins and that further experiments are needed to confirm the "neurotransmitter" nature of these substances. If the two substances discovered in the nerves (on the adrenal gland) of the trout are indeed neurotransmitters, suggest two predictions, and for each an experiment, to test the hypothesis that the VIP and PACAP are neurotransmitters involved in the secretion of the adrenal tissue adrenaline in trout. (6 points) -20 mV+55 mV-63 mV-80 mV