1 Quiz 04 Results Question 1: Multiple Choice Average Score 0.41135 points Which one of the following statements about the electrical properties of axons is false? Correct Percent Answered All else being equal, the higher the density of voltage-gated Na+ channels in the membrane of an unmyelinated axon, the faster will be the conduction velocity of the action potential. 9.929% Unmyelinated axons with long (large) length constants will conduct action potentials more slowly than axons with short (small) length constants. 82.27% In a myelinated axon, the capacitance of the nodal membrane is higher than the capacitance of the internodal membrane. 6.383% Along the length of an axon, the electrical resistance of the axoplasm is lower for a large diameter axon compared to a small diameter axon. 1.418% In myelinated axons, nearly all the voltage-gated Na+ channels are located at nodes of Ranvier. 0% Unanswered 0% Question 2: Multiple Choice Average Score 0.43617 points The resting potential of a neuron is -70 mV, ENa = +60 mV, EK = -80 mV, and ECl = -70 mV. What would happen if a synapse onto that neuron was activated that caused a postsynaptic increase in Cl- permeability? Correct Percent Answered Cl- ions would move into the neuron and depolarization would occur. 0% Cl- ions would move into the neuron and hyperpolarization would occur. 8.511% Cl- ions would move out of the neuron and depolarization would occur. 2.837% Cl- ions would move out of the neuron and hyperpolarization would occur. 1.418% There would be no net movement of Cl- ions across the membrane. 87.234% Unanswered 0% Question 3: Multiple Choice Average Score 0.4078 points Refer to the attached diagram (Quiz04 Q3 Fig.jpg). Imagine a synapse in the brain, at which release of transmitter from the presynaptic terminal (Terminal A) opens a channel in the postsynaptic neuron (Neuron C). This postsynaptic channel is equally permeable to Na+ and K+. Now, imagine there is a second presynaptic terminal (Terminal B) that does not synapse directly onto neuron C, but instead onto terminal A. Release of transmitter from terminal B binds to a metabotropic receptor on terminal A that results in a decrease in the opening probability of voltage-gated K+ channels in the membrane of terminal A. For this situation, which one of the following statements is correct? Correct Percent Answered2 Activity in terminal B would affect neuron C regardless of whether terminal A was active or inactive. 4.965% Activity in terminal B would tend to increase the effect of terminal A on neuron C. 81.56% Activity in terminal A would hyperpolarize neuron C. 4.255% Activity in terminal B hyperpolarizes terminal A. 9.22% Unanswered 0% Question 4: Essay Average Score 0 points The tissues of rough-skinned newts (Taricha granulosa) contain large amounts of a deadly neurotoxin, one that irreversibly blocks voltage-gated Na+ channels. Garter snakes (Thamnophis sirtalis) that overlap geographically hunt the newts and eat them. This sickens the snakes but does not kill them. However, garter snakes of the same species, but taken from an area where newts do not occur, will die rapidly if they eat a newt. Provide a plausible hypothesis to explain these observations, using your knowledge of ion channel function and mechanisms of evolution. Given Answers From an evolutionary perspective in order for the garter snakes to have survived in this area with a lot of newts, the garter snakes that survived must have developed an antitoxin to degrade the toxin from blocking the NA channels. Also, they may be able to flush the toxin out quickly. Their liver may be adapted to the toxin and another chemical in their liver could bind to the toxin and inactivate it. Temporaliy NA+ will not be able to enter cells which would cause a hyperpolarizing effect, less likely for an action potential to happen. This is what sickens them. Howver, the snakes may quickly be able to digest and inactivate the toxin therefore survining the influx of toxin. Natural selection explains the ability of the Grater snakes that overlap geographically with newts to eat them but not die. Initially the population of Grater snakes contained individuals that would die from eating newts, and also individuals that had a certain character that enabled them to eat the newts and only get sick. Over generations, the more successful character was passed and became dominant in the population of Grater snakes. The ability of the Grater snakes to eat the newts without dying and only getting sick lies within the mechanism of generating action potential. the neurotoxin can block the Na channels which causes a significant increase in the threshold due to the dysfunctional Na voltage gated channels. it is likely that these snakes rely on another type of channels that enables Na influx and triggers depolarization in response to stimulus. For instance these snakes could rely on non-selective channels for Na influx. Due to the higher driving force of Na, it can outpace the K efflux through these channels and generate action potential. Becuase of the increase in K efflux that will result from this mechanism, the threshold for action potential will be high, this will result in disturbance in the ability of the snakes to control certian physiological functions due to the requirement of more stimuli than in the normal snakes. The snakes that are from a different area died because of exposure to the neurotoxin produced by the newts that blocks voltage gated Na+ channels. Blocking these channels prevents the formation of an action potential, which can affect numerous signaling and communication methods that lead to death. The snakes that are away from this area experienced different evolutionary pressures than the snakes in the newt area; the away snakes did not need to evolve measures to eat poisonous newts (possibly they were not in their diet). Contrastingly, there are strong selective pressures in the area for snakes that have these newts in their diet. These snakes in the area could have evolved more Na+ channels, learned to utilize another pathway for signaling, or evolved protection methods against the specific poison. It is likely that garter snakes that live in the same area as the newts have developed an immunity to death by newt. This would be a result of evolution and adapting to the environment for that population of garter3 snakes. The foreign