Zoo Psych Neuro 523 Answers to Practice questions for exam 3 2016 la At a neuromuscular junction the average mepp amplitude is O 5mV When the nerve is stimulated 50 times the average response per stimulus is 1 15 mV How many times did neuromuscular transmission fail m 1 15 0 5 2 3 pO e m 0 1 nO N pO 5 lb If the nerve to a neuromuscular junction is stimulated 100 times no response is seen 10 times What is the quantal content This is another version of the same question just to give you practice in handling the math p O nO N 0 1 ln pO m so m 2 3 2 The end plate potential is affected in different ways by d tubocurarine curare and eserine an acetylcholinesterase inhibitor Why Does TTX affect the responsiveness of a vertebrate muscle cell to acetylcholine Please explain your answer Curare blocks the ACh receptor so the endplate potential is reduced in amplitude or abolished Eserine by blocking the ACh esterase reduces the rate of removal of ACh from the synaptic cleft after release diffusion is now the only mechanism that gets rid of ACh The EPP is larger and prolonged TTX does not affect the responsiveness of the muscle to ACh but prevents the generation of an action potential by blocking the voltage sensitive sodium channels 3 Two different cholinergic synapses are stimulated One is inhibited by atropine Explain Where would you expect to find synapses with these properties The two major categories of ACh receptors are nicotinic and muscarinic receptors Muscarinic receptors are blocked by atropine but nicotinic receptors are not Muscarinic receptors are found in cells innervated by parasympathetic cholinergic neurons 4 When an inhibitory pathway is stimulated a hyperpolarizing PSP is seen in the postsynaptic cell The inhibitory mechanism is due to an increase in chloride conductance How would you make this same pathway cause an action potential to fire in the postsynaptic cell Give a brief explanation If ECl is changed so that it is more positive than threshold an increase in gCl will be excitatory ECl can be made more positive by injecting chloride into a cell 5 Analysis of purified synaptic vesicles show that compounds X Y and Z are present When the synapse is stimulated and perfused with the aim of identifying the released compounds only X is found Suggest three possible explanations A X is a soluble component inside the vesicle and Y and Z are in the vesicle membrane X would be released and could be collected but Y and Z would stay with the presynaptic membrane and then be re internalized for recycling B X Y and Z are all soluble components and are released but Y and Z are removed by uptake or destruction e g by hydrolysis and cannot be detected even though they are released C Y and Z are present in different vesicles than X and need different stimulation parameters to be released e g high frequencies 6 How would you measure quantal content and quantal size at a neuromuscular junction If you selectively opened non voltage sensitive K channels in the muscle cell membrane with that newly discovered toxin from the Loch Mendota Monster how would this affect quantal content and quantal size Please explain your answer briefly There are 2 methods for measuring quantal content One is to stimulate the nerve multiple times and measure the amplitude of the PSP Take the average response and divide it by the average amplitude of the mepp which is quantal size The second is to count the number of failures zero responses From the first term of the Poisson distribution Po e m so ln Po m These methods only work if the quantal content is low so you can see the fluctuations in the responses The toxin will not affect quantal content since it only opens channels in the muscle the postsynaptic cell It will decrease quantal size because each quantum turns on a certain number of ACh gated channels and a certain amount of postsynaptic current will flow The depolarization caused by this current depends on the input resistance of the postsynaptic cell Since the toxin is opening K channels it will reduce the muscle input resistance and the quantal size will go down