Zoo Psych Neuro 523 Probset 4 answers 2017 1 What are the similarities and differences between the molecular structures of the voltage gated sodium channel and the voltage gated potassium channel The voltage gated potassium channel has a sequence that predicts 6 trans membrane TM helices One of these S4 includes several positively charged amino acids that are required for sensitivity to membrane depolarization Four of these proteins assemble to form a channel The voltage gated sodium channel has four of these 6 TM domains in the polypeptide chain they are sequence related to each other and are linked by sequences that are cytoplasmic One of these between regions III and IV is responsible for the slow depolarization activated inactivation properties of the Na channel The K channel of the squid does not inactivate Other V gated K channels e g the Shaker channel inactivates due to a region at the N terminus If this region is removed the channel no longer inactivates inactivation is restored when the N terminal peptide is replaced in the cytoplasm It is believed to act as a ball and chain that can plug the open channel 2 A frog muscle fiber has a membrane potential of 90 mV If the internal concentration of sodium ions is 10 mM and of potassium ions is 150 mM and the external concentration of sodium ions is 110 mM and of potassium is 2 5 mM what is the ratio of the sodium and potassium conductance of the muscle fiber Ignore any contribution from chloride First calculate EK and ENa ENa 58 log 110 10 60 4 mV EK 58 log 2 5 150 103 1 mV Then solve the EIE for gNa gK Vm Ek Vm ENa 90 103 1 90 60 4 13 1 150 4 0 087 3 If in the future someone discovers a weird toxin that makes the properties of the voltage sensitive potassium channel in the squid axon identical to those of the sodium channel Now the total sodium and potassium conductance of the axon are identical at all times during the action potential What would the maximum amplitude of the action potential be The action potential would never exceed ENa EK 2 putting gNa gK in the EIE 4 If the resting membrane conductance to potassium ions is increased what effect does this have on the membrane potential of a squid giant axon If gK is increased Vm would go closer to EK i e the membrane would hyperpolarize Inspection of the EIE tells you this 5 How would you change the concentration of potassium ions in the saline solution surrounding a squid giant axon in order to hyperpolarize the membrane To hyperpolarize the membrane by changing potassium concentration you would need to decrease the external concentration of K