A rather tame ltxnsp was recently found on Easter Island by a retired neurophysiologist. He reported the following measurements of ECF and ICF concentrations (in millimoles):
ICF ECF
Pr+ 0 150
Na+ ? ?
K+ 10 100
Cl- 109 250
HCO3- 1 10
a) He lacked the proper instruments for determining Na+ concentrations, but was rather certain that no other major ions were present in the ECF and ICF. Can you calculate either concentration for him? (Show all calculations)
Na+ in ICF is 100 (109+1-10) and ECF is 10 (250+10-100-150)
b) He measured Em (membrane potential) as -58 mv at 20°C (K = 58 mv at 20°C). What ions should be in a passive equilibrium across the membrane? Why?
K+ is in passive equilibrium because they it has potential differences of +58 mv.
c) He determined that the rising phase of the action potential in ltxnsp neurons was due to a sudden increase in PK. What should be the sign and approximate amplitude of the voltage change of these action potentials? Why?
d) Which ions are actively transported? Why?
Pr+ because it is impermeable otherwise, Na+, Cl-, and HCO3- because all of these ions have a resting membrane potential different than -58 mv.
e) Suppose that in other ltxnsp cells the HCO3- in the ICF = 10mM, 1mM in the ECF, and is passively distributed across the membrane. What should be the sign and magnitude of their membrane potential (Em)?