Zoo Psych Neuro 523 Probset 5 answers 2016 1 An axon has a radius of 20 micrometers an internal resistivity of 50 ohm cm and a specific membrane resistance of 2 500 ohm cmE2 If its membrane is hyperpolarized by 30 mV at a discrete point at what distance will the magnitude of the hyperpolarization fall to 10 mV Lambda sqrt a Rm 2 Ri remember units are cm sqrt 20 X 10 4 X 2 500 2 X 50 cm 0 22 cm V Vo e x lambda so ln V Vo x 0 22 ln 10 30 x 0 22 1 099 x 0 22 x 0 24 cm 2 The inside of a squid axon is treated with pronase a protease that destroys the sodium inactivation mechanism but leaves the rest of the sodium channel intact What happens when the axon is brought above threshold and why The membrane potential will depolarize towards Ena The amplitude of the AP will be a bit higher than normal because the repolarizing phase usually has 2 components the turn off of Na current due to Na inactivation and the turn on of K current If the Na current does not inactivate then the falling phase of the AP will be longer than usual 3 Why does the myelin sheath make the action potential propagate faster Myelin has 2 effects one on the membrane resistance of the fiber and the other the capacitance Resistance goes up and capacitance goes down The effect of the increased resistance is to make it harder for current to leak across the membrane so the spread of inward current from the axonal membrane at an active node of Ranvier goes further down the fiber through the axoplasmic resistance Lambda is increased The decrease in capacitance means that the charge brought in from the current flow at the active node does not get mopped up very much in charging up the membrane capacitance of the myelin sheath Both of these effects lead to there being more charge available to bring the membrane of the next node to threshold more quickly than if the myelin is absent 4 An invertebrate photoreceptor has an axon that is 2 cm long with a diameter of 60 micrometers This axon does not fire action potentials What properties of the cell might enable the sensory information to reach the CNS The fact that this is a photoreceptor is incidental to the issue The question is how relatively long distance signaling can occur without action potentials The answer is that if the Rm is increased a lot lambda is increased a lot and current is funneled down the axon as per the role of myelin discussed above This example is a real one Barnacle photoreceptors signal to the CNS without APs and they are specialized by having extremely high values of Rm