1 3 2 Neurons Action Potentials and Cable Properties BIO 361T Fall 2014 1 If the axon membrane decreases its number of leak channels by 4x but otherwise stays the same how does this affect the distance that an electrical signal can travel purely by electrotonic conduction 4x fewer leak channels 4x more resistant Rm According to the equation for this will double the length constant The signal will be able to travel twice as far 2 Why is the length constant particularly relevant in myelinated axons Nodes of Ranvier must be less than one length constant apart otherwise the voltage gated Na channels at the next node will not be brought to threshold and the action potential will fail to be reinitiated 3 Why are neurons with small time constants less likely to reach threshold via temporal summation of graded potentials They will decay before they get to the axon hillock where summation occurs 4 Why do giant axons like those in the squid which can be 1 cm across have higher conduction speeds than those of smaller diameter sometimes 1000x smaller Because there is more space for the current to flow through the axon so Ri decreases which increases There is also more space for it to cross the membrane so Rm also decreases but a factor slower as it is related to surface area not volume 5 Draw an action potential with time on the x axis and Vm on the y axis Draw dashed lines at the following potentials resting potential threshold potential and Keq for K and Na Label the following events depolarization repolarization hyperpolarization absolute and relative refractory periods voltagegated Na channels open close and inactivate voltage gated K channels open and close and Na and K flow into and out of the cell respectively 2 6 What would happen at 30 mV if voltage gated K channels did not open Draw an action potential in which voltage gated K channels fail to open The membrane would very slowly repolarize back to rest via leak channels and the Na K pump Closing Na channels does not repolarize the membrane it just stops depolarization 7 Electrotonic conduction is bidirectional a change in voltage will spread in both directions along the membrane However action potentials are unidirectional they travel only from axon hillock to axon terminal How does the axon ensure unidirectionality of an action potential The refractory period Because voltage gated Na channels inactivate the potential cannot flow backwards to a recently depolarized patch of membrane
View Full Document
Unlocking...