Injection of a 5 pA (picoamp) hyperpolarizing current into a spherical cell caus
ID: 56086 • Letter: I
Question
Injection of a 5 pA (picoamp) hyperpolarizing current into a spherical cell causes a 15 mV change in membrane potential. Calculate the input resistance of the cell. What is the expected change in membrane potential for a similar current injection into another spherical cell (made of the same material) that was half the diameter? (Hint: check up formula for the surface area of a sphere). Using this knowledge, explain why cell size affects the threshold stimulus (i.e. magnitude of current injected) needed to initiate an action potential in an excitable cell);in other words, explain the differences in threshold stimulus for a large vs a small cell made of same material.
Answer the following questions:
a) Subthreshold potentials decay in amplitude over distance, but action potentials do not. Explain.
b) What is the effect of increasing the density of Na+ channels on the action potential threshold and action potential peak in the squid giant axon?
c) The recovery or repolarizing phase of the action potential is due to the action of the Na+-K+ ATPase, which essentially extrudes the Na+ that entered, and returns the K+ that left, the cell during the rising or spike phase. Explain whether you agree or disagree with this statement.
d) Two action potentials approach each other from opposite ends of the squid giant axon (say 5 cms long). What happens when they collide and why?
e) In demyelinating diseases such as Multiple Sclerosis, loss of myelin in the internodal regions may lead to conduction block, i.e. failure of the action potential to propagate. Explain.
f) What do you think would happen if the lengths of the intermodal regions in a myelinated axon were made too long?
Explanation / Answer
1. Input resistance of the cell R equal to voltage applied divided by the current flow V/I , V is 5 pico amp , current is 15 so resistance is 5/15 equal to 0.33 .
2. Threshold stimulus ; that causes a neuron to fire over 50 percent and its amplitude depends on the size of neuron because it effects elevtrical resistance of the cell
a . subthreshold potentials ; it is stimulus which is very weak which is very small in magnitude to produce the action potential in excitable cells ,this leads to the depolarization of the memberane it is not enough to reach the threshold voltage there fore subthresh hold stimuli do not elicit action potential .
b. the generation of the action potential requires the sodium influx via voltage dependent sodium channels that drive upstroke of the action potential ,it requires high density of sodium channels at axon intital segment which is matained by the tight anchoring to cytoskeleton ,it causes the mebrane to depolarize which causes the increase in the threshold stimulus which makes the axon to fire ,when potential of the cell is more or higher than the resting potential ,sodium channels close at the action potential ,when the threshold changes to 15mv then sodium channels current dominates
c. yes i will agree with the statement ,repolarisation refers to the change in memberane potential tthat return it to a negative value after depolarisation state of action potential has changed to the positive value ,repolarization state brings the memberane back to resting phase ,the effulx of potassium channels is the fallign phase of the action potential ,the potassium channels selectively of the potasium pore .
d. since the opposite charges attarct each other because of the concentration gradients the excess of the positive ions
e. the failure of the action potential contains the high density of the sodium channels that are requred for the rapid and effiecnt saltatory conditions ,this conditon causes the disruption in axon glial interactions .
f. the excitation and axon potential of the memebrane changes there will be delayed thershold stimulus this furthur lead to rapid depolarization and hyperpolarisation state .