What are the sources of increased variation in threshold for somatic versus axonal spikes?
To examine this question we injected noise into the soma of real layer 5 pyramidal cells maintained in vitro. Then we examined the membrane potential response of this noise injection in the soma and the axon, recorded simultaneously. Close examination of the membrane potential in the axon (red) and soma (black) during noise injection into the soma reveals that they are highly correlated, as expected. However, there are periods of significant difference between the two. These differences can be several millivolts. Three points in which the membrane potential is significantly different in the axon than in the soma are illustrated by the arrowheads. Since action potentials are intiated in the axon and back-propagate into the soma, any membrane potential differance variation between the two components of the neuron will appear as variance in membrane potential of spike threshold in the soma.