Bill Connelly wrote:Am I right in thinking that a section of dendrite between points A and B is reciprocal if injection a X pA of current at location A creates Y mV of deflection at location B AND injection of X pas of current at location B creates Y mV of deflection at location A?
A two port system has the property of reciprocity if application of a current I to one port elicits the same potential V at the other port, even if the "current injection" and "voltage observation" ports are swapped. So the answer to your question is "yes."
And if the connection between a dendritic site and a somatic site is reciprocal, what does that mean for the physical/electrical properties of your typical tapering dendrite?
Not much except that (1) it happens and (2) it's always a surprise when it happens.
Keivan wrote:to my knowledge, this is not happening in the CA1 and layer 5 cortical pyramidal neurons in reality.
Quite the contrary. Fairly striking examples of reciprocity are presented in Fig. 9 A and B of
Magee, J.C.
Dendritic hyperpolarization-activated currents modify the integrative properties of hippocampal CA1 nyramidal neurons
J. Neurosci. 18: 7613 - 7624, 1998
which is obtainable for free from the journal's WWW site. The figure shows membrane potential recorded in a CA1 pyramidal cell patched at the soma and apical dendrite. Current steps were injected at one site, and observed at the other. Note that the membrane potential traces overlie each other almost exactly, regardless of where the current was injected, even though there was a prominent sag in all responses (compare "I inject to dend / propagated" with "I inject to soma / propagated"). Adding cesium to the bath eliminated the sag, but the traces were still nearly identical. An impressive demonstration of reciprocity in at least a portion of the apical dendritic tree of a cell whose membrane was full of voltage-gated, time-dependent currents, including a fairly large h current. The injected currents were large enough to elicit 10 - 15 mV changes of membrane potential at the injection and "remote" sites--fairly substantial perturbations.
The paper makes no mention of reciprocity. It might be worth asking Jeff if he noticed it, and if he did, whether reciprocity was also found in other cells. Or, if he didn't, whether he can easily dig up the data from those old experiments to take another look . . .
I wouldn't be surprised if others have done similar experiments and gotten similar results (but may not have recognized them for what they were, or what they imply). Certainly there are lots of papers that present IV curves obtained by injecting and recording current at the same site--almost always the soma--under control conditions (no drugs or ionic manipulations), in which the slope is nearly linear over membrane potential ranges that span 10-20 mV or more. This suggests that, except in the near vicinity of spike threshold, neurons are operating in a more or less linear manner, and that reciprocity is quite common if not the rule.
Experimentalists and theoreticians who work on a systems level find it convenient to treat cells as essentially linear integrators of synaptic inputs, but those who work at the cellular level tend to focus on the complexities of ion channels and their nonuniform distribution over the cell surface. It might be expected that papers by the latter would seem to ignore accidentally revealed linearities.