The spatial distribution of ionic conductances in normal and axotomized motorneurons

Abstract

In a previous paper, a model was developed to study and contrast the relations between cell geometry and repetitive firing in large and small motorneurons. This model is extended and revised in the present paper and used to analyze certain experimental findings in normal and axotomized motorneurons. Our major conclusions are as follows: 1. (1) For normal motorneurons, in order to reproduce the experimentally observed difference in interspike voltage trajectory (concave vs convex) in the primary and secondary ranges respectively of the current-firing rate curve, it is necessary to localize the major part of a slow potassium conductance on the motorneuron dendrites. This slow potassium conductance leads to the afterhyperpolarization following an action potential. 2. (2) In order to activate the slow potassium conductance after an action potential, we have postulated a calcium conductance in the motorneuron membrane, as suggested by the work of Barrett and Barrett, and Krnjevid and Lisiewicz. In normal motorneurons, the proximal dendrites must display a certain amount of sodium conductance (gNa) in order to activate the calcium conductance (gCa) sufficiently to 'trigger' the slow potassium channel. 3. (3) With the above mechanism for calcium and slow potassium conductances, the model shows that the firing rate in the primary range is mainly determined by the decay of the slow potassium conductance which generates the long afterhyperpolarization. In the secondary range, this slow potassium conductance is essentially constant between spikes, so that the firing rate is determined by fast potassium recovery. 4. (4) The model is further able to display several features of the partial responses seen in axotomized motorneurons: for example, their all-or-nothing character following different levels of synaptic excitation, and the hump on the trailing part of an action potential. The model achieves this when a patch of membrane with an increased sodium and fast potassium conductance (a 'hot spot') is localized on a dendritic branch about 0.4-0.5 or more space constants away from the soma. 5. (5) For an axotomized motorneuron, we were able to obtain simultaneously the correct current-firing rate curve (a single, high-slope region), and also the experimentally observed convex interspike trajectories, by manipulating slow potassium and 'hot spot' gNa parameters; this could be done by moving the 'hot spots' distally (to 0.6-0.7 space constants from the somas With a 'hot spot' close to the soma, shifting the Hodgkin-Huxley rate functions αm and αh (which lowers the voltage threshold) tended to accentuate the amount of primary range present; this was not observed with 'hot spots' located more distally. 6. (6) An important variable in determining the nature of repetitive firing in both normal and axotomized motorneurons is the height of the dendritic potential (in the region in which the afterhyperpolarization is developed) immediately following the soma action potential. © 1977.