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NEUROSCIENCE |
1 Department of Biomedical Engineering
2 Centre for Neuroscience, Faculty of Medicine and Dentistry
3 Faculty of Rehabilitation Medicine, University of Alberta, Edmonton, Alberta, Canada
The goal of this study was to investigate the nature of activation of the dendritic calcium persistent inward current (Ca2+ PIC) and its contribution to the enhancement and summation of synaptic inputs in spinal motoneurones. A compartmental cable model of a cat 
-motoneurone was developed comprising the realistic dendritic distribution of Ia-afferent synapses and low-voltage-activated L-type calcium (Cav1.3) channels distributed over the dendrites in a manner that was previously shown to match a wide set of experimental measurements. The level of synaptic activation was systematically increased and the resulting firing rate, somatic and dendritic membrane potentials, dendritic Cav1.3 channel conductance, and dendritic Ca2+ PIC were measured. Our simulation results suggest that during cell firing the dendritic Ca2+ PIC is not activated in an all-or-none manner. Instead, it is initially activated in a graded manner with increasing synaptic input until it reaches its full activation level, after which additional increases in synaptic input result in minimal changes in the Ca2+ PIC (PIC saturated). The range of graded activation of Ca2+ PIC occurs when the cell is recruited and causes a steep increase in the firing frequency as the synaptic current is increased, coinciding with the secondary range of the synaptic frequency–current (F–I) relationship. Once the Ca2+ PIC is saturated the slope of the F–I relationship is reduced, corresponding to the tertiary range of firing. When the post-spike afterhyperpolarization (AHP) is blocked, either directly by blocking the calcium-activated potassium channels, or indirectly by blocking the sodium spikes, the PIC is activated in an all-or-none manner with increasing synaptic input. Thus, the AHP serves to limit the depolarization of the cell during firing and enables graded, rather than all-or-none, activation of the Ca2+ PIC. The graded activation of the Ca2+ PIC with increasing synaptic input results in a graded (linear) enhancement and linear summation of synaptic inputs. In contrast, the saturated Ca2+ PIC enhances synaptic inputs by a constant amount (constant current), and leads to less-than linear summation of multiple synaptic inputs. These model predictions improve our understanding of the mode of activation of the dendritic Ca2+ PIC and its role in the enhancement and integration of synaptic inputs.
(Received 21 September 2005;
accepted after revision 15 November 2005;
first published online 24 November 2005)
Corresponding author V. K. Mushahwar: 513 Heritage Medical Research Center, Department of Biomedical Engineering, Centre for Neuroscience, University of Alberta, Edmonton, Alberta, Canada T6G 2S2. Email: vivian.mushahwar{at}ualberta.ca
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