Size and Remoteness: Two Relatively Independent Parameters of Dendrites, As Studied for Spinal Motoneurones of the Cat

1. The spatial extent of motoneuronal dendrites was analysed using data from the fifty-two dendritic trees of four completely reconstructed cat motoneurones that had been labelled with intracellularly injected horseradish peroxidase. The cells belonged to m. triceps surae, and their physiological properties covered much of the known range for this muscle. 2. The percentage of total dendritic area extending beyond a radial somatofugal distance of 750 microns (PA750) was selected as an index of 'dendritic remoteness'. 3. The value of our standard remoteness index PA750 was well correlated with a number of other possible remoteness measures, such as the percentage of total dendritic area beyond 500 or 1000 microns or the percentage of cumulative dendritic length beyond 750 or 1000 microns. 4. Most of the dendritic parameters could be classified as belonging either to a cluster of remoteness-related properties or to a cluster of size-related properties. Remoteness-related properties were significantly correlated with PA750 but not with stem diameter. Size-related properties were significantly correlated with stem diameter but not with the remoteness index PA750. The remoteness-related properties included the mean somatofugal distance to branch points, the mean and maximum distance to terminal ending and, somewhat less distinctly, the mean length of terminal branches. The size-related properties included the total dendritic area, the cumulative dendritic length, and the total number of branch points and terminal endings. 5. The extent of area expansion at branch points and the overall extent of tapering were not systematically related to differences in remoteness. 6. The relative degree of remoteness was greater for medial than for lateral dendrites. 7. Even in the present limited sample of cells, there were marked and systematic differences between different neurones with respect to the relative remoteness of their dendrites. The slowest and most high-resistance motoneurone was the one with the most remote dendrites. On the whole there was, however, a considerable degree of independence between the variation in dendritic remoteness and the variation in physiologically determined properties (axonal conduction velocity, input resistance, after-hyperpolarization). 8. In the discussion it is suggested that differences in dendritic remoteness might lead to a selectivity between the various members of a given motoneurone pool with respect to their reception of topographically distributed systems of synapses. It is proposed that such differences might be of interest in relation to, for instance, the organization of innervation to motoneurones belonging to different task groups within the same muscle.