A comparison of the structures of α‐ and γ‐spinal motoneurones of the cat

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Abstract
The structures of 7 .gamma.-motoneurons (axonal conduction velocities of 15-48 m/s) were compared with those of 9 .alpha.-motoneurons (axonal conduction velocities of 71-91 m/s) by using histochemical methods to reveal horseradish peroxidase previously injected intracellularly into indentified motoneurons in the cat lumbosacral spinal cord. The size of cell bodies of the motoneurons and the diameters of their intramedullary axons were related to their axonal conduction velocities over the whole range studied. Despite the smaller size of the cell bodies of the .gamma.-motoneurons, their dendritic trees extended as far as those of the .alpha.-motoneurons. .gamma.-Motoneurons had fewer main dendrites than the .alpha.-motoneurons, and these branched much less so that the dendritic trees of the .gamma.-motoneurons were much simpler than those of .alpha.-motoneurons. Although the extents of the dendritic trees were not related to axonal conduction velocity, the complexity of the dendritic trees was clearly related to axonal conduction velocity and to cell body size. The total surface area of each cell, taken as an indication of the area available for synaptic contact, was much smaller for .gamma.- than for .alpha.-motoneurons and was related to axonal conduction velocity. Only 1 of the 7 .gamma.-motoneurons studied had axon collaterals, whereas 5 of the 9 .alpha.-motoneurons had well developed collaterals. This finding is consistent with the relative contribution that each group of motoneuron axons makes to recurrent inhibition. One of the .gamma.-motoneurons had 2 axons, of different diameter, which emerged from the spinal cord at the same level but in different ventral rootlets. These features of motoneuron structure are related to aspects of their physiological properties.