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Department of Physiology, The Medical School, Birmingham B15 2TJ

1. The structures of seven -motoneurones (axonal conduction velocities of 15-48 m/sec) were compared with those of nine -motoneurones (axonal conduction velocities of 71-91 m/sec) by using histochemical methods to reveal horseradish peroxidase which had previously been injected intracellularly into indentified motoneurones in the cat lumbosacral spinal cord.

2. The size of the cell bodies of the motoneurones, and the diameters of their intramedullary axons, were related to their axonal conduction velocities over the whole range studied.

3. Despite the smaller size of the cell bodies of the -motoneurones, their dendritic trees extended as far as those of the -motoneurones. However, -motoneurones had fewer main dendrites than the -motoneurones and these branched much less, so that the dendritic trees of the -motoneurones were much simpler than those of -motoneurones. 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.

4. The total surface area of each cell, taken as an indication of the area available for synaptic contact, was much smaller for - than for -motoneurones, and was related to axonal conduction velocity.

5. Only one of the seven -motoneurones studied had axon collaterals whereas five of the nine -motoneurones had well developed collaterals. This finding is consistent with the relative contribution that each group of motoneurone axons makes to recurrent inhibition.

6. One of the -motoneurones had two axons, of different diameter, which emerged from the spinal cord at the same level but in different ventral rootlets.

7. These features of motoneurone structure are related to aspects of their physiological properties.