Does alpha-motoneurone size correlate with motor unit type in cat triceps surae?

The cell bodies and first-order dendrites of alpha-motoneurones supplying different functional types of muscle units in the cat gastrocnemius (type FF, FR and S units) and soleus (type SOL-S units) muscles, were studied after intracellular injection of horseradish peroxidase. The SOL-S neurones had smaller values for cell body diameter in comparison with both the FF and FR neurones. The SOL-S neurones also had significantly thinner first-order dendrites than the FF, FR and S neurones. In the gastrocnemius pool the S neurones had smaller values for dendritic diameters than the FF and FR cells. The values for combined diameter of the first-order dendrites indicated that the dendritic trees of the FF and FR neurones are, on the average, larger than those of the S and SOL-S neurones. Furthermore, the relationship between the combined dendritic diameter and the mean soma diameter, indicated that a difference in relative scaling of soma and dendrites exists between the FF and FR neurones on the one hand and the S and SOL-S neurones on the other. Similar results were obtained also when relating the combined dendritic parameter sigma d3/2 to the soma surface area. Although a certain statistical relation seems to exist between motoneurone size and motoneurone type, it should be emphasized, however, that the range of values for each parameter studied overlapped considerably between the different types of motoneurones.