A quantitative morphological study of HRP-labelled cat alpha-motoneurones supplying different hindlimb muscles.

Cat alpha-motoneurones supplying the quadriceps (Q), posterior biceps (PB), gastrocnemius (G), soleus (SOL) and short intrinsic plantar foot (SP) muscles were studied after retrograde or intracellular labelling with HRP. The average soma sizes were rather similar for the different pools, the SOL cells being the smallest. The median number of first-order dendrites ranged from 10 (PB) to 12 (SOL). The median diameters of the first-order dendrites ranged from 6 (SOL) to 8.5 (PB, G) micrometer. The dendritic projection patterns were rather similar for the different motoneurone groups, except for a prominent dorsomedial projection of SP dendrites. A considerable fraction of the dendrites extended into the white matter. The diameter of the first-order dendrite correlated positively to the number of end branches as well as to the combined length, surface area and volume of the whole dendrite. These relations appeared to be independent of motoneurone group and dendritic orientation. The combined diameter of the first-order dendrites, which reflects the total dendritic size of a motoneurone, exhibited median values between 82 micrometers (SOL) and 112 micrometers (Q). With respect to the relative scaling of soma and dendrites, motoneurones with large somas tended to have proportionally larger dendritic trees. The distribution of dendritic diameters, number of branches, dendritic surface area and volume, and the combined dendritic parameter (epsilon d3/2) at various distances from the soma were quite similar for the different motoneurone groups.