Compartmentalization of the cat lateral gastrocnemius motor nucleus.

The spatial and size distributions of motoneurons which supply the cat lateral gastrocnemius (LG) neuromuscular compartments were examined to determine their anatomical organization. Individual primary muscle nerve branches (PMNB) to LG were isolated by microdissection, cut, and soaked in horseradish peroxidase (HRP). As a control in each case, the entire contralateral lateral gastrocnemius-soleus (LG-S) nerve was similarly cut and soaked. Retrogradely labeled motoneurons were identified, their positions plotted, and their sizes measured on both sides of the cord. Results show that extensive overlap exists in the spatial distribution of motoneurons innervating different PMNBs. Labeled cells supplying each PMNB also vary considerably in their sizes. However, both a highly significant topography-like organization and a preferential size distribution are found between different groups of motoneurons. Neurons which supply proximal compartments occupy more rostral portions of the LG motor nucleus and are among the largest in the pool. Very few small motoneurons innervate proximal compartments. Neurons supplying more distal compartments are distributed in more caudal parts of the pool and contain both large and small cells in relatively equal numbers. If the large cells are alpha and the small cells gamma motoneurons, then proximal compartments are relatively poor in gamma innervation and presumably muscle spindles, and distal compartments are rich in fusimotor innervation.