To determine the capacity of motoneurons to increase their motor unit (MU) size by collateral sprouting and to assess this capacity in relation to the size of the motor nerve, we partially denervated soleus, lateral gastrocnemius (LG), and medial gastrocnemius (MG) muscles in adult and neonatal cats. Isometric force and extracellular nerve potentials were recorded from > or = 7% of the remaining MUs, 2.5-18 mo later. S1 or L7 roots were sectioned unilaterally and the number of remaining MUs was quantified by use of charge and force measurements. 2. The mean unit force increased inversely with MU number in partially denervated muscles, but the increase was less than predicted for extensive denervations (> or = 90%). The same enlargement of MU size occurred whether muscles were partially denervated in neonatal or adult animals. 3. The force distribution of MUs in partially denervated muscles was similar to normal but was shifted to larger force values in direct proportion to the extent of partial denervation (PD). All MUs increased in size by the same factor to preserve the normal force distribution. 4. Normal size relationships among force, contractile speed, and axon potential amplitude were observed for MUs in partially denervated muscles. Because changes in muscle fiber size could not account for the increase in unit force, these data show that increase in MU size, with respect to unit force and innervation ratio (muscle fibers per motoneuron), is proportional to the size of the motor nerve. 5. Enlargement of MU size in partially denervated muscles did not have a retrograde effect on nerve fiber caliber because axon potential amplitude and conduction velocity were not changed after PD. 6. Under conditions of extensive PD (> 85%), regenerated nerves from the cut spinal root reinnervated the gastrocnemius muscles. It is likely that nerves supplied muscle fibers that were not innervated by sprouts from nerves in the uncut root as well as displacing sprouts from terminals in extensively enlarged MUs. 7. We conclude that all motoneurons within a motor pool compensate for partial nerve injuries by collateral sprouting and that enlargement of MU size is a function of motor nerve size, consistent with Henneman's size principle.
