Lack of topography in the spinal cord projection to the rabbit soleus muscle

Several mammalian muscles in the limb and trunk receive topographically organized innervation from spinal cord motor neurons. Some muscles in which topographic innervation has been demonstrated have a sheet-like architecture; others are compartmentalized and/or have more than one origin. An interesting question is whether topography is related to these anatomical features, or whether it occurs as a general consequence of the development of innervation. To address this question, we examined the pattern of projections to the soleus muscle, which lacks these anatomical features. Intracellular recordings of endplate potentials in early and intermediate age rabbits were used to assess the spinal origin of inputs to two distinct regions of the muscle. Both regions were innervated by both rostral and caudal portions of the motor pool. These experiments also showed that individual muscle fibers frequently receive separate inputs arising from widely separated regions of the motor pool. In another set of experiments, physiological measurements of tension overlap in young, polyinnervated muscles showed that the relative positions of motor neurons in the spinal cord do not correlate with the extent to which motor units share muscle fibers. In a third set of experiments, motor neurons were retrogradely labeled following local injections of tracer into muscle. Small and large local injections resulted in comparably dispersed labeling of motor neurons within the motor pool. Moreover, the rostrocaudal position of labeled neurons was not correlated with the position of the injection site within the muscle. Together, these results provide evidence that the soleus muscle is not topographically innervated. Furthermore, an examination of several age groups suggests that the innervation pattern in this muscle is not altered by postnatal synapse elimination.