Motor Neuron Projection Patterns and Maturation of Motor Unit Types in the Rabbit Soleus Muscle
<p> The nervous system contains specific connections reflecting different types of projection patterns. Some of these projections have a topographic arrangement, while others have a diffuse pattern. Projections from spinal cord motor pools to different muscles are topographic. At a finer scale...
| Summary: | <p> The nervous system contains specific connections reflecting different types of projection patterns. Some of these projections have a topographic arrangement, while others have a diffuse pattern. Projections from spinal cord motor pools to different muscles are topographic. At a finer scale, patterns of projections within several muscles are topographic. These muscles tend to be flat and/or segmented. This thesis presents an investigation of the projection pattern in the innervation of the rabbit soleus muscle, which is compact and has only one tendon of origin and one tendon of insertion. Using intracellular recording of endplate potentials, tension overlap between pairs of ventral root filaments, and retrograde labeling of motor neurons following small injections of horseradish peroxidase into different regions within the muscle, it was shown that the soleus receives diffuse innervation from the spinal cord. It is thus likely that topography is related to muscle function, and that it correlates with spatial
heterogeneity within muscles.</p>
<p> Another type of specificity in connections to muscles is that between fast and slow motor neurons and their corresponding muscle fiber types. These connections form distinct types of motor units. We have investigated the
maturation of motor unit types during postnatal synapse elimination.</p>
<p> The ratio of motor unit tension at polyinnervated ages to that at singly innervated ages has been used to estimate the degree of polyinnervation for fast versus slow muscle fibers. Twitch and tetanic tension yield conflicting results. This contradiction was resolved using latencies to endplate potentials as an indicator of muscle fiber type. We found that fast and slow muscle fibers are polyinnervated to the same extent during both early and intermediate stages of synapse elimination, implying that specific tension, and not polyinnervation, changes differently in fast versus slow muscle fibers. These changes are consistent with those found in twitch/tetanus ratios. During synapse elimination, the twitch/tetanus ratios for fast motor units increase while those for slow motor units decrease. Furthermore, these intracellular recordings suggest a high degree of specificity at birth, which is further refined during synapse elimination.</p>
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