Somatotopy in the segmental innervation of the cat diaphragm

A somatotopic organization in the segmental innervation of the cat diaphragm (DIA) was determined using evoked electromyographic responses and glycogen depletion of stimulated type II muscle fibers. With the use of the glycogen depletion method, the specific location and proportion of muscle fibers innervated by the fifth (C₅) or sixth (C₆) cervical ventral roots were determined for different regions of the DIA. The sternal and ventral portions of the costal and crural DIA regions were innervated primarily by C₅. The dorsal portions of both the costal and crural regions were innervated primarily by C₆. Thus the somatotopic organization in the segmental innervation of the DIA was not correlated with the anatomic division of the sternal, costal, and crural regions. Instead, the somatotopic projections of cervical ventral roots were organized in the ventrodorsal axis of each DIA region. This topographical pattern resulted in an extensive overlap of the DIA territories innervated by C₅ and C₆. Within a region, the fibers innervated by a specific ventral root were not randomly distributed but often followed fascicle divisions. This frequently resulted in a wide range in the proportion of fibers innervated by a ventral root even within a specific region.