The cat cervical dorsal root ganglia: general cell-size characteristics and comparative study of neck muscle, neck cutaneous and phrenic afferents

Sizes of neuronal somata in the cat cervical dorsal root ganglia were determined at different levels (C1–C8). The average value and class distribution of mean cell diameter were analyzed. The ganglia from C1 to C5 could be clearly distinguished from those at levels of brachial plexus afferents (C6–C8) with respect to cell size range, distribution and average. The size distribution, most often limited to 70 μm from C1 to C5, skewed to more than 90 μm from C6 to C8. Cells in the 35–50 μm range of diameter constituted the main portion of the cell population (49–52%) at the C1–C5 levels, whereas from C6 to C8 51–77% of the ganglion cell bodies were more than 50 μm in diameter. The cell size distribution of afferents projecting from C1 to C5 and supplying different muscle or cutaneous targets was studied following retrograde labeling with horseradish peroxidase conjugated to wheatgerm agglutinin. Sizes of cell bodies of biventer cervicis (postural muscle), phrenic (purely respiratory muscle) and cutaneous afferents were all similar. The labelled cell bodies were in the majority (51–64%) less than 35 μm in diameter and ranged towards smaller diameters than counterstained cells in the corresponding ganglia. In spite of similarities in cell size distribution it was estimated from the fiber caliber spectra of the labelled afferents that both unmyelinated and myelinated cutaneous afferents originate from larger cell bodies than muscle afferents in the same diameter range.