Morphometrical study of the cat thoracic dorsal root ganglion cells in relation to muscular and cutaneous afferent innervation.

The sizes of neuronal somata in cat dorsal root ganglia were determined at the different thoracic segmental levels (T1-T13). The intersegmental variations in the average value and class distribution of diameters were analysed. The maximal and minimal average mean cell diameters were 51.1 and 43.3 microns at the T1 and T2 levels, respectively. Caudally, this value gradually increased from T2 to T8 (47.8 microns) and thereafter decreased progressively to T12 (44.7 microns). At T1, large cells (greater than 50 microns in diameter) were 3.3-fold in excess compared to small ones (less than 35 microns in diameter). The proportion of large to small cells strongly decreased to a 0.9 ratio from T1 to T2, then increased again from T2 (0.9) to T8 (2.3). The size distributions of the overall cell populations were compared to those of neurones supplying muscular targets via the external intercostal nerves or cutaneous targets via the lateral branch of the internal intercostal nerves, identified following the retrograde transport of horseradish peroxidase. The size distribution of cells serving cutaneous nerves was similar to that exhibited by the overall population of ganglion cells. In contrast, the size distributions of cells giving rise to muscle afferents tended towards smaller values. In the thoracic dorsal root ganglia, the cell body sizes of the muscular primary afferents were close to those previously reported for the visceral primary afferents.