Topography and numerical distribution of neurons of the thoraco‐lumbar intermediolateral nucleus in the cat

Abstract Serial transverse sections of the spinal cord of eight adult cats were stained with thionin and examined to determine the topography of the thoraco‐lumbar intermediolateral nucleus (ILN) and the numerical segmental distribution of its neurons. The ILN was found to extend from C 8 —T 1 to L 4 and to be located within the lateral horn of the grey matter except in segments C 8 —T 1 and L 1 —L 2 where neurons were observed scattered throughout finger‐like projections of the grey matter. The total number of neurons ranged from 32,790 to 53,340. The mean counts of neurons on the left and right sides were not significantly different. A statistically significant (p < 0.02) difference, however, was demonstrated between the mean counts in four female cats (35,543 ± 1,411) and those in four males (45,765 ± 2,556). The number of ILN neurons was found to vary considerably from segment to segment, with the highest counts occurring in segments T 1 —T 2 and L 3 —L 4 . Quantitative estimates of cell sizes were also obtained from one male cat. The modal cross‐sectional area was found to be 290 μ 2 . The data reported provide a useful map for neurophysiological studies of this nucleus. In addition, the demonstration of a sex difference in the number of ILN neurons suggests that the number of neurons in the mammalian central nervous system may be different in the two sexes.