Evidence that two sizes of ventromedial hypothalamic neurones project to the mesencephalic central grey matter in rats.

The spatial spread of the extracellular antidromic action potentials was measured in eighty-three neurones in the ventromedial nucleus region of the female rat hypothalamus following electrical stimulation of the mesencephalic central grey matter. The positive-negative configuration of the antidromic action potentials across the extracellular field suggested that the potentials were generated predominantly by neuronal soma with simple geometries. When represented by the distance travelled by the electrode, at which peak-to-peak spike amplitude exceeded the half maximum value, the spatial spread of the extracellular field ranged from 25 to 174 micron. The frequency distribution for the field size was distinctively bimodal and could be divided into large and small groups at 85 micron. Antidromic action potentials with larger extracellular fields had significantly larger maximum spike amplitude and shorter duration, indicating that differences in field size were associated with neuronal size. At least 55% of the central grey projection of the ventromedial nucleus originated from small neurones. Taking into account the sampling bias, a much greater proportion of the central grey projection may arise from small neurones. The lack of a systematic difference in the antidromic spike latencies between large and small cells indicated that axonal thickness is not the major factor in determining the latency of the responses of the ventromedial hypothalamic neurones to stimulation of the central grey matter.