Horseradish peroxidase injection of identified low threshold cutaneous mechanoreceptor (LTCM) primary afferent axons was used to assess the somatotopic organization of hindlimb projections to laminae III and IV of cat dorsal horn. Multiple injections in the same animals were used to assess bilateral symmetry and precision. Thirty-one axons were injected, with more than 1 axon injected in each of 8 animals (25 axons). Somatotopic relations between their receptive field (RF) centers and the centers of their dorsal horn projections were similar to the somatotopic relations between dorsal horn cell RF centers and cell locations. Very few reversals of mediolateral somatotopic gradients (proximodistal RF location as a function of mediolateral projection center) were observed. Two afferents with nearly identical RFs in 1 animal had nearly identical projections. These observations held for many different combinations of receptor types. A simple mathematical model was used to demonstrate that assembly of dorsal horn cell RFs via passive sampling of the presynaptic neuropil by dorsal horn cell dendrites cannot account for the sizes of dorsal horn cell LTCM RFs. Hypothesized mechanisms for assembly of dorsal horn cell RFs must take into account the functional selectivity of connections required to produce RFs smaller than those predicted by the passive assembly model.

A permanent decrease in spontaneous locomotor activity of the rat can be produced by destruction of either of two specific regions in the ventral hypothalamus at the level of the tuber cinereum. One is a lateral region which lies between the fornix and cerebral peduncle and extends from the level of the fornix downward to the base of the brain. Its rostrocaudal extent seems to be from about the level of the posterior border of the paraventricular nucleus back to, but not including, the premammillary area. The other is a medial region which seems to lie adjacent to the ventral half of the third ventricle, extending laterally from this structure only about .5–1 mm. This region, too, appears to reach to the base of the brain. The exact rostrocaudal extent of this medial region is uncertain, but at least it does not extend rostrally into the preoptic area nor caudally into the premammillary area. Activity was decreased to a much lower level after lateral lesions than after medial lesions. Lesions in other parts of the hypothalamus did not produce hypoactivity. Hypoactivity seemed to occur independently of other signs of hypothalamic impairment.

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