Evoked responses of pyramidal tract neurons to stimulation of the lateral hypothalamus

The evoked responses of identified pyramidal tract neurons of the pericruciate region of the cortex arising on stimulation of the posterior, tuberal, and anterior sections of the lateral hypothalamus were investigated in acute experiments on cats anesthetized with chloralose. THe pyramidal tract neurons recorded were situated at a depth of 0.75-2.5 mm and on stimulation of the hypothalamus discharged with a latent period of 1.6-52.5 msec. They discharged most efficiently in the case of stimulation of the caudal half of the lateral hypothalamus. The pyramidal tract neurons activated by the hypothalamus were also tested by electrocutaneous stimulation of the four limbs. Of the neurons 75% responded to stimulation of several limbs, i.e., they had a broad bilateral receptive field, while 15% reacted to stimulation of one contralateral limb (either the anterior limb or the posterior limb), i.e., they had a small contralateral receptive field. Comparison of the latent periods of the anti- and orthodromic responses of the pyramidal tract neurons did not reveal any relationship between the magnitude of the latent period of the orthodromic response (on hypothalamic or electrocutaneous stimulation) and the type of pyramidal tract neuron (according to the axonal transmission velocity). No relationship between the latent period of hypothalamic stimulation and the magnitude of the latent period of the response to a peripheral stimulus was detected either.