Effects of hypothalamic thermal stimuli on sympathetic neurones innervating skin and skeletal muscle of the cat hindlimb.

1. Postganglionic neurones supplying hairless and hairy skin of the cat hindlimb were analysed for their responses to thermal stimuli applied to the anterior hypothalamus and spinal cord in anaesthetized and artificially ventilated cats. Activity was recorded from multi- and single-unit bundles which were isolated from peripheral nerves. The neurones were functionally identified as cutaneous vasoconstrictor (CVC) and muscle vasoconstrictor (MVC) neurones. Activity in sudomotor (SM) neurones was either monitored indirectly by recording the phasic negative deflections of the skin potential from the surface of the hairless skin, or in some experiments additionally by recording activity directly from the SM axons. 2. The activity in forty-one out of forty-four multi-unit and six out of six single-unit CVC bundles was inhibited, in a graded manner, by hypothalamic warming. An increase in the temperature of the surface of hairless skin followed the decrease in activity of the CVC neurones supplying it. Large changes in skin temperature only followed decreases in CVC activity of more than 40%. Cooling of the hypothalamus had only weak transient effects on CVC neurones. 3. Simultaneous warming of hypothalamus and spinal cord had multiplicative effects on the activity in CVC neurones. Subthreshold warming of one structure increased the response to warming of the other one and reduced the threshold temperature. 4. SM neurones were not affected by hypothalamic warming, but activated during hypothalamic cooling. 5. MVC neurones were weakly activated during hypothalamic warming only if arterial blood pressure decreased, otherwise they were unaffected. It is likely that this activation was due to secondary unloading of arterial baroreceptors. 6. Two silent postganglionic neurones projecting to skin were activated during hypothalamic warming. These neurones may have had a vasodilatory function. 7. Rhythmicity of the activity in CVC neurones, related to the cycle of artificial ventilation, increased during hypothalamic warming whereas that of MVC neurones was unchanged. 8. The functionally highly specific responses to hypothalamic warming in CVC neurones indicate a pathway from the hypothalamus that is specific for CVC neurones, in contrast to MVC and SM neurones. This central pathway is integrated with other spinal and supraspinal reflex pathways that determine the characteristic reflex pattern of CVC neurones to somatic and visceral stimuli and possibly with pathways that generate other physiological changes during hypothalamic warming (e.g. increase in respiratory drive).