Activation and inhibition of muscle and cutaneous postganglionic neurones to hindlimb during hypothalamically induced vasoconstriction and atropine-sensitive vasodilation.

1. Discharge patterns in postganglionic neurones to muscle and to hairy skin of the hindlimb of chloralose anaesthetized cats were investigated during electrical hypothalamic stimulation which induced either vasoconstriction or atropine sensitive vasodilation in the skeletal muscle. 2. Spontaneously active postganglionic neurones to muscle were activated both during hypothalamically induced vasoconstriction and active vasodilation. Stimulation of the hypothalamic vasodilator area induced mostly a sequence of activation-depression-activation in these neurones. Stimulation of cutaneous Group IV afferents elicited reflexes in these neurones; repetitive high frequency stimulation of large diameter afferents in the vago-depressor nerve produced depression of spontaneous activity followed by a post-inhibitory excitation. The characteristics of these neurones fit those that would be expected of vasoconstrictors. 3. Normally inactive postganglionic neurones to skeletal muscle could only be activated during hypothalamically induced atropine sensitive vasodilation. These neurones exhibit no reflexes on somatic stimulation. The axons of these neurones conduct faster than those of the spontaneously active postganglionic neurones. It is likely that they are cholinergic vasodilator neurones. 4. Most of the cutaneous postganglionic neurones to hairy skin were activated during stimulation of both the hypothalamic vasoconstrictor and the vasodilator areas. These neurones have the characteristics of cutaneous vasoconstrictor neurones. Part of the cutaneous not spontaneously active postganglionic neurones could neither be activated from the hypothalamus nor by somatic stimuli.