Pathways from medullary nuclei to spinal cardioacceleratory neurons in the cat

In 15 chloralosed vagotomized cats medullary sites of origin of descending autonomic fibres projecting to spinal sympathetic neurons were identified by activating fibre terminals in the intermediolateral nucleus (ILN) at histologically and functionally identified cardioacceleratory sites on the right side of spinal segment T2 and exploring the ipsilateral medulla for antidromically evoked field potentials. The cardiovascular function of positive sites of recording was then determined by observing the effects on heart rate and arterial pressure of electrical stimulation of these sites. Finally, precise localization of spinal pathways of descending fibres from these sites was determined by observing the effects on the medullary field potentials of restricted, histologically identified surgical or electrolytic lesions in spinal segments C5–C7.Evoked field potentials recorded at 10 medullary sites from which electrical stimulation elicited cardiac slowing were eliminated by selective lesion of either the ventral (8 animals) or the dorsolateral funiculus (2 animals). Evoked field potentials at five medullary sites from which stimulation elicited cardioacceleration were eliminated by lesion of the dorsolateral funiculus.Transection of the right dorsolateral funiculus in eight animals caused a decrease in heart rate and in arterial pressure; transection of the right ventral funiculus in 12 animals caused no change in heart rate or arterial pressure.It is concluded that: a) inhibitory fibres from medullary nuclei to ipsilateral spinal Cardioacceleratory neurons descend in the dorsolateral and ventral funiculi of the cervical spinal cord; b) excitatory fibres descend in the ipsilateral dorsolateral funiculus; c) excitatory, but not inhibitory, medullary fibres projecting to cardioacceleratory and vasoconstrictor neurons in the ILN are tonically active in the chloralosed cat.