Cardiovascular neurons of brain stem with projections to spinal cord.

In urethan-anesthetized rats a portion of the rostral ventrolateral medulla, the intermediate portion of the nucleus paragigantocellularis lateralis, was explored electrophysiologically for spinally projecting cardiovascular neurons. Spinal projections were demonstrated for 48 of 300 units tested for antidromic activation following spinal cord stimulation. The cardiovascular nature of units was tested by monitoring unit activity during transient changes in arterial pressure. A total of 47 cardiovascular neurons were identified; of these, 43 showed activities inversely correlated with arterial pressure and changes in activity closely coupled in time to arterial pressure changes. At high arterial pressures these neurons also displayed activities phasically linked to the cardiac cycle. Of these 43 neurons, 13 projected to or through the thoracic cord and 5 to or through the cervical cord. Axonal conduction velocities averaged 4.7 m/s. A second type of cardiovascular neuron (4 units of 47) was found that increased activity as arterial pressure increased. Three of these neurons projected to the cord. These results provide evidence for tonically active, baroreceptor-inhibited neurons with axonal projections to the thoracic spinal cord.