Ascending spinal pathways for somatoautonomic reflexes in the anesthetized dog.

The ascending spinal pathways mediating pressor and heart rate responses to somatic afferent stimulation and induced exercise were studied in pentobarbital- and alpha-chloralose-anesthetized dogs. Bilateral sciatic nerve stimulation and induced exercise, via lumbosacral ventral root stimulation, produced pressor and heart rate responses that were blocked by bilateral dorsolateral sulcus (DLS) lesions of the lumbar spinal cord (L1-L3). Baroreceptor-mediated bradycardia were attenuated by sciatic stimulation but not by induced exercise. This attenuation was blocked by combined dorsolateral funiculus (DLF) and DLS lesions. Induced exercise with vascular occlusion to the muscle augmented the pressor and heart rate responses to exercise. These responses were blocked by combined DLS and DLF lesions. Such lesions did not influence responses to bilateral carotid occlusion, indicating that descending autonomic pathways were intact. Therefore, ascending spinal pathways mediating somatocardiovascular reflexes in anesthetized dogs are located in the lateral funiculus, extending from the dorsal root entry zone to a position somewhat ventral to the dentate ligament.

Ascending pathways mediating somatoautonomic reflexes in exercising dogs.

The ascending spinal pathways mediating somatocardiovascular reflexes during exercise were studied in unanesthetized dogs by placing lesions in the lumbar spinal cord. After training to run on a treadmill with hindlimbs only, 20 dogs were anesthetized and instrumented using sterile surgical techniques. To chronically record heart rate and arterial blood pressure, the aorta was cannulated via the omocervical artery. To test the intactness of descending spinal sympathetic pathways, reflex pressor responses to baroreceptor hypotension were produced by bilateral carotid arterial occlusion using pneumatic vessel occluders placed around the common carotid arteries. To generate transient ischemic exercise (120 s), a pneumatic occluder was placed around the left iliac artery. Eight to 10 days after instrumentation, blood pressure and heart rate were monitored at rest and during hindlimb running with and without simultaneous iliac arterial occlusion. The modest pressor response and tachycardia elicited by hindlimb exercise were markedly augmented by simultaneous hindlimb ischemia (i.e., iliac arterial occlusion). Lesion placement in the dorsolateral sulcus area and the dorsolateral funiculus at L2 significantly reduced the blood pressure and heart rate responses to simultaneous exercise occlusion. The cardiovascular responses to nonischemic exercise and bilateral carotid arterial occlusion were not altered by such spinal sections. It is concluded that in the dog the ascending spinal pathways mediating cardiovascular responses to ischemic exercise are located in the lateral funiculus, including the dorsolateral sulcus area and dorsolateral funiculus.

Effect of clonidine on blood pressure alterations induced in spinal cats.

Blood pressure alterations were evoked by stimulation of spinal pathways descending in the dorsolateral funiculus of chloralose-anesthetized cats, spinalized at the C1 level. Intravenous clonidine was found to attenuate threshold, intermediate, and maximal pressor responses (elicited by increments in current delivery) in a dose-related manner. The pressor responses following norepinephrine administration (0.2 micron/kg, i.v.) were somewhat potentiated by clonidine in these animals. Furthermore, this agent was found ineffective in blocking blood pressure alterations occurring in response to T3 or T4 white ramus stimulation at several frequencies. It is concluded that clonidine has a significant inhibitory action at the spinal level.

Ascending spinal pathways mediating somato-cardiovascular reflexes.

The influence of spinal somatic afferent pathways upon cardiovascular parameters were studied in anesthetized and conscious dogs. Previous studies on anesthetized cats indicated that activation of somatic A afferent fibres results in a depressor response mediated by ascending spinal pathways in the dorsolateral funiculus (DLF). Additional activation of somatic C afferent fibers results in a pressor response mediated by pathways in the region of the dorsolateral sulcus (DLS). In anesthetized dogs sympathetic-mediated cardiovascular responses to somatic afferent stimulation are found to be conducted in the DLS. To test the possibility that these afferent pathways may mediate cardiovascular responses to exercise, dogs instrumented for blood pressure and heart rate monitoring were trained to run on a treadmill on all 4 legs and on their hindlegs only. Lesions on the DLS and a portion of the DLF significantly altered heart rate responses to treadmill running on all 4 legs. These data suggest that somato-autonomic reflexes participate in the cardiovascular adjustments which occur during exercise.

Organization and reflex control of vagal cardiomotor neurons.

The anatomical and functional organization of cardiac vagal preganglionic somata and the organization of ascending spinal pathways regulating the cell bodies were studies in the dog and cat. Horseradish peroxidase was injected subepicardially in cats after left or right cervical vagotomy to retrogradely label cardiomotor neurons. Somata were ipsilateral to the intact vagus in the dorsal motor nucleus (DMN) of the vagus, the nucleus ambiguus (NA) and an intermediate zone (IZ) between the DMN and NA. The NA contained the maximum number of cell bodies (72%) while successively fewer somata were located in the DMN (19%) and IZ (9%). NA somata were heterogeneously distributed along the longitudinal neuroaxis while DMN and IA somata were homogeneously distributed. Cell bodies of the NA were larger than those of the DMN and IZ. To investigate the possibility that the NA and DMN regulate different cardiac functions, ventricular contractility (LV dP/dt) and heart rate (HR) were recorded in beta-blocked cats during NA and DMN stimulation with and without cardiac pacing. DMN stimulation produced decreases in LV dP/dt but no HR changes, while NA stimulation produced decreases in HR and increases in LV dP/dt. Since cardiac pacing eliminated the LV dP/dt increase, the contractility response was secondary to the bradycardia. Thus, cardiac vagal somata are organized for differential cardiac control. Ascending spinal pathways affecting vagal control of the heart were identified in beta-blocked cats by comparing cardiac responses to nerve stimulation before and after making spinal lesions. The carotid sinus nerve was stimulated to elevate cardiac vagal activity. Somatic afferent stimulation inhibited the negative chronotropic and inotropic responses produced by carotid sinus stimulation. Bilateral lesions placed in the dorsolateral sulcus area abolished the effects of somatic afferent stimulation. Thus, an ascending pathway regulating somata of the DMN and NA is located bilaterally in the dorsolateral sulcus. Similar studies in the dog suggested the pathway extends into the dorsolateral funiculus as well.

Somato-autonomic reflexes in anesthetized and unanesthetized dogs.

Blood pressure and heart rate alterations were induced in anesthetized and unanesthetized mongrel dogs. Pressor responses were brought about in the anesthetized group by bilateral, high intensity stimulation of the sciatic nerves. In these animals, bilateral section of the dorsolateral sulcus area (DLS) in the lumbar spinal cord completely eliminated both blood pressure and heart rate responses to this stimulation. Baroreceptor-mediated bradycardia induced by pressor doses of phenylephrine was attenuated by sciatic nerve stimulation. Obliteration of this baroreceptor-somatic afferent interaction required ventrolateral extension of the bilateral spinal lesions to include both the DLS and the dorsolateral funiculus (DLF). Blood pressure and heart rate increments in unanesthetized dogs were induced by treadmill running. The pressor response to exercise was markedly increased by transient hind limb arterial occlusion during the course of the run. Surgical interruption of the ascending limb of the somato-autonomic reflex in the spinal cord of these animals (L1-L2 combined DLS and DLF lesion) significantly reduced the blood pressure response to simultaneous exercise-occlusion. These spinal lesions also reduced the heart rate response to treadmill running without occlusion. The descending pathways involved in autonomic reflexes appeared intact as the spinal lesions did not alter the blood pressure or heart rate response to bilateral carotid artery occlusion.