Fig. 6.

Schematic illustration of the proposed mechanisms underlying somato-autonomic reflexes induced by nociceptive stimulation of paraspinal muscles. a In intact conditions, stimulation of paraspinal muscles with hypertonic saline inhibits tonic sympathetic activity in the brainstem and activates descending inhibitory pathways. This produces a generalised deactivation of sympathetic vasomotor neurons of spinal origin, which in turn induces vasodilation and a depressor response. Besides, descending pathways also inhibit spinal reflexes involving the gastric sympathetic motor neurons, preventing the inhibition of gastric motility by somatic-sympathetic reflexes when the spinal cord is intact. b Nociceptive inputs from T2 and L6 project, in part, to spinal neurons that also receive inputs from the limbs. This partial convergence of back afferents on spinal neurons that receive limb afferents and project to pressor regions in the brainstem may trigger a pressor response. In turn, this response may compete with the depressor response induced by the inhibition of the tonic sympathetic activity in the brainstem. The resulting response, integrated in the brainstem, may then be smaller or abolished compared with the depressor response induced by the HS injection at T13, which nociceptors project to spinal segments devoid of neurons that receive limb afferent inputs. c In spinalised conditions, descending pathways are interrupted, leading to no change in blood pressure. However, the existing connection between somatic afferents from the back and preganglionic sympathetic neurons is disinhibited and allows somato-gastric reflexes to be evoked by hypertonic saline injections. It should be noted that vasodilation was not measured directly in this study, but was rather extrapolated from the GBF and MAP responses. Considering that a strong depressor response was produced by HS, it must be associated with some vasodilation, although it was not monitored. CNS central nervous system, GSNA gastric sympathetic nerve activity, MAP mean arterial pressure, GP gastric pressure