Convergence Of The Inputs Of Visceral And Somatic Nociception In Anterior Cingulate Gyrus Of Cats

Pain perception is one of important defense functions in human body. Loss of pain perception makes human body to suffer from serious injury. But pain is a vast distress of patients in clinic, which is a puzzle to treat. Recent pain researches focus on effect of anterior cingulate gyrus (ACG). Many researchers confirm ACG plays an important role in pain sensation. ACG is an important center which can receive and modulate pain signals, and be involved in emotional response of pain. Visceral and somatic pain have the differences in the perceptual qualities. Visceral pain is poorly localized and is often referred to somatic regions. The perception of pain intensity and affect in visceral and somatic pain syndromes is often different, with visceral pain reported as more unpleasant. Visceral pain results in quiescence whereas somatic pain results in a flight and fight response. The precise reasons for the differences in perception between visceral and somatic pain remain unclear up to now. It has very important theoretical value for exploitation of new pain-killer and analgesia, to elucidate the mechanism of the differences in perception between visceral and somatic pain. Saphenous nerve (SN) conducts impulses of somatic pain from lower limb. Greater splanchnic nerve(GSN)conducts impulses of visceral pain from upper abdomen. No studies have been done to examine whether ACG is one of the representative areas of the SN or GSN afferent pathways at present. Neurons are basic structural and functional units of nervous system. The analysis function of sensory cortex mostly depends on the properties of different cortical neurons. The integration of afferent messages in a single sensory neuron is a basic for sophisticated sense analysis function of the cerebrum. So it is necessary to study response patterns and functional properties of a single visceronociceptive and somatonociceptive neurons for exploring the cerebral cortex mechanism of visceral and somatic pain sensation, and mechanism of the differences in perception between visceral and somatic pain. Strongly electrically stimulating the left SN was used as an experimental model of somatic pain.Strongly electrically stimulating the left GSN was used as an experimental model of visceral pain. With intracellular recording techniques of glass microelectrode in vivo, at the first time we discovered that ACG is one of the representative areas of the SN and GSN afferent pathways, and there are visceronociceptive neurons and somatonociceptive neurons in ACG, and visceronociceptive neurons and somatonociceptive neurons in ACG had different electrophysiological properties, which might be electrophysiological basis and one of reasons for the differences in perception between visceral and somatic pain.The data of our study confirmed that ACG received inputs of both GSN and SN. However, at present, little is available with respect to research on the convergence of inputs of both GSN and SN in the contralateral ACG neurons and its properties. Therefore, in the present study we further investigated the convergence of inputs of both GSN and SN in the contralateral ACG neurons and its properties, and clarified the integration between visceral and somatic nociceptive inputs in the contralateral ACG neurons, which might provide new experimental data for Convergence Theory of Referred Pain.With intracellular recording techniques of glass microelectrode in vivo, in the present study, we found that there were SN-stimulus-irrelative VNNs only responsed to noxious stimulation of GSN(52.21%, n=136), GSN-stimulus-irrelative SNNs only responsed to noxious stimulation of SN(13.23%), and convergence viscerosomatic nociceptive neurons ( CVSNNs,34.56%) responsed to noxious stimulation of both GSN and SN in the contralateral ACG. The existence of both SN-stimulus-irrelative VNNs and GSN-stimulus-irrelative SNNs indicated that some afferent fibers of GSN and SN projected to different neurons in the ACG via different pathways. Inputs of the afferent fibers of both GSN and SN did not converge in the ACG neurons. SN-stimulus-irrelative VNNs only received inputs of the afferent fibers of GSN. GSN-stimulus-irrelative SNNs only received inputs of the afferent fibers of SN. The findings indicated that the ACG could sense and distinguish inputs of the afferent fibers of both GSN and SN, and respond to the inputs. The outcome further demonstrated that the different afferent fibers of both GSN and SN ascended along the different locations of the spinal cord to the different areas in the central nervous system. The existence of CVSNNs indicated that some afferent fibers of GSN and SN projected to the same neurons in the ACG. Inputs of the afferent fibers of both GSN and SN converged in the same neurons of the ACG. The results might provide new experimental data for Convergence Theory of Referred Pain.Compared the evoked responses of CVSNNs in the ACG to GSN noxious stimulation with those to SN noxious stimulation, we found there were two types of CVSNNs in the ACG. One was CVSNNs with the similar evoked responses to GSN and SN noxious stimulation (44.68%). Another was CVSNNs with the dissimilar evoked responses to GSN and SN noxious stimulation (55.32%). We thought that the similar evoked responses were because the noxious inputs of GSN and SN converged in the central nervous system, and then were transmitted to the ACG, and converged in the same neurons in the ACG through a common pathway. Whereas the dissimilar evoked responses were because the noxious inputs of GSN and SN ascended and converged in the same neurons in the ACG via different pathways in the central nervous system. The results indicated that the convergence and functional relationship between visceral nociceptive afferent signals from GSN and somatic nociceptive afferent informations from SN in the ACG were complex and multiple.In the present study, modes, frequencies, and amplitudes of spontaneous discharges of SN-stimulus-irrelative VNNs and GSN-stimulus-irrelative SNNs and CVSNNs in the ACG were not obvious distinct.The modes of spontaneous biological electric activities of these neurons were found as follows: no-discharges in resting state, synaptic activities, occasional discharges, continuous tonic discharges and rhythmic discharges. These results were similar to those in ACG reported previously by our laboratory. But the percentage of CVSNNs had spontaneous firing is significantly higher than those in SN-stimulus-irrelative VNNs and GSN-stimulus-irrelative SNNs. The result indicated that the excitability of the CVSNNs was higher than those of the SN-stimulus-irrelative VNNs and GSN-stimulus-irrelative SNNs, and the CVSNNs were more susceptible to input of synaptic impulses than the SN-stimulus-irrelative VNNs and GSN-stimulus-irrelative SNNs.In the present study, patterns of the evoked responses of the three kinds of neurons in the ACG induced by stimulating GSN and/or SN were divided into excitatory, inhibitory and mixed groups, which were similar to those in ACG reported previously by our laboratory and in somatosensory cortex reported previously by other authors.Visceral and somatic pain have the differences in the perceptual qualities. There are the differences in electrophysiological properties between SNNs and VNNs in ACG reported by our laboratory. In the present study, 33.80% of SN-stimulus-irrelative VNNs had spontaneous firing, which is significantly lower than 55.56% in GSN-stimulus-irrelative SNNs(P<0.01). The mean value of RP of the former was -52.79±9.91mV(n=71), which is significantly higher than that of the latte(r-46.33±7.24mV, n=18,P<0.01). The result indicated that the excitability of the GSN-stimulus-irrelative SNNs was higher than that of SN-stimulus-irrelative VNNs, and were more susceptible to input of synaptic impulses than the SN-stimulus-irrelative VNNs. Compared with the evoked response of the GSN-stimulus-irrelative SNNs, the evoked response patterns of the SN-stimulus-irrelative VNNs were more complex. The percentage of mixed evoked responses of the ACG SN-stimulus-irrelative VNNs was 15.49%, which was higher than that of the ACG GSN-stimulus-irrelative SNNs(11.11%). Moreover, the percentage of the SN-stimulus-irrelative VNNs was 52.21%, significantly bigger than that of the GSN-stimulus-irrelative SNNs in the ACG(13.23%,P<0.01). Therefore, we thought that the SN-stimulus-irrelative VNNs had the specificities with low excitability and complex reactive patterns, and bigger percentage in ACG. The differences in electrophysiological properties between the GSN-stimulus-irrelative SNNs and the SN-stimulus-irrelative VNNs in ACG might be one of reasons for the differences in perception between visceral and somatic pain. The results might provide new electrophysiological experimental evidence for expounding the mechanism of the differences in perception between visceral and somatic pain.