Distribution of primary afferent neurons associated with the porcine inferior mesenteric ganglion (IMG)

Abstract

The existence of afferent neurons supplying IMG nerve cells by their collaterals has previously been demonstrated in laboratory animals, where these fibers constitute an important part of intraganglionic nerve input involved in the regulation of autonomic neurons activity. The domestic pig is thought to be a reliable animal model for medical and biomedical studies, especially those dealing with the organization of nerve pathways contributing to the neural control of the bowel and lower urinary tract functions. However, there is no data dealing with the existence and distribution of spinal afferent neurons that may be involved in the integration of various stimuli at the level of IMG, one of the most important nerve centers controlling these organs. Therefore, the present study was aimed at disclosing the distribution and morphological features of sensory neurons associated with the porcine IMG by means of retrograde tract-tracing technique. To this purpose, the right IMG was injected with the retrograde neuronal tracer Fast Blue (FB) in five immature female piglets of Large Polish White race and the distribution of FB-positive neurons in dorsal root ganglia (DRG) of interest was studied three weeks later. Retrogradely labelled perikarya were found in both left and right DRG Th15-L5, however, with a clear lateralization in the bilateral projections pattern. The number of contralaterally located IMG-projecting perikarya distinctly decreased towards the caudal direction, constituting approximately 23%, 20%, 16%, 6.5%, 4% and 0% of all FB-positive neurons found in both left and right DRG at neuromeres Th15-L5, respectively. The majority of FB-positive neurons (approximately 90% of all traced cells) were found in bilateral L2 and L3 DRG. The largest part of IMG-projecting sensory neurons (68.9 +/- 5.0%), irrespective of their location, belonged to the class of medium-sized perikarya (diameter from 30 to 80 microm). The present study provides a detailed description of distribution and morphological characteristics of spinal afferent neurons projecting to the porcine IMG, simultaneously demonstrating typical features of their inter- and intraganglionic distribution pattern. Furthermore, the present results strongly suggest that spinal afferents contribute to the co-ordination and integration of peripheral reflexes involving sympathetic IMG neurons.