Sympathetic and afferent neurons projecting in the splenic nerve of the cat

Afferent and sympathetic postganglionic nerve cell bodies projecting in the splenic nerve of the cat have been labeled retrogradely with horseradish peroxidase (HRP) in order to study numbers and locations of these neurons. Labeled somata were found bilaterally in dorsal root and sympathetic paravertebral ganglia T3-L2 with a maximum in T10-T13 and in the coeliac superior mesenteric ganglion complex. About 13,200 neurons project into the splenic nerve of the cat, 12,550 being postganglionic and 650 afferent. About 98% of the postganglionic neurons were located in prevertebral and 2% in paravertebral sympathetic ganglia. About 73% of the afferent and 66% of the postganglionic paravertebral neurons were located on the left side. The density of the splenic innervation is about 3-4 times higher than that of the kidneys when normalized to the weight of both organs. It is discussed whether the afferent and sympathetic innervation of the spleen is not only involved in cardiovascular functions but also in regulation of the immune response of this organ.