The autonomic innervation of the rat diaphragm

The innervation of the diaphragm has been studied by three methods--cobalt tracing of the nerves, demonstration of cholinesterase activity and fluorescence microscopy for catecholamines and VIP. The cobalt method reveals the peripheral nerve fibers with a sharpness similar to that shown at the level of the central nervous system where this method has so far been more widely applied. The cobalt method helps to outline the distribution pattern of the nerve fibers and it can be of particular interest at the level of the viscera in order to show the different sources of the axons. Fibers giving a positive response to cholinesterase staining are shown at the level of the motor end plates and surrounding the blood vessels. It is suggested that the axons of phrenic origin contribute to the motor end plates while those coming from the vagus are distributed along the connective tissue surrounding the vascular system. Noradrenergic innervation is scarce, appearing as fine varicosities around the vascular beds. The VIPergic fibers are probably, together with the cholinergic ones, the most widespread. They are distributed among the muscle fascicules as well as being in close connection with the blood vessels.