Intraganglionic portal sinus located between small intensely fluorescent (SIF) cells and principal ganglionic neurons in the inferior mesenteric ganglion of the guinea pig.

The vascular system in the inferior mesenteric ganglion of the guinea pig was studied to clarify the transport pathway of transmitters released by the small intensely fluorescent (SIF) cells to the principal ganglionic neurons. Reconstruction of about 1500 1-micron-thick serial sections of the ganglion demonstrated its portal system. SIF cells were tightly packed and formed two or three clusters under the capsule of the ganglion. Branches from the inferior mesenteric artery ran directly toward these clusters and broke up into a number of coiled and looped sinusoid capillaries among the SIF cells. They then drained into a large sinus surrounding the clusters in the ganglion. Capillaries were derived from this sinus and ramified among the principal ganglionic neurons. After supplying the neurons, these vessels drained into veins surrounding the ganglion. Therefore, as we observed two distinct groups of capillaries, we call this sinus the "intraganglionic portal sinus". All the transmitters secreted from the SIF cells are collected into this intraganglionic portal sinus and are then conveyed through the capillaries to the principal ganglionic neurons.