Sensory nerve fiber terminals in the arachnoid granulations of non-human primates.

Myelinated and unmyelinated axons terminate within the arachnoid granulations. Serial section analysis by light and transmission electron microscopy reveals that the nerve fibers terminate at different locations or tissue compartments of the arachnoid granulation. 1. Myelinated axons ramify and terminate as free axon terminals like slowly adapting type II mechanoreceptors in the leptomeningeal connective tissue core of the granulation. 2. Myelinated and unmyelinated axons terminate in the transitional spongy zone which represents a tissue compartment for the diffusion of the cerebrospinal fluid from the subarachnoidal space to the venous sinus. This zone is composed of arachnoidal cells, dural neurothelial cells and fibrocytes. 3. Myelinated and unmyelinated axons terminate within lymphoid cell aggregation associated to the arachnoid granulation.