Nerve cells in the human ciliary muscle: ultrastructural and immunocytochemical characterization

Intrinsic nerve cells in the human ciliary muscle were identified and characterized by immunohistochemical and ultrastructural methods.

Serial sections through the ciliary muscle of 10 human donors (age range, 53 to 91 years) were investigated by electron microscopy, NADPH-diaphorase (NADPH-d) staining, and immunohistochemistry. Antibodies against nitric oxide synthase (NOS), protein gene product 9.5 (PGP 9.5), neurofilament proteins, tyrosine hydroxylase (TH), neuropeptide Y (NPY), vasoactive intestinal peptide (VIP), substance P (SP) and calcitonin gene-related peptide (CGRP) were used. Nerve cell density per millimeter of circumferential width was analyzed in three eyes, and in one eye the total number of neurons in the entire ciliary muscle was evaluated.

Small (70% of the total; longitudinal diameter 10 to 14 microns) and large (longitudinal diameter 20 to 30 microns) ganglion cells were identified in the inner parts of the muscles' reticular and circular portions. No nerve cells were observed in the anterior longitudinal portion. The cells were in contact with unmyelinated axons and synaptic boutons containing small agranular and large granular vesicles. Axo-somatic and axo-dendritic synapses were observed. Histochemically and ultrahistochemically, the neurons stained intensely for NADPH-d. Both cell types were multipolar and expressed long filamentous processes. Axonal processes with periodic swellings suggesting varicosities ran close and parallel to neighboring muscle bundles. Some nerve cells were connected with each other by axonal processes. No perivascular NADPH-d-positive nerves were seen around ciliary muscle vessels, but they were present in the wall of the major arterial circle of the iris. A small number of ganglion cells contributed to this perivascular network. NADPH-d-positive neurons stained for PGP 9.5 and NOS. No TH, NPY, or VIP-positive ciliary muscle neurons were observed. In double labeling experiments, 70% of the nerve cells were in contact with nerve endings expressing SP-like and CGRP-like immunoreactivity. Seventeen to 32 NADPH-d-positive neurons were counted per millimeter of ciliary muscle circumferential width, with 923 in the entire ciliary muscle of one donor eye.

The presence of intrinsic NOS-positive nerve cells concentrated in the inner parts of the ciliary muscle might indicate a physiological role of nitric oxide for disaccommodation or fluctuations during accommodation.