Abstract
The fine structure, origin, and distribution density of the autonomic nerve endings in the sphincter and dilator muscles of the pupil of the mouse were studied by histochemistry and electron microscopy. Using histochemical methods, the fine nerve plexus in both the muscles shows both catecholamine-positive varicose fibers and acetyicholinesterase-active varicose fibers. In electron micrographs, the axonal varicosities appear as expansions containing many synaptic vesicles. The axonal expansions partly lack the Schwann sheath and directly face the muscle cells. In the sphincter, many of the axonal expansions come in contact with muscle cells, having a gap of 20 to 50 nm in width, while in the dilator a relatively wide space,0.1 to 1.0 μm in width, lies between most of the axonal expansions and muscle cells. The expansions can be classified into two types: Type I having small granular synaptic vesicles, and Type II having agranular vesicles instead of small granular synaptic vesicles. Both Type I acrd II often come in contact with the same smooth muscle cell in the sphincter. In the sphincter, superior cervical ganglionectomy leads to a disappearance of catecholamine-positive nerve endings and an irregular clumping and fragmentation of Type-I expansions followed by their disappearance. Type I in both the sphincter and dilator corresponds to the synaptic ending of the adrenergic fiber originating from the superior cervical ganglion. Type I and II are in the ratio 1: 9 in the sphincter and 3: 2 in the dilator.
