Innervation of the iris by individual parasympathetic axons in the adult mouse

Ciliary neurones were identified electrophysiologically in isolated ganglia and the projection of individual axons to the intrinsic muscles of the eye determined by labelling identified cells with horseradish peroxidase (HRP). The ciliary ganglion of the adult mouse contains 80-100 parasympathetic neurones clustered at the point where the oculomotor nerve branches to supply the medial and inferior rectus. These nerve cells show varying degrees of dendritic complexity (zero to six primary dendrites) and differing degrees of innervation by preganglionic axons (one to seven inputs). Cells with more complex dendritic arborization generally receive synaptic input from commensurately larger numbers of preganglionic axons. The axons of individually labelled cells give rise to varicose terminals in the iris and ciliary body; in most cases a single parent axon branches only after entering the iris, producing a network of terminals in the region of both the sphincter and the antagonist dilator muscle. Cells with both simple and complex dendritic geometry show this pattern of axonal arborization. The usual net effect of stimulating a single ciliary ganglion neurone through an intracellular electrode is constriction of the pupil. Since the terminals of individual ciliary ganglion neurones are commonly distributed to both the sphincter and dilator regions of the iris, impulses invading the terminals of a single ciliary neurone cause a contraction of the sphincter muscle, perhaps accompanied by a simultaneous relaxation of the dilator. Direct examination of individually labelled neurone has revealed details of the pattern of parasympathetic axon distribution to the iris, thus accounting for the origin of cholinergic terminals in the dilator, classically considered to be primarily a target of adrenergic sympathetic neurones.