Gross Anatomy

The gross anatomy of the iris can be divided into four layers. The first is the anterior border layer. The second contains the stroma and the sphincter muscle. The third is the anterior epithelium, which differentiates into the dilator muscle. The fourth is the posterior pigmented epithelium, which gives most of the color to the iris. The further development of a brown iris depends on the progressive increase in stromal melanosomes. When there are few melanosomes, the shorter blue wavelengths pass through the stroma and are reflected as blue from the posterior pigmented layer, which does not absorb them. As the melanosomes of the stroma increase, the color progresses to gray and then brown. The development of the melanosomes occurs during the first year of life and depends on sympathetic innervation. Lack of this sympathetic stimulation leads to the heterochromia that is a part of congenital or neonatal sympathetic paralysis (Homer syndrome).

Parasympathetic System

The ocular motor complex in the mesencephalon contains a paired group of cells called the Edinger-Westphal nucleus. These are the visceral nuclei and are located dorsal medial to the somatic nuclei. They contain preganglionic parasympathetic cell bodies that send processes to synapse in the ciliary body. Between these two paired columns of somatic cells is the nucleus of Perlia. The anterior medial nuclei are located ventral to the complex and are near the midline. Preganglionic pupil fibers not only arise from this complex but also may come from Perlia's nucleus. The exact part of the preganglionic fibers contributed by Perlia's nucleus is not certain. There is still a controversy over which part of the Edinger-Westphal nucleus supplies the pupil and which part accommodation. The parasympathetic fibers follow the course of the oculomotor nerve (see Figs. 6.15, 6.16, 6.17, and 6.18) from the brainstem, exit in the interpeduncular fossa, and travel with the oculomotor nerve all the way into the cavernous sinus. During its course, the parasympathetic fibers are located in the peripheral superior part of the oculomotor nerve. In the cavernous sinus, the parasympathetic fibers travel with the inferior division of the third nerve into the orbit and then enter the ciliary ganglia, where most of the fibers are preganglionic parasympathetic. Other fibers from a branch of the oculomotor nerve to the inferior oblique muscle also enter and synapse with cell bodies in the ciliary ganglion. All these fibers now become post-ganglionic fibers and become the short ciliary nerves that pass forward between the sclera and the choroid and innervate the iris sphincter and ciliary body.

Other fibers pass through the ciliary ganglion without synapsing. These are post-ganglionic sympathetic fibers from the superiorcervical ganglion. They pass through the ciliary ganglion without synapsing and continue on also as short ciliary nerves. These fibers then act as vasomotor fibers for the iris vessels, and a very small number innervate the dilator muscle.

Ninety-four percent of the fibers that synapse in the ciliary ganglion go to the ciliary body. The ratio of pupillary fibers to the light reaction to those to accommodation appears to explain the preference of fiber regeneration for the near reflex over that to the light reflex, as seen in Adie's pupil.

The short ciliary nerves that contain post-ciliary ganglion parasympathetic fibers pass forward in the space between the choroid and sclera mix and form into a complex that innervates the iris sphincter, ciliary muscle, and vasomotor responses in the iris vessel walls. This complex appears to innervate the muscles in a random manner. However, Thompson and others feel that there is a more direct and specific fiber to muscle cell relationship. In general, the postganglionic fibers from the ciliary ganglion innervate the ciliary muscle and the iris sphincter.

Sympathetic System

The first neuron of the sympathetic fibers begins in the posterior hypothalamus, traverses the midbrain and reticular substance of the pons, and ends in the anterior lateral gray substance of the spinal cord (Fig. 3.6). It synapses somewhere between C-8 and T-2, in what is referred to as the ciliospinal center (Budge's center). The second neuron begins when the fibers leave the spinal cord via the white rami communicantes of C-8 to T-2, traveling through the stellate ganglion and vertical sympathetic trunk to synapse at the superior cervical ganglion, where the second neuron is completed. The work of Palumbo suggests that the preganglionic neurons controlling the pupil enter the upper one-half of the stellate ganglion by a separate paravertebral root. The stellate ganglion contains the fibers from the lower cervical and first thoracic ganglia. He noted that unlike taking the entire stellate ganglion, When he resected the preganglionic root and only the lower one-half of the ganglion, he did not produce a Homer's pupil in 93% of the cases. He concluded that the pupillary fibers must travel by a separate root to only the upper half of the ganglion.

The third neuron is composed of fibers dial form a plexus around the carotid artery. As the carotid artery bifurcates, the fibers subserving sweating follow the external eurotid artery, a fact that is important in the localization of the lesion producing Homer's Nyndrome. The main fibers go with the internal carotid artery into the carotid canal, in which a thin-walled structure separates the carotid artery from the tympanic cavity. The sympathetic fibers penetrate the carotid tympanic wall and form, the caroticotympanic nerve, which runs submucosally in the middle ear. They then pass through the cranial vault near the pterygoid canal and enter the cavernous sinus. Passing over the gasserian  ganglion, they enter the orbit via the superior orbital fissure. The sympathetic fibers that innervate the dilator muscle branch with the nasal ciliary nerve, bypass the ciliary ganglion, and then branch into the long ciliary nerves and innervate the dilator muscle. These enter the suprachoroidal space nasally and temporally beneath the horizontal recti muscles to join the anterior neural plexus. The other sympathetic fibers pass through the ciliary ganglion without synapsing and travel with the short ciliary nerves to become vasomotor fibers for the iris vessels.

Another group of fibers that pass through the ganglion without synapsing is the long sensory root that leaves the ganglion to travel with the nasociliary nerve of the ophthalmic division of the trigeminal nerve.

The afferent pupillary fibers are not retina specific and are believed to begin with photoreceptor cells. They travel with the axons of the optic nerve and branch off just short or the lateral geniculate body. The fibers subserving the pupillary light reflex branch off the optic tract before the lateral geniculate body and travel in the brachium of the superior colliculus, where they synapse in the pretectal region of the mesencephalon. Stereotaxic stimulation studies by Ranson and Magoun demonstrated pupillary constriction when this area was stimulated and lack of constriction when this same area was bilaterally destroyed.

Change in accommodation is produced by movement of the ciliary body with a resultant change in shape of the lens from contraction or relaxation of the zonules. The ciliary muscle is divided into three segments: meridional, radial, and circular portions. Convincing evidence demonstrates a dual innervation for the ciliary muscle. One of our group, R. Fenton, examined the ciliary muscle of a patient with a congenital oculomotor nerve palsy and found only the circular muscle hypoplastic. Similar findings are seen in cases of sympathetic paralysis in the longitudinal and radial portions of the ciliary muscle. These parasympathetic fibers appear to originate in the caudal portion of the parasympathetic nucleus in the mesencephalon, as suggested by the stereotaxic work of Bender. There is some evidence put forth by Burde that there may also be a more direct parasympathetic route to the ciliary muscle, bypassing the ciliary ganglion.