Iris

Iris is the most anterior part of the choroid, visible through a transparent cornea. It has the form of a disk with a thickness of about 0.4 mm, placed in the frontal plane. In the center of the iris there is a round hole - the pupil (pupilla). The diameter of the pupil is unstable. The pupil tapers under strong light and expands in the dark, performing the role of the diaphragm of the eyeball. The pupil is confined to the pupil margin (margo pupillaris) of the iris. The outer ciliary edge (margo ciliaris) connects with the ciliary body and the sclera with the help of a comb (lig. Pectinatum indis - NBA). This ligament fills the iris-corneal angle formed by the iris and cornea (angulus iridocornealis). The front surface of the iris is facing toward the anterior chamber of the eyeball, and the posterior to the posterior chamber and lens.

Blood vessels are located in the connective tissue stroma of the iris. Cells of the posterior epithelium are rich in pigment, the amount of which determines the color of the iris (eye). If there is a lot of pigment, the eye color is dark (brown, brown) or almost black. If the pigment is small, then the iris will have a light gray or light blue color. In the absence of pigment (albinos), the iris is of a reddish color, as through it the blood vessels appear through. In the thickness of the iris are two muscles. Around the pupil, there are circular beams of smooth muscle cells - the sphincter pupillae, and radially from the ciliary edge of the iris to its pupillary margin, there are thin bundles of muscle dilating the pupil (dilatator pupillae), the dilator of the pupil.

Pupillary innervation

The size of the pupil of the person is controlled by two smooth muscles - the dilator and the sphincter of the pupil. The first receives sympathetic innervation, the second - parasympathetic.

Sympathetic innervation of the muscle that dilates the pupil (dilator)

The descending path proceeds from the hypothalamus through the brain stem and the cervical part of the spinal cord, then exits the spinal canal together with the front roots (CVIII-ThI-ThII) and returns to the skull.

For ease of description, the pathway between the hypothalamus and the cervical ciliospinal center (see below) is called the first neuron (although it is probably interrupted by several synapses in the bridge and midbrain areas); the region from the ciliospinal center to the upper cervical node - the second neuron; a segment from the upper node to the muscle that widens the pupil, the third neuron.

Preganglionic fibers (second neuron). The cells of the cells lie in the gray intermediolateral columns of the lower and upper thoracic segments of the spinal cord, forming the so-called Buccian ciliospinal center.

In humans, most preganglionic fibers innervating the eye leave the spinal cord along with the front roots of the thoracic segment I. A small part can also go in the composition of the rootlets CVIIII and ThIII. From here through the white connecting branches of the fiber pass to the near-vertebral sympathetic chain. They then, without forming synapses, continue upward and pass through the lower and middle cervical nodes, eventually reaching the upper cervical node.

The upper cervical node, which is the fusion of the first four cervical sympathetic nodes, is located between the internal jugular vein and the internal carotid artery, below the base of the skull (i.e., somewhat higher than commonly thought). The oculosympathic and navigational fibers of the face form here synapses.

Postganglionic fibers (third neuron). Fibers that innervate the muscle dilating the pupil leave the node and accompany the internal carotid artery in the carotid canal and the torn hole, reaching the area of the trigeminal node. Sympathetic fibers closely prilensat to the internal carotid artery in the cavernous sinus. Most of them connect with the ocular part of the trigeminal nerve, penetrating into the orbit with its nasociliary branch. Long ciliary nerves leave this branch, bypass the ciliary node, perforate the sclera and vascular membrane (both nasally and temporally) and eventually reach the muscle that expands the pupil.

Postganglionic sympathetic fibers also pass to other structures of the eye. Those of them that innervate blood vessels or uveal chromatophores of the iris participate in the formation of the initial part of the postganglionic pathway. They leave the nasociliary nerve as the "long root" of the ciliary node, passing through these structures (without the formation of synapses) on their way to their effector organs.

Most shipborne and pilo-rectal fibers innervating the face area leave the upper cervical node and reach their destination by passing through a plexus located along the outer carotid artery and its branches. The sudomotor fibers that go to the forehead can again return to the skull and then most of the way to accompany the fibers that go to the muscle that expands the pupil, eventually reaching the gland together with the eye artery and its upper glabular branch.

Parasympathetic innervation of the muscle, narrowing the pupil (sphincter)

The descending pathways to the pupil sphincter go through two systems of neurons.

The first (preganglionic) neuron begins in the nucleus of Yakubovich-Edinger-Westphal in the rostral part of the midbrain. It goes in the III cranial nerve, its branches to the lower oblique muscle and the short root of the ciliary node. This node is located in the loose fat tissue of the apex of the orbit, between the optic nerve and the external rectus muscle.

The second (postganglionic) neuron begins from the cage body cells. The fibers go in the composition of short ciliary nerves and reach the sphincter of the pupil. On their way, these fibers perforate in the region of the posterior pole of the eyeball, then go anteriorly, first directly in the sclera, and then in the interweaving of the subchoroidal space. Damages in these areas are more common than most neurologists believe. The overwhelming majority of these patients are found in ophthalmologists.

All fibers that fit the muscle that narrows the pupil are likely to reach the iris, forming a synapse in the ciliary node. The assumption that the cholinergic fibers that innervate the muscle, the narrowing pupil, pass the ciliary knot or form synapses in the episcleral cells, sometimes found along the short ciliary nerves, has no anatomical basis.

It is important to emphasize that the overwhelming majority (94%) of parasympathetic postganglionic fibers leaving the ciliary node are not related to pupillary narrowing. They are scattered in the ciliary muscle and are associated with accommodation. These observations are crucial for a modern understanding of the pathogenesis of the syndrome of Adi.

Pupillary reflexes

The pupil has reciprocal innervation from the side of the parasympathetic and sympathetic systems. Parasympathetic influences result in pupil constriction, sympathetic ones lead to dilatation. With a complete block of parasympathetic and sympathetic innervation, pupillary reflexes are lost, but the pupil size remains normal. There are many different stimuli that cause changes in the size of the pupil.

Psychic reflex pupils - the expansion of pupils in various emotional reactions (joyful or unpleasant news, fear, surprise, etc.). Reflex is associated with the state of the brain, affecting the sympathetic innervation of the pupils. Impulses from the cerebral hemispheres through the brain stem and cervical spinal cord enter the ciliospinal centers, and then along the efferent fibers of the latter to the dilator pupil. This makes clear the dysfunction of the pupils at various brain lesions (epilepsy, meningitis, swelling, encephalitis).

Trigeminal pupillary reflex: short-term irritation of the cornea, conjunctiva of the eyelids or tissues surrounding the eyes, first gives pupil dilatation, then a rapid contraction. Reflex arc: I branch of trigeminal nerve, triple node, nuclear center of ophthalmic nerve branch, posterior longitudinal fascicle, nucleus of sphincter of pupil (Yakubovich-Edinger-Westphal), efferent pathways to pupillary sphincter. In case of a disease (inflammation) of the eye sclera, conjunctivitis, etc., the pupils very often become narrower, and sometimes there is a noticeable decrease in the amplitude of their reaction to light. This is because the inflammatory process leads to irritation of the trigeminal fibers of the eyeball, and this entails a reflex change in the parasympathetic pupillary innervation.

The nasophacial pupillary reflex consists in dilating the pupil on the side of irritation in the nostril (with tamponade, tickling, etc.). Any intense stimulation in one nostril is accompanied by a two-way vigorous dilatation of the pupils. The arc of this reflex is constructed of sensitive fibers of the trigeminal nerve and sympathetic pupillary tracts.

Respiratory pupillary reflex - dilated pupils with deep inspiration and constriction when exhaled. This reflex has extreme inconstancy and constitutes the vagotonic reaction of the pupils, since it is mainly associated with the excitation of the vagus nerve.

Among the reflexes of the pupils on physiological stress are the cervical reflex of the pupils (dilatation of the muscles of the neck or the nerve muscle) and the dilatation of the pupils with a handshake.

In the differential diagnosis of pupillary disorders, neuropharmacological tests based on the detection of denervation hypersensitivity are widely used. They allow you to differentiate ptosis and miosis due to damage to the third neuron of sympathetic innervation of the muscle, dilating the pupil, from disorders in which the basis of Horner's symptom is the more proximal damage to the pathways to this muscle. They are used for the differential diagnosis of the syndrome of Adi (the cause of which, as mentioned above, is currently considered to be damage to postganglionic parasympathetic fibers innervating the muscle, narrowing pupil) from disorders in which large pupil sizes are caused by injuries of preganglionic fibers innervating the sphincter of the pupil. Such studies make it possible to study the easily accessible visual observation in a manner of interest to the neurologist for impaired pupillary functions.

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