Rainbow

The iris is the most anterior part of the vascular tunic, visible through the transparent cornea. It has the form of a disk about 0.4 mm thick, placed in the frontal plane. In the center of the iris there is a round opening - the pupil (рupilla). The diameter of the pupil is variable. The pupil narrows in strong light and expands in the dark, acting as a diaphragm of the eyeball. The pupil is limited by the pupillary edge (margo pupillaris) of the iris. The outer ciliary edge (margo ciliaris) is connected to the ciliary body and to the sclera by means of the pectineal ligament (lig. pectinatum indis - NBA). This ligament fills the iridocorneal angle (angulus iridocornealis) formed by the iris and cornea. The anterior surface of the iris faces the anterior chamber of the eyeball, and the posterior surface faces the posterior chamber and lens.

The connective tissue stroma of the iris contains blood vessels. The cells of the posterior epithelium are rich in pigment, the amount of which determines the color of the iris (eye). If there is a large amount of pigment, the eye is dark (brown, hazel) or almost black. If there is little pigment, the iris will be light gray or light blue. In the absence of pigment (albinos), the iris is reddish, since the blood vessels shine through it. There are two muscles in the thickness of the iris. Bundles of smooth muscle cells are located circularly around the pupil - the sphincter of the pupil (m. sphincter pupillae), and thin bundles of the muscle that dilates the pupil (m. dilatator pupillae) - the pupil dilator - extend radially from the ciliary edge of the iris to its pupillary edge.

Innervation of the pupil

The size of the human pupil 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 pathway goes from the hypothalamus through the brainstem and the cervical part of the spinal cord, then exits the spinal canal along with the anterior roots (CVIII-ThI-ThII) and returns to the skull again.

For convenience of description, the section of 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 region of the pons and tegmentum of the midbrain); the section from the ciliospinal center to the superior cervical ganglion, the second neuron; the section from the superior ganglion to the muscle that dilates the pupil, the third neuron.

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

In humans, most of the preganglionic fibers that innervate the eye leave the spinal cord with the anterior roots of the first thoracic segment. A small portion may also go with the roots of CVIIII and ThIII. From here, the fibers pass through the white connecting branches to the paravertebral sympathetic chain. Then, without forming synapses, they continue upward and pass through the lower and middle cervical ganglia, eventually reaching the superior cervical ganglia.

The superior cervical ganglion, which is the fusion of the first four cervical sympathetic ganglia, is located between the internal jugular vein and the internal carotid artery, below the base of the skull (i.e., somewhat higher than is usually believed). The oculosympathetic and sudomotor fibers of the face form synapses here.

Postganglionic fibers (third neuron). The fibers that innervate the dilator pupillae muscle leave the ganglion and accompany the internal carotid artery in the carotid canal and foramen lacerum, reaching the region of the trigeminal ganglion. The sympathetic fibers closely adhere to the internal carotid artery in the cavernous sinus. Most of them join the ophthalmic part of the trigeminal nerve, penetrating the orbit with its nasociliary branch. The long ciliary nerves leave this branch, bypass the ciliary ganglion, pierce the sclera and choroid (both nasally and temporally), and finally reach the dilator pupillae muscle.

Postganglionic sympathetic fibers also pass to other structures of the eye. Those that innervate blood vessels or the 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 roots" of the ciliary ganglion, passing through these structures (without forming synapses) on their way to their effector organs.

Most of the sudomotor and piloerection fibers that innervate the face leave the superior cervical ganglion and reach their destination by passing through a plexus along the external carotid artery and its branches. The sudomotor fibers that go to the forehead may return to the skull and then accompany the fibers that go to the muscle that dilates the pupil for most of the way, eventually reaching the gland along with the ophthalmic artery and its superior orbital branch.

Parasympathetic innervation of the muscle that constricts the pupil (sphincter)

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

The first (preganglionic) neuron originates in the Yakubovich-Edinger-Westphal nucleus in the rostral midbrain. It is part of the third cranial nerve, its branch to the inferior oblique muscle and the short root of the ciliary ganglion. This ganglion is located in the loose fatty tissue of the orbital apex, between the optic nerve and the lateral rectus muscle.

The second (postganglionic) neuron originates from the cell bodies of the ciliary ganglion. The fibers travel as part of the short ciliary nerves and reach the sphincter of the pupil. On their way, these fibers pierce the area of the posterior pole of the eyeball, then go forward, first directly in the sclera, and then in the plexus of the subchoroidal space. Damage in these areas is more common than most neurologists believe. The overwhelming majority of such patients are referred to ophthalmologists.

All fibers supplying the constrictor pupillae muscle probably reach the iris, synapsing in the ciliary ganglion. The suggestion that the cholinergic fibers that innervate the constrictor pupillae muscle bypass the ciliary ganglion or synapse in the episcleral cells sometimes found along the short ciliary nerves has no anatomical basis.

It is important to emphasize that the vast majority (94%) of parasympathetic postganglionic fibers leaving the ciliary ganglion are not related to pupillary constriction. They disperse in the ciliary muscle and are related to accommodation. These observations are crucial for the current understanding of the pathogenesis of Adie syndrome.

Pupillary reflexes

The pupil has reciprocal innervation from the parasympathetic and sympathetic systems. Parasympathetic influences lead to constriction of the pupil, sympathetic influences - to dilation. With 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 pupil size.

The mental reflex of the pupils is the dilation of the pupils during various emotional reactions (happy or unpleasant news, fear, surprise, etc.). The reflex is associated with the state of the brain, which affects the sympathetic innervation of the pupils. Impulses from the hemispheres of the brain through the brain stem and the cervical spinal cord enter the ciliospinal centers, and then along the efferent fibers of the latter - to the dilator of the pupil. This makes it clear that the pupil function is impaired in various brain lesions (epilepsy, meningitis, tumor, encephalitis).

Trigeminal pupillary reflex: short-term irritation of the cornea, conjunctiva of the eyelids or tissues surrounding the eye causes first dilation of the pupils, then rapid constriction. Reflex arc: 1st branch of the trigeminal nerve, trigeminal ganglion, nuclear center of the ophthalmic branch of the nerve, posterior longitudinal fasciculus, nucleus of the sphincter of the pupil (Yakubovich-Edinger-Westphal), efferent pathways to the sphincter of the pupil. In case of disease (inflammation) of the sclera of the eye, 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 explained by the fact that 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 nasofacial pupillary reflex consists of dilation of the pupil on the side of irritation in the nostril (during tamponade, tickling, etc.). Any intense irritation in one nostril is accompanied by bilateral vigorous dilation of the pupils. The arc of this reflex is constructed from the sensory fibers of the trigeminal nerve and sympathetic pupillary pathways.

The respiratory pupillary reflex is the dilation of the pupils during a deep inhalation and their constriction during exhalation. This reflex is extremely variable and constitutes a vagotonic reaction of the pupils, since it is mainly associated with the excitation of the vagus nerve.

The pupillary reflexes to physiological stress include the cervical reflex of the pupils (dilation when the muscles of the neck or sternocleidomastoid muscle are compressed) and dilation of the pupils when shaking hands.

Neuropharmacological tests based on the detection of denervation hypersensitivity are widely used in the differential diagnosis of pupillary disorders. They allow one to differentiate ptosis and miosis due to damage to the third neuron of the sympathetic innervation of the muscle that dilates the pupil from disorders in which Horner's symptom is based on more proximal damage to the conducting pathways to this muscle. They are used for the differential diagnosis of Adie's syndrome (the cause of which, as noted above, is currently considered to be damage to the postganglionic parasympathetic fibers that innervate the muscle that constricts the pupil) from disorders in which large pupil sizes are caused by damage to the preganglionic fibers that innervate the sphincter of the pupil. Such studies allow one to study pupillary dysfunctions of interest to a neurologist in a way that is easily accessible to visual observation.

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