Pupillary Fibers

PUPILLARY FIBERS, REFLEXES, CENTERS. The pupil (pupilla) is the opening in the iris of the eye serving to allow light to pass into the eye cavity. In external appearance, the pupil is a regular circle with a smooth edge; its width in daylight in adults equals 1.5–2.0 mm, and in darkness reaches 10 mm. The diameter of the pupil can fluctuate quite significantly in size due to the interaction of smooth muscle fibers laid down in the iris: the muscles arranged radially dilate the pupil (m. dilatator pupillae), while the circular muscles constrict it (m. sphincter pupillae). The m. dilatator pupillae is innervated by the sympathetic system, and the m. sphincter pupillae by the parasympathetic [see separate table (st. 507–508), fig. 3]. The parasympathetic fibers going to the sphincter pupillae originate 1) in the Edinger-Westphal nucleus, situated in the tectum of the midbrain, inside and behind the main large-cell nucleus of the oculomotor nerve (n. oculomotorius) beneath the aqueduct of Sylvius, and 2) in the solitary median or central nucleus of Perlia, located there, between the two main nuclei. The fibers from these nuclei, forming preganglionic fibers, proceed with the n. oculomotorius into the orbital cavity and terminate there in the ciliary ganglion (g. ciliare), located on the n. opticus; from the g. ciliare begin the short ciliary nerves (pp. ciliares breves), or postganglionic fibers, which penetrate the thickness of the eyeball and terminate in the mm. sphincter pupillae and m. ciliaris. Stimulation of the g. ciliare, nn. ciliares breves, and n. oculomotorius causes maximum constriction of the pupil. The fibers dilating the pupil originate in the spinal cord, in the cells of the lateral horns (nucleus sympathicus lateral, superior) of the C8 and D1 segments, in the so-called cilio-spinal center, exit the spinal cord together with the anterior roots, and then through the white rami communicantes and the cervical part of the sympathetic trunk proceed to the superior cervical ganglion, where they terminate (preganglionic fibers); in the ganglion also originate postganglionic fibers, which, forming the carotic plexus around the internal carotid artery, penetrate together with the artery into the cranial cavity, then pass into the ramus naso-ciliaris of the n. trigemini, reach the orbit together with it, and then, like the fibers of the n. trigemini, terminate in the g. ciliare; the sympathetic fibers only permeate it and proceed into the thickness of the eye to the m. dilatator. In lesions of the C8 and D1 segments of the spinal cord, as well as of the cervical sympathetic nerve, constriction of the pupil and other symptoms are observed (see Horner's symptom complex). Stimulation of these departments causes dilation of the pupil. The sympathetic center (cilio-spinal center) is dependent on the corpus Luysi; stimulation of the medial-frontal part of this formation causes excitation of the sympathetic nerve—dilation of the pupil and palpebral fissure, especially on the opposite side. Besides the subcortical center, the existence of a cortical center located in the anterior parts of the frontal lobe is recognized by some. The conductors starting in the cortical center go to the subcortical one, where they are interrupted, and from there a new system of conductive fibers arises, going to the spinal cord and undergoing an incomplete decussation, as a result of which the n. sympathicus is connected with the centers of both sides. Stimulation of certain areas of the occipital and parietal lobes causes constriction of the pupil, but it has not yet been definitely established what significance should be attributed to these areas of the cerebral cortex, as well as by what path the irritation is transmitted to the subcortical center of the m. sphincter pupillae. Movements of the pupil are involuntary; part of them belong to pure reflexes: reaction to light, to pain; another part belongs to conjugate movements: reaction to accommodation, to convergence. The reaction to light is detected by alternating illumination and darkening of the eye; at the moment of light falling on the eye, constriction of the pupil occurs, and at the moment of darkening, dilation of it. Constriction of the pupil depends on the transmission of light irritation through the n. opticus to the anterior quadrigeminal bodies, from there to the parasympathetic nuclei of the n. oculomotorius, where the fibers innervating the m. sphincter pupillae originate. Since the optic fibers undergo a decussation in the chiasma, the light irritation of each eye separately leads to simultaneous constriction of both pupils. The constriction of the pupil of the eye being illuminated is called the direct reaction, and the constriction of the other pupil upon illumination of the first is called the consensual reaction. The path for transmitting light irritation to the n. sympathicus is not yet sufficiently studied; there is a hypothesis that part of this path does not go to the nuclei of the n. oculomotorius, but to the corpus Luysi. On painful irritation in any part of the body, the pupil reacts with dilation; the reflex center for transmitting these irritations to the m. dilatator pupillae is the corpus Luysi, receiving fibers from the lemniscus. The reaction of the pupil to accommodation and convergence occurs when viewing some object at a close distance or when bringing the eyeballs inward. Besides these reactions, there are reactions to winking—constriction of the pupils upon winking. A whole series of mental moments (fright, fear, attention, etc.) causes dilation of the pupil; this reaction is considered a cortical reflex. In examining the pupils, the face of the subject must be turned directly to the light source, both eyes must be evenly illuminated. First of all, attention is paid to the size of both pupils, to their shape; the size fluctuates depending on age (in elderly persons the pupils have a narrower caliber), and on the degree of illumination of the eye (the weaker the illumination, the wider the diameter of the pupil). Then the examination of the pupillary reaction to light, pain, and accommodation of the eye proceeds. In examining the reaction to light, the examiner covers both eyes of the patient with his hands—one eye remains covered all the time, while the other eye is alternately uncovered and then covered again, i.e., alternately illuminated and darkened. In the absence of daylight, a candle or electric lamp is used, moving the source of light toward or away from the eye; the other eye remains closed all the time. Thus the direct reaction to light is examined. In examining the consensual reaction, the eye for which this reaction is determined remains open; the other eye is alternately illuminated and darkened; at the moment of illumination of the closed eye, the pupil of the open eye constricts, and conversely—upon darkening, it dilates. In the moment of examining the painful reaction, the patient remains in the same position relative to the light; then a prick is made on some area of the skin and the pupils are observed, which begin to dilate noticeably. The reaction of the pupils during accommodation of the eye is determined as follows: the examiner places himself in front of the patient and, holding some object before his eyes, moves it, either removing it or approaching it; the patient must continuously follow the object; at the moment of removing the object, the pupils dilate, and at the moment of approaching, they constrict. In various diseases of the nervous system affecting the pupillary centers or fibers, changes in the shape of the pupil, its magnitude, and reaction are observed. The pupil may be dilated (mydriasis) depending on two moments: from paralysis of the m. sphincter pupillae (disruption of conductivity in the parasympathetic system—mydriasis paralytica) or from spasm of the m. dilatator pupillae (irritation of the sympathetic system in the departments relating to the pupil)—mydriasis spastica; or, conversely, constriction of the pupil (miosis) may be observed, depending on the opposite causes: irritation of the parasympathetic system (miosis spastica) and paralysis of the sympathetic (miosis paralytica). Change in the width of the pupil may be observed simultaneously in both eyes or only in one; in one eye the pupil may be dilated, in the other constricted; inequality of the pupils is called anisocoria (see). The contours of the pupil may be uneven, the phenomenon of hippus may be observed, the pupil may be jumping, i.e., change its magnitude, one dilating and the other constricting, and vice versa. The reaction of the pupils may be sluggish, partially absent [only to light (Argyll Robertson symptom, see) or only to accommodation and convergence]; hemianopic reaction of the pupils is observed (see); change in the painful reaction, paradoxical reaction—dilation in light and constriction in darkness—and finally complete immobility of the pupil.

E. Kononova.