▃Normal Pupil (PY10.17)
Pupil is the circular aperture in the iris diaphragm. Characteristic features of pupil are given below.
Number: Normally, there is one pupil in each eye. Rarely, there may be more than one pupil in one eye. This congenital anomaly is known as polycoria.
Location: Normally, pupil is slightly nasal. Rarely, it may be congenitally eccentric. An eccentric pupil is called corectopia.
Size: Normal size is 2.5 to 4 mm.
Miotic pupils are <2 mm.
Mydriatic pupils are >7 mm.
During sleep, it is smaller due to parasympathetic dominance. Normally, the two pupils are equal in size. This condition is known as isocoria. A difference in pupillary diameter of the two eyes of the same individual by 0.3 mm or more is known as anisocoria. The pupillary size is controlled by two muscles of ectodermal origin:
•Sphincter pupillae—It constricts the pupil (miosis) and is supplied by parasympathetic fibers via the oculomotor (III) nerve originating from the Edinger–Westphal (EW) nucleus. It is arranged in a circular fashion around the pupillary margin.
•Dilator pupillae—It dilates the pupil (mydriasis) and is supplied by sympathetic fibers. It is arranged radially near the root of the iris.
Shape: Normally, it is circular in shape.
Color: Normally, it is grayish black.
Functions of the pupil
•It regulates the amount of light entering the eye by miosis in light and mydriasis in darkness.
•It improves the visual acuity because it prevents the irregular refraction by the periphery of the cornea and lens, and increases the depth of focus.
•It allows passage of aqueous humour from the posterior chamber to the anterior chamber.
▃Afferent Pathway of Pupil
The afferent pupillary pathway consists of the afferent input from the retina, optic nerve, chiasma optic tract, and midbrain pathways. Afferent pupillary defects in any of the retinal layers, up to midbrain pretectal areas, interfere with the input of light to the pupillomotor system. Thus, defect in afferent pupillary pathway results in decrease in contraction of both pupils to light when given to the damaged eye.
■First Order Neuron
It commences at the retinal photoreceptors. The fibers responsible for pupillomotor excitation pass through optic nerve, partially decussate in the chiasma, and enter the optic tracts. The pupillary afferents separate from visual fibers in the posterior third of the optic tract and terminate in pretectal nucleus.
The impulses originating in nasal retina will be conducted by the fibers to contralateral pretectal nucleus, and impulses from temporal retina will be conducted by uncrossed fibers to ipsilateral pretectal nucleus.
■Second Order Neuron
This connects each pretectal nucleus to the EW nucleus on each side. It explains constriction of both pupils on light stimulation of one pupil. The neurons joining pretectal nucleus to EW nucleus are called internuncial neurons. These cross dorsal to the aqueduct. Internuncial neurons are damaged in syphilis and by pinealomas, resulting in light-near dissociation.
▃Efferent Pathways of Pupil
The efferent pupillary pathway is the pupillary motor output from the pretectal nucleus in the midbrain to the ciliary sphincter muscle of the iris. Efferent pupillary defects interfere with contraction or dilatation of the pupil due to damage in the midbrain, and in the peripheral nerve that supplies the iris muscles, causing asymmetrical pupils (anisocoria).
Efferent pathway includes parasympathetic and sympathetic pathways. Efferent pupillary defects may be associated with either damage to the parasympathetic or sympathetic nerves.
The fibers start in the EW nucleus in the midbrain and consist of two neurons:
•EW nucleus to ciliary ganglion via oculomotor (III) nerve.
•Ciliary ganglion to sphincter pupillae via short ciliary nerves.