These control the pupillary aperture, allowing the pupil size to vary from 1 to 9 mm.

The sphincter and dilator muscles are located within the stroma.

The posterior epithelial layer is derived from the internal layer of the cup.

The anterior epithelial layer is derived from the external layer of the cup.

Both the dilator and sphincter pupillae muscles are derived from the anterior epithelial layer [3].

The iris stroma consists of fibroblasts, melanocytes, blood vessels, and nerves in a collagen-rich extracellular matrix.

The anterior surface is velvety and lacks epithelium. It is not a barrier for solutes or fluid.

Stromal vessels have non-fenestrated endothelium that maintains the blood-aqueous barrier [6].

Stromal melanocyte pigment concentration determines iris color [7].

A lightly pigmented iris is blue/green as longer wavelengths are absorbed and shorter reflected.

The sphincter is a 1-mm-wide ring of smooth muscle within the pupillary border.

Smooth muscle cells are connected by gap junctions and innervated by parasympathetic nerves.

Uniform pupillary constriction is achieved through simultaneous stimulation of each muscle segment and spread of current through gap junctions [8].

The dilator is a thin peripheral layer of myoepithelium extending from the iris root.

The muscle fibers are long basal processes extending from cells in the anterior iris epithelial layer.

These are interconnected by gap junctions and innervated by sympathetic nerves.

Like pupillary constriction, uniform pupillary dilation is achieved through simultaneous stimulation of each muscle segment and spread of current through gap junctions [8].

The pupil constricts in light and dilates in the dark to help our eye function optimally at different background luminance levels (see Chap. 21) [10].

The pupil’s aperture restricts light rays to the central cornea and lens [11].

This reduces optical blur from refractive error, spherical aberration, and chromatic aberration [11, 12].

The aperture is not too small to cause image degradation from diffraction and reduced illumination [13].

Pupil constriction accompanies accommodation to increase depth of focus [14].

Approaching objects remain relatively well focused as only radially oriented light enters the eye.

The pupil acts as a aqueous conduct channel between the posterior and anterior chambers [15].

This prevents a dangerous buildup of pressure in the posterior chamber.

The light reflex involves photoreceptor cells used in visual perception [17].

Rods provide input for pupillary contractions in scotopic conditions [5].

Cones provide input for large pupillary constrictions in photopic conditions [17, 18].

A specific ganglion cell type (the γ-cell) mediates the light reflex to midbrain pretectal nuclei [19].

As well as receiving rod- and cone-cell input, these ganglion cells contain the photopigment melanopsin and have intrinsic photosensitivity used for nonimage-projecting visual functions (see Chap. 8, The Retina) [20].

Each pretectal nucleus receives:

These travel via the optic nerve, chiasm, and then tract.

Pretectal nuclei neurons send equal bilateral projections to the Edinger-Westphal (E-W) nuclei.

The E-W nuclei are located in the midbrain on either side of the periaqueductal gray matter [21].

The E-W nucleus sends preganglionic parasympathetic fibers in the oculomotor nerve to the ciliary ganglion where they synapse with postganglionic parasympathetic neurons [1, 22].

Factors affecting the light reflex

Stimulus factors (e.g., retinal adaptation, stimulus duration, light intensity, retinal location) that influence visual perception produce a comparable change in pupil responsiveness [16, 23].

With greater intensity stimuli, the amplitude of constriction increases and latency decreases [5].

With long duration stimuli, the pupil may undergo oscillations (hippus) or slow dilation (pupil escape) after initial contraction due to light adaptation [24].

Visual blur and crossed diplopia stimulate the near response.

This resulting in signal generation in cortical visual association areas and frontal eye fields (see Chap. 18, Neural Control of Eye Movements) [26].

These areas control the efferent pathways of the near reflex [27].

Each arm of the triad is conveyed by distinct oculomotor nerve fibers

When awake, centrally mediated sympathetic activity inhibits the E-W nuclei [30].

This inhibitory activity is overcome by the light or near reflexes [31].