Visual accommodation in vertebrates: mechanisms, physiological response and stimuli

Abstract

The mechanism and stimulation of the accommodative reflex in vertebrate eyes are reviewed. Except for lampreys, accommodation is brought about by intraocular muscles that mediate either a displacement or deformation of the lens, a change of the corneal radius of curvature or a combination of these mechanisms. Elasmobranchs have little accommodation and are emmetropic in water rather than hyperopic as commonly stated. Accommodation in teleosts and amphibians is well understood and achieved by lens displacement. The accommodative mechanism of amniotes is of considerable diversity and reflects different lifestyles rather than phylogenetical relationships. In all amniotes, the ciliary muscle never has a direct impact on the lens. It relaxes the tension applied to the lens by zonular fibers and/or ligaments. In birds and reptiles the ciliary muscle is usually split into two parts, of which the anterior portion changes the corneal radius of curvature. The deformation of the lens is generally achieved either by its own elasticity (humans, probably other mammals and sauropsids) or by the force of circular muscle fibers in the iris (reptiles, birds, aquatic mammals). In the second part of the paper, some of the current hypotheses about the accommodative stimulus are reviewed together with physiological response characteristics.

Collagen organization in the chicken cornea and structural alterations in the retinopathy, globe enlarged (rge) phenotype—An X-ray diffraction study

“…Differences in corneal collagen arrangement between animals are common, and may be marked even within the same taxonomic class (Hayes et al, in press). Furthermore, unique to birds and reptiles is the ability of the cornea to change its curvature as part of the accommodation process (Ott, 2006), suggesting the avian cornea may have specialized biomechanical requirements not shared with mammals.…”

Suspension and optical properties of the crystalline lens in the eyes of basal vertebrates

“…This suggestion is based on morphological and histological observations without the knowledge of the entire lens suspension apparatus as described in the present study. Furthermore, accommodation by protaction of the lens in fishes is, except for Munk's interpretation, described only in Chondrichthyes (Sivak, ; Ott, ). Our results therefore challenge Munk's suggestion that bichirs possess a protractor lentis muscle.…”

The relationship between accommodative amplitude and the ratio of central lens thickness to its equatorial diameter in vertebrate eyes

“…[2][3][4][5][6][7][8][9][10][11] Accommodation induced by a change in corneal curvature, forward or backward movement of the lens, or which involved a system of force mediated by a structure other than the ciliary muscle were not included. Statistical analysis was performed using two binomial distributions: one for the vertebrates with high accommodative amplitudes and one for the vertebrates with low accommodative amplitudes.…”

“…1 This latter means, particularly in man, is the most efficient accommodative mechanism of all. [2][3][4][5][6][7] Diving birds and primates, most notably humans, exemplify the efficiency and speed of accommodation induced by deformation of the lens. Diving birds and primates, such as the Rhesus monkey, accommodate up to a remarkable 40 to 60 diopters.…”