Physiology - Image formation

Image formation

Formation of a clear image is achieved through three important processes: light refraction, accommodation and pupil diameter.

Light refraction

Figure 1: Image formation in the eye. Light refraction at the air-cornea interface is greater than refraction at the aqueous humour-lens inferface.
Source: Sjaastad OV, Sand O, Hove K. 2010. Physiology of domestic animals. 2nd ed. Oslo: Scandinavian Veterinary Press.

• Cornea and lens refracts light
• Refraction at air-cornea interface > refraction at aqueous humour-lens interface
• Cornea responsible for most of eye's refractive power
• Inverted image (upside down) focused onto rods and cones in retina

Accommodation

Lens changes shape and increase refractive power to focus objects at different distances on the retina.

Figure 2: Accomodation. a: Ciliary body is relaxed when viewing distant objects. Zonular fibres are taut and lens is flattened. b: Ciliary body is contracted when viewing nearby objects. Zonular fibres are relaxed and lens is more spherical.
Source: Sjaastad O.V., Sand O. and Hove K. (2010) Physiology of domestic animals, 2nd edn., Oslo: Scandinavian Veterinary Press.

Far vision

• Ciliary muscles relaxed
• Zonular fibres tightened
• Lens flattened
• Focus distant object on retina

Near vision

• Ciliary muscles contracted
• Zonular fibres relaxed
• Lens becomes more spherical and convex (natural resting shape of lens)
• Refraction of light increases à focus nearby object on retina

Pupil diameter

Image 3: Variation in diameter of pupil depending on light intensity. Radial muscle fibres contract in dim light while circular muscle fibres contract in strong light.
Source: Sjaastad O.V., Sand O. and Hove K. (2010) Physiology of domestic animals, 2nd edn., Oslo: Scandinavian Veterinary Press.

• Controls amount of light entering eye
• Controlled by circular muscle fibers of sphincter pupillae muscle and radial muscle fibres of radial dilator pupillar muscle

Dim light

• Increased activity in sympathetic nerve fibres à radial muscle contraction
• Pupil dilates à allow more light to enter eye 

Strong light

• Increased stimulation by parasympathetic nerve fibres à circular muscle contraction
• Pupil narrows à  restrict amount of light entering eye
• Constriction prevents divergent light rays from entering eye à rays fall on periphery of retina where they would not be focused properly

References

1. Akers R.M. and Denbow D.M. (2013) Anatomy and physiology of domestic animals, 2nd edn., Iowa: John Wiley & Sons, Inc.
2. Dyce K.M., Sack W.O. and Wensing C.J.G. (2010) Textbook of veterinary anatomy, 4th edn., Missouri: Saunders Elsevier.
3. Sjaastad O.V., Sand O. and Hove K. (2010) Physiology of domestic animals, 2nd edn., Oslo: Scandinavian Veterinary Press.