Module 1 Lesson 2 - 2

Lesson 2 — Photoreception: The Eye

The Structures of the Eye

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Read pages 410 - 415

The eye is an amazing organ! It contains many structures that must all function properly for you to see. Several of these structures refract and focus light before it even reaches the various photoreceptors that make up the structure called the retina. When these photoreceptors are stimulated by light energy, they convert this energy into an electrochemical nerve impulse. Sensory neurons in the optic nerve communicate this nerve impulse to interneurons in the occipital lobe of the brain where visual information is processed.

The inner layer of the eye consists of the delicate retina that contains two types of photoreceptors called rods and cones. Directly in line with the middle of the lens is an area of the retina called the fovea centralis, or fovea. This small, depressed area is where the cones are most highly concentrated. Cones are the photoreceptors that are sensitive to the various colours; rods are sensitive to light intensity. Rods are more numerous on the periphery of the retina.

The neural fibres from the retina form the optic nerve, which is the sensory pathway out of the eye. The spot where the fibres come together is the optic disc or blind spot. It is called the 'blind' spot because the point where the optic nerve exits the posterior of the eye has no photoreceptors. Therefore, no stimulus can be detected at that point. Because the optic nerves from both eyes meet at the base of the brain before entering the brain, the blind spot is not in the exact centre of the eyeball. Normally, we do not notice the blind spot because one eye compensates for the other eye by filling in the missing information. Try to find your blind spot in the ‘Try This’ activity later in the lesson.

In front of the lens is the anterior chamber filled with aqueous humour. This transparent, watery fluid is produced by the ciliary body. It maintains the shape of the cornea and provides oxygen to the surrounding cells. The aqueous humour is produced constantly and drains through ducts. If the aqueous humour is not drained successfully from the anterior chamber, its buildup eventually will result in the eye disorder called glaucoma.

Behind the lens is the posterior chamber filled with vitreous humour, which is a clear, amber-coloured gel-like fluid. It maintains the shape of the eyeball and supports the surrounding cells.

Both the aqueous and vitreous humours aid the refraction of light to a small degree.

The Anatomy of the Human Eye Summary

Structure	Function
External layer (Sclera)
Sides and back of sclera	Protects and supports eyeball
Cornea	Bends light rays into the eye
Intermediate layer (Choroid)
Sides and back of choroid	Absorbs scattered light and contains blood vessels
Iris	Regulates the amount of light that enters the eye
Pupil	The opening for light to enter the inner eye
Ciliary muscles	Changes the shape of the lens to focus
Internal Layer (Retina)
Rods	Photoreceptors sensitive to dim light
Cones	Photoreceptors sensitive to various wavelengths of light (colour vision)
Fovea centralis	Contains a high density of cones and provides acute vision
Other
Lens	Focuses light rays onto the fovea centralis
Humours	Support the eyeball with the pressure of the fluids within the eye
Optic nerve	Transmits sensory information to the brain

Watch and Listen

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Watch this section of Biologix-06 that deals with the eye structures. After watching the video, make a diagram complete with summary notes for review purposes.

Self-Check

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Complete the following crossword puzzle to consolidate your understanding of the various structures of the eye and their functions: