The Eye: Structure and Function

• The eye module is MCP 08.10
• The three learning outcomes are: to label a diagram of the eye, to describe the structure and function of the different eye parts, and to explain the roles of rods and cones in eye physiology.

Conjunctiva

• Is a mucous membrane
• Lubricates the eye
• Prevents drying

Fibrous Layer: Sclera

• It's made of fibrous CT
• Protects and shapes the eye
• Provides an anchor for eye muscles
• Modified to form the cornea

Fibrous Layer: Cornea

• It's transparent
• Both sides are covered with epithelia
• Lets light enter the eye
• Is involved in light bending

Vascular Layer: Choroid

• A vascular pigmented membrane
• Contains blood vessels that provide nutrition
• Contains pigments that prevents light scattering
• Anteriorly, it forms the ciliary body and the iris

Vascular Layer: Ciliary Body

• Composed of smooth muscle
• It encircles the lens

Vascular Layer: Ciliary Zonules

• Ciliary muscles attach to them, which impacts the lens
• When the ciliary muscle contracts the tension on the zonules is released
• As a result of the tension being released the lens fattens

Vascular Layer: Iris

• Consists of coloured smooth muscle
• Regulates the amount of light entering the pupil by changing size
• Radial muscles contract which dilates the pupil
• Circular muscles contract which constricts the pupil

Sensory Layer: Retina

• Lies inside choroid

Internal Chambers and Fluids

• The lens and ciliary zonules divide the eye
• Anterior to the lens is the aqueous humor
• Posterior to the lens is the vitreous humor

Vitreous Humor

• It transmits light
• Supports the lens
• Contributes to intraocular pressure

Aqueous Humor

• Supports the eye internally
• Supplies nutrients to the lens, cornea & some retinal cells
• Carries away wastes

Lens

• Biconvex, transparent disk of protein (crystallins)
• Held in place by ciliary zonules
• Focuses light on to the retina

Sensory Layer - The Retina

• Light entering the eye strikes the two layered retina
• The two layers: inner neural layer & outer pigmented layer
• The outer pigmented layer is a single layer of epithelial cells
• This layer absorbs light and prevent it from scattering in the eye
• It stores Vitamin A for photoreceptor cells
• It removes dead or damaged photoreceptor cells by phagocytosis

The Retina: Neurons

• The inner transparent layer of neurons relays visual information from photoreceptors to the optic nerve and then the brain
• Photoreceptors, rods and cones, detect light
• Bipolar cells receive info (signals) from rods and cones
• Ganglion cells gather input (signals) from bipolar cells, action potentials are generated
• Horizontal and amacrine cells integrate information across retina

Rods

• Contain Rhodopsin
• Consists of retinal, a light absorbing molecule derived from Vit A bound to an opsin membrane protein
• They do not distinguish between colour
• They are more sensitive to light
• Key to seeing at night but only in black and white

Cones

• Contain Photopsins
• retinal but three distinct opsin proteins
• The three distinct opsin proteins are:
• Blue sensitive cones (max 430nm)
• green sensitive cones (max 530nm)
• red sensitive cones (max 562nm)
• Absorption spectra overlap given the perception of intermediate hues

Photoreceptors: Electrical Signals

• Retinal has two isomers and absorption of light shifts one bond from 11-cis to all-trans
• Trans de-stabilises the retinal, and activates the opsin protein by changing its shape
• Results in a series of chemical and electrical reactions that produce an electrical signal
• Trans is enzymatically converted back to cis