Sense of Vision Learning Outcomes

Sense of Vision Learning Outcomes

• Students should be able to describe the accessory structures of the eye and their functions
• Students should be able to describe the anatomy of the eye
• Students should be able to describe the anatomy of eye cavities and their chambers
• Students should be able to discuss the refraction of light and image formation
• Students should be able to describe the stimulation of photoreceptors
• Students should be able to discuss the visual pathway to the brain

Introduction

• More than half of the sensory receptors in the human body are located in the eyes
• A large portion of the cerebral cortex is devoted to processing visual information
• The eye is the organ of sight, located in the orbital cavity and supplied by the optic nerve (cranial nerve 2)

Accessory Organs of the Eyes

• Eyebrows/eyelashes - protect the eyeballs from foreign objects, sweat and direct sunlight
• Eyelids - protect the eyes from excessive light and injury, and keep the cornea moist. The orbicularis muscle closes the lids, and the levator palpebrae superioris muscle raises the upper eyelids.
• Conjunctiva - a thin mucous membrane that lines the inner aspect of the eyelids and is reflected onto the anterior surface of the eyeball.
• Lacrimal apparatus - produces and drains tears. It consists of the lacrimal gland, lacrimal canal, lacrimal sac, and nasolacrimal ducts. Tears contain salt, water, and lysozyme.
• Extraocular muscles - six muscles connected to the sclera which move the eyeballs in all directions

Anatomy of the Eye

• The walls of the eyeball consist of three layers of tunics: fibrous tunic (sclera and cornea), vascular tunic (choroid, ciliary body, and iris), and nervous tunic (retina)
• The internal cavity is filled with fluids called humors. The lens separates the internal cavity into posterior and anterior segments. The anterior segment contains the aqueous humor. The posterior segment contains the vitreous humor.
• Fibrous tunic is the outer coat of the eyeball and protects the intraocular contents. It is divided into sclera and cornea. At the junction of the sclera and cornea is the scleral venous sinus (canal of Schlemm).
• The sclera is the "white" of the eye, maintaining its shape and providing attachment to the extrinsic muscles.
• The cornea is part of the fibrous tunic; it is continuously from the sclera and refracts light rays. LASIK is a common visual corrective procedure on the cornea.

Vascular Tunic (Uvea)

• The middle layer composed of choroid, ciliary body, and iris.
• Choroid lines between the sclera and retina, contains blood vessels to nourish the retina, and melanocytes to produce melanin to absorb light rays, preventing scattering within the eyeball.
• Ciliary body consists of ciliary processes and ciliary muscles which secrete aqueous humor and change the shape of the lens to adapt for near/far vision.
• Iris is the colored portion, and the pupil is the black hole. The iris regulates the amount of light entering the eye.

Sensory Tunic (Retina)

• The innermost layer, a delicate, transparent membrane, receives light and converts it into nerve impulses.
• Divided into optic disc (blind spot), macula lutea, and peripheral retina.
• The fovea centralis is the point of sharpest vision in the retina, characterized by a high concentration of cones.
• Rods are photoreceptors sensitive to dim light for black and white vision and shapes and movement. Rhodopsin is the photopigment in rods.
• Cones are specialized for color vision and sharpness in bright light. There are three types of cones (blue, green, and red). Intermediate colors are perceived by activation of more than one type of cone.

Visual Pathway

• After light stimulation, rods and cones trigger electrical signals to ganglion cells.
• The impulses are conveyed through the retina to the optic nerve, the optic chiasma, then optic tracts, and to the thalamus.
• From the thalamus, impulses travel along the optic radiations to the visual cortex in the occipital lobe of the brain.

Clinical Applications (various sections)

• Conjunctivitis: inflammation of the conjunctiva
• Watery eyes: obstruction of the nasolacrimal ducts
• Glaucoma: improper flow of aqueous humor
• Cataracts: a clouding of the lens
• Retinal detachment: occurs when the retina peels away from its support tissue
• Diabetic retinopathy: damage to the retinal blood vessels caused by diabetes.
• Age-Related Macular Degeneration (AMD): vision loss in the central visual field (macula) due to retinal damage

Pathway of Light and Refraction

• Refraction is the bending of light rays.
• The refractive media of the eye (cornea, aqueous humor, lens, and vitreous humor) bend light rays to focus on the retina.
• Images formed on the retina are upside down and backward, but the brain corrects this.

Accommodation

• Accommodation is adjusting the lens shape to focus images properly on the retina.
• For near vision, the ciliary muscle contracts, suspensory ligaments relax, and the lens thickens (bulges).
• For far vision, the ciliary muscle relaxes, suspensory ligaments become taut, and the lens flattens.

Pupillary Response

• The pupil, an opening in the iris, is regulated by radial and circular muscles.
• Contraction of the circular muscles causes the pupil to constrict.
• Contraction of the radial muscles causes the pupil to dilate.

Convergence

• Convergence is the inward movement of the eyes to focus on a close object.
• The nearer the object, the greater the degree of convergence needed.
• Extrinsic eye muscles bring about convergence.

Refraction Abnormalities

• Myopia (nearsightedness): image focuses in front of the retina (corrected with concave lenses).
• Hyperopia (farsightedness): image focuses behind the retina (corrected with convex lenses).
• Astigmatism: irregular curvature of the cornea or lens causing blurry vision (corrected with specialized lenses).

Recap/Quiz (various sections)

• Contains questions about the material covered to assess understanding. Questions cover structures, functions, clinical conditions, and measurements.