BIO261 Class 07/06 Part 2

Microscopic Structure and Layers of the Retina

  • The Pigment Portion of the Retina

    • Composition: This layer contains melanin and serves as a critical storage site for vitamin A.

    • Role of Vitamin A: A small portion of vitamin A is stored in this pigmented layer. It functions as a key precursor for the chemicals within the light receptors (photoreceptors) that react to light stimuli. Without vitamin A, these essential chemicals are not produced, leading to an inability to see properly.

    • Clinical Significance: Vitamin A deficiency is directly linked to vision impairment because the receptors cannot function without the chemical derived from it.

  • Photoreceptor Layer: Rods and Cones

    • Receptors are classified based on the morphology (shape) of their outer segments.

    • Rods: Characterized by a tubular, elongated outer segment.

    • Cones: Characterized by a tapered, triangular, or cone-shaped outer segment.

  • Hierarchical Layers of the Retina

    1. Pigmented Layer: The outermost layer containing melanin and Vitamin A.

    2. Layer of Rods and Cones: The photoreceptor layer.

    3. Outer Synaptic Layer: The site where rods and cones synapse with the subsequent neurons.

    4. Bipolar Cell Layer: Contains the bipolar neurons that connect receptors to the ganglion cells.

    5. Inner Synaptic Layer: The site of synapse between bipolar cells and ganglion cells.

    6. Ganglion Cell Layer: The final layer of neurons within the retina.

    7. Axonal Layer: Composed of the long axons of the ganglion cells. This is the innermost layer relative to the center of the eye.

  • The Optic Nerve

    • The axons from the axonal layer exit the eye at a specific point called the optic disc.

    • Once these axons exit the eye, they converge to form the optic nerve, also known as Cranial Nerve II.

The Path of Light and Stimulation Sequence

  • Light Filtration: Light must pass through almost all layers of the retina to reach and stimulate the receptors (rods and cones), with the exception of the pigmented layer.

  • Sequence of Light Entry: To stimulate the photoreceptors, light travels through the layers in the following order:

    1. Axonal Layer

    2. Ganglion Cell Layer

    3. Inner Synaptic Layer

    4. Bipolar Cell Layer

    5. Outer Synaptic Layer

    6. Rods and Cones (where the stimulus is captured)

Fluid Dynamics: Aqueous Humor and Glaucoma

  • Aqueous Humor Production and Drainage

    • Formation: Produced by the ciliary body.

    • Circulation: It circulates through the eye and is eventually drained at the corneal limbus.

    • Drainage Structure: The fluid drains into a specialized vein called the scleral venous sinus, commonly known as the Canal of Schlemm.

  • Glaucoma

    • Definition: A condition characterized by increased intraocular pressure (pressure inside the eye).

    • Pressure Thresholds: Normal intraocular pressure ranges between 12 mm Hg12\,mm\,Hg and 22 mm Hg22\,mm\,Hg. If the pressure exceeds 22 mm Hg22\,mm\,Hg, it is considered pathologically high.

    • Pathophysiology: Increased pressure can compress the retina, leading to retinal damage and potential blindness.

    • Causes: Glaucoma often results from a blockade or narrowing at the limbus, which decreases the drainage rate of aqueous humor.

The Vitreous Body

  • Character: A gel-like substance located in the posterior part of the eye, behind the lens.

  • Functions:

    • Helps support the retina by holding it in place.

    • Maintains the structural spherical shape of the eye.

The Lens: Physics and Anatomy

  • Optical Properties

    • The human eye contains a biconvex lens, which functions as a converging lens.

    • Refraction: This is the bending of light rays as they pass through a medium. In a convex lens, light rays are refracted toward a central focal point.

    • Comparison: A concave lens (biconcave) is a diverging lens, which bends light rays away from the focal point.

  • Structural Composition

    • The lens is comprised of layers of protein fibers enclosed in a thick capsule.

    • The growth pattern is described as being "like an onion," with layers of protein fibers continuing to build up throughout a person's life.

  • Age-Related Changes

    • Flexibility: In youth, the lens is malleable and flexible. With age, the accumulation of tissue layers makes the lens stiff and less capable of flexing.

    • Accommodation Loss: Because the lens loses flexibility, older individuals have difficulty changing the thickness of the lens to focus on near objects.

    • Cataracts: The continuous addition of protein layers, combined with denaturing of proteins by sunlight (UV exposure), makes the lens opaque (loss of transparency). This opaqueness is called a cataract and is treated by surgically replacing the natural lens with an artificial one.

Vision Mechanics and Accommodation

  • The Focal Point and Retina

    • To see a clear image, light rays must converge and meet exactly on the retina. If the image falls in front of or behind the retina, it appears blurry.

    • Refractive Power: A thicker lens bends light rays more (higher refraction), bringing the focal point closer to the lens. A thinner lens bends light rays less, pushing the focal point further back.

  • Accommodation Reflex

    • Definition: The process by which the lens changes its thickness to maintain a focused image on the retina as an object moves.

    • Mechanics:

      • As an object moves closer, the image naturally tends to fall behind the retina. The lens automatically becomes thicker (more rounded) via the action of the ciliary muscles to pull the image forward onto the retina.

      • As an object moves further away, the lens becomes thinner (flatter) to ensure the image does not fall in front of the retina.

    • Convergence: Part of the accommodation reflex involves the eyes moving inward (converging) when focusing on a close object, such as a pencil brought toward the face.

Visual Anomalies and Clinical Terms

  • Retinal Image Characteristics

    • Images formed on the retina are mirror images: they are completely reversed (left to right) and upside down. The brain is responsible for readjusting this perception.

  • Refractive Conditions

    • Emetropia: A normal eye with perfect vision.

    • Myopia (Nearsightedness): Light focuses in front of the retina, often because the eye is too long or the lens is too thick. These individuals see near objects clearly but distant objects are blurry.

    • Hyperopia (Farsightedness): Light focuses behind the retina, often because the eye is too short or the lens is too thin. These individuals see distant objects clearly but near objects are blurry.

    • Presbyopia: Age-related hyperopia caused by the lens losing flexibility and staying relatively thin. This makes it difficult for elderly individuals to read or see objects up close.

    • Astigmatism: Distorted vision caused by unequal curvature or unevenness in the lens/cornea, resulting in unequal refraction of light across different axes.

    • Anopia: The loss of vision.

      • Partial Anopia: Incomplete loss of sight.

      • Hemianopia: Loss of half of the visual field.

      • Complete Anopia: Total blindness.