Physiology of Vision and the Visual Pathway

Review of Eye Anatomy and Internal Structures

  • Internal Ocular Topography (Right Eye View):     * Optic Disc: The anatomical location where the optic nerve forms and exits the posterior aspect of the eye. It is the point where the central artery and vein enter and exit the globe.     * Macula Lutea: A yellowish area near the center of the retina.     * Central Fovea: Located within the macula lutea. It contains a high concentration of cones. Is characterized by having very few ganglion and bipolar cells covering it, which minimizes light scattering. This allowed uninterrupted light to strike the photoreceptors, resulting in a clean, high-resolution image.     * Central Artery, Nerve, and Vein: These structures enter and exit through the optic disc.

  • The Lens and Ciliary Apparatus:     * Lens Properties: The lens is described as highly flexible and elastic. Its shape can be altered to bend light effectively.     * Ciliary Body: Composed of the ciliary muscle (a smooth muscle) and ciliary processes.     * Suspensory Ligaments: Attached to the ciliary processes and connected directly to the lens.     * Mechanical Function:         * The shape of the lens is adjusted by altering the tension in the suspensory ligaments.         * Increasing tension on the ligaments pulls the lens, causing it to flatten out.         * Releasing tension allows the elastic lens to recoil into a more oval or spherical (round) shape.

Image Formation and Refraction

  • Concept of Refraction:     * Refraction is the bending of light rays. This occurs when light travels from one medium (e.g., air) to another medium with a different density (e.g., water or the structures of the eye).     * Light rays bend most significantly when they strike a surface at an angle rather than perpendicularly.

  • Refractory Media of the Eye:     * Cornea: The primary and major refractory medium. It accounts for approximately 75%75\% of the light refraction. The cornea is a fixed structure; its shape does not change to adjust focus.     * Lens: Responsible for the remaining 25%25\% of light refraction. Unlike the cornea, the lens is capable of changing shape to adjust focus, a process called accommodation.

  • Image Inversion:     * Images passing through the refractory media are inverted.     * On the retina (specifically the central fovea), the image appears upside down and reversed from right to left (e.g., an object in the lower right becomes an image in the upper left).     * The brain learns early in development to flip and interpret these images correctly.

Accommodation and Focal Points

  • Standard Vision (Objects > 20 feet20\,feet away):     * Light rays from objects 20 feet20\,feet or further away are considered parallel when they strike the cornea.     * Minimal refraction is needed. Most of the refraction occurs at the cornea, and the light passes relatively straight through the lens.     * Lens Shape: The lens remains flat.     * Mechanism: The ciliary muscle relaxes, pulling the choroid toward the back of the eye, which increases tension on the suspensory ligaments.

  • Near Vision (Objects < 20 feet20\,feet away):     * Light rays from close objects are diverging when they hit the eye.     * The focal point must remain just in front of the central fovea to maintain focus.     * Accommodation Reflex: The process of changing the lens shape for near vision.     * Lens Shape: The lens must become more spherical to bend the diverging light rays more sharply.     * Mechanism: The ciliary muscle contracts. This pulls the ciliary body and choroid forward (toward the anterior pole), which releases tension on the suspensory ligaments. Because the lens is elastic, it recoils into a spherical shape.     * Limit of Accommodation: The lens can only become so spherical. The "near point" of vision is approximately 4 inches4\,inches in front of the face; objects closer than this cannot be focused because the lens cannot bend light any further.

Physiology of Vision: Phototransduction

  • Synaptic Mechanics:     * The synapse between photoreceptors (rods/cones) and bipolar cells is an inhibitory synaptic event.     * Neurotransmitter: Glutamic acid (also known as glutamate).     * When glutamic acid is released, it hyperpolarizes the bipolar cell, preventing the propagation of an action potential to the ganglion cells.

  • Structure of Photoreceptors:     * Rods: Rod-shaped; contain the photopigment rhodopsin.     * Cones: Cone-shaped; contain the photopigment iodopsin.     * Outer Segment: Contains a high concentration of highly folded membranes embedded with integral proteins called photopigments.

  • Components of Photopigments:     * Opsin: The protein/amino acid sequence. (Rhodopsin in rods, Iodopsin in cones).     * Retinal: A derivative of Vitamin A.

  • Biochemical Pathway in Darkness (Below Threshold):     * Retinal is in the cis-retinal (bent) shape.     * Transducin (a G-protein) and Phosphodiesterase (PDE) are inactive.     * High levels of cyclic GMP (cGMP) are present in the cell. cGMP acts as a "key" that holds sodium (Na+Na^+) channels open.     * Sodium diffuses into the cell, depolarizing the membrane to approximately −30 mV-30\,mV.     * This depolarization causes the constant release of glutamic acid.     * Glutamic acid hyperpolarizes the bipolar cell, shutting down the visual pathway. No signal is sent to the brain.

  • Biochemical Pathway in Light (Above Threshold):     * Light strikes the photopigment, causing Isomerization: cis-retinal changes shape into trans-retinal.     * Trans-retinal activates Transducin, which then activates Phosphodiesterase (PDE).     * PDE converts cGMP into GMP.     * As cGMP levels drop, they fall off the sodium channels, causing the channels to close.     * Sodium entry stops (though some leak channels remain), and the membrane potential drops to its resting state of approximately −70 mV-70\,mV.     * At this potential, the release of glutamic acid ceases.     * The absence of the inhibitory neurotransmitter allows the bipolar cell to depolarize (to approximately −30 mV-30\,mV).     * This initiates a nerve impulse that propagates through the ganglion cells and along the optic nerve.

The Visual Pathway

  • Neural Sequence:     1. Photoreceptors (Rods and Cones).     2. Bipolar cells.     3. Ganglion cells (axons form the optic nerve).     4. Optic Nerve.     5. Optic Chiasm (site of partial decussation/crossing over).     6. Optic Tract.     7. Geniculate Nucleus (located in the Thalamus).     8. Optic Radiations.     9. Visual Cortex (Occipital Lobe).

  • Bilateral Interpretation and Crossing:     * The brain interprets information from both eyes on both sides through the Optic Chiasm.     * Medial (Nasal) Aspect of Retina: Light hitting the medial/nasal side of the retina produces impulses that cross over at the optic chiasm to be interpreted by the contralateral (opposite) side of the brain.     * Lateral (Temporal) Aspect of Retina: Light hitting the lateral/temporal side of the retina produces impulses that stay on the same side (ipsilateral) of the brain for interpretation.