8.3

Physiology of Vision

  • The organ responsible for sight is the eye.

  • Vision is a special sense based on the transduction of light stimuli received through the eyes.

  • Key Structures in the Eye:

    • Retina: The inner sensory layer of the eye that detects light.

    • Light enters through the cornea, travels through:

    • Aqueous humor

    • Lens

    • Vitreous humor

    • Reaches the neural layer of the retina.

  • Photoreceptors:

    • Detect light and turn the stimulus into electrical potentials across the cell membrane.

    • Signals are carried by the optic nerves to the brain for interpretation as vision.

  • Types of Photoreceptors:

    • Rods:

    • More numerous; sensitive to dim light.

    • Cannot generate sharp or color images.

    • Cones:

    • Operate in bright light; generate sharp, color images.

Eye Structures

  • Ciliary Body

  • Ciliary Zonule

  • Cornea

  • Iris

  • Pupil

  • Aqueous Humor (anterior segment)

  • Lens

  • Scleral Venous Sinus (canal of Schlemm)

  • Vitreous Humor (posterior segment)

  • Sclera

  • Choroid

  • Fovea Centralis

  • Optic Nerve

  • Optic Disc (blind spot)

  • Central Artery and Vein of Retina

Vision Focusing Mechanisms

  • For Close Vision:

    • Ciliary muscles contract; lens is rounded.

    • Focal point is on the fovea centralis.

  • For Distant Vision:

    • Ciliary muscles relax; lens is flattened.

    • Focal point is again on the fovea centralis.

Refraction and Accommodation

  • Refraction: The process of bending light as it passes through a medium (cornea, lens, aqueous and vitreous humors).

  • Accommodation: Changing the shape of the lens to maintain the focal point on the retina, allowing for sharp vision directly at the fovea centralis.

Retinal Structure and Visual Acuity

  • At the fovea centralis, the retina contains only photoreceptors, leading to greatest visual acuity.

  • Peripheral vision is less defined; images are blurry and vague, as peripheral retina cannot focus as sharply.

  • Eye and head movements help to center visual stimuli on the fovea for optimal viewing.

Image Formation on the Retina

  • Images on the retina are miniature, reversed, and inverted:

    • Top of an object focused on the bottom of the retina.

    • Bottom of an object focused on the top of the retina.

    • Closer portions focused nearer to the retina, resulting in inversion.

  • The brain corrects this reorientation, allowing for proper perception of the images.

Visible Light

  • Visible light is a segment of electromagnetic radiation with wavelengths between 380 and 720 nm.

  • Wavelength determines color; example:

    • 500 nm = blue; 720 nm = dark red.

  • Pigments in Human Eyes:

    • Sensitive to red, green, and blue lights (420 nm, 530 nm, 560 nm respectively).

    • Color perception arises from the comparatives in activity of these cones.

Phototransduction

  • Process of Phototransduction:

    • Converting light stimuli into action potentials.

    • Composed of several layers within the retina, with specialized cells:

    • Pigmented Layer: Absorbs non-detectable light; recycles photoreceptor fragments.

    • Photoreceptors: Rods and cones, embedded in the pigmented layer.

  • Signal Transmission:

    • Light alters membrane potential in rods and cones, affecting neurotransmitter release to bipolar cells.

    • Bipolar cells connect to ganglion cells, which converge to form the optic nerve at the optic disc, creating a blind spot.

Structure of Photoreceptor Cells

  • Photoreceptor cells consist of:

    • Inner Segment: Contains nucleus and organelles.

    • Outer Segment: Specialized for light detection, containing visual pigments in membrane discs.

  • Rhodopsin: The visual pigment in rods, consists of opsin and retinal (dependent on Vitamin A); involved in sensory conversion.

Rhodopsin Activation and Regeneration

  • When photons hit rhodopsin, it converts 11-cis retinal to 11-trans retinal, starting the visual transduction process.

  • Bleaching: Occurs when rhodopsin is broken down; retinal must convert back to 11-cis form.

  • Rhodopsin regeneration occurs via ATP energizing enzymes to convert retinal shapes.

Photoreception: Dark and Light States

  • Dark State:

    • Rhodopsin inactive; high levels of cGMP maintain sodium channels open, leading to depolarization and continuous glutamate release, keeping bipolar and ganglion cells inactive.

  • Light State:

    • Light reduces cGMP, closing sodium channels leading to hyperpolarization.

    • Glutamate release decreases, causing depolarization of bipolar cells and increased action potentials in ganglion cells sent to the brain.

Vision Pathway

  • Photoreceptive signals travel through:

    • Optic Nerve → Optic Chiasm (where medial fibers cross) → Optic Tracts → Lateral Geniculate Body (Thalamus) → Optic Radiations → Primary Visual Area (Occipital Lobe).

  • Visual Field: Encompasses total area visible while focusing on a central point.

    • Consistent overlap, necessary for 3D/ Binocular vision.

Visual Field Description

  • Each retina comprises:

    • Nasal Hemiretina (closer to the nose) and Temporal Hemiretina (closer to the temple).

  • Visual signals are inverted; right visual fields project to the left hemiretinas, and vice versa.

  • Optic Chiasm Functionality:

    • Crossing of medial fibers allows appropriate hemisphere processing for accurate vision interpretation.

Homeostatic Imbalances of Vision

  • Cataracts: Hardened, cloudy lenses causing blurry vision. Treatment involves replacement with an artificial lens.

  • Age-Related Macular Degeneration (AMD): Deterioration of the macula affecting sharp vision, common in older adults.

  • Diabetic Retinopathy: Damage from diabetes leads to impaired visual function through damaged retinal blood vessels, causing nutrient and oxygen deficiencies.

  • Visual Field Deficits: Damage to the brain may result in specific types of vision loss (e.g., hemianopia). Example: Damage to right optic tract results in left visual field loss.