CAP 50 2/5: Acomodación del cristalino y dilatación pupilar l Fisiología de Guyton

Optics of Vision Part 2: The Eye as a Camera

In this section, the speaker discusses the eye as a camera, highlighting its components such as lenses, variable aperture system, and the retina.

System of Lenses in the Eye

• The eye has four surfaces of refraction:

• Between air or external world and the anterior surface of the cornea.

• Between the posterior surface of the cornea and aqueous humor.

• Between aqueous humor and anterior surface of the lens.

• Between posterior surface of lens and vitreous humor.

Refractive Indices

• Different structures in the eye have varying refractive indices:

• Cornea: 1.38

• Aqueous humor: 1.33

• Lens: 1.4

• Vitreous humor: 1.34

Power of Refraction

• Understanding refractive indices helps comprehend light deviation:

• Higher refractive index implies greater light deviation.

• The total dioptric power of the eye is approximately 52 diopters.

Understanding Accommodation Mechanism

This part delves into how accommodation works in the eye, focusing on changes in lens shape for image formation on the retina.

Image Formation on Retina

• Light rays from objects reach different parts of our retina creating an inverted image:

• Central nervous system corrects this inversion for perception accuracy.

Accommodation Capability

• The lens can change shape to adjust focus:

• Children can alter their lens curvature to achieve different levels of accommodation (e.g., from moderate to high convexity).

• By modifying lens curvature, individuals can vary their dioptric power for enhanced vision capabilities.

Lens Suspension Mechanism

• Ligaments called suspensory ligaments hold and position the lens within the eye:

Eye Anatomy and Presbyopia

In this section, the speaker discusses the anatomy of the eye, focusing on the role of ligaments and muscles in shaping the lens for vision. The concept of presbyopia, age-related loss of lens flexibility, is also explained.

Ligaments and Muscles in Eye Function

• Ligaments suspend specifically in the retina and choroides, affecting the shape of the lens. -

• Suspensory ligaments are relaxed by ciliary muscles, causing a convex lens shape. -

• Ciliary muscles contract to relax suspensory ligaments, allowing for lens accommodation. -

Lens Accommodation and Presbyopia

• Ciliary muscle contraction leads to a convex lens for near vision; relaxation results in normal concavity. -

• Aging causes presbyopia as the lens loses elasticity and thickness increases. -

• Presbyopia correction involves bifocal glasses with lenses for both near and far vision. -

Mechanism of Vision Adjustment

• Sympathetic stimulation contracts ciliary muscles, relaxing suspensory ligaments for close-up focus. -

Presbyopia Correction and Pupillary Diameter

In this section, the discussion revolves around correcting presbyopia with bifocal glasses and understanding the pupillary diameter's role in regulating light entering the eye.

Presbyopia Correction with Bifocal Glasses

• Bifocal glasses are used to correct presbyopia by enabling clear vision for both near and far distances.

Understanding Pupillary Diameter

• The pupil, surrounded by the iris, adjusts its diameter based on light conditions: dilating in low light (midriasis) and constricting in bright light (miosis).

• Midriasis and miosis are regulated by the sympathetic and parasympathetic nervous systems respectively, influencing pupillary diameter changes.

Function of Pupillary Diameter

• The pupillary diameter regulates the amount of light reaching the retina, impacting visual acuity.

• Variations in pupillary diameter affect depth of focus, determining the area where images appear sharp on the retina.

Depth of Focus and Retinal Function

This section delves into depth of focus concerning retinal function and how variations in pupillary diameter influence image clarity on the retina.

Depth of Focus Explained

• Depth of focus refers to an area on the retina where images are sharply focused.

Impact of Pupillary Changes on Image Clarity

• In miosis (pupil constriction), light rays converge sharply onto the retina, enhancing image clarity even if retinal movement occurs.

• Retinal movement within a focused area maintains image sharpness due to concentrated light rays during miosis.

Parasympathetic vs. Sympathetic Actions

This section contrasts parasympathetic and sympathetic actions regarding pupillary changes and their effects on visual acuity at different distances.

Parasympathetic Actions (Miosis)

• Parasympathetic actions induce miosis, increasing depth of focus for near vision by relaxing ciliary muscles and altering lens shape for close object viewing.

Sympathetic Actions (Midriasis)