Visual System Overview Chapter 5

Vision Overview

• The visual system's objectives include describing eye components, explaining refraction, image formation, and accommodation, and describing the neural pathway, descending control, reflex functions, and degenerative conditions associated with the visual system.
• The readings for this topic are located in Chapter 5, pages 157-172.

Retinal Degeneration

• The prevalence of macular degeneration is estimated at approximately 1.4 million Canadians, glaucoma affects ~450,000 Canadians, and retinitis pigmentosa impacts ~10,000 Canadians.
• Approximately 50,000 Canadians go blind annually.
• The cost of vision loss to the Canadian health system is estimated to be ~$9 billion per year.

Visual System Anatomy and Circuits

• The eye has basic anatomical structures and visual system circuits.
• The retina is responsible for phototransduction and retinal physiology.
• The eye also has non-visual functions.

Light Refraction and Accommodation

• The eye refracts light to form images.
• Light passes through the cornea, pupil, and lens.
• Accommodation involves changing the lens shape to focus on near or far objects.
• The ciliary muscle and suspensory ligaments adjust lens curvature. (e.g. relaxed ciliary muscle leads to a flattened weak lens; contracted ciliary muscle leads to a rounded strong lens).

Visual Pathways

• Oculomotor nerve (III) ≠ Optic nerve (II).
• Information from the visual system travels through the optic nerve, optic chiasm, and optic tracts to the brain.

Phototransduction

• The retina involves phototransduction.
• The retina contains 120 million rod cells and 6 million cone cells.
• Rods are for night vision, cones for high acuity color vision; cones have three types (red, green, blue).

Visual Cycle

• The visual cycle involves isomerization of 11-cis-retinal to all-trans-retinal in response to light.
• Vitamin A plays a role in this process.
• The process is crucial for phototransduction.
• R/G colour blindness can occur due to variations in cone populations.

Retina Structure and Layers

• The retina contains approximately 1 million retinal ganglion cells; 120 million rod cells and 6 million cone cells are also present.
• A variety of cellular layers exist in the retina that are necessary for function.
• The human retina is highly organized and functions across different parts of the visual spectrum.

Retinotopy

• Retinal interneurons, in the retina, integrate information from photoreceptors to compute visual features.
• Ganglion cells respond in a specific manner to the spatial location of light in their receptive field.
• The retina is not a camera, but functions as a processor.

Non-Visual Functions of the Eye

• The eye participates in pupillary light reflex, vestibulo-ocular reflex, oculocephalic reflex, palpebral oculogyric reflex.
• The eye has circadian rhythms impacted by light input.
• This input influences the brain region impacting the production of melatonin and other body functions.

Intrinsically Photosensitive RGCs (ipRGCs)

• ipRGCs are retinal ganglion cells that respond directly to light.
• They play a role in non-image-forming visual functions, such as circadian entrainment and pupillary light reflexes.
• ipRGCs contain melanopsin, a photopigment.