Describe Visual Receptors and Their Physiology

Retina

internal or neural tunic

• optic disc, macula lutea, peripheral retina

Pigmented layer

• attached to choroid (internal to it)

• provides vitamin A for photoreceptors

• absorbs stray light to precent light scatter

Neural layer

• houses photoreceptors and neurons

• receives light and converts to nerve signals

Optic disc

• not photoreceptors → blind spot

• ganglion exit toward brain

Macula lutea

• rounded, yellowish region lateral to optic disc

• contains fovea centralis

• highest proportion of cones (no rods)

• sharpest vision

Peripheral retina

• contains rods

• most effectively in low light

Refraction of light

• sharp vision = light rays to bend as the go towards retina

• refraction occurs when light passes between media of different densities such as air and cornea

• through curved surfaces such as lens

Focusing light: objects closer than 20ft

• eyes medially, image of object directed onto fovea centrialis

• ciliary contract → decrease tension on suspensory ligaments → lens becomes spherical

• extrinsic eye muscles weaker in one eye = diplopia (double vision)

  • light refracted to greater extent = accommodation

  • sphincter pupillae contract, pupil constricts

Focusing light: object further than 20 feet

• eyes forward

• ciliary muscles relax, tensing suspensory ligaments flatted lens, dilator pupillae contracts, dilating pupil

Visual Impairments

emmetropia: normal, parallel light rays focused on retina
hyperopia: far-sighted, can’t see up close, eyeball too short, convergent rays from distant points to focus, corrected convex lens
myopia: near-sighted, can’t see far away, eyeball too long, rays close to eye focus retina, corrected with concave lens
astigmatism: unequal focusing, unequal curvatures in 1+ refractive surface
presbyopia: age-related vision change, lens less about to = spherical, reading up close difficult, corrective convex lens, treated with surgical techniques

Photoreceptors

Rods and Cones

Rods

• longer and narrower than cones

• more numerous

• sensitive to dim light, blurry image (periphery of retina contains rods)

• converge on few bipolar cells which converge on fewer ganglion cells

Cones

• concentrated at fovea centralis

• activated high intensity light

• COLOR VISION

• one-to-one relationship with bipolar cells and ganglion cells

• sharp image but in bright light

Color blindness

• x-linked recessive condition more common in males

• absence or deficit in 1 types of cone cell

  • red and green most commonly affected