Copyright © 2010 Pearson Education, Inc. THE EYE: PART B.

Copyright © 2010 Pearson Education, Inc.

THE EYE: PART B


Light and Optics

• Refraction-Bending of a light ray

• Occurs when a light ray meets the surface of a different medium at an oblique angle

• The greater this angle, the greater the bending


Focusing Light on the Retina

• Light is refracted (bent) as it passes through 3 areas of the eye

• Entering the cornea

• Entering the lens

• Leaving the lens

• Since the lens shape can be changed, it can be used to:

• Increase refractory power above that of cornea alone

• Bend light rays so that they converge at a single point (focal point)

• Fine focus on an image


• The image formed at the focal point is upside-down and reversed right to left

Focusing Light on the Retina

Copyright © 2010 Pearson Education, Inc. Figure 15.12

Point sources

(a) Focusing of two points of light.

(b) The image is inverted—upside down and reversed.

Focal points


Focusing for Distant Vision

• Light rays from distant objects are parallel and need little refraction, so ciliary muscles are relaxed and the lens is stretched flat by tension in the ciliary zonule

Copyright © 2010 Pearson Education, Inc. Figure 15.13a

Lens

Invertedimage

Ciliary zonule

Ciliary muscle

Nearly parallel raysfrom distant object

(a) Lens is flattened for distant vision. Sympatheticinput relaxes the ciliary muscle, tightening the ciliary zonule, and flattening the lens.

Sympathetic activation


Focusing for Close Vision

• Light from a close object diverges as it approaches the eye; Three processes must occur to produce clear vision:

• Accommodation— increases refractory power of lens

• Ciliary muscles contract

• Tension in ciliary zonule decreases

• The lens bulges

Copyright © 2010 Pearson Education, Inc. Figure 15.13b

Divergent raysfrom close object

(b) Lens bulges for close vision. Parasympathetic input contracts the ciliary muscle, loosening the ciliary zonule, allowing the lens to bulge.

Invertedimage

Parasympathetic activation


Focusing for Close Vision

• Constriction of the pupils— Constriction of pupils prevents the most divergent rays from entering eye

• Sphincter pupillae muscles of iris decrease pupil size

• Mediated by parasympathetic NS

• Convergence—medial rotation of the eyeballs toward the object being viewed


Problems of Refraction

• Emmetropia- normal vision

• Myopia (nearsightedness)—focal point in front of retina

• Hyperopia (farsightedness)—focal point behind retina

Copyright © 2010 Pearson Education, Inc. Figure 15.14 (1 of 3)

Focalplane

Focal point is on retina.

Emmetropic eye (normal)


Concave lens moves focalpoint further back.

Eyeballtoo long

UncorrectedFocal point is in front of retina.

Corrected

Myopic eye (nearsighted)


Eyeballtoo short

UncorrectedFocal point is behind retina.

CorrectedConvex lens moves focalpoint forward.

Hyperopic eye (farsighted)


Photoreception and the Retina

• Structure of the Retina

• Pigmented Layer-single cell thick, abuts the choroid; stores vitamin A

• Neural Layer –

• Innermost layer of retina

• Extends anteriorly to the ora serrata


• Composed of three layers:

• Photoreceptors- rods and cones

• The rods and cones are adjacent to: the pigmented layer

• Rods

• Sensitive to dim light

• Best suited for night and peripheral vision

• Perceived input is in gray tones only & images are fuzzy and indistinct

• Outer segment: rod shaped, contains -molecules that change shape as they absorb light (visual pigments )

• Inner segment: of each joins the outer segment to the cell body

Photoreception and the Retina


Functional Anatomy of Photoreceptors

• Cones

• Operate in bright light

• Provide high-acuity, color vision


Process ofbipolar cell

Outer fiber

Apical microvillus

Discs containingvisual pigments

Melaningranules

Discs beingphagocytized Pigment cell nucleus

Inner fibers

Rod cell body

Cone cell body

Synaptic terminals

Rod cell body

Nuclei

Mitochondria

Connectingcilia

Basal lamina (borderwith choroid)

The outer segments of rods and cones are embedded in the pigmented layer of the retina.

Pig

men

ted

layer

Ou

ter

seg

men

tIn

ner

seg

men

t


Neurons of the Retina

• Bipolar Cells

• Receive signals from photoreceptor cells and do not generate action potentials

• Link photoreceptors to ganglion cells

• Ganglion Cells

• Receive input from bipolar cells

• Only neurons of retina that generate action potentials


Visual Pigments and Stimulation of Photoreceptors

• Retinal -

• Light-absorbing molecule that combines with one of four proteins (opsins) to form visual pigments

• Synthesized from vitamin A

• Two isomers: 11-cis-retinal (bent form) and all-trans-retinal (straight form)

• Conversion of 11-cis-retinal to all-trans-retinal initiates a chain of reactions leading to transmission of electrical impulses in the optic nerve

Copyright © 2010 Pearson Education, Inc. Figure 15.15b

Rod discs

Visualpigmentconsists of• Retinal• Opsin

(b) Rhodopsin, the visual pigment in rods, is embedded in the membrane that forms discs in the outer segment.


Stimulation of Photoreceptors• Excitation of Rods

• The visual pigment of rods is rhodopsin (opsin + 11-cis-retinal)

• In the dark, rhodopsin forms and accumulates

• When light is absorbed, (11-cis-retinal is converted to all-trans-retinal)

• all-trans-retinal separates from opsin and rhodopsin breaks down (bleaching of the pigment)

• Enzymes slowly convert the all-trans-retinal back to11-cis retinal in the pigmented epithelium

• Retinal returns to outer segment of rods, rejoins with opsin, forming rhodopsin


• Excitation of Cones

• There are three types of cones:

• Named for the colors of light absorbed: blue, green, and red

• Intermediate hues are perceived by activation of more than one type of cone at the same time

• 100 x less sensitive than rods (takes brighter light to activate them)

Stimulation of Photoreceptors


• Breakdown and regeneration of visual pigments: Light absorption by visual pigments triggers pigment breakdown and then pigments are regenerated in the dark

• The threshold for activation of cones is: 420 nm (blue), 530 nm (green) and 560 nm (red)

Stimulation of Photoreceptors

Copyright © 2010 Pearson Education, Inc. Figure 15.16

11-cis-retinal

Bleaching ofthe pigment:Light absorptionby rhodopsintriggers a rapidseries of stepsin which retinalchanges shape(11-cis to all-trans)and eventuallyreleases fromopsin.

1

Rhodopsin

Opsin and

Regenerationof the pigment:Enzymes slowlyconvert all-transretinal to its11-cis form in thepigmentedepithelium;requires ATP.

Dark Light

All-trans-retinal

Oxidation

2H+

2H+

Reduction

Vitamin A

2

11-cis-retinal

All-trans-retinal


1 cGMP-gated channelsopen, allowing cation influx;the photoreceptordepolarizes.

Voltage-gated Ca2+

channels open in synapticterminals.

2

Neurotransmitter isreleased continuously.

3

4

Hyperpolarization closesvoltage-gated Ca2+ channels,inhibiting neurotransmitterrelease.

5

No EPSPs occur inganglion cell.6

No action potentials occuralong the optic nerve.7

Neurotransmitter causesIPSPs in bipolar cell;hyperpolarization results.

Na+

Ca2+

Ca2+

Photoreceptorcell (rod)

Bipolarcell

Ganglioncell

In the dark


1 cGMP-gated channelsare closed, so cation influxstops; the photoreceptorhyperpolarizes.

Voltage-gated Ca2+

channels close in synapticterminals.

2

No neurotransmitteris released.3

Lack of IPSPs in bipolarcell results in depolarization.4

Depolarization opensvoltage-gated Ca2+ channels;neurotransmitter is released.

5

EPSPs occur in ganglioncell.6

Action potentialspropagate along theoptic nerve.

7

Photoreceptorcell (rod)

Bipolarcell

Ganglioncell

Light

Ca2+

In the light


Photoreceptor Imbalances

• Nyctalopia (night blindness) – rod function is seriously hampered

• Caused by: prolonged vitamin A deficiency (poor nutrition) or retinitis pigmentosa (areas of good nutrition)

• This leads to the degeneration of rods


• Color blindness

• Due to lack of: one or more of the cone types; inherited as an X-linked condition

• More common in males than females

Photoreceptor Imbalances


Right eye Left eye

Fixation point

Optic radiation

Optic tractOptic chiasma

Optic nerve

Lateral geniculatenucleus ofthalamus

Superior colliculus Occipital lobe (primary visual cortex)


Cortical Processing

Optic Nerve Optic chiasma fibers from medial region of each eye cross over to the opposite side optic tracts lateral geniculate body of the thalamus optic radiations primary visual cortex in the occipital lobe of the brainconscious perception of the image

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