Physics The cornea and lens refract light rays coming into the eye. The image projected onto the...

Physics

• The cornea and lens refract light rays coming into the eye.

• The image projected onto the retina is upside down and backwards.

• If the focal plane for the lens/cornea is on the retina, the image will be in focus (emmetropia).

Fig. 16.30

Fig. 8-4 Ganong

Out-of-focus Images

• hyperopia– farsightedness– focal plane posterior to the

retina

• myopia– nearsightedness– focal plane anterior to retina

• astigmatism– no single focal plane

• irregularities in cornea and lens

• cataracts– cloudy lens

Fig. 16.33

HarperCollins A&P Laserdisc

Retina

• 3 cell layers– photoreceptor

cell layer• rods and cones

– bipolar cell

layer• plus interneurons (in “synaptic layers”)

– horizontal cells (outer synaptic layer)

– amacrine cells (inner synaptic layer)

– ganglion cell layer• axons of the ganglion cells form the optic nerve fibers

Note: The light signal and nerve signal travel in opposite directions.

Fig. 16.34

Photoreceptor Structure

• outer segment– many membrane infoldings

(cones) or discs / cisternae (rods)

– photosensitive pigments are transmembrane proteins

• inner segment– mitochondria

• cell body– nucleus

• synaptic regionFig. 16.35

Photosensitive Pigments• transmembrane protein

– opsin• detachable, vitamin A derivative

– retinal• In rods the protein / retinal conjugated

protein is called rhodopsin (“visual purple”).

Fig. 16.36

Rhodopsin1-3. activation• light received by retinal• cis-retinal converted to

trans-retinal• activated rhodopsin: For a

second or two the trans-retinal remains attached to the opsin.

4. inactivation• retinal detaches• rhodopsin is inactivated or

“bleached”5&6. regeneration

• retinal returns to cis configuration and reattaches to the opsin, restoring rhodopsin to its resting state

restingresting

activated

inactivated

Fig. 16.37

(bleaching)

(regeneration)

Transduction

• overview– Light activates rhodopsin which results in a

hyperpolarization of the cell and a decrease in the release of neurotransmitter.

• details– Activated rhodopsin activates transducin (a G

protein) which activates a phosphodiesterase which catalyzes the breakdown of cGMP.

[cGMP]cytosol closure of cGMP-gated Na+ channels hyperpolarization decreased release of neurotransmitter from photoreceptor to bipolar cells

Fig. 16.38

Fig. 8-18

Ganong

Transduction Mechanisms

Amplification by a G-protein System

• 1 photon• 1 activated rhodopsin• 500 activated transducins (activated

rhodopsin like a pinball in the membrane)• 500 activated phosphodiesterases• 105 cGMP hydrolyzed• 250 Na+ channels closed• 1 million fewer Na+ enter• 1 mv hyperpolarization

Alberts, et al., Molecular Biology of the Cell

(The entire amplification cascade lasts about one second.)

Adaptation

• Vision functions over a wide range of light intensities due to adaptation. – more than a 1012 difference between the dimmest

light detectable by rods and brightest light detectable by cones (see Fig. 8-27, Ganong)

• adaptation mechanisms (in increasing order of importance):– pupil diameter – neural circuitry – photoreceptor physiology

Adaptation by Photoreceptor Cells

• both rods and cones involved

• light adaptation (decreased sensitivity with exposure to light)– bleaching of photopigment

[cGMP]cytosol

• dark adaptation (increased sensitivity with exposure to darkness)– recovery of photopigment

[Ca++]cytosol activated guanylate cyclase

[cGMP]cytosol

Adaptation

Fig. 8-28

Ganong

Sensitivity to light increases during time in the dark.

Color

• three sets of cones with different absorption maxima– blue (420 nm)– green (531 nm)– “red” (558 nm)

Fig. 16.40

Color

• color blindness– one or two sets of cones missing

Fig. 16.41

• color constancy– Colors are not just interpreted by wavelength, but

also by context.• Your eye can let you see colors that are not really there.

– e.g., When you wear sunglasses, you can still distinguish colors.

– e.g., The color differences between fluorescent light and incandescent light are very obvious on film; in visual perception there is very little difference because interpretation by the eye and the brain eliminates most of the differences.

Color

http://www.uni-mannheim.de/fakul/psycho/irtel/color/kodak.html http://dragon.uml.edu/psych/colors1.html

Visual Pathways• photoreceptor bipolar

cell ganglion cell (optic nerve fibers)– typically: 100

photoreceptors / optic nerve fiber (e.g. of convergence)

– in fovea: 1 cone / optic nerve fiber

• allowing acute vision

• With its two neuronal pools (synaptic layers), interpretation begins in the retina.– at least 15 different neurotransmitters– horizontal cells

• increase contrast by lateral inhibition

– amacrine cells• phasic response – increase sensitivity to movement

Fig. 16.34

Eye to Brain• optic nerve fibers– at optic chiasm

• medial fibers cross over to opposite side

• lateral fibers remain ipsilateral

– as a result• The left side of the brain

receives information about the right half of the visual field from both eyes.

• The right side of the brain receives information about the left half of the visual field from both eyes.

For example, cut “C” on the left optic tract prevents information from the right half of the visual field of both eyes from reaching the brain.Fig. 8-4, Ganong

Eye to Brain

Fig. 16.43

• fibers of optic nerve / optic tract– synapse in thalamus– projection to primary visual area

of occipital lobe• interpretation of lines / edges and

movements– visual association area

• shapes interpreted• 3D perceived

– eye and head reflexes• via collaterals from optic tracts to

superior colliculi– Pupillary reflexes and

accommodation• via collaterals to pretectal nucleus