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Lecture 16

BIOC32H3 Lecture Notes - Lecture 16: Depth Perception, Guanosine Monophosphate, Mitochondrion


Department
Biological Sciences
Course Code
BIOC32H3
Professor
Catherine Nash
Lecture
16

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Sensory Physiology 4
Anatomy of the Eye
-the eye is a hollow sphere divided into two compartments: aqueous humor and vitreous humor
-the two compartments are separated by the lens; the lens and cornea focus light on the retina
- ciliary muscles and zonules (ligaments) control bending of the lens
-retina (back of eye) is the sensory “wall” and contains blood vessels, photoreceptors, optic disk, and
fovea
*Note: the fovea is the area of highest visual sharpness while the optic disk is the blindspot
Pathway to Vision
1. Light enters cornea
2. Pupil controls amount of light allowed to enter;
-can change from 1.5mm- 8mm in diameter; 28 fold difference in pupil area
3. ciliary muscles and zonules (ligaments) modify shape of lens Lens focuses light waves onto retina
4. Photocereptors of retina transduce light energy into an electrical signal
5. Electrical signal is then processed through neural pathways to the visual cortex
Accommodation
-light converges onto retina at the focal point
-distance from centre of lens to the focal point focal length; for focal length to change, the lens must
change shape
- ciliary muscles and zonules (ligaments) muscles contract and relax to round and flatten the lens to
accommodate the rays to hit the retina
*Note: when ciliary muscles are relaxed, zonule ligaments are pulled tight flattening of lens; when ciliary
muscles are contracted, zonule ligaments are loosened  rounding of lens
Distant Objects (>20 ft): light entering eyes is parallel
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Close Objects: light rays will fall behind the retina
-ciliary muscles must contract, zonules must loosen, lens must round
-rounding of lens shortens focal length to hit the retina
Common Visual Defects:
Hyperopia (far sightedness)- occurs when focal point falls behind retina (shorter eyeball)
-corrected with a convex lens
*Note: typically for old age
Myopia (near sightedness)- occurs when focal point falls in front of the retina (longer eyeball)
-corrected with a concave lens
*Note: typically for children
Astigmatism- when eyeball is not perfectly dome shaped thus image may be distorted
*Note: these people sometimes cannot use contact lens because it would require the dome shaped
eyeball
*Note: presbyopia is the loss of ability of lens to change shape; often due to lens losing plasticity or muscles
unable to contract as well
Phototransduction at the Retina
-information transfer is convergent: 15-45 photoreceptors  3 bipolar cells  1 ganglion cell  optic nerve
*Note: when the light reaches retina it is sent all the way to photoreceptors posterior to the retinal epithelium
first thus bypassing optic nerve, ganglion cells, an bipolar cells; the signal is then sent back in direction of optic
nerve
-the reason why the fovea has the highest acuity of
vision is due to the fact that there is no convergence
that occurs at the fovea:
1 photoreceptor 1 bipolar cell 1 ganglion cell optic
nerve
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