Sensory and Motor Mechanisms

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Compound eyes are very effective at detecting movement.

Slide 40

Compound eyes

Rhabdom

(a) Fly eyes

Crystalline cone

Lens

(b) Ommatidia

Ommatidium

Photoreceptor

Axons

Cornea

2 mm

Slide 41

Single-lens eyes are found in some jellies, polychaetes, spiders, and many molluscs.

They work on a camera-like principle: the iris changes the diameter of the pupil to control how much light enters.

In vertebrates the eye detects color and light, but the brain assembles the information and perceives the image.

Slide 42

Structure of the Eye

Main parts of the vertebrate eye:

The sclera: white outer layer, including cornea

The choroid: pigmented layer

The iris: regulates the size of the pupil

The retina: contains photoreceptors

The lens: focuses light on the retina

The optic disk: a blind spot in the retina where the optic nerve attaches to the eye.

Slide 43

The eye is divided into two cavities separated by the lens and ciliary body:

The anterior cavity is filled with watery aqueous humor

The posterior cavity is filled with jellylike vitreous humor

The ciliary body produces the aqueous humor.

Slide 44

Vertebrate Eye

Optic nerve

Fovea = center of visual field

Lens

Vitreous humor

Optic disk (blind spot)

Central artery and vein of the retina

Iris

Retina

Choroid

Sclera

Ciliary body

Suspensory ligament

Cornea

Pupil

Aqueous humor

Slide 45

Humans and other mammals focus light by changing the shape of the lens.

Ciliary muscles relax.

Retina

Choroid

(b) Distance vision

(a) Near vision (accommodation)

Suspensory ligaments pull against lens.

Lens becomes flatter.

Lens becomes thicker and rounder.

Ciliary muscles contract.

Suspensory ligaments relax.

Slide 46

The human retina contains two types of photoreceptors: rods and cones

Rods are light-sensitive but don’t distinguish colors.

Cones distinguish colors but are not as sensitive to light.

In humans, cones are concentrated in the fovea, the center of the visual field, and rods are more concentrated around the periphery of the retina.

Slide 47

Sensory Transduction in the Eye