light stimulus

Receptors A receptor receives information from the

environment. They can be classified according to the type pf

stimulus they respond to.

Receptors are traducers: they convert the Energy of the stimulus into the kind of energy the nervous system can deal with (electrical impulses).

Receptor type Responds to stimulus

Example in humans

Photoreceptor Light Rod cells in retina of eye

Chemoreceptor Chemicals Taste buds

Thermoreceptor Changes in temperature

Thermoreceptors in skin

Mechanoreceptor Mechanical changes Hair cells in ear.

LIGHT STIMULUS:

PHOTORECEPTORS

How living beings sense light?

Unicellular organisms and plants.Photoreceptor pigment inside the cell.

Worms

PlanarianLight sensitive

eyespots (ocelli)

Only sense variations in light´s intensity.

They can detect where the light is coming from.

MollusksDiscriminates simple shapes.

Lens

Sense organ

Receptors may need help in receiving the stimulus. They are often grouped together with other tissues to form a sense organ. The other tissues allow the receptor cells to work efficiently.

Simple eyeProcesses some

images and detects shapes.

InsectsCompound eyes: consists of numerous

separate light-sensitive units (ommatidia). All these images generate an overall picture.

Very useful in detecting movement.

Vertebrates

The position of the eyes is important

Monocular vision

Laterally located eyes: the animal perceives a wide picture of its surroundings.

Binocular vision

Frontally located eyes. 3D imaging allows better asessment of target´s position and field´s depth.

Activity Find the function for the underlined

structures: