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Natural Science 2. Secondary Education
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Sense organs

• Sense organs contain receptor cells that capture
  a particular type of stimulus and send a signal
  to the nervous system, which interprets it as a
  sensation.

2 The cats ears capture the vibrations of the
birds chirp.
1 A bird chirps (producing vibrations in air
particles) and reflects light.
3 The cats eyes capture the light reflected
from the bird.
A cats sense organs capturing stimuli.
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Photoreceptor organs

• The photoreceptor organs of animals are the
  eyes.
• They capture light and permit sight.
• They are normally on the head.

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Types of eyes (I)

• Depending on their complexity, eyes can be
  classified into different categories
• Simple eyes. Many invertebrates have these. They
  only detect the presence or absence of light.
• Compound eyes. Arthropods have these. They are
  made up of thousands of individual photoreceptor
  units called ommatidia. Together, these units
  provide the animals with a mosaic image.

Simple eye
Compound eye
Ommatidium
The compound eye in an insect.
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Types of eyes (II)

• Camera-type eyes. Some invertebrates, such as
  cephalopods, have these eyes, and almost all
  vertebrates have them. Their structure is similar
  to that of a camera and they provide clear images
  of the animals surroundings.

The camera-type eye of a bird
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Camera-type eyes
Retina This works like the sensor in a digital
camera. It is the innermost layer of the eye.
This is where the image received by the eye is
formed. This image is formed upside-down, but
the brain corrects this and turns it to become an
upright image.
Choroid membrane The middle layer of the eye. It
is dark red because it is full of blood
capillaries.
Lens This works just like the lens in a camera,
focusing objects.
Pupil This serves the same purpose as the
diaphragm of a camera. It opens and closes to
control the amount of light that enters the eye.
Fovea The part of the retina with the most cones.
Iris A disc made of muscle. Its function is to
open and close the pupil. It gives the eye its
colour.
Sclera The outermost layer. It is white, except
for the front part, the cornea, which is
transparent.
Optic nerve This carries the information
received by the retina to the brain.
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Mechanoreceptor organs (I)

• There are various types of mechanoreceptor
  organs which differ depending on the stimulus
  they capture
• Hearing organs (ears) These are usually holes
  that capture vibrations in the air or water.
• Balance organs Cells that are sensitive to
  gravity and indicate the position of the animals
  body.

Ear opening
Structures that detect vibrations
A reptiles ear
Ear opening
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Mechanoreceptor organs (II)

• Lateral line Found in fish. This line detects
  vibrations and movements in the water.
• Skin The skin has receptor cells that respond
  to pressure. It provides the sense of touch.

The lateral line of a fish
Skin
Receptor cell
Channel
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Chemoreceptor organs

• These capture substances which are in the air,
  water and foods.
• They provide two closely related senses smell
  and taste.

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Chemoreceptor organs

• Usually, these receptors are located near the
  mouth. For example
• Arthropods on the antennae.
• Vertebrates in the nostrils or on the tongue.

Antennae These contain many receptors that
respond to substances in the air, working as
olfactory organs.
Tongue The tongue captures substances from its
surroundings and brings them to an olfactory
receptor organ
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Natural Science 2. Secondary Education