Sense Organs

• Sense organs are groups of receptor cells responding to specific stimuli: light, sound, touch, temperature and chemicals.

Eye structure

• The cornea refracts light.
• The lens changes its shape to focus light onto the retina.
• The suspensory ligaments hold the lens in place.
• The pupil is a small opening in the center of the eye. It regulates the amount of light entering the eye to enable vision.
• The iris controls the intensity of light entering through the pupil.
• The ciliary muscle changes the thickness of the lens when focusing.
• The retina is the inner light-sensitive layer that contains:
  • rod cells for dim light
  • cone cells for deteting color and details.
• The yellow spot or fovea is the most sensitive part of the retina that is responsible for sharp, detailed and color-rich central vision.
• The blind spot is the point where the optic nerve attaches to the eye. No light-sensitive cells here.
• The optic nerve carries nerve impulses away to the brain.

Pupil reflex

• The pupil reflex is an involuntary, automatic contraction of the iris muscle causing the pupil to constrict in response to bright light, and dilate in dim light.
  • In bright light, eye receptors detect high light intensity and send signals via the optic nerve to the brain. The circular muscles contract and the radial muscles relax. The pupil constricts (becomes smaller).
  • IN dim light, eye receptors detect low light intensity and send signals to the brain. The circular muscles relax and the radial muscles contract. The pupil dilates (becomes larger).
  • These muscles never contract at the same time. This makes them antagonistic.