1. Lesson Overview
31.4 The Senses

2. Touch, pressure, & vibration
Sensory receptors are categorized by the type of stimuli to which they respond.
Sensory Receptors
Type
Responds to
Locations
Chemoreceptor
Chemicals
Mouth & nose
Photoreceptor
Light
Eyes
Mechanoreceptor
Touch, pressure, & vibration
Skin & ears
Thermoreceptor
Temperature
Skin
Pain receptor
Tissue injury

3. Touch
Human skin contains at least seven types of mechanoreceptors that respond to different levels of pressure and creates the sensation of touch.
Not all parts of the body are equally sensitive to touch. For example, the skin on your fingers has a much higher density of touch receptors than the skin on your back.

4. Temperature
Thermoreceptors respond to heat and cold and are found throughout the skin
The hypothalamus is part of the brain that senses blood temperature

5. Pain
Pain receptors in the skin respond to physical injuries like cutting or tearing.
Many tissues also have pain receptors that respond to chemicals released during infection or inflammation.
The brain does not have pain receptors.
For this reason, patients are often kept conscious during brain surgery, enabling them to tell surgeons what sensations are produced when parts of the brain are stimulated.

6. Smell and Taste
Your senses of taste and smell involve the ability to detect chemicals.
Chemoreceptors = chemical-sensing cells
Taste buds = the sense organs in the mouth that detect taste
Olfactory receptors = the sense organs in the nose that detect smell

7. Hearing and Balance
The human ear has two sensory functions—hearing and detecting movement
Mechanoreceptors found in parts of the ear transmit impulses to the brain.
The brain translates the impulses into sound and information about balance.
Hearing = cochlea (see the notes below)
Balance = semicircular canals

8. Hearing
Sound is a result of vibrations moving through the air around us.
Vibrations enter the ear cause the eardrum to vibrate.

9. Hearing
Three tiny bones, called the hammer, anvil, and stirrup, transmit these vibrations to a membrane that creates pressure waves in the fluid-filled cochlea of the inner ear.

10. Hearing
The cochlea is lined with tiny hair cells that are pushed back and forth by these pressure waves.
The motion of these sensitive hair cells produces nerve impulses that travel to the brain which processes these nerve impulses as sound.

11. Balance
Semicircular canals = Three tiny canals within the inner ear that monitor the position of your body, especially your head, in relation to gravity.

12. The semicircular canals are filled with fluid and lined with hair cells.
As the head changes position, the fluid in the canals also changes position, causing the hair on the hair cells to bend.

13. Balance
This bending sends impulses to the brain that enable it to determine body motion and position.

14. Vision
How do the eyes and brain produce vision?
Vision occurs when photoreceptors in the eyes transmit impulses to the brain, which translates these impulses into images.

15. Order that light passes through the eye:
Cornea = a tough transparent layer of cells.

16. Iris = colored part of the eye
Pupil = small opening in the middle of the iris

17. Lens = focuses the eyes to see near or distant objects clearly
Small muscles attached to the lens change its shape helping to adjust the eyes’ focus

18. Retina = inner layer of the eye where the lens focuses
Photoreceptors arranged in a layer in the retina convert light energy into nerve impulses that are carried to the brain

19. There are two types of photoreceptors: rods and cones.
Rods = only sense in black and white
Cones = respond to different colors, producing color vision

20. Label the eye: