Chapter 12

1. Chapter 12 Senses

2. Chapter Outcomes • Explain the difference between sensory reception, sensation, and perception • Describe the process of sensory adaptation • Distinguish between the major sensory receptors in the human body • Describe the principal structures of the human eye and their functions

3. Chapter Outcomes • Observe the principal features of the mammalian eye and perform experiments that demonstrate the functions of the human eye • Describe several eye disorders and treatments • Describe how the structures of the human ear support the functions of hearing and balance

4. Chapter Outcomes • Explain how humans sense their environment through taste, smell, and touch • Explain how small doses of neurotoxins can be used as painkillers

5. 12.1- Sensory Reception, Sensation & Perception • What is the difference between these three? • Reception • Sensation • Perception

6. Sensory Receptors • Our sensory neurons are attached to receptors that are activated by specific stimuli • These sensory receptors are highly modified ends (dendrites) of sensory neurons • We have a number of different types of receptors in our body

8. Groups of Receptors • Often receptors are grouped in specific organs which are specialized to respond to a single stimuli (such as organs for taste, smell, hearing and vision) • The sensations that we receive from these receptors are actually produced in the brain – if transmission from the sensory neuron is blocked, the sensation stops

9. In general, the stimuli that we respond to are those most relevant to our survival • For example, our range of hearing and vision is limited compared to other animals, even though the other stimuli are present • Our senses can also undergo sensory adaptation • This occurs when a receptor becomes accustomed to a particular stimulus being present

10. Sensory Adaptation Examples • Ever notice that some strong smells, over time, seem to disappear? • However, if you leave that environment and return, the smell has seemingly reappeared • This phenomenon is due to your sense of smell becoming accustomed to that strong smell

11. We can also become accustomed to temperature changes • For instance, before you step into a warm shower, the bathroom might seem relatively warm • However, after the shower, you step out and feel very cold • This is because your body becomes accustomed to the warmer temperatures of the shower

12. 12.2 - The Eye

13. The Eye • The eye consists of three layers: • The Sclera • Outermost portion of the eye • Includes the cornea & aqueous humor

14. Choroid Layer • Contains pigments that prevent light from scattering • Includes the: • Iris • Pupil • Ciliary Muscles

15. Retina • Composed of three layers of cells: • Rods & Cones • Bipolar Cells • Ganglion Cell Layer

16. The Retina

17. The Fovea Centralis • This is a region in the center of the retina that contains a dense bundle of cones • The lens of the eye focuses the majority of the light on this area • The fovea produces sharp colour images • Surrounding this area are rods which pick up low-intensity black & white light

18. Vision – The Lens • Images form on the retina because of the focal length of the lens • Unlike plastic or glass lenses, the lens of the eye can change its shape, which makes it able to focus on near and far objects • Objects 6 m (20 ft) from the eye should be focused without any change to the lens’ normal shape

19. The Chemistry of Vision • Rods and cones contain a light-sensitive pigment known as rhodopsin • In the absence of light, rods release inhibitory neurotransmitters that inhibits nearby nerve cells • When light hits this pigment it is split into two components: Opsin (a protein) and retinene (a form of vitamin A) • This division stops the release of the inhibitory transmitter, allowing transmission of an impulse to the optic nerve

21. Regeneration • As indicated by the previous diagram, the breakdown of rhodopsin is much faster than its regeneration • This is responsible for the afterimages that are often seen after looking at a single object for a long time or at a bright light • Bright light can cause temporary blindness because the rhodopsin is not regenerated in sufficient amounts to maintain vision

22. Colour Vision • The cones used for colour vision come in three varieties – red, green and blue • Slight changes in the opsin component of rhodopsin are responsible for the various cones’ sensitivities to different colours of light • The following diagram shows the subtle differences in the opsin molecules

23. As you can see, there are subtle changes to the amino acids that make up these proteins

24. Colour Blindness • Colour blindness is caused when one or more of the colour cones are defective • This is caused by a mutation in the genes that create the opsin molecules • These mutations alter the sequence of amino acids that make up the opsin, and therefore change its shape and function

25. Types of Colour Blindness • There are a number of different types of colour blindness • A rare case, known as monochromacy, occurs when a person lacks all three colour pigments and can distinguish no colour at all • More common is dichromacy, where one of the pigments is absent – this is often inherited and affects males more often than females

26. Types of Colour Blindness • A third type of colour blindness is anomalous trichromacy, where all three pigments are present, but have altered spectral sensitivity • It often results in a difficulty in distinguishing between red and green hues (most common) or yellow and blue hues (very rare)

27. Types of Dichromacy • Protanopia – an absence of red colour receptors; red will appear dark • Deuteranopia – green photoreceptors are absent, and it affects red-green colour distinction • Tritanopia – total absence of blue receptors What does dichromacy look like?

28. Other Common Vision Defects • There are a number of other common vision defects • Glaucoma • Caused by increased pressure in the aqueous humor • This pressure causes the blood vessels in the retina to collapse • The rods and cones die because of a lack of oxygen and other nutrients

29. Glaucoma can be treated with medication or surgery • Medications aim at reducing the pressure within the aqueous humor by either helping it drain or reducing the production of the aqueous humor • Laser or microsurgery can be used to cut a small hole to relieve the fluid pressure, but this is not a permanent solution

30. 2. Cataracts • Cataracts are caused by the lens becoming more opaque • This prevents light from coming through and reaching the retina • Cataracts can be treated by replacing the damaged lens with an artificial one using surgery http://upload.wikimedia.org

31. Astigmatism • Astigmatism occurs when the lens is irregularly-shaped and only correctly focuses in one plane • This can be countered by using an external lens to compensate for the irregular shape of the lens in the eye

32. Myopia (Nearsightedness) • Myopia occurs when the eyeball is “too long” and the image from the lens focuses in front of the retina • This is treated by using a biconcave lens to diverge the light rays before they reach the lens

33. Hyperopia (farsightedness) • The main contributing factor to hyperopia is an eye that is “too short”, resulting in the image being focused behind the retina • A convex lens can be used to converge the light rays before they reach the lens, which refocuses the light on the retina • As well, as we age, our lens becomes less elastic and we lose the ability to focus on near objects

34. The Blind Spot • Where the ganglion cells merge, they form the optic nerve • At the point where the optic nerve enters the retina, it creates a region that has no rods or cones • This is known as the blind spot

35. Visual Interpretation • Messages from the eyes travel through the optic nerves to the brain • Once in the brain, the pieces of visual information are sorted, processed, and integrated to produce a 3-D image • Aspects of sight such as movement, colour, depth, and shape are handled by different parts of the occipital lobe • This speeds up the processing of the visual image

36. Note that images from the right eye are interpreted on the left side of the occipital lobe

37. 12.3 - The Ear • The ear carries out two functions – it is used for balance and for hearing • Both of these senses use specialized hair cells that are very tiny and respond to the movement of fluids in the ear

38. Anatomy of the Ear

39. The Outer Ear • The outer ear consists of: • The pinna • The auditory canal

40. The Middle Ear • The middle ear produces the sound nerve impulses that are sent to the brain • It consists of several parts: • The tympanic membrane (tympanum) • The ossicles

41. The oval window • The Eustachian tube

42. The Inner Ear • The inner ear contains: • The cochlea • The semicircular canals • The vestibule

43. Hearing and Sound • Our hearing can detect sound energy as low as 1.0×10-12 Watts • Sound travels as pressure waves through a material, and therefore will not pass through a vacuum • Sounds travel most rapidly through solids, and most slowly through gases

44. The Organ of Corti • The Organ of Corti consists of three structures: • The basilar membrane, which contains many hair cells • The hair cells, which have many tiny projections known as stereocilia • The tectorial membrane, into which are embedded the ends of the stereocilia

46. Production of a Sound Impulse • The tympanic membrane vibrates as pressure waves hit it • These vibrations are passed on to the ossicles, which amplify the sound • The vibrations of the ossicles move the oval window; the round window moves as well, producing waves of fluid in the inner ear

47. These waves of fluid travel through the cochlea • The movement of fluid causes a thin membrane known as the basilar membrane to move. This membrane is attached to hair cells located in the organ of Corti • The movement of the cilia of the hair cells against the tectorial membrane produces a nerve impulse which is sent to the brain Production of a Sound Impulse - Animation

48. Hearing and Pitch • Different pitches of sound can be heard by the human ear (a range of about 20 – 20,000 cycles per second) • Low-pitched sounds stimulate the hair cells near the far end of the cochlea, while high-pitched sounds stimulate hair cells near to the oval window

49. Hearing Loss • Hearing loss generally results from nerve damage (generally damage to the hair cells) or damage to the sound-conduction system of the outer and middle ear • Repeated loud noise destroys stereocilia • Any noise over 80 dB can damage hair cells

50. Hearing Loss Treatment • For people who have conduction deafness, hearing aids are often used • However, patients with nerve deafness can have a device implanted in the ear that picks up sounds and transmits them directly to the auditory nerve • Scientists have also been able to use viruses to insert genes that allow the growth of new stereocilia in guinea pigs