1. Chapter 50
The Senses

2. Concept 50.1: Sensory receptors transduce stimulus energy and transmit signals to the central nervous system
All stimuli represent forms of energy
Sensation involves converting energy into a change in the membrane potential of sensory receptors
Sensations are action potentials that reach the brain via sensory neurons
The brain interprets sensations, giving the perception of stimuli

3. Sensory Pathways
Functions of sensory pathways: sensory reception, transduction, transmission, and integration
For example, stimulation of a stretch receptor in a crayfish is the first step in a sensory pathway

4. Fig. 50-2
Weak receptor potential
Action potentials
Membrane potential (mV)
–50
Membrane potential (mV)
–70
Slight bend: weak stimulus
–70
Brain perceives slight bend.
Dendrites
Stretch receptor
Time (sec) 1 2 4
Axon 3
Brain
Muscle
Brain perceives large bend.
Action potentials
Large bend: strong stimulus
Strong receptor potential
Membrane potential (mV)
Membrane potential (mV)
–50
–70 1
Reception
Figure 50.2 A simple sensory pathway: Response of a crayfish stretch receptor to bending
–70
Time (sec) 2
Transduction 3
Transmission 4
Perception

5. Sensory Reception and Transduction
Sensations and perceptions begin with sensory reception, detection of stimuli by sensory receptors
Sensory receptors can detect stimuli outside and inside the body
Sensory transduction is the conversion of stimulus energy into a change in the membrane potential of a sensory

6. Transmission
After energy has been transduced into a receptor potential, some sensory cells generate the transmission of action potentials to the CNS
Integration of sensory information begins when information is received

7. Perception
Perceptions are the brain’s construction of stimuli
Stimuli from different sensory receptors travel as action potentials along different neural pathways
The brain distinguishes stimuli from different receptors by the area in the brain where the action potentials arrive

8. Specialized sensory receptors detect five categories of stimuli
There are five categories of sensory receptors.
1. Pain receptors detect dangerous stimuli including high heat and pressure.
2. Thermoreceptors detect heat or cold.
3. Mechanoreceptors respond to
mechanical energy,
touch,
pressure, and
sound.
Teaching Tips
In elementary school, students often learn that there are five senses (taste, smell, touch, sight, and hearing). Consider matching these five senses to the types of specialized sensory receptor described in Module 29.3.
© 2012 Pearson Education, Inc. 8

9. Connective tissue Hair movement
Figure 29.3A
Heat
Light touch
Pain
Cold
Hair
Epidermis
Figure 29.3A Sensory receptors in the human skin
Dermis
Nerve
to brain
Connective
tissue
Hair
movement
Strong
pressure 9

10. 29.3 Specialized sensory receptors detect five categories of stimuli
4. Chemoreceptors
include sensory receptors in our nose and taste buds and
respond to chemicals.
5. Electromagnetic receptors respond to
electricity,
magnetism, and
light (sensed by photoreceptors).
Teaching Tips
In elementary school, students often learn that there are five senses (taste, smell, touch, sight, and hearing). Consider matching these five senses to the types of specialized sensory receptor described in Module 29.3.
© 2012 Pearson Education, Inc. 10

11. Heat Gentle touch Pain Cold Hair Epidermis Dermis Hypodermis Nerve
Fig. 50-3
Heat
Gentle touch
Pain
Cold
Hair
Epidermis
Dermis
Figure 50.3 Sensory receptors in human skin
Hypodermis
Nerve
Connective tissue
Hair movement
Strong pressure

12. Chemoreceptors
General chemoreceptors transmit information about the total solute concentration of a solution
Specific chemoreceptors respond to individual kinds of molecules

13. Fig. 50-4
Figure 50.4 Chemoreceptors in an insect
0.1 mm

14. Electromagnetic Receptors
Electromagnetic receptors detect electromagnetic energy such as light, electricity, and magnetism
Photoreceptors are electromagnetic receptors that detect light

15. Eye Infrared receptor (a) Rattlesnake Fig. 50-5a
Figure 50.5a Specialized electromagnetic receptors
(a) Rattlesnake

16. (b) Beluga whales Fig. 50-5b
Figure 50.5b Specialized electromagnetic receptors
(b) Beluga whales

17. Concept 50.2: The mechanoreceptors responsible for hearing and equilibrium detect moving fluid or settling particles
Hearing and perception of body equilibrium are related in most animals
Settling particles or moving fluid are detected by mechanoreceptors

18. Sensing Gravity and Sound in Invertebrates
Most invertebrates maintain equilibrium using sensory organs called statocysts
Statocysts contain mechanoreceptors that detect the movement of granules called statoliths

19. Ciliated receptor cells
Fig. 50-6
Ciliated receptor cells
Cilia
Statolith
Figure 50.6 The statocyst of an invertebrate
Sensory axons

20. Many arthropods sense sounds with body hairs that vibrate or with localized “ears” consisting of a tympanic membrane and receptor cells

21. Hearing and Equilibrium in Mammals
In most terrestrial vertebrates, sensory organs for hearing and equilibrium are closely associated in the ear

22. Auditory nerve to brain
Fig. 50-8a
Middle ear
Outer ear
Inner ear
Skull bone
Stapes
Semicircular canals
Incus
Malleus
Auditory nerve to brain
Figure 50.8 The structure of the human ear
Cochlea
Oval window
Eustachian tube
Pinna
Auditory canal
Round window
Tympanic membrane

23. Vestibular canal Tympanic canal
Fig. 50-8b
Cochlear duct
Bone
Auditory nerve
Vestibular canal
Tympanic canal
Figure 50.8 The structure of the human ear
Organ of Corti

24. Axons of sensory neurons To auditory nerve
Fig. 50-8c
Tectorial membrane
Hair cells
Figure 50.8 The structure of the human ear
Basilar membrane
Axons of sensory neurons
To auditory nerve

25. Fig. 50-8d
Hair cell bundle from a bullfrog; the longest cilia shown are about 8 µm (SEM).
Figure 50.8 The structure of the human ear

26. Hearing
Vibrating objects create percussion waves in the air that cause the tympanic membrane to vibrate
Hearing is the perception of sound in the brain from the vibration of air waves
The three bones of the middle ear transmit the vibrations of moving air to the oval window on the cochlea

27. These vibrations create pressure waves in the fluid in the cochlea that travel through the vestibular canal
Pressure waves in the canal cause the basilar membrane to vibrate, bending its hair cells
This bending of hair cells depolarizes the membranes of mechanoreceptors and sends action potentials to the brain via the auditory nerve

28. The fluid waves dissipate when they strike the round window at the end of the tympanic canal

29. Axons of sensory neurons Apex
Fig a
Axons of sensory neurons
Apex
Oval window
Vestibular canal
Stapes
Vibration
Figure 50.10a Transduction in the cochlea
Basilar membrane
Tympanic canal
Base
Fluid (perilymph)
Round window

30. The ear conveys information about:
Volume, the amplitude of the sound wave
Pitch, the frequency of the sound wave
Outer Ear
Middle Ear
Inner Ear
Auditory
canal
Ear-
drum
Hammer,
anvil, stirrup
Oval
window
Cochlear canals
Pinna
Upper and middle
Lower
Pressure
Concentration
in the
middle ear
Organ of
Corti
stimulated
Time
Figure 29.4E The route of sound wave vibrations through the ear
One
vibration
Amplitude 30

31. The cochlea can distinguish pitch because the basilar membrane is not uniform along its length
Each region vibrates most vigorously at a particular frequency and leads to excitation of a specific auditory area of the cerebral cortex

32. Flexible end of basilar membrane
Fig b
500 Hz (low pitch)
1 kHz
Flexible end of basilar membrane
Apex
2 kHz
Basilar membrane
4 kHz
Figure 50.10b Transduction in the cochlea
8 kHz
Base (stiff)
16 kHz (high pitch)

33. Several organs of the inner ear detect body position and balance:
Equilibrium
Several organs of the inner ear detect body position and balance:
The utricle and saccule contain granules called otoliths that allow us to detect gravity and linear movement
Three semicircular canals contain fluid and allow us to detect angular acceleration such as the turning of the head

34. Semicircular canals Flow of fluid Vestibular nerve Cupula Hairs
Fig
Semicircular canals
Flow of fluid
Vestibular nerve
Cupula
Hairs
Hair cells
Vestibule
Axons
Figure Organs of equilibrium in the inner ear
Utricle
Body movement
Saccule

35. Hearing and Equilibrium in Other Vertebrates
Unlike mammals, fishes have only a pair of inner ears near the brain
Most fishes and aquatic amphibians also have a lateral line system along both sides of their body
The lateral line system contains mechanoreceptors with hair cells that detect and respond to water movement

36. Opening of lateral line canal Cupula Epidermis
Fig
Lateral line
Surrounding water
Scale
Lateral line canal
Opening of lateral line canal
Cupula
Epidermis
Sensory hairs
Hair cell
Figure The lateral line system in a fish
Supporting cell
Segmental muscles
Lateral nerve
Axon
Fish body wall

37. Concept 50.4: Similar mechanisms underlie vision throughout the animal kingdom
Many types of light detectors have evolved in the animal kingdom

38. Most invertebrates have a light-detecting organ
One of the simplest is the eye cup of planarians, which provides information about light intensity and direction but does not form images

39. Two major types of image-forming eyes have evolved in invertebrates: the compound eye and the single-lens eye
Compound eyes are found in insects and crustaceans and consist of up to several thousand light detectors called ommatidia
Compound eyes are very effective at detecting movement

40. (a) Fly eyes Cornea Lens Crystalline cone Rhabdom Photoreceptor Axons
Fig
2 mm
(a) Fly eyes
Cornea
Lens
Crystalline cone
Rhabdom
Figure Compound eyes
Photoreceptor
Axons
Ommatidium
(b) Ommatidia

41. Single-lens eyes are found in some jellies, polychaetes, spiders, and many molluscs
They work on a camera-like principle: the iris changes the diameter of the pupil to control how much light enters

42. The Vertebrate Visual System
In vertebrates the eye detects color and light, but the brain assembles the information and perceives the image

43. Main parts of the vertebrate eye:
Structure of the Eye
Main parts of the vertebrate eye:
The sclera: white outer layer, including cornea
The choroid: pigmented layer
The iris: regulates the size of the pupil
The retina: contains photoreceptors
The lens: focuses light on the retina
The optic disk: a blind spot in the retina where the optic nerve attaches to the eye

44. The ciliary body produces the aqueous humor
The eye is divided into two cavities separated by the lens and ciliary body:
The anterior cavity is filled with watery aqueous humor
The posterior cavity is filled with jellylike vitreous humor
The ciliary body produces the aqueous humor

45. Fovea (center of visual field)
Fig
Sclera
Choroid
Retina
Ciliary body
Suspensory ligament
Fovea (center of visual field)
Cornea
Iris
Optic nerve
Pupil
Aqueous humor
Figure Structure of the vertebrate eye
Lens
Central artery and vein of the retina
Vitreous humor
Optic disk (blind spot)

46. Humans and other mammals focus light by changing the shape of the lens

47. Visual acuity is the ability of the eyes to distinguish fine detail.
29.9 CONNECTION: Artificial lenses or surgery can correct focusing problems
Visual acuity is the ability of the eyes to distinguish fine detail.
Visual acuity is measured by reading standardized eye charts from a distance of 20 feet.
The ability to see normally at 20 feet is 20/20 vision.
Student Misconceptions and Concerns
Many common visual phenomena may have been noticed but not understood by students. Students have experienced or know about floating specks in the visual field, difficulty focusing on text late at night, and colorblindness. However, few students have the ability to accurately explain these and many other phenomena related to vision. These familiar subjects of curiosity can be used in your class to encourage reflective critical thought using the information provided in Modules 29.7– Insight into their explanations and other questions can be found in the Teaching Tips directly below.
Teaching Tips
Challenge students to explain why an image appears clearer as we move closer to it. In general, it has to do with the number of rods and cones in the retina that are used to form the image. When we see an object at a distance, perhaps using only 10% of our field of vision, we use a proportional amount of rods and cones to form the image (about 10%). When we move closer, the image forms a larger percentage of our field of view and a proportionally higher number of rods and cones paint the picture. Like the images displayed on computer monitors or printed in newspapers, this image is formed by a series of dots: the more dots used to form the picture, the clearer the image.
© 2012 Pearson Education, Inc. 47

48. Three vision problems are common.
29.9 CONNECTION: Artificial lenses or surgery can correct focusing problems
Three vision problems are common.
1. Nearsightedness is the inability to focus on distant objects, usually caused by an eyeball that is too long.
2. Farsightedness is the inability to focus on close objects, usually caused by an eyeball that is too short.
3. Astigmatism is blurred vision caused by a misshapen lens or cornea.
Corrective lenses can bend light rays to compensate for each of these problems.
Student Misconceptions and Concerns
Many common visual phenomena may have been noticed but not understood by students. Students have experienced or know about floating specks in the visual field, difficulty focusing on text late at night, and colorblindness. However, few students have the ability to accurately explain these and many other phenomena related to vision. These familiar subjects of curiosity can be used in your class to encourage reflective critical thought using the information provided in Modules 29.7– Insight into their explanations and other questions can be found in the Teaching Tips directly below.
Teaching Tips
Challenge students to explain why an image appears clearer as we move closer to it. In general, it has to do with the number of rods and cones in the retina that are used to form the image. When we see an object at a distance, perhaps using only 10% of our field of vision, we use a proportional amount of rods and cones to form the image (about 10%). When we move closer, the image forms a larger percentage of our field of view and a proportionally higher number of rods and cones paint the picture. Like the images displayed on computer monitors or printed in newspapers, this image is formed by a series of dots: the more dots used to form the picture, the clearer the image.
© 2012 Pearson Education, Inc. 48

49. Figure 29.9A A nearsighted eye (eyeball too long)
Diverging
corrective
lens
Focal
point
Shape of
normal
eyeball
Lens
Focal
point
Figure 29.9A A nearsighted eye (eyeball too long)
Retina 49

50. Figure 29.9B A farsighted eye (eyeball too short)
Converging
corrective
lens
Focal
point
Shape of
normal
eyeball
Figure 29.9B A farsighted eye (eyeball too short)
Focal
point 50

51. The human retina contains two types of photoreceptors: rods and cones
Rods are light-sensitive but don’t distinguish colors
Cones distinguish colors but are not as sensitive to light
In humans, cones are concentrated in the fovea, the center of the visual field, and rods are more concentrated around the periphery of the retina

52. Sensory Transduction in the Eye
Each rod or cone contains visual pigments consisting of a light-absorbing molecule called retinal bonded to a protein called an opsin
Rods contain the pigment rhodopsin (retinal combined with a specific opsin), which changes shape when absorbing light

53. In humans, three pigments called photopsins detect light of different wave lengths: red, green, or blue

54. Processing of Visual Information
Processing of visual information begins in the retina
Absorption of light by retinal triggers a signal transduction pathway

55. Bipolar cell either depolarized or hyperpolarized
Fig
Dark Responses
Light Responses
Rhodopsin inactive
Rhodopsin active
Na+ channels open
Na+ channels closed
Rod depolarized
Rod hyperpolarized
Figure Synaptic activity of rod cells in light and dark
Glutamate released
No glutamate released
Bipolar cell either depolarized or hyperpolarized
Bipolar cell either hyperpolarized or depolarized

56. In the dark, rods and cones release the neurotransmitter glutamate into synapses with neurons called bipolar cells
Bipolar cells are either hyperpolarized or depolarized in response to glutamate
In the light, rods and cones hyperpolarize, shutting off release of glutamate
The bipolar cells are then either depolarized or hyperpolarized

57. Three other types of neurons contribute to information processing in the retina
Ganglion cells transmit signals from bipolar cells to the brain; these signals travel along the optic nerves, which are made of ganglion cell axons
Horizontal cells and amacrine cells help integrate visual information before it is sent to the brain
Interaction among different cells results in lateral inhibition, a greater contrast in image

58. Retina Choroid Photoreceptors Neurons Retina Cone Rod Light To brain
Fig
Retina
Choroid
Photoreceptors
Neurons
Retina
Cone
Rod
Light
To brain
Optic nerve
Light
Figure Cellular organization of the vertebrate retina
Ganglion cell
Amacrine cell
Horizontal cell
Optic nerve axons
Bipolar cell
Pigmented epithelium

59. The optic nerves meet at the optic chiasm near the cerebral cortex
Here, axons from the left visual field (from both the left and right eye) converge and travel to the right side of the brain
Likewise, axons from the right visual field travel to the left side of the brain

60. Most ganglion cell axons lead to the lateral geniculate nuclei
The lateral geniculate nuclei relay information to the primary visual cortex in the cerebrum
Several integrating centers in the cerebral cortex are active in creating visual perceptions

61. Lateral geniculate nucleus
Fig
Right visual field
Optic chiasm
Right eye
Left eye
Figure Neural pathways for vision
Left visual field
Optic nerve
Primary visual cortex
Lateral geniculate nucleus