1. 35
Sensors

2. Concept 35.1 Sensory Systems Convert Stimuli into Action Potentials
Sensory receptor cells, or sensors or receptors, transduce physical and chemical stimuli into a change in membrane potential.
The change in membrane potential may generate an action potential that conveys the sensory information to the CNS for processing.
Sensory transduction—begins with a receptor protein that can detect a specific stimulus.
The receptor protein opens or closes ion channels in the membrane, changing the resting potential.
See Concept 34.2

3. Concept 35.1 Sensory Systems Convert Stimuli into Action Potentials
Receptor potentials—graded membrane potentials that travel a short distance.
Receptor potentials can generate action potentials in two ways:
Can generate action potentials in the receptor cell
Can trigger release of neurotransmitter so that a postsynaptic neuron generates an action potential
LINK Review the mechanics of graded membrane potentials and action potentials in Concept 34.2 and of synaptic transmission in Concept 34.3

4. Concept 35.1 Sensory Systems Convert Stimuli into Action Potentials
Stretch receptors in crayfish cause receptor potentials when the attached muscle is stretched.
Receptor potentials spread to the base of the axon and generate action potentials.
The rate of firing depends on the magnitude of the receptor potential, which depends on the amount of stretching.

5. Figure 35.1 Stimulating a Sensory Cell Produces a Receptor Potential

6. Concept 35.1 Sensory Systems Convert Stimuli into Action Potentials
Different sensory receptors respond to particular stimuli:
Mechanoreceptors detect physical forces such as pressure (touch) and variations in pressure (sound waves).
Thermoreceptors respond to temperature.
Electrosensors are sensitive to changes in membrane potential.

7. Concept 35.1 Sensory Systems Convert Stimuli into Action Potentials
Chemoreceptors respond to the presence or absence of certain chemicals.
Photoreceptors detect light.
Some sensory receptor cells are organized with other cells in sensory organs, such as eyes and ears.
Sensory systems include sensory cells, associated structures, and neural networks that process the information.

8. Figure 35.2 Sensory Receptor Proteins Respond to Stimuli by Opening or Closing Ion Channels

9. Concept 35.1 Sensory Systems Convert Stimuli into Action Potentials
Sensation depends on which part of the CNS receives the sensory messages.
Intensity of sensation is coded as the frequency of action potentials.
Some sensory cells transmit information to the brain about internal conditions, without conscious sensation.

10. Concept 35.1 Sensory Systems Convert Stimuli into Action Potentials
Adaptation—diminishing response to repeated stimulation.
Enables animals to ignore background conditions but remain sensitive to changing or new stimuli.
Some sensory cells don’t adapt (e.g., mechanoreceptors for balance).

11. Concept 35.2 Chemoreceptors Detect Specific Molecules or Ions
Chemoreceptors—receptor proteins that bind to various molecules, responsible for taste and smell.
Also monitor internal environment, such as CO2 levels in blood.
Olfaction—sense of smell; depends on chemoreceptive neurons embedded in epithelial tissue at top of nasal cavity (in vertebrates).

12. Figure 35.3 Olfactory Receptors Communicate Directly with the Brain (Part 1)

13. Figure 35.3 Olfactory Receptors Communicate Directly with the Brain (Part 2)

14. Concept 35.2 Chemoreceptors Detect Specific Molecules or Ions
Axons from olfactory sensors extend to the olfactory bulb in the brain—dendrites end in olfactory hairs on the nasal epithelium.
Odorant—a molecule that activates an olfactory receptor protein
Odorants bind to receptor proteins on the olfactory cilia.
Olfactory receptor proteins are specific for particular odorants.

15. Concept 35.2 Chemoreceptors Detect Specific Molecules or Ions
When an odorant binds to a receptor protein, it activates a G protein, which activates a second messenger (cAMP).
The second messenger causes an influx of Na+ and depolarizes the olfactory neuron.
Many more odorants can be discriminated than there are olfactory receptors.
In the olfactory bulb, axons from neurons with the same receptors converge on glomeruli.

16. Concept 35.2 Chemoreceptors Detect Specific Molecules or Ions
Pheromones—chemical signals used by insects to attract mates.
Example: Female silkworm moth releases bombykol. Male has receptors for bombykol on the antennae.
One molecule of bombykol is enough to generate action potentials.

17. Figure 35.4 Some Scents Travel Great Distances (Part 1)

18. Figure 35.4 Some Scents Travel Great Distances (Part 2)

19. Concept 35.2 Chemoreceptors Detect Specific Molecules or Ions
Vomeronasal organ (VNO) is found in many vertebrates—specialized for pheromones
It is a paired tubular structure embedded in the nasal epithelium.
When animal sniffs, the VNO draws a sample of fluid over chemoreceptors in walls.
Information goes to an accessory olfactory bulb and on to other brain regions.
APPLY THE CONCEPT Chemoreceptors detect specific molecules or ions

20. Concept 35.2 Chemoreceptors Detect Specific Molecules or Ions
Gustation is the sense of taste.
Taste buds—clusters of chemoreceptors.
Some fish have taste buds on the skin; the duck-billed platypus has taste buds on its bill.
Human taste buds are embedded in the tongue epithelium, on papillae. The sensory cells generate action potentials when they detect certain chemicals.

21. Figure 35.5 Taste Buds Are Clusters of Sensory Cells (Part 1)

22. Figure 35.5 Taste Buds Are Clusters of Sensory Cells (Part 2)

23. Concept 35.2 Chemoreceptors Detect Specific Molecules or Ions
Humans taste salty, sour, sweet, bitter, and umami—a savory, meaty taste.
“Salty” receptors respond to Na+ depolarizing the cell.
“Sour” receptors detect acidity as H+, and “sweet” receptors bind different sugars.
Umami receptors detect the presence of amino acids, as in MSG.
Bitterness is more complicated and involves at least 30 different receptors.

24. Concept 35.3 Mechanoreceptors Detect Physical Forces
Mechanoreceptors are cells that detect physical forces.
Distortion of their membrane causes ion channels to open and a receptor potential to occur.
This may lead to the release of a neurotransmitter.

25. Concept 35.3 Mechanoreceptors Detect Physical Forces
The skin has diverse mechanoreceptors:
Free nerve endings detect heat, cold, and pain.
Merkel’s discs: Adapt slowly, give continuous information.
Meissner’s corpuscles: Adapt quickly, give information about change.
INTERACTIVE TUTORIAL 35.1 Sensory Receptors

26. Concept 35.3 Mechanoreceptors Detect Physical Forces
Ruffini endings: Deep, adapt slowly, react to vibrating stimuli of low frequencies.
Pacinian corpuscles: Deep, adapt rapidly, react to vibrating stimuli at high frequencies.

27. Figure 35.6 The Skin Feels Many Sensations

28. Concept 35.3 Mechanoreceptors Detect Physical Forces
Muscle spindles: Mechanoreceptors in muscle cells, called stretch receptors.
When muscle is stretched, action potentials are generated in neurons.
CNS adjusts strength of contraction to match load on muscle.

29. Concept 35.3 Mechanoreceptors Detect Physical Forces
Golgi tendon organ: Another mechanoreceptor, in tendons and ligaments.
Provides information about the force generated by muscle; prevents muscle tearing.

30. Figure 35.7 Stretch Receptors (Part 1)

31. Figure 35.7 Stretch Receptors (Part 2)

32. Concept 35.3 Mechanoreceptors Detect Physical Forces
Hair cells—mechanoreceptors in organs of hearing and equilibrium.
Hair cells have projections called stereocilia that bend in response to pressure.
Bending of stereocilia can depolarize or hyperpolarize the membrane.

33. Figure 35.8 Hair Cells Have Mechanosensors on Their Stereocilia (Part 1)

34. Figure 35.8 Hair Cells Have Mechanosensors on Their Stereocilia (Part 2)

35. Concept 35.3 Mechanoreceptors Detect Physical Forces
Auditory systems use hair cells to convert pressure waves to receptor potentials.
Outer ear:
Pinnae collect sound waves and direct them to the auditory canal.
The tympanic membrane covers the end of the auditory canal and vibrates in response to pressure waves.

36. Figure 35.9 Structures of the Human Ear (Part 1)

37. Concept 35.3 Mechanoreceptors Detect Physical Forces
Middle ear—air filled cavity:
Open to the throat via the eustachian tube. Eustachian tubes equilibrate air pressure between the middle ear and the outside.
Ossicles—malleus, incus, stapes— transmit vibrations of tympanic membrane to the oval window.
VIDEO 35.1 Human ear drums and bones

38. Figure 35.9 Structures of the Human Ear (Part 2)

39. Concept 35.3 Mechanoreceptors Detect Physical Forces
Inner ear has two sets of canals—the vestibular system for balance and the cochlea for hearing.
The cochlea is a tapered and coiled chamber composed of three parallel canals separated by Reissner’s membrane and the basilar membrane.

40. Figure 35.9 Structures of the Human Ear (Part 3)

41. Concept 35.3 Mechanoreceptors Detect Physical Forces
The organ of Corti sits on the basilar membrane—transduces pressure waves into action potentials.
Contains hair cells with stereocilia—tips are embedded in the tectorial membrane.
Hair cells bend and create a graded potential that can alter neurotransmitter release.
VIDEO 35.2 Hair cells of the cochlea responding to music

42. Concept 35.3 Mechanoreceptors Detect Physical Forces
Upper and lower canals of the cochlea are joined at distal end.
The round window is a flexible membrane at the end of the canal.
Traveling pressure waves of different frequencies will produce flexion of the basilar membrane.

43. Concept 35.3 Mechanoreceptors Detect Physical Forces
Different pitches, or frequency of vibration, flex the basilar membrane at different locations.
Action potentials stimulated by mechanoreceptors at different positions along the organ of Corti are transmitted to regions of the auditory cortex via the auditory nerve.
ANIMATED TUTORIAL 35.1 Sound Transduction in the Human Ear

44. Figure 35.10 Sensing Pressure Waves in the Inner Ear

45. Concept 35.3 Mechanoreceptors Detect Physical Forces
Conduction deafness: Loss of function of tympanic membrane or ossicles.
Nerve deafness: Damage to inner ear or auditory nerve pathways.
Hair cells in the organ of Corti can be damaged by loud sounds. This damage is cumulative and irreversible.

46. Concept 35.3 Mechanoreceptors Detect Physical Forces
The vestibular system in the mammalian inner ear has three semicircular canals at angles to each other, and two chambers—the saccule and the utricle.
Hair cells sense position and orientation of head by shifting of endolymph.
Cupulae in canals contain hair cell stereocilia—otoliths in membrane exert pressure and bend stereocilia.

47. Figure 35.11 Organs of Equilibrium (Part 1)

48. Figure 35.11 Organs of Equilibrium (Part 2)

49. Figure 35.11 Organs of Equilibrium (Part 3)

50. Concept 35.4 Photoreceptors Detect Light
Photosensitivity—sensitivity to light
A range of animal species from simple to complex can sense and respond to light.
All use same pigments—rhodopsins.
ANIMATED TUTORIAL 35.2 Photosensitivity

51. Concept 35.4 Photoreceptors Detect Light
Rhodopsin molecule consists of opsin (a protein) and a light-absorbing group, 11- cis-retinal.
Rhodopsin molecule sits in plasma membrane of a photoreceptor cell.
11-cis-retinal absorbs photons of light and changes to the isomer all-trans-retinal— changes the conformation of opsin.

52. Concept 35.4 Photoreceptors Detect Light
In vertebrate eyes, the retinal and opsin eventually separate, called bleaching.
A series of enzymatic reactions is required to return all-trans-retinal back to 11-cis- retinal, which recombines with opsin to become photosensitive rhodopsin again.

53. Figure 35.12 Light Changes the Conformation of Rhodopsin

54. Concept 35.4 Photoreceptors Detect Light
Rod cells are modified neurons with:
An outer segment with discs of plasma membrane containing rhodopsin to capture photons
An inner segment that contains the nucleus and organelles
A synaptic terminal where the rod cell communicates with other neurons

55. Figure 35.13 A Rod Cell Responds to Light (Part 1)

56. Figure 35.13 A Rod Cell Responds to Light (Part 2)

57. Concept 35.4 Photoreceptors Detect Light
Stimulation of rod cells by light makes the membrane potential more negative (hyperpolarized)—the opposite of other sensory cells responding to their stimuli.
The dark current is a flow of Na+ ions that continually enters the rod cell in the dark.
Rod cell is depolarized and releases neurotransmitter continually.
Hyperpolarizing effect of light decreases neurotransmitter release.

58. Concept 35.4 Photoreceptors Detect Light
When rhodopsin absorbs a photon of light, a cascade of events begins, starting with the activation of a G protein, transducin.
Transducin activates PDE which converts cGMP to GMP—the Na+ channels close, and the membrane is hyperpolarized.

59. Figure 35.14 Light Absorption Closes Sodium Channels

60. Concept 35.4 Photoreceptors Detect Light
Rhodopsin in a variety of visual systems:
Flatworms—photoreceptor cells in paired eye cups.
Arthropods—compound eyes. Each eye consists of units called ommatidia.
Each ommatidium has a lens to focus light onto photoreceptor cells.
INTERACTIVE TUTORIAL 35.2 Visual Receptive Fields

61. Figure 35.15 Ommatidia: The Functional Units of Insect Eyes (Part 1)

62. Figure 35.15 Ommatidia: The Functional Units of Insect Eyes (Part 2)

63. Concept 35.4 Photoreceptors Detect Light
Vertebrates have image-forming eyes— bounded by sclera, connective tissue that becomes transparent cornea on front of eye.
Iris (pigmented)—controls amount of light reaching photoreceptors; opening—pupil.
Lens—crystalline protein, focuses image, allows accommodation, can change shape.
Retina—photosensitive layer, back of eye.
VIDEO 35.3 Human iris responding to changes in light

64. Figure 35.16 The Human Eye (Part 1)

65. Concept 35.4 Photoreceptors Detect Light
The retina has five layers of neurons including photoreceptors (rods and cones) at the back.
Photoreceptors send information to bipolar cells, which send information to the ganglion cell layer.
Axons from ganglion cells conduct information to the brain.
VIDEO 35.4 Human retina

66. Figure 35.16 The Human Eye (Part 2)

67. Concept 35.4 Photoreceptors Detect Light
Two other cell types communicate laterally across the retina:
Horizontal cells form synapses with bipolar cells and photoreceptors.
Amacrine cells form local synapses with bipolar cells and ganglion cells.
Ultimately, all information converges on ganglion cells.

68. Concept 35.4 Photoreceptors Detect Light
A receptive field—a group of photoreceptors that receive information from a small area of the visual field and activate one ganglion cell.
The receptive field of a ganglion cell results from a pattern of synapses between photoreceptors, bipolar cells and lateral connections.

69. Concept 35.4 Photoreceptors Detect Light
Receptive fields have two concentric regions, a center and a surround.
A field can be either on- or off-center.
Light falling on an on-center receptive field excites the ganglion cell, while light falling on an off-center receptive field inhibits the ganglion cell.
The surround area has the opposite effect so ganglion cell activity depends on which part of the field is stimulated.

70. Concept 35.4 Photoreceptors Detect Light
Neurons of the visual cortex, like retinal ganglion cells, have receptive fields.
Cortical neurons are stimulated by bars of light in a particular orientation, corresponding to rows of circular receptive fields of ganglion cells.
The brain assembles a mental image of the world by analyzing the edges in patterns of light and dark.

71. Concept 35.4 Photoreceptors Detect Light
Vertebrate photoreceptors consist of rod cells and cone cells.
Rod cells are responsible for night vision; cone cells are responsible for color vision.
Fovea—area where cone cell density is highest.

72. Figure 35.17 Rods and Cones (Part 1)

73. Concept 35.4 Photoreceptors Detect Light
Humans have three types of cone cells with slightly different opsin molecules—they absorb different wavelengths of light.
This allows the brain to interpret input from the different cones as a full range of color.
Color blindness is the loss of function of a type of cone cell—the result of a nonfunctional gene.

74. Figure 35.17 Rods and Cones (Part 2)

75. Answer to Opening Question
All of these animals make use of other senses besides vision to perceive their surroundings in the dark.
Information is also conveyed through tactile stimuli, olfaction, heat-detection, and auditory input.