1. Sensory Systems

2. Mechanoreceptors Transform mechanical stimuli into electrical signals
All organisms (and most cells) sense and respond to mechanical stimuli
Two main types of mechanoreceptor proteins:
ENaC
Epithelial sodium channels
TRP channels
Transient receptor potential channels
Channels are linked to extracellular matrix
Mechanical stimuli alter channel permeability

3. Touch and Pressure Three classes of receptors Baroreceptors
Interoceptors detect pressure changes
Tactile receptors
Exteroceptors detect touch, pressure, and vibration
Proprioceptors
Monitor the position of the body

4. Vertebrate Tactile Receptors
Widely dispersed in skin
Receptor structure
Free nerves endings
Nerve endings enclosed in accessory structures
(e.g., Pacinian corpuscle)

5. Vertebrate Proprioceptors
Monitor the position of the body
Three major groups
Muscle spindles
Located in skeletal muscles
Monitor muscle length
Golgi tendon organs
Located in tendons
Monitor tendon tension
Joint capsule receptors
Located in capsules that enclose joints
Monitor pressure, tension, and movement

6. Insect Tactile Receptors
Two common types of sensilla
Trichoid
Hairlike projection of cuticle
Bipolar sensory neuron
TRP channel
Campaniform
Dome-shaped bulge of cuticle

7. Insect Proprioceptors
Scolopidia
Bipolar neuron and complex accessory cell (scolopale)
Can be isolated or grouped into chordotonal organs
Most function in proprioception
Can be modified into tympanal organs for sound detection

8. Equilibrium and Hearing
Utilize mechanoreceptors
Equilibrium (“balance”)
Detect position of the body relative to gravity
Hearing
Detect and interpret sound waves
Vertebrates
Ear is responsible for equilibrium and hearing
Invertebrates
Organs for equilibrium are different from organs of hearing

9. Statocysts Organ of equilibrium in invertebrates
Hollow, fluid filled cavities lined with mechanosensory neurons
Statocysts contain statoliths
Dense particles of calcium carbonate
Movement of statoliths stimulate mechanoreceptors

10. Insect Hearing Strong vibrations sensed by trichoid sensilla
Weak vibrations and sounds are detected by chordotonal organs
Clusters of scolopidia
Located on leg
Mechanosensitive ion channels
Tympanal organs
Thin layer of cuticle (tympanum) overlays chordotonal organ

11. Vertebrate Hair cells Mechanoreceptor for hearing and balance
Modified epithelial cells (not neurons)
Cilia on apical surface
Kinocilium (a true cilium)
Stereocilia (microvilli)
Tips of stereocilia are connected by proteins (tip links)
Mechanosensitive ion channels in stereocilia
Movement of stereocilia  change in permeability
Change in membrane potential
Change in release of neurotransmitter from hair cell

12. Signal Transduction in Hair Cells
Figure 6.18

13. Fish and Amphibian Hair Cells
Hair cells detect body position and movement
Neuromast
Hair cells and cupula
Stereocilia embedded in gelatinous cap
Detect movement of water
Lateral line system
Array of neuromasts within pits or tubes running along the side of the body
Fish Neuromast

14. Vertebrate Ears Function in both equilibrium and hearing Outer ear
Not in all vertebrates
Pinna
Auditory canal
Middle ear
Interconnected bones in air-filled cavity
Inner ear
Present in all vertebrates
Series of fluid-filled membranous sacs and canals
Contains mechanoreceptors (hair cells)
Mammalian Ear

15. Inner Ear: Vestibular Apparatus and Cochlea
Vestibular apparatus detects movements
Three semi-circular canals with enlarged region at one end (ampulla)
Two sacklike swellings (utricle and saccule)
Lagena
Extension of saccule
Extended in birds and mammals into a cochlear duct or cochlea for hearing
Hair cells present in vestibular apparatus and lagena (cochlea)

16. Vestibular Apparatus (1)
Mechanoreceptors of the inner ear
Macula
Present in utricle and saccule
Mineralized otoliths suspended in a gelatinous matrix
Stereocilia of hair cells embedded in matrix
>100,000 hair cells
Detect linear acceleration and tilting of head

17. Vestibular Apparatus (2)
Cristae
Present in ampullae of semicircular canals
Gelatinous matrix (cupula) lacks otoliths
Stereocilia of hair cells embedded in matrix
Detect angular acceleration (turning) of head

18. Maculae Detect Linear Acceleration and Tilting
Figure 6.23

19. Cristae Detect Angular Acceleration
Figure 6.24

20. Sound Detection by Inner Ear
Fish
Sound waves cause otoliths to move
Displacement of cilia on hair cells
Some fish use the swim bladder to amplify sounds
Figure 6.25

21. Sound Detection by Inner Ear
Terrestrial Vertebrates
Hearing involves the inner, middle, and outer ear
Sound transfers poorly between air and the fluid-filled inner ear
Amplification of sound waves
Pinna acts as a funnel to collect more sound
Middle ear bones increase the amplitude of vibrations from the tympanic membrane to the oval window

22. Mammalian Middle and Inner Ear
Figure 6.26a

23. Mammalian Inner Ear Specialized for sound detection Perilymph
Fills vestibular and tympanic ducts
Similar to extracellular fluids (high Na+ and low K+)
Endolymph
Fills cochlear duct
Different from extracellular fluid (high K+ and low Na+)
Organ of Corti
Hair cells on basilar membrane
Inner and outer rows of hair cells
Stereocilia embedded in tectorial membrane in cochlear duct (filled with endolymph)

24. Mammalian Inner Ear
Figure 6.26a,b

25. Sound Transduction Round window serves as a pressure valve
Sound waves vibrate tympanic membrane
Middle ear bones transmit vibration to oval window
Oval window vibrates
Pressure waves in perilymph of vestibular duct
Basilar membrane vibrates
Stereocilia on the inner hair cells bend
Hair cells depolarize
Hair cells release neurotransmitter (glutamate)
Glutamate excites sensory neuron
Round window serves as a pressure valve

26. Encoding Sound Frequency
Frequency Detection
Basilar membrane is stiff and narrow at the proximal end and flexible and wide at distal end
High frequency sound vibrates stiff end
Low frequency sound vibrates flexible end
Specific regions of brain respond to specific frequencies
Place coding

27. Encoding Sound Amplitude and Amplification
Amplitude Detection
Loud sounds cause larger movement of basilar membrane than quiet sounds
 depolarization of inner hair cells
 AP frequency
Outer hair cells amplify quiet sounds
Change shape in response to sound
Do not release neurotransmitter
Change in shape increases movement of basilar membrane
Increased stimulus to inner hair cells

28. Detecting Sound Location
Brain uses time lags and differences in sound intensity to detect location of sound
Sound in right ear first
Sound located to the right
Sound louder in right ear
Rotation of head helps localize sound

29. Photoreception Ability to detect visible light
A small proportion of the electromagnetic spectrum from ultraviolet to near infrared
Ability to detect this range of wavelengths supports idea that animals evolved in water
Visible light travels well in water; other wavelengths do not

30. Electromagnetic Spectrum
Figure 6.27a,b

31. Photoreceptors
Range from single light-sensitive cells to complex, image-forming eyes
Two major types of photoreceptor cells:
Ciliary photoreceptors
Have a single, highly folded cilium
Folds form disks that contain photopigments
Rhabdomeric photoreceptors
Apical surface covered with multiple outfoldings called microvillar projections
Microvillar projections contain photopigments
Photopigments
Molecules that absorb energy from photons

32. Phylogeny of Photoreceptors
Figure 6.28

33. Vertebrate Photoreceptors
Vertebrates have ciliary photoreceptors
Rods
Cones
Both have inner and outer segments
Inner and outer segments connected by a cilium
Outer segment contains photopigments
Inner segment forms synapses with other cells

34. Characteristics of Rods and Cones
Table 6.1

35. Diversity in Rod and Cone Shape
Diverse shapes of rods and cones among vertebrates
Shape does not determine properties of photoreceptor
Properties of photoreceptor depend on its photopigment
Figure 6.30

36. Photopigments Photopigments have two covalently bonded parts
Chromophore
Derivative of vitamin A
For example, retinal
Contains carbon-carbon double bonds
Absorption of light converts bond from cis to trans
Opsin
G-protein-coupled receptor protein
Opsin structure determines photopigment characteristics
For example, wavelength of light absorbed

37. Phototransduction Steps in photoreception
Chromophore absorbs energy from photon
Chromophore changes shape
Double bond isomerizes from cis to trans
Activated chromophore dissociates from opsin
“Bleaching”
Opsin activates G-protein
Formation of second messenger
Ion channels open or close
Change in membrane potential

38. Phototransduction
Figure 6.32

39. The Eye Eyespots Eyes are complex organs
Cells or regions of a cell that contain photosensitive pigment
For example, protist Euglena
Eyes are complex organs
Detect direction of light
Light-dark contrast
Some can form an image

40. Types of Eyes Flat sheet eyes
Some sense of light direction and intensity
Often in larval forms or as accessory eyes in adults
Figure 6.33a

41. Types of Eyes Cup-shaped eyes (e.g., Nautilus)
Retinal sheet is folded to form a narrow aperture
Discrimination of light direction and intensity
Light-dark contrast
Image formation
Poor resolution
Figure 6.33b

42. Types of Eyes Vesicular Eyes (present in most vertebrates)
Lens in the aperture improves clarity and intensity
Lens refracts light and focuses it onto a single point on the retina
Image formation
Good resolution
Figure 6.33c

43. Types of Eyes Convex Eye (annelids, molluscs, arthropods)
Photoreceptors radiate outward
Convex retina
Figure 6.33d

44. Compound Eyes of Arthropods
Composed of ommatidia (photoreceptor)
Each ommatidium has its own lens
Images formed in two ways
Apposition compound eyes
Ommatidia operate independently
Each one detects only part of the image
Afferent neurons interconnect to form an image
Superposition compound eyes
Ommatidia work together to form image
Resolving power is increased by reducing size and increasing the number of ommatidia

45. Structure of The Vertebrate Eye
Sclera
“White” of the eye
Cornea
Transparent layer on anterior
Retina
Layer of photoreceptor cells
Choroid
Pigmented layer behind retina
Tapetum
Layer in the choroid of nocturnal animals that reflects light

46. Structure of the Vertebrate Eye
Iris
Two layers of pigmented smooth muscle
Pupil
Opening in iris allows light into eye
Lens
Focuses image on retina
Ciliary body
Muscles that change lens shape
Aqueous humor
Fluid in the anterior chamber
Vitreous humor
Gelatinous mass in the posterior chamber

47. Image Formation Refraction – bending of light rays
Cornea and lens focus light on the retina
In terrestrial vertebrates, most of the refraction occurs between air and cornea
Lens does fine focusing
Lens changes shape to focus on near or far objects
Accommodation

48. Image Accommodation Accommodation Focal point Focal distance
Light rays must converge on the retina to produce a clear image
Focal point
Point at which light waves converge
Focal distance
Distance from a lens to its focal point

49. Image Accommodation Distant objects Close objects
Light rays are parallel when entering the lens
Ciliary muscles contract
Lens is pulled and becomes thinner
Little refraction of light by lens
Close objects
Light rays are not parallel when entering the lens
Ciliary muscles relax
Lens becomes thicker
More refraction of light by lens

50. Image Accommodation
Figure 6.36

51. The Vertebrate Retina Arranged into several layers
Rods and cones are are in the retina and their outer segments face backwards
Other cells are in front of rods and cones
Bipolar cells, ganglion cells, horizontal cells, amacrine cells
Axons of ganglion cells join together to form the optic nerve
Optic nerve exits the retina at the optic disk (“blind spot”)

52. The Fovea Region in center of retina Image is focused on the fovea
Overlying bipolar and ganglion cells are pushed to the side
Contains only cones
Color vision
Provides the sharpest images
Image is focused on the fovea

53. Cephalopod Eye and Retina
Photoreceptors are on the surface of the retina
Project forward
Supporting cells are located between photoreceptor cells
No other layers of cells associated with photoreceptors
Axons of photoreceptors form optic nerve

54. Signal Processing in the Retina
Rods and cones form different images
Rods
Convergence
Many rods synapse with a single bipolar cell
Many bipolar cells synapse with a single ganglion cell
Ganglion cells has large receptive field
Poor resolution (fuzzy image)
Cones
Each cone synapses with a single bipolar cell
Each bipolar cells connects to a single ganglion cell
Ganglion cell has small receptive field
High resolution

55. Convergence in the Vertebrate Retina
Figure 6.38a,b

56. Signal Processing in the Retina
Complex “on” and “off” regions of the receptive fields of ganglion cells improve their ability to detect contrasts between light and dark
Figure 6.39

57. Signal Processing in the Retina
“On” and “off” regions of the receptive field of ganglion cells improve contrast of light and dark
“Center-surround” organization of receptive field
“On-center” ganglion cells
Stimulated by light in center of receptive field
Inhibited by light in periphery of receptive field
“Off-center” ganglion cells
Stimulated by dark in center of receptive field
Inhibited by dark in periphery of receptive field
Photoreceptors in center and periphery inhibit each other by lateral inhibition

58. Lateral Inhibition in the Retina
Figure 6.40

59. The Brain Processes the Visual Signal
Action potentials from retina travel to brain
Optic nerves  optic chiasm  optic tract  lateral geniculate nucleus  visual cortex
Binocular vision
Eyes have overlapping visual fields
Binocular zone
Combine and compare information from each eye to form a three-dimensional image
Depth perception

60. Color Vision Detecting different wavelengths of visible light
Requires photopigments with different light sensitivities
Most mammals: see two (dichromatic) colors
Humans: see three (trichromatic) colors
Birds, reptiles and fish: see three, four (tetrachromatic), or five (pentachromatic) colors

61. Color Vision
Retina and brain compare output from each type of receptor and infer the color
Figure 6.42

62. Thermoreception Central thermoreceptors Peripheral thermoreceptors
Located in the hypothalamus and monitor internal temperature
Peripheral thermoreceptors
Monitor environmental temperature
Warm-sensitive
Cold-sensitive
Thermal nociceptors – detect painfully hot stimuli
ThermoTRPs
Thermoreceptor proteins
TRP ion channel

63. Specialized Thermoreception
Specialized organs for detecting heat radiating objects at a distance
Pit organs
Pit found between the eye and the nostril of pit vipers
Can detect 0.003°C changes (humans can detect only 0.5°C changes)
Figure 6.43

64. Magnetoreception Ability to detect magnetic fields
For example, migratory birds, homing salmon
Neurons in the olfactory epithelium of rainbow trout contain particles that resemble magnetite
Responds to magnetic field