1. Sensory Receptors – Part II
Based on type of stimuli the receptors can detect (stimulus modality)
Chemoreceptors – chemicals, e.g., smell and taste
Mechanoreceptors – pressure and movement, e.g., touch, hearing, balance, blood pressure
Photoreceptors – light, e.g., vision; detect photons
Electroreceptors – electrical fields
Magnetoreceptors – magnetic fields
Thermoreceptors - temperature

2. Mechanoreceptors Transform mechanical stimuli into electrical signals
All organisms and cells can sense and respond to mechanical stimuli
Two main types
ENaC – epithelial sodium channels
TRP – transient receptor potential

3. Touch and Pressure Three classes
Baroreceptors – interoceptors that detect pressure changes
Tactile receptors – exteroceptors that detect touch, pressure, and vibration on the body surface
Proprioceptors – monitor the position of the body

4. Insects
Two types of mechanoreceptors

5. Type 1 – External Surface
Two common types of sensilla
Trichoid – hairlike
Campaniform – bell-shaped
Figure 7.13

6. Type 1 – Internal Surface
Scolopidia – bipolar neuron and complex accessory cell (scolopale)
Can be isolated or grouped to form chordotonal organs
Most function in proprioception
Can be modified into tympanal organs for sound detection
Figure 7.14

7. Vertebrate Tactile Receptors
Widely dispersed
Function as isolated sensory cells
Free nerves endings or enclosed in accessory structures (e.g., Pacinian corpuscle)
Figure 7.15

8. Proprioceptors
Monitor the position of the body
Three major groups
Muscle spindles – located on the surface of the muscle and monitor muscle length
Golgi tendon organs – located at the junction between skeletal muscles and tendons and monitor tendon tension
Joint capsule receptors – located in the capsules that enclose joints and detect pressure, tension, and movement in the joint

9. Equilibrium and Hearing
Utilize mechanoreceptors
Equilibrium or balance – detecting position of the body relative to gravity
Hearing – detecting and interpreting sound waves
Vertebrates: ear is responsible for both equilibrium and hearing
Invertebrates: organs for equilibrium are different from organs of hearing (e.g., tympanal organs)

10. Statocysts Organ of equilibrium in invertebrates
Hollow, fluid filled cavities lined with mechanosensory neurons
Contain statoliths – dense particles of calcium carbonate
Figure 7.16a

11. Hair cells
Mechanoreceptor cells used for hearing and balance in vertebrates
Modified epithelial cells
Have extensive extracellular structures and cilia that extend from the apical end

12. Signal Transduction in Hair Cells
Can detect movement and direction

13. Fish
Use hair cells in ears for hearing and for detecting body position and orientation
Have neuromasts that detect water movement
Neuromast – hair cell and accessory cupula
Lateral line system – array of neuromasts within pits or tubes running along the side of the body

14. Vertebrate Ears
Function in both equilibrium and hearing

15. Equilibrium Vestibular apparatus detects movements
Vestibular apparatus – three semi-circular canals with enlarged region at one end (ampulla) and two sacklike swellings (utricle and saccule)
All regions contain hair cells

16. Vestibular Apparatus
Utricle and saccule contain mineralized otoliths suspended in a macula covering >100,000 hair cells
Ampullae lack otoliths and contain cristae (hair cells located in a cupula)

17. Maculae Detect Linear Acceleration and Tilting
Figure 7.23

18. Cristae Detect Angular Acceleration
Figure 7.24

19. Sound Detection Inner ear detects sound
In fish, incoming sound waves cause otoliths to move which bend cilia of hair cells
Some fish use the swim bladder to amplify sounds
Figure 7.25

20. Terrestrial Vertebrates
Hearing involves the inner, middle, and outer ears
Problem: sound transfers poorly between air and the fluid-filled inner ear
Solution: amply sound
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
Figure 7.26a

21. Mammalian Inner Ear Specialized for sound detection
Cochlea is coiled in mammals
Perilymph – fills vestibular and tympanic ducts and is similar to extracellular fluids
Endolymph – fills cochlea duct and is high in K+ and low in Na+
Organ of Corti contains hair cells and sits on basilar membrane
Two types of hair cells
Inner hair cells detect sound
Outer hair cells amplify sounds
Figure 7.26b

22. Sound Transduction Steps Incoming sound Oval window vibrates
Waves in perilymph of vestibular duct
Basilar membrane vibrates
Stereocilia on the inner hair cells bend
Depolarization
Release of neurotransmitter (glutamate)
Excite sensory neuron
Round window serves as a pressure valve

23. Sound Encoding
Basilar membrane is stiff and narrow at the proximal end and flexible and wide at distal end
Frequency
High  stiff end vibrates
Low  flexible end vibrates

24. Amplification Loudness
Loud sounds   movement of basilar membrane   depolarization of inner hair cells   AP frequency
Outer hair cells
Change shape in response to sound instead of releasing neurotransmitter
Change in shape causes basilar membrane to move more and causes a larger stimulus to the inner hair cells
Amplifies sound

25. Sound Location
Brain uses information on time lags and differences in sound intensity
Sound to right ear first  sound located to the right
Sound louder in right ear  sound located to the right

26. Photoreception
Ability to detect a small proportion of the electromagnetic spectrum from ultraviolet to near infrared
Concentration on this range or wavelengths supports idea that animals evolved in water
Figure 7.27

27. Photoreceptors
Organs range from single light-sensitive cells to complex, image forming eyes
Two major types
Ciliary photoreceptors – have single, highly folded cilium; folds form disks that contain photopigments
Rhabdomeric photoreceptors – apical surface is covered with multiple outfoldings called microvillar projections
Photopigments - molecules that absorb energy from photons

28. Vertebrate Photoreceptors
All are ciliary photoreceptors
Two types
Rods
Cones
Figure 7.29

29. Characteristics of Rods and Cones
Nocturnal animals have relatively more rods

30. Photopigments Photopigments have two covalently bonded parts
Chromophore – pigment that is a derivative of vitamin A, e.g., retinal
Opsin – G-protein-coupled receptors
Steps in photoreception
Chromophore absorbs energy from photon
Chromophore changes shape
Photoreceptor protein changes shape
Signal transduction cascade
Change in membrane potential
Bleaching – process where activated retinal no longer bonds to opsin, thereby activating opsin

31. Phototransduction
Transduction cascades differ in rhabdomeric and ciliary photoreceptors

32. The Eye
Eyespots are single cells or regions of a cell that contain photosensitive pigment, e.g., protist Euglena
Eyes are complex organs
Figure 7.33

33. Flat-sheet Eyes Provide some sense of light direction and intensity
Most often seen in larval forms or as accessory eyes in adults
Figure 7.33a

34. Cup-shaped Eyes Retinal sheet is folded to form a narrow aperture
Better discrimination of light direction and intensity
Seen in the Nautilus

35. Vesicular Eyes
Use a lens in the aperture to improve clarity and intensity
Lens refracts light and focuses it onto a single point on the retina
Present in most vertebrates
Figure 7.33c

36. Convex Eye Photoreceptors radiate outward forming a convex retina
Present in annelids, molluscs, and arthropods (eeeeeeeeeek)

37. Compound Eyes Most complex convex eyes found in arthropods
Composed of ommatidia
Form images in two ways
Apposition compound eyes – ommatidium operate independently; afferent neurons make interconnection to generate an image
Superposition compound eyes – ommatidium work together to form an image on the retina

38. The Vertebrate Eye Forms bright, focused images Parts
Sclera – white of the eye
Cornea – transparent layer
Choroid – pigmented layer
Tapetum – layer in the choroid of nocturnal animals that reflects light
Figure 7.35

39. The Vertebrate Eye, Cont.
Parts
Iris – two layers of pigmented smooth muscle
Pupil – opening in iris
Lens – focuses image
Ciliary body – muscles for changing lens shape
Aqueous humor – fluid in the anterior chamber
Vitreous humor – gelatinous mass in the posterior chamber
Figure 7.35

40. Image Formation Refraction – bending light rays
Both the cornea and the lens act as converting lens to focus light on the retina
In terrestrial vertebrates, most of the refraction occurs between the air and the cornea
Figure 7.36a

41. Image Accommodation
Accommodation - incoming 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
Distant object: light rays are parallel when entering the lens
Close object: light rays are not parallel when entering the lens and must be refracted more
Light rays are focused on the retina by changing the shape of the lens

42. The Retina Arranged into several layers
Rods and cones are are at the back and their tips face backwards
Axons of ganglion cells join together to form the optic nerve
Optic nerve exits the retina at the optic disk (“blind spot”)
Figure 7.37a

43. The Fovea
Small depression in the center of the retina where overlying bipolar and ganglion cells are pushed to the side
Contains only cones
Provides the sharpest images
Figure 7.37a

44. Signal Processing in the Retina
Rods and cones form different images
Rods
Principle of convergence – as many as 100 rods synapse with a single bipolar cell  many bipolar cells synapse with a ganglion cell
Large visual field
Fuzzy image
Cones
One cone synapses with one bipolar cell which connects to one ganglion cell
Small visual field
High resolution image

45. Signal Processing in the Retina, Cont.
Complex “on” and “off” regions of the receptive fields of ganglion cells improve their ability to detect contrasts between light and dark
Figure 7.39

46. The Brain Processes the Visual Signal
Optic nerves  optic chiasm  optic tract  lateral geniculate nucleus  visual cortex
Figure 7.41

47. Color Vision Detecting different wavelengths of light
Requires multiple types of photoreceptors with different maximal sensitivities
Humans: three (trichromatic)
Most mammals: two (dichromatic)
Some bird, reptiles and fish: three, four, or five (pentachromatic)
Figure 7.42

48. Thermoreception
Central 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 – TRP ion channel thermoreceptor proteins

49. 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 (0.5°C for humans)

50. Magnetoreception Ability to detect magnetic fields
e.g., migratory birds, homing salmon
Neurons in the olfactory epithelium of rainbow trout contain particles that resemble magnetite