1. Sensory Receptors
8th ed 50.1 to 50.4
7th ed 49.1 to 49.4

2. Physiological basis for all animal activity: processing sensory information and generating motor output in response to that information

6. This is a continuous cycle
Sensory information can be from external or internal environment.

7. Sensory pathway:
Sensory reception
Transduction
Transmission
Perception
Amplification and adaptation

8. Sensory pathway:
Sensory reception
Transduction
Transmission
Perception
Amplification and adaptation

9. Sensory reception 1st step of sensory pathway
Sensory receptors: Specialized neurons or epithelial cells; Single cells or a collection of cells in organs
Very sensitive

10. Sensory pathway:
Sensory reception
Transduction
Transmission
Perception
Amplification and adaptation

11. Receptor potential: Sensory transduction:
Conversion of physical, chemical and other stimuli to change in membrane potential
Receptor potential:
change in membrane potential itself

12. Sensory pathway:
Sensory reception
Transduction
Transmission
Perception
Amplification and adaptation

13. Transmission:
Sensory information is transmitted through the nervous system as nerve impulses or action potential to the Central Nervous System (CNS).
Some axons can extend directly into the CNS and some form synapses with dendrites of other neurons
Sensory neurons spontaneously generate action potential without stimulus at a low rate

14. Magnitude of receptor potential controls the rate at which action potentials are generated (larger receptor potential results in more frequent action potentials)
Weak
muscle stretch
Strong
muscle stretch
Muscle
Dendrites
–50
Receptor potential
–50
–70
–70
Stretch
receptor
potential (mV)
Membrane
Action potentials
Axon
–70
–70 1 2 3 4 5 6 7 1 2 3 4 5 6 7
Time (sec)
Time (sec)

15. Sensory pathway:
Sensory reception
Transduction
Transmission
Perception
Amplification and adaptation

16. Perception:
Action potential reach the brain via sensory neurons, generating perception of a stimulus
All action potentials have the same property, what makes the perceptions different are the part of the brain they link to.

17. Sensory pathway:
Sensory reception
Transduction
Transmission
Perception
Amplification and adaptation

18. Amplification and adaptation:
Strengthening of stimulus energy during transduction (involves second messengers)
Continued adaptation: decrease in responsiveness upon prolonged stimulation

19. Types of sensory receptors:
Mechanoreceptors
Chemoreceptors
Electromagnetic receptors
Thermoreceptors
Pain receptors

20. Mechanoreceptors:
sense physical deformation caused by pressure, touch, stretch, motion, sound

21. Stretch receptors are mechanoreceptors are dendrites that spiral around small skeletal muscle fibers
Weak
muscle stretch
Strong
muscle stretch
Muscle
Dendrites
–50
Receptor potential
–50
–70
–70
Stretch
receptor
potential (mV)
Membrane
Action potentials
Axon
–70
–70 1 2 3 4 5 6 7 1 2 3 4 5 6 7
Time (sec)
Time (sec)
Crayfish stretch receptors have dendrites embedded in abdominal muscles. When the abdomen bends,
muscles and dendrites stretch, producing a receptor potential in the stretch receptor. The receptor potential triggers action potentials
in the axon of the stretch receptor. A stronger stretch produces a larger receptor potential and higher frequency of action potentials.

22. Hair
Light
touch
Strong
pressure
Touch receptors (light and deep touch) are embedded in connective tissue
Epidermis
Dermis
Hypodermis
Nerve
Connective
tissue

23. General receptors: respond to total solute concentrations
LE 49-4
Chemoreceptors:
General receptors: respond to total solute concentrations
Specific receptors: respond to concentrations of specific molecules

24. Electromagnetic receptors:
Detect various forms of electromagnetic energy like visible light, electricity, magnetism
Snakes can have very sensitive infrared receptors – detect body heat of prey
Animals can use earth’s magnetic field lines to orient themselves during migration (magnetite in body) – orientation mechanism
Eye
Infrared
receptor
This rattlesnake and other pit vipers have a pair of infrared receptors, one between each eye and nostril. The organs are sensitive enough to detect the infrared radiation emitted by a warm mouse a meter away.
Some migrating animals, such as these beluga whales, apparently sense Earth’s magnetic field and use the information, along with other cues, for orientation.

25. Thermoreceptors: Detect heat and cold
Hair
Thermoreceptors:
Detect heat and cold
Located in skin and anterior hypothalamus
Mammals have many thermoreceptors each for a specific temperature range
Nerve
Connective
tissue

26. Capsaicin triggers the same thermoreceptors as high temperature
Menthol triggers the same receptors as cold (<28oC)

27. Pain receptors (nociceptors):
Hair
Pain receptors (nociceptors):
Stimulated by things that are harmful – high temperature, high pressure, noxious chemicals, inflammations
Defensive function
Nerve
Connective
tissue

28. Sensing gravity and sound in invertebrates:
Hair of different stiffness and length vibrate at different frequencies and pick up sound waves and vibrations

29. Statocyts:
organ with ciliated receptor cells surrounding a chamber containing statoliths in invertebrates – sense gravity
Ciliated
receptor cells
Cilia
Statolith
Sensory
nerve fibers

30. Tympanic membrane stretched over their “ear” help sense vibrations
1 mm
Tympanic membrane stretched over their “ear” help sense vibrations

31. Sensing gravity and sound in humans:
Human ear: sensory organ for hearing and equilibrium
Our organ for hearing “hair cells” are mechanoreceptors because they respond to vibrations
Moving air pressure is converted to fluid pressure

32. Ear structure:
Outer ear: pinna, auditory canal, tympanic membrane (separates outer and middle ear)

33. Middle ear: three small bones malleus (hammer), incus (anvil) and stapes (stirrup) transmit vibrations from tympanic membrane to the oval window.
Eustachian tube connects middle ear to the pharynx and equalizes pressure
Inner ear: consists of fluid filled chambers including semicircular canals (equilibrium) and cochlea (hearing)

34. Moving air travels through the air canal and causes the tympanic membrane to vibrate
Three bones of the middle ear transmit the vibrations to the oval window, a membrane on the cochlear surface
That causes pressure waves in the fluid inside the cochlea

35. The cochlea:
Upper vestibular canal and inner tympanic canal filled with perilymph; middle cochlear duct filled with endolymph

36. Organ of Corti:
Floor of the cochlear duct is the basilar membrane
Organ of corti is located on the basilar membrane, with hair cells which has hair projecting into the cochlear duct.
Many of the hairs are attached to the overhanging tectorial membrane.
Sound waves cause the basilar membrane to vibrate. This results in displacement and bending of the hair cells within the bundle.
This activates the mechanoreceptors, changes the hair cell membrane potential (sensory transduction) which generates action potential in the sensory neuron.

37. Cochlea
Stapes
Axons of
sensory
neurons
Vestibular
canal
Oval
window
Perilymph
Apex
Base
Round
window
Tympanic
canal
Basilar
membrane

39. Equilibrium:
In the vestibule behind the oval window are urticle, saccule and three semicircular canals

40. The hair cells form a cluster. They have a gelatinous capula.
Three semicircular canals arranged in three spatial planes detect angular movements of the head
The hair cells form a cluster. They have a gelatinous capula.
Fluid in the semicircular canals pushes against the capula deflecting the hairs, stimulates the neurons
Semicircular canals
Ampulla
Flow
of endolymph
Flow
of endolymph
Vestibular nerve
Cupula
Hairs
Hair
cell
Vestibule
Nerve fibers
Utricle
Body movement
Saccule

41. Dizziness: false sensation of angular motion
Utricle (oriented horizontally), saccule (oriented vertically) tell the brian which way is up and the position of the body and acceleration
Sheet of hair cells project into a gelatinous capula embedded with otoliths (ear stones). Movement of the head causes otoliths to in different directions against the hair protruding from the hair cells. This movement is detected by the sensory neurons
Dizziness: false sensation of angular motion

42. Hearing and equilibrium in fish:
Vibrations in the water – conducted by skeleton to inner ear canals, move otoliths which stimulate hair cells
Swim bladder: air filled, responds to sound
Lateral line sense organ

43. Lateral line sense organ:
Water flows through the system
Bends hair cells; generates receptor potential
Nerve carries action potential to the brain
Helps them sense water currents, moving objects, low frequency sounds
Lateral line
Lateral line canal
Scale
Opening of
lateral line canal
Epidermis
Neuromast
Segmental muscles of body wall
Lateral nerve
Cupula
Sensory
hairs
Supporting
cell
Hair cell
Nerve fiber

44. Planarians: Ocelli or eye spots in the head region
Vision
Photoreceptors
Planarians: Ocelli or eye spots in the head region
Light stimulates photoreceptors
Brain compares rate of action potential coming form the two ocelli
Brain directs the body to turn until sensation form both ocelli are equal and minimal
Animal can move to shade, under a rock away from predators
Light
Light shining from
the front is detected
Photoreceptor
Nerve to
brain
Visual pigment
Screening
pigment
Ocellus
Light shining from
behind is blocked
by the screening pigment

45. Compound eyes:
Very good at detecting movement
Very good at detecting flickering light (6 times faster than human eye)
Some bees can see in the ultraviolet range of light

46. Several thousand omatidia (facets) in every eye
Cornea and crystalline cone form the lens which focuses light on the rhabdom
Light stimulates the photoreceptors to generate receptor potential which generates action potential
Cornea
Lens
Crystalline
cone
Rhabdom
Photoreceptor
Axons
2 mm
Ommatidium

47. Vertebrate eye
Single lens system (very different from invertebrate single eyes)

48. Eyeball or globe consists of
Sclera
Choroid
Eyeball or globe consists of
Sclera: tough white outer connective tissue
Cornea: clear part of sclera in the front of the eye – lets light into the eye, acts as a fixed lens
Choroid: pigmented inner layer: forms iris (doughnut shaped) – can change size to regulate the amount of light coming in
Iris
Cornea
Optic
nerve
Pupil
Central artery and
vein of the retina
Optic disk
(blind spot)

49. Retina: innermost layer with neurons and photoreceptors
Aqueous humor: fluid that fills the anterior cavity (blockage of ducts increases pressure and causes glaucoma)
Vitreous humor: jellylike, fills the posterior chamber
Retina
Fovea (center
of visual field)
Aqueous
humor
Vitreous humor

50. Lens: clear disk of protein
Ciliary body
Suspensory
ligament
Lens

51. Humans and other mammals
Front view of lens
and ciliary muscle
Humans and other mammals
spherical ( ciliary muscles contract, sensory ligaments relax – near objects)
flatter (ciliary muscles relax, edge of choroid moves away from lens, suspensory ligaments contract and pull the lens – distant objects)
Choroid
Lens (rounder)
Retina
Ciliary
muscle
Suspensory
ligaments
Near vision (accommodation)
Lens (flatter)
Distance vision

52. Fishes, squids and octopuses focus by moving lens forward and backward

53. Photoreceptors
Rods: sensitive to light, do not distinguish colors
Cones: detect color, not very sensitive to light
Nocturnal animals have a higher proportion of rods

55. Rods and cones have stacks of disks
rhodopsin (retinal - vitamin A derivative + opsin) in the membrane
get activated and cause sensory transduction
Rod
Outer
segment
Disks
Inside
of disk
Cell body
Synaptic
terminal
Cytosol
Retinal
Rhodopsin
Opsin

56. Optic nerves meet cross at the optic chiasm
Information from the each eye is carried by the optic nerve (each with about a million axons)
Optic nerves meet cross at the optic chiasm
Information from right visual field of both eyes goes to the left side of the brain
Information from the left visual field of both eyes goes to the right side of the brain
Synapse with interneurons which take the information to the primary visual cortex
Left
visual
field
Right
visual
field
Left
eye
Right
eye
Optic chiasm
Optic nerve
Lateral
geniculate
nucleus
Primary
visual cortex

57. Perception of gustation (taste) and olfaction (smell)
Insects
In insects taste sensation is located within sensory hairs called sensilla (on feet and mouthparts)
Olfactory odorants are detected by olfactory hairs
Sensillum

58. In mammals specialized epithelial cells form taste buds
Tastants detect five perceptions of taste: sweet, sour, salty, bitter and savory (MSG)
Chemoreceptors generate receptor potential by triggering a chain of reactions involving different proteins for different tastes in the receptor cells
Taste pore
Sugar
molecule
Sensory
receptor
cells
Taste
bud
Sensory
neuron
Tongue

59. Sensory neurons lining the nasal cavity and extending into the mucus layer get stimulated by odorants. The stimulus is transmitted directly to the olfactory bulb of the brain
Brain
Action
potentials
Olfactory bulb
Nasal cavity
Bone
Odorant
Epithelial cell
Odorant
receptors
Chemoreceptor
Plasma
membrane
Cilia
Odorant
Mucus