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Sensory Receptors – Part II

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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

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