BIO3420.2010.6 Sensory Systems 15 and 18Oct10 2

Sensory Systems

Chapter 6; 15-18.10.2010

Sensory Receptors  Range from single cells to complex sense organs  Types of receptors  Chemoreceptors, mechanoreceptors, photoreceptors, electroreceptors, magnetoreceptors, thermoreceptors

 All receptors transduce incoming stimuli into changes in membrane potential  Receptor protein detects stimulus  Opening or closing of ion channel  Change in membrane potential  Signal sent to integrating center (central nervous system) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Sensory Receptors

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Figure 6.1

Graded Potentials Generator potential  Sensory receptor is also the primary afferent neuron  Change in membrane potential spreads along membrane

Receptor potential  Sensory receptor is separate from the afferent neuron  Change in membrane potential triggers release of neurotransmitter

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Graded Potentials

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Figure 6.2

Classification of Sensory Receptors Based on stimulus location  Telereceptors  Detect distant stimuli  For example, vision and hearing

 Exteroceptors  Detect stimuli on the outside of the body  For example, pressure and temperature

 Interoceptors  Detect stimuli inside the body  For example, blood pressure and blood oxygen

Tells little or nothing about how receptors work Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Classification of Sensory Receptors Based on type of stimulus (stimulus modality) the receptors detect  Chemoreceptors  Chemicals  For example, smell and taste

 Mechanoreceptors  Pressure and movement  For example, touch, hearing, balance, blood pressure

 Photoreceptors  Light  For example, vision Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Classification of Sensory Receptors  Electroreceptors  Electrical fields

 Magnetoreceptors  Magnetic fields

 Thermoreceptors  Temperature

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Sensitivity to Multiple Modalities Adequate stimulus  Preferred (most sensitive) stimulus modality

Many receptors can be excited by other stimuli, if sufficiently strong  For example, pressure on eyelid  perceive light

Polymodal receptors  Sensitive to more than one stimulus modality  For example, nociceptors; polymodal receptors for multiple types of pain

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Stimulus Encoding  All stimuli are ultimately converted into action potentials in a primary afferent neuron  How can organisms differentiate among stimuli or detect the strength of the signal?  Sensory receptors and sensory neurons must encode four types of information  Stimulus modality  Stimulus location  Stimulus intensity  Stimulus duration

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Stimulus Modality and Location  Receptor location encodes stimulus modality and location  Integrating center interprets modality and location  Modality  Theory of labeled lines  Discrete pathway from sensory cell to integrating center

 Polymodal receptors are exceptions  Encode modality via temporal patterns of APs

 Location  Theory of labeled lines

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Receptive Field and Location of Stimulus  Receptive field  Region of the sensory surface that causes a response when stimulated  Smaller receptive field allows more precise location of the stimulus (i.e., greater acuity)

 Improved ability to localize stimuli by  Using more than one sensory receptor cell  Lateral inhibition  Signals from neurons at the center of the receptive field inhibit neurons on the periphery

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Receptive Field and Location of Stimulus

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Figure 6.3

Dynamic Range  Sensory neurons code stimulus intensity by changes in action potential frequency  For example, strong stimuli  high frequency

 Dynamic range  Range of stimulus intensities over which a receptor exhibits an increased response  Threshold of detection  Weakest stimulus that produces a response in a receptor 50% of the time

 Saturation  Top of the dynamic range (maximal response) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Dynamic Range

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Figure 6.4a

Dynamic Range and Discrimination  Trade-off between dynamic range and discrimination  Large dynamic range  Large change in stimulus causes a small change in AP frequency  Large dynamic range  Poor sensory discrimination

 Narrow dynamic range  Small change in stimulus causes a large change in AP frequency  Small dynamic range  Good sensory discrimination Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Dynamic Range and Discrimination

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Figure 6.4b

Range Fractionation  Range fractionation  Groups of receptors work together to increase dynamic range without decreasing sensory discrimination

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Figure 6.4c

Encoding Logarithmically Encode a wide range of stimulus intensities using a single receptor cell  Good discrimination at certain intensities  Poor discrimination at other intensities

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Figure 6.4d

Tonic and Phasic Receptors Two classes of receptors encode stimulus duration:  Phasic  Produce APs at the beginning or end of the stimulus  Encode change in stimulus, but not stimulus duration

 Tonic  Produce APs as long as the stimulus continues  Encode duration of stimulus  Receptor adaptation – AP frequency decreases if stimulus intensity is maintained at the same level

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Tonic and Phasic Receptors

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Figure 6.5

Chemoreception  Most cells can sense chemical stimuli  Animals have many types of chemoreceptors  Olfaction (smell)  Detection of chemicals in air

 Gustation (taste)  Detection of chemicals emitted from food

 Olfaction and gustation are distinguished by structural criteria  Performed by different sense organs  Different signal transduction mechanisms  Processed in different integrating centers

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The Olfactory System Evolved independently in vertebrates and insects Vertebrate olfactory system    

Can distinguish thousands of odorants Located in the roof of the nasal cavity Mucus layer to moisten olfactory epithelium Odorant binding proteins  Allow lipophilic odorants to dissolve in mucus

 Receptor cells are bipolar neurons with cilia  Odorant receptor proteins are located in the cilia Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Olfactory System

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Figure 6.6

Odorant Receptors  Each olfactory neuron expresses only one odorant receptor protein  There are 1000s of different receptor proteins

 Each receptor can recognize more than one odorant  Each odorant can stimulate more than one receptor  Odorant receptor is linked to G-protein  Odorant binding causes formation of cAMP  Opening of ion channels  Depolarization

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Odorant Receptors

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Figure 6.7

Pheromones  Vomeronasal organ  Detects pheromones  Chemical signals between animals

 Structurally and molecularly distinct from the olfactory epithelium  Located in base of nasal cavity in mammals  Located in palate in reptiles  Receptor is linked to G-protein  Activates phospholipase C transduction system  Opening of ion channels  Depolarization

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Pheromones

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Figure 6.8

Invertebrate Olfactory Systems  Located in many parts of the body  Most near the head

 Primarily on antennae in Arthropods  Sensilla  Hair-like projections of cuticle  Sensilla contain odorant receptor neurons  Odorant receptor is linked to G-protein  Odorant binding causes formation of cAMP  Opening of ion channels  Depolarization

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Invertebrate Olfactory Systems

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Figure 6.9

The Gustatory System  Five classes of tastants  Salty  Sweet  Bitter  Sour  Umami (savory or meaty)

 Sweet, umami, and salty indicate carbohydrates, proteins, and ions  Bitter and sour indicate potentially toxic substances

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Taste Buds in Vertebrates Taste receptors are epithelial cells that release neurotransmitter  vertebrate taste receptors are not neurons

Each taste receptor expresses more than one kind of taste receptor protein Taste receptor cells clustered in groups  On tongue, soft palate, larynx, and esophagus  On external surface of the body in some fish

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Vertebrate Taste Bud

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Figure 6.10

Taste Receptor Transduction Pathways

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Figure 6.11a,b

Taste Receptor Transduction Pathways

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Figure 6.1c,d

Taste in Invertebrates  Located on sensilla  Inside and outside the mouth  Along the wing margin  Ends of the legs

 Receptors  Bipolar sensory neurons  G-protein coupled  Express only a single receptor protein

 Differences between vertebrates and invertebrates suggest independent evolution Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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

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Mechanosensory Protein Receptors

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Figure 6.12

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

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

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Vertebrate Tactile Receptors

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Figure 6.13

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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Insect Tactile Receptors Two common types of sensilla  Trichoid  Hairlike projection of cuticle  Bipolar sensory neuron  TRP channel

 Campaniform  Dome-shaped bulge of cuticle  Bipolar sensory neuron

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Insect Tactile Receptors

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Figure 6.14

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

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Scolopidia

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Figure 6.15

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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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

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Statocysts

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Figure 6.16a.b

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

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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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Hair cells

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Figure 6.17

Signal Transduction in Hair Cells

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Figure 6.18

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

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Fish Neuromast

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Figure 6.19

Vertebrate Ears  Function in both equilibrium and hearing  Outer ear  Not in all vertebrates  Pinna  Auditory canal

 Middle ear  Not in all vertebrates  Interconnected bones in air-filled cavity

 Inner ear  Present in all vertebrates  Series of fluid-filled membranous sacs and canals  Contains mechanoreceptors (hair cells) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Mammalian Ear

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Figure 6.20

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)

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Vertebrate Inner Ears

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Figure 6.21

Vestibular Apparatus  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

 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Vestibular Apparatus

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Figure 6.22a.b

Maculae Detect Linear Acceleration and Tilting

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Figure 6.23

Cristae Detect Angular Acceleration

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Figure 6.24

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

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Figure 6.25

Sound Detection by Inner Ear  Terrestrial Vertebrates  Hearing involves the inner, middle, and outer ear

 Sound transfers poorly between air and the fluidfilled 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

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Mammalian Middle and Inner Ear

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Figure 6.26a

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) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Mammalian Inner Ear

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Figure 6.26a,b

Sound Transduction  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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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

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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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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  Sound located to the right

 Rotation of head helps localize sound

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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

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Electromagnetic Spectrum

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Figure 6.27a,b

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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Phylogeny of Photoreceptors

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Figure 6.28

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

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Vertebrate Photoreceptors

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Figure 6.29

Characteristics of Rods and Cones

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Table 6.1

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

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Figure 6.30

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

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Isomerization of Retinal

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Figure 6.31

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

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Phototransduction

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Figure 6.32

The Eye  Eyespots  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

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Types of Eyes Flat sheet eyes  Some sense of light direction and intensity  Often in larval forms or as accessory eyes in adults

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Figure 6.33a

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

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Figure 6.33b

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

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Figure 6.33c

Types of Eyes  Convex Eye (annelids, molluscs, arthropods)  Photoreceptors radiate outward  Convex retina

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Figure 6.33d

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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Compound Eyes

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Figure 6.34

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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Structure of The Vertebrate Eye

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Figure 6.35

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

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Image Accommodation  Accommodation  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

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Image Accommodation  Distant 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

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Image Accommodation

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Figure 6.36

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”)

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Vertebrate Retina

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Figure 6.37a

The Fovea  Region in center of retina  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

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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

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Cephalopod Eye and Retina

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Figure 6.37b

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

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Convergence in the Vertebrate Retina

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Figure 6.38a,b

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

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Figure 6.39

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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Lateral Inhibition in the Retina

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Figure 6.40

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

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The Brain Processes the Visual Signal

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Figure 6.41

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

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Color Vision Retina and brain compare output from each type of receptor and infer the color

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Figure 6.42

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  Thermoreceptor proteins  TRP ion channel Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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)

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Figure 6.43

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

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