Sensory Systems

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

PowerPointPowerPoint®® Lecture Slides prepared by Lecture Slides prepared by Stephen Gehnrich, Salisbury UniversityStephen Gehnrich, Salisbury University

6C H A P T E R

Sensory SystemsSensory Systems


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


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


Vertebrate Tactile Receptors

Widely dispersed in skin Receptor structure

Free nerves endings Nerve endings enclosed

in accessory structures (e.g., Pacinian corpuscle)


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


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


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


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


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


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


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


Signal Transduction in Hair Cells

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

Fish Neuromast


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) Mammalian Ear


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)


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


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


Maculae Detect Linear Acceleration and Tilting

Figure 6.23

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsFigure 6.24

Cristae Detect Angular Acceleration


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


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

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsFigure 6.26a

Mammalian Middle and Inner Ear


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 CummingsFigure 6.26a,b

Mammalian Inner Ear


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


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


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


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


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


Electromagnetic Spectrum


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


Phylogeny of Photoreceptors


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

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsTable 6.1

Characteristics of Rods and Cones


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


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


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


Phototransduction


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

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsFigure 6.33a

Types of Eyes

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

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsFigure 6.33b

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

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsFigure 6.33c

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

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsFigure 6.33d

Types of Eyes

Convex Eye (annelids, molluscs, arthropods) Photoreceptors radiate outward

Convex retina


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


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


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


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


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


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


Image Accommodation


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”)


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


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


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


Convergence in the Vertebrate Retina



Complex “on” and “off” regions of the receptive fields of ganglion cells improve their ability to detect contrasts between light and dark



“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


Lateral Inhibition in the Retina


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


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


Color Vision

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


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


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)


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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Sensory Systems