June 2, 2023 • 34 mins
This textbook is designed specifically for Kansas State's Biology 198 Class. The course is taught using the studio approach and based on active learning. The studio manual contains all of the learning objectives for each class period and is the record of all student activities. Hence, this textbook is more of a reference tool while the studio manual is the learning tool.
Authors: Robert Bear, David Rintoul, Bruce Snyder, Martha Smith-Caldas, Christopher Herren, and Eva Horne
Kansas State University Libraries
New Prairie Press
Bear, Robert; Rintoul, David; Snyder, Bruce; Smith-Caldas, Martha; Herren, Christopher; and Horne, Eva, "Principles of Biology" (2016). Open Access Textbooks. 1. https://newprairiepress.org/textbooks/1
The textbook was originally published and is also available to download at http://cnx.org/contents/db89c8f8-a27c-4685-ad2a-19d11a2a7e2e@24.1.It is licensed under a Creative Commons Attribution License 4.0 license.
Authors: Robert Bear, David Rintoul, Bruce Snyder, Martha Smith-Caldas, Christopher Herren, and Eva Horne
Kansas State University Libraries
New Prairie Press
Bear, Robert; Rintoul, David; Snyder, Bruce; Smith-Caldas, Martha; Herren, Christopher; and Horne, Eva, "Principles of Biology" (2016). Open Access Textbooks. 1. https://newprairiepress.org/textbooks/1
The textbook was originally published and is also available to download at http://cnx.org/contents/db89c8f8-a27c-4685-ad2a-19d11a2a7e2e@24.1.It is licensed under a Creative Commons Attribution License 4.0 license.
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(00:00):
Welcome to Principles of Biology. Thisbook was written by the Open Alternative Textbook
Initiative at Kansas State University and isbeing released as a podcast and distributed under
the terms of the Creative Commons AttributionLicense. Today's episode is Chapter twenty eight
point one. Sensory Systems. Allhyperlinks, images and sources can be found
(00:22):
at the link to the book.In the description, the act of smelling
something anything is remarkably like the actof thinking. Immediately at the moment of
perception, you can feel the mindgoing to work, sending the odor around
from place to place, setting offcomplex repertories through the brain, pulling one
center after another for signs of recognition, for old memories and old connection Lewis
(00:45):
Thomas on smell, nineteen eighty five. Senses provide information about the body and
its environment. Humans have five specialsenses all faction, smell, gustation,
taste, equilibrium, balance and bodyposition, vision and hearing. Additionally,
we possess general senses, also calledsomatisensation, which respond to stimuli like temperature,
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pain, pressure, and vibration.Vestibular sensation, which is an organism's
sense of spatial orientation and balance,proprio section, position of bones, joints,
and muscles, and the sense oflimb position that is used to track
kinesthesia limb movement are part of somtisensation. Although the sensory systems associated with these
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senses are very different, all sharea common function to convert a stimulus such
as light or sound, or theposition of the body into an electrical signal
in the nervous system. This processis called sensory transduction. We are going
to focus on the five special sensesof humans. There are three types of
stimuli that are detected by the humansensory systems. The first is chemical stimulus,
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where molecules stimulate a sensory neuron.Chemical stimuli are detected by the olfactory
system when molecules in the airbine tosensory cells in the nasal epithelia, and
in the gustation taste system when moleculesin your food stimulate your taste buds.
The second is electromagnetic radiation. Lightinteracts with molecules in the sensory cells rods
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and cones of your retina, andthose sensory cells send a signal to your
brain. The third is mechanical stimulation, where the sensory cells are activated by
movement or touch. Mechanical stimuli aredetected by the cells in the interior that
help you detect balance and body position, and by other cells in your interior
detect sound the sense of hearing.Additionally, mechanical stimuli are involved in other
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somatosensory systems, such as pressure,pain, or vibration. In proprioception position
of legs, arms, and otherbody parts, and in kinesthesia detection of
motion of those same body parts.Sensory perception reception. The first step in
sensation is reception, which is theactiveation of sensory receptors by stimuli such as
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mechanical stimuli being bent or squished,for example, chemicals or temperature. The
receptor can then respond to the stimuli. The region in space in which a
given sensory receptor can respond to astimulus, be it far away or in
contact with the body, is thatreceptor's receptive field. Think for a moment
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about the differences in receptive fields forthe different senses. For the sense of
touch, a stimulus must come intocontact with body. For the sense of
hearing, a stimulus can be amoderate distance away. Some bellin whale sounds
can propagate for many kilometers. Forvision, a stimulus can be very far
away. For example, the visualsystem perceives light from stars at enormous distances.
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Transduction, the most fundamental function ofa sensory system, is the translation
of a sensory signal to an electricalsignal. In a nervous system, this
takes place at the sensory receptor,and it produces a change an electrical potential
in response to the stimulus. Thisis called the receptor potential. How is
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sensory input, such as pressure onthe skin, changed to a receptor potential.
In one example, a type ofreceptor called the mechaner receptor, as
shown in figure, possesses specialized membranesthat respond to pressure. Disturbance of these
dendrites by compressing them or bending themopens gated ion channels in the plasma membrane
of the sensory neuron changing its electricalpotential. Recall that in the nervous system,
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a positive change of a neuron's electricalpotential, also called the membrane potential,
depolarizes the neuron. Receptor potentials aregraded potentials. The magnitude of these
graded receptor potentials varies with the strengthof the stimulus. If the magnitude of
depolarization is sufficient, that is,if membrane potential reaches a threshold, the
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neuron will fire an action potential.In all cases, the appropriate stimulus WI
will cause a change in the membranepotential of the sensory cell. The exact
mechanism for changing the membrane potential willbe different for different sensory calls. Illustration
A shows a closed gated ion channelembedded in the plasma membrane. A hair
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like tether connects the channel to theextracellular matrix outside the cell, and another
tether connects the channel to the innercytoskeleton. When the extracellular matrix is deflected,
the tether tugs on the gated ionchannel, pulling it open. Ions
may now enter or exit the cell. Illustration beach shows stereocilia hair like projections
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on outer hair cells that attach tothe tectorial membrane of the inner ear.
The outer hair cells are connected tothe cochlear nerve. A Mechanosensitive ion channels
are gated ion channels that respond tomechanical deformation of the plasma membrane. A
mechanosensitive channel is connected to the plasmamembrane and the cytoskeleton by hair like tethers.
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When pressure causes the extracellular matrix tomove, the channel opens, allowing
ions to enter or exit the cell. B Stereocilia in the human ear are
connected to mechano sensitive ion channels.When a sound causes the stereocilia to move,
mechano sensitive ion channels transduce the signalto the cochlear nerve Sensory receptors for
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different senses are very different from eachother, and they are specialized according to
the type of stimulus they sense.They have receptor specificity. For example,
touch receptors, light receptors, andsound receptors are each activated by different stimuli.
Touch receptors are not sensitive to lightor sound. They are sensitive only
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to touch or pressure. However,stimuli may be combined at higher levels in
the brain, as happens with allfaction contributing to our sense of taste perception.
Perception is an individual's interpretation of asensation. Although perception relies on the
activation of sensory receptor, perception happensnot at the level of the sensory receptor,
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but at higher levels in the nervoussystem in the brain. The brain
distinguishes sensory stimuli through a sensory pathway. Action potentials from sensory receptors travel along
neurons that are dedicated to a particularstimulus. These neurons are dedicated to that
particular stimulus and synapse with particular neuronsin the brain or spinal cord. All
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sensory signals, except those from theolfactory system, are transmitted though the central
nervous system and are routed to thethalamus and to the appropriate region of the
cortex. Recall that the thalamus isa structure in the forebrain that serves as
a clearinghouse and relay station for sensoryas well as motor signals. When the
sensory signal exits the thalamus, itis conducted to the specific area of the
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cortex figure dedicated to processing that particularsense. How are neural signals interpreted?
Interpretation of sensory signals between individuals ofthe same species is largely similar, owing
to the inherited similarity of their nervoussystems. However, barrisome individual differences.
A good example of this is individualtolerances to a painful stimulus such as dental
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pain, which certainly differ. Interestingly, studies have shown that the allele that
results in red hair in humans homozygousfor that allele, known as MC one
R, is a member of thefamily of sensory receptors that detect pain,
and redheads are more sensitive to painand require about twenty percent more anesthetic during
surgery or dental work. So benice to your red headed friends. Illustration
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A shows side view of a humanbrain. The thalamus is in the inner
middle part. Illustration beach shows thelocation of sensory processing regions in the brain.
The visual processing region is at theback of the brain, the auditory
processing region is in the middle ofthe brain, and the somatosensory processing region
is located in a sliverlike region inthe upper part of the brain and extending
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halfway down. In humans, withthe exception of all faction, all sensory
signals are routed from the athalamus tobe final processing regions in the cortex of
the brain. Credit be modification ofwork by Paulina Tishina. Taste and smell.
Taste also called gustation and smell alsocalled all faction, are the most
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interconnected senses in that both involve moleculesof the stimulus entering the body and bonding
to receptors. Smell lets an animalsense the presence of food or other animals,
whether potential mates, predators, orprey, or other chemicals in the
environment that can impact their survival.Similarly, the sense of taste allows animals
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to discriminate between types of foods.While the value of a sense of smell
is obvious, what is the valueof a sense of taste? Different tasting
foods have different attributes, both helpfuland harmful. For example, sweet tasting
substances tend to highly chloric, whichcould be necessary for survival in lean times.
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Bitterness is associated with toxicity, andsourness is associated with spoiled food Salty
foods are valuable in maintaining homeostasis byhelping the body retain water and by providing
irons necessary for cells to function.Tastes and odors, both taste and odor
stimuli, are molecules taken in fromthe environment. The primary tastes detected by
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humans are sweet, sour, bitter, salty, and numami. The first
four tastes need little explanation. Theidentification of amammy as a fundamental taste occurred
fairly recently. It was identified innineteen oh eight by Japanese scientist Kukuni Acada
while he worked with seaweed broth,but it was not widely accepted as a
taste that could be physiologically distinguished untilmany years later. The taste of umammy,
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also known as savoriness, is attributableto the taste of the amino acid
ilglutamate, in fact, monosodia glutamate, where MSG is often used in cooking
to enhance the savory taste of certainfoods. What is the adaptive value of
being able to distinguish humami? Savorysubstances tend to be high in protein.
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All odors that we perceive are volublechemicals in the air we breathe. If
a substance does not release molecules intothe air from its surface, it has
no smell, and if a humanor other animal does not have a receptor
that recognizes a specific molecule, thenthat molecule has no smell. Humans have
about three hundred and fifty olfactory receptorsubtypes that work in various combinations to allow
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us to sense about ten thousand differentodors. Compare that to mice, for
example, which have about one thousand, three hundred olfactory receptor types and therefore
probably sends more odors. Both odorsand tastes involve molecules that stimulate specific chemoreceptors.
Although humans commonly distinguish taste as onesense and smell as another, they
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work together to create the perception offlavor. A person's perception of flavor is
reduced if he or she has congestednasal passages. Reception and transduction taste.
Detecting a taste gustation is fairly similarto detecting an odor of faction, given
that both taste and smell rely onchemical receptors being stimulated by certain molecules.
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The primary organ of taste is thetaste bud. A taste bud is a
cluster of gustatory receptors taste cells thatare located within the bumps on the tongue,
called papilli singular papilla illustrated in figure. An illustration shows small philiform papilli
scattered across the front two thirds ofthe tongue. Larger circumvallate popilli form an
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inverted v at the back of thetongue. Medium sized fungiform papili are shown
scattered across the back two thirds ofthe tongue. Foliate popilli form ridges on
the back edges of the tongue.A micrograph shows a cross section of a
tongue in which the foliate papilli canbe seen as square protrusions about two hundred
microns across and deep i. Foliatecircumvallate and fungiform papili are located on different
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regions of the tongue. B Foliatepapili are prominent protrusions on the slight micrograph.
Credit a modification of work by NCIscale bar data from Matt Russell.
Each taste bud's taste cells are replacedevery ten to fourteen days. These are
elongated cells with hair like processes calledmicrovilli at the tips that extend into the
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taste bud pore illustrate in figure foodmolecules. Tastints are dissolved in saliva and
they bind with and stimulate the receptorson the microvilli. The receptors for tastins
are located across the outer portion andfront of the tongue, outside of the
middle area where the filiform papili aremost prominent. A taste bud is shaped
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like a garlic bulb and is embeddedin the epidermis of the tongue. Together,
the two types of cells that makethe taste bud, taste cells and
supporting cells, resemble cloves. Hairlike microvilli extend from the tips of the
taste cells into a taste porre onthe surface of the tongue. Nerve endings
extend into the bottom of the tastebud from the dermis. Pores in the
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tunnel allow tastings to enter taste poresin the tongue credit modification of work by
Vincenzo Rizzo. In humans, thereare five primary tastes, and each taste
has only one corresponding type of receptor. Thus, like all faction, each
receptor is specific to its stimulus tastened. Both tasting abilities and sense of smell
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change with age. In humans,the senses decline dramatically by age fifty and
continue to decline. A child mayfind a food to be too spicy,
whereas an elderly person may find thesame food to be bland and unappetizing.
Hearing and equilibrium audition or hearing isimportant to humans and to other animals for
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many different interactions. It enables anorganism to detect and receive information about danger,
such as an approaching predator, andto participate in communal exchanges like those
concerning territories or mating. On theother hand, although it is physically linked
to the auditory system, the vestibularsystem is not involved in hearing. Instead,
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an animal's vestibular system detects its ownmovement, both linear and angular acceleration
and deceleration, and balance. Soundauditory stimuli are sound waves, which are
mechanical pressure waves that move through amedium such as air or water. There
are no sound waves in a vacuum, since there are no air molecules to
move in waves. The speed ofsound waves differs based on altitude, temperature,
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and medium, but at sea leveland a temperature of twenty degree C
sixty eight degrees a sound waves travelin the air at about three hundred and
forty three meters per second, Asis true for all waves. There are
four main characteristics of a sound wave, frequency, wavelength, period, and
amplitude. Frequency is the number ofwaves per unit of time, and in
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sound is heard as pitch. Highfrequency greater than are equal to fifteen point
zero zero zero herts. Sounds arehigher pitched short wavelength than low frequency long
wavelengths less than are equal to onehundred herts. Sounds frequency is measured in
cycles per second, and for sound, the most commonly used unit is herts
hc or cycles per second. Mosthumans can perceive sounds with frequencies between thirty
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and twenty thousand herts. Women aretypically better at hearing high frequencies, but
everyone's ability to hear high frequencies decreaseswith age. Dogs detect up to about
forty thousand herts, cats sixty thousandherts, bats one hundred thousand herts,
and dolphins one hundred and fifty thousandherts. And American shad a loosasapidissima of
fish can hear one hundred and eightythousand herts. Those frequencies above the human
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range are called ultrasound. Reception ofsound in mammals, sound waves are collected
by the external cartilachinus part of theear called the pinna, then travel through
the auditory canal and cause vibration ofthe thin diaphragm called the tympanum or ear
drum. The innermost part of theouter ear illustrated in figure. Interior to
the tympanum is the middle ear.The middle ear holds three small bones called
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the oscles, which transfer energy fromthe moving tympanum to the inner ear.
The three osscles are the malleus alsoknown as the hammer, the incus,
the anvil, and stapies the stirrup. The aptly named stapies looks very much
like a stirrup. The three ossclesare unique to mammals and each plays a
role in hearing. The malleus attachesat three points to the interior surface of
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the tympanic membrane. The incus attachesthe malleus to the stapies. In humans,
the stepies is not long enough toreach the tympanum. If we did
not have the malleus and the incus, then the vibrations of the tympanum would
never reach the inner ear. Thesebones also function to collect force and amplify
sounds. The ear osicles are homologousto bones in a fish mouth. The
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bones that support gills and fish arethought to have been adapted for use in
the vertebrate ear over evolutionary time.The illustration shows the parts of the human
ear. The visible part of theexterior ear is called the pinna. The
ear canal extends inward from the pinnato a circular membrane called the tympanum.
On the other side of the tympanumis the Eustachian tube. Inside the Eustacean
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tube, the malleus, which touchesthe inside of the tympanum, is attached
to the incas, which is inturn attached to the horseshoe shaped stepies.
The steepies is attached to the roundwindow, a membrane. In the snail
shell shaped cocchlea. Another window,called the round window, is located in
the wide part of the cocchlea.Ring like semicircular canals extend from the cocchlea.
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The cochlear nerve and vestibular nerve ofboth connect to the cocchlea. Sound
travels through the outer ear to themiddle ear, which is bounded on its
exterior by the tympanic membrane. Themiddle ear contains three bones called aussicles that
transfer the sound wave to the ovalwindow. The exterior boundary of the inner
ear. The organ of cordy,which is the organ of sound transduction,
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lies inside the COCCHLEA credit modification ofwork by Lars Chica Axle Brochman vestibular information.
The stimuli associated with the vestibular systemare linear acceleration, gravity, and
angular acceleration and deceleration. Gravity accelerationand deceleration are detected by evaluating the inertia
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on receptive cells in the vestibular system. Gravity is detected through head position.
Angular acceleration and deceleration are expressed throughturning or tilting of the head. The
vestibular system has some similarities with theauditory system. It utilizes as hair cells
just like the auditory system, butit excites them in different ways. There
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are five vestibular receptor organs in theinner ear, the utricle, the saccule,
and three semicircular canals. Together theymake up what's known as the vestibular
labyrinth that is shown in figure.The utricle and saccule respond to acceleration in
a straight line, such as gravity. The roughly thirty thousand hair cells and
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the utricle and sixteen thousand hair cellsand the saccule lie below a gelatinous layer
with their stereocilia projecting into the gelatin. Embedded in this gelatin are calcium carbonate
crystals, like tiny rocks. Whenthe head is tilted, the crystals continue
to be pulled straight down by gravity, but the new ankle of the head
causes the gelatin to shift, therebybending the stereocilia. The bending of the
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stereocilia stimulates the neurons and they signalto the brain that the head is tilted,
allowing the maintenance of balance. Itis the vestibular branch of the vestibulo
cochlear cranial nerve that deals with balans. This illustration shows the snail shell shaped
cochlea which widens into the vestibule.Two circular organs, the utricle and the
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saccule, are located in the vestibule. Three ring like canals, the horizontal
canal, the posterior canal, andthe superior canal, extend from the top
of the vestibule. Each canal projectsin a different direction. The structure of
the vestibular labyrinth is shown credit modificationof work by nih. The fluid filled
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semicircular canals are tubular loops set atoblique angles. They are arranged in three
spatial plains. The base of eachcanal has a swelling that contains a cluster
of hair cells. The hair's projectinto a gelatinous cap called the cupula,
and monitor angular acceleration and deceleration fromrotation. They would be stimulated by driving
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your car around a corner, turningyour head, or falling forward. One
canal lies horizontally, while the othertwo lie at about forty five degree angles
to the horizontal axis, as illustratedin figure. When the brain processes input
from all three canals together, itcan detect angular acceleration or deceleration in three
dimensions. When the head turns,the fluid in the canal shifts, thereby
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bending stereocilia and sending signals to thebrain. Upon cessation, accelerating or decelerating,
or just moving, the movement ofthe fluid within the canal slows or
stops. For example, imagine holdinga glass of water. When moving forward,
water may splash backwards onto the hand, and when motion has stopped,
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water may splash forward onto the fingers. While in motion, the water settles
in the glass and does not splash. Note that the canals are not sensitive
to velocity itself, but to changesin velocity. So moving forward at sixty
miles per hour with your eyes closedwould not give the sensation of movement,
but suddenly accelerating or breaking with stimulatethe receptors. Vision. Vision is the
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ability to detect light patterns from theoutside environment and interpret them into images.
Animals are bombarded with sensory information,and the sheer volume of visual information can
be problematic. Fortunately, the visualsystems of species have evolved to attend to
the most important stimuli. The importanceof vision to humans is further substantiated by
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the fact that about one third ofthe human cerebral cortex is dedicated to analyzing
and perceiving visual information light. Aswith auditory stimuli, light travels in waves.
The compression waves that composed sound musttravel in a medium a gas,
a liquid, or a solid.In contrast, light is composed of electromagnetic
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waves and needs no medium. Lightcan travel in a vacuum figure. The
behavior of light can be discussed interms of the behavior of waves, and
also in terms of the behavior ofthe fundamental unit of light, a packet
of a tromagnetic radiation called a photon. A glance at the electromagnetic spectrum shows
that visible light for humans is justa small slice of the entire spectrum,
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which includes radiation that we cannot seeas light because it is below the frequency
of visible red light and above thefrequency of visible violet light. Certain variables
are important when discussing perception of light. Wavelength, which varies inversely with frequency,
manifests itself as hue. Light atthe red end of the visible spectrum
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has longer wavelengths and is lower frequency, while light at the violet end has
shorter wavelengths and is higher frequency.The wavelength of light is expressed in nanimeters,
and one nanometer is one billionth ofa meter. Humans perceive light that
ranges between approximately three hundred and eightynanimeters and seven hundred and forty nanimeters.
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Some other animals, though, candetect wavelengths outside of the human range.
For example, b s naraltraviolet lightin order to locate nectar guides on flowers,
and some non avian reptiles sense infraredlight heat that prey gives off.
The illustration shows the electromagnetic spectrum,which consists of different wavelengths of electromagnetic radiation.
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Radio waves have the longest wavelength aboutone hundred and three meters. Wavelength
gets increasingly shorter for microwave, infrared, visible ultraviolet, X rays, and
gamma rays. Gamma rays have awavelength of about ten to twelve meters.
Frequency is inversely proportional to wavelength.In the electromagnetic spectrum, visible light lies
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between three hundred and eighty nanimeters andseven hundred and forty nanimeters credit modification of
work by NASA. Wave amplitude isperceived as luminous intensity or brightness. The
standard unit of intensity of light isthe candela, which is approximately the luminous
intensity of a one common candle.Light waves travel two hundred and ninety nine
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thousand, seven hundred and ninety twokilometers per second in a vacuum and somewhat
slower in various medias such as airand water, and those waves arrive at
the eye as long red, mediumgreen, and short blue waves. What
is termed white light is light thatis perceived as white by the human eye.
This effect is produced by light thatstimulates equally the color receptors in the
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human eye. The apparent color ofan object is the color or colors that
the object reflects Thus a red objectreflects the red wavelengths in mixed white light
and absorbs all other wavelengths of light. Anatomy of the eye. The photoreceptive
cells of the eye where a transductionof light to nervous impulses occurs, are
located in the retina shown in figureon the inner surface of the back of
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the eye. But light does notimpinge on the retina unaltered. It passes
through other layers that process it sothat it can be interpreted by the retina
Figure EBB. The cornea, thefrat transparent layer of the eye, and
the crystalline lens, a transparent convexstructure behind the cornea, both for fract
bend light to focus the image onthe retina. The iris, which is
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conspicuous as the colored part of theeye, is a circular muscular ring lying
between the lens and cornia that regulatesthe amount of light entering the eye.
In conditions of high ambient light,the iris contracts, reducing the size of
the pupil at its center. Inconditions of low light, the iris relaxes
and the pupil enlarges. The leftillustration shows a human eye which is round
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and filled with vitreous humor. Theoptic nerve and retinal blood vessels exit the
back of the eye. At thefront of the eye is the lens with
a pupil in the middle. Thelens is covered by the iris, which
in turn is covered by the cornea. The aqueous humor is a jelic substance
between the cornea and iris. Theretina is the lining of the inner eye.
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A second illustration is a blow upwhich shows that the optic nerve is
at the surface of the retina.Beneath the optic nerve is a layer of
ganglian cells, and beneath this isa layer of bipolar cells. Both ganglia
and bipolar cells are nerve cells withroot like appendages. Beneath the bipolar cell
layer are the rods and cones.Rods and cones are similar in structure and
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column like a The human eye isshown in cross section B. A blow
up shows the layers of the retina. The main function of the lens is
to focus light on the retina andphobous entrallis. The lens is dynamic,
focusing and refocusing light as the eyerests on near and far objects. In
the visual field. The lens isoperated by muscles that stretch it flat or
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allow it to thicken, changing thefocal length of light coming through it to
focus it sharply on the retina.With age comes the loss of the flexibility
of the lens, and a formof far sidedness called presbiopia results. Presbiopia
occurs because the image focuses behind theretina. Presbiopia is a deficit similar to
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a different type of far sightedness calledhyperopia, caused by an eyeball that is
too short. For both defects,images in the distance are clear, but
images nearby are blurry. Myopia nearsightedness occurs when an eyeball is elongated and
the image focus falls in front ofthe ratina. In this case, images
in the distance are blurry but imagesnearby are clear. There are two types
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of photoreceptors in the ratina, rodsand cones, named for their general appearance
as illustrated in figure. Rods arestrongly photosensitive and are located in the outer
edges of the ratina. They detectdim light and are used primarily for peripheral
and night time vision. Cones areweakly photosensitive and are located near the center
of the ratina. They respond tobright light and their primary role is in
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daytime color vision. This illustration showsthat rods and cones are both long column
like cells, with the nucleus locatedin the bottom portion. The rod is
longer than the cone. The outersegment of the rod contains rhodopsin the outer
segment of the rod contains other photopigments. An oil droplet is located beneath the
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outer segment. Rods and cones arephotoreceptors in the retina. Rods respond in
low light and can detect only shadesof gray. Cones respond in intense light
and are responsible for color vision.Credit modification of work by Pyotr Sliwa.
The phovia is the region in thecenter back of the eye that is responsible
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for acute vision. The phovia hasa high density of cones. When you
bring your gaze to an object toexamine it intently in bright light, the
eyes orients so that the object's imagefalls on the phovia. However, when
looking at a star in the nightsky or other object in dim light,
the object can be better viewed bythe peripheral vision because it is the rods
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at the edges of the retina ratherthan the cones at the center that operate
better in low light. In humans, cones far outnumber rods in the phobia
transduction of light. The rods andcones are the site of transduction of light
to a neural signal. Both rodsand cones contain photopigments. Invertebrates, the
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main photopigment, rhodopsin, has twomain parts, an opsin, which is
a membrane protein in the form ofa cluster of alpha helicies that span the
membrane, and retinal, a moleculethat absorbs light. When light hits a
photoreceptor, it causes a shape changein the retinal, altering its structure from
a bent cis form of the moleculeto its linear trans isomer. This isomerization
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of retinal activates the rhodopsin, startinga cascade of events that ends with a
change in the membrane potential of therod or cone cell trichromatic coding. There
are three types of cones with differentphotopsins, and they differ in the wavelength
to which they are most responsive,as shown in figure. Some cones are
maximally responsive to shore light waves offour hundred and twenty nanimeters, so they
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are called s cones S for short. Others respond maximally to waves of five
hundred and thirty nanimeters m cones formedium. A third group responds maximally to
light of longer wavelengths at five hundredand sixty nanimeters. Al or long cones.
With only one type of cone,color vision would not be possible,
and a two cone dichromatic system haslimitations. Primates use a three cone trichromatic
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system, resulting in full color vision. The color we perceive is a result
of the ratio of activity of ourthree types of cones. The colors of
the visual spectrum, running from longwavelength light to short are red seven hundred
nanimeters, orange six hundred nanimeters,yellow five hundred and sixty five nanimeters,
green four hundred and ninety seven nanimeters, blue four hundred and seventy nanimeters,
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indigo four hundred and fifty nanimeters,and violet four hundred and twenty five nanimeters.
Humans have very sensitive perception of colorand can distinguish about five hundred levels
of brightness, two hundred different huesand twenty steps of saturation or about two
million distinct colors. Graph plots normalizedabsorbents for rods and s, M and
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L cons against wavelength For all fourcell types. The trend is an approximately
bell shaped curve with a steeper decreasethan increase. For AS cons, the
peak absorbents is four hundred and twentyn animators. For rods, the peak
absorbents is four hundred and ninety eightnnimators. For M cons, the peak
absorbents is five hundred and thirty fournanimators. For L cones, the peak
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absorbents is five hundred and sixty fournanimators. Human rod cells and the different
types of cone cells each have anoptimal wavelength. However, there is considerable
overlap in the wavelengths of light detected. This podcast will be released episodically and
follow the sections of the textbook inthe description for a deeper understanding. We
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encourage you review the text version ofthis work voice by voicemaker Dotayne. This
was produced by Brandon Casturo as acreative Common Sense production.
Welcome to Principles of Biology. Thisbook was written by the Open Alternative Textbook
Initiative at Kansas State University and isbeing released as a podcast and distributed under
the terms of the Creative Commons AttributionLicense. Today's episode is Chapter twenty eight
point one. Sensory Systems. Allhyperlinks, images and sources can be found
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at the link to the book.In the description, the act of smelling
something anything is remarkably like the actof thinking. Immediately at the moment of
perception, you can feel the mindgoing to work, sending the odor around
from place to place, setting offcomplex repertories through the brain, pulling one
center after another for signs of recognition, for old memories and old connection Lewis
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Thomas on smell, nineteen eighty five. Senses provide information about the body and
its environment. Humans have five specialsenses all faction, smell, gustation,
taste, equilibrium, balance and bodyposition, vision and hearing. Additionally,
we possess general senses, also calledsomatisensation, which respond to stimuli like temperature,
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pain, pressure, and vibration.Vestibular sensation, which is an organism's
sense of spatial orientation and balance,proprio section, position of bones, joints,
and muscles, and the sense oflimb position that is used to track
kinesthesia limb movement are part of somtisensation. Although the sensory systems associated with these
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senses are very different, all sharea common function to convert a stimulus such
as light or sound, or theposition of the body into an electrical signal
in the nervous system. This processis called sensory transduction. We are going
to focus on the five special sensesof humans. There are three types of
stimuli that are detected by the humansensory systems. The first is chemical stimulus,
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where molecules stimulate a sensory neuron.Chemical stimuli are detected by the olfactory
system when molecules in the airbine tosensory cells in the nasal epithelia, and
in the gustation taste system when moleculesin your food stimulate your taste buds.
The second is electromagnetic radiation. Lightinteracts with molecules in the sensory cells rods
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and cones of your retina, andthose sensory cells send a signal to your
brain. The third is mechanical stimulation, where the sensory cells are activated by
movement or touch. Mechanical stimuli aredetected by the cells in the interior that
help you detect balance and body position, and by other cells in your interior
detect sound the sense of hearing.Additionally, mechanical stimuli are involved in other
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somatosensory systems, such as pressure,pain, or vibration. In proprioception position
of legs, arms, and otherbody parts, and in kinesthesia detection of
motion of those same body parts.Sensory perception reception. The first step in
sensation is reception, which is theactiveation of sensory receptors by stimuli such as
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mechanical stimuli being bent or squished,for example, chemicals or temperature. The
receptor can then respond to the stimuli. The region in space in which a
given sensory receptor can respond to astimulus, be it far away or in
contact with the body, is thatreceptor's receptive field. Think for a moment
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about the differences in receptive fields forthe different senses. For the sense of
touch, a stimulus must come intocontact with body. For the sense of
hearing, a stimulus can be amoderate distance away. Some bellin whale sounds
can propagate for many kilometers. Forvision, a stimulus can be very far
away. For example, the visualsystem perceives light from stars at enormous distances.
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Transduction, the most fundamental function ofa sensory system, is the translation
of a sensory signal to an electricalsignal. In a nervous system, this
takes place at the sensory receptor,and it produces a change an electrical potential
in response to the stimulus. Thisis called the receptor potential. How is
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sensory input, such as pressure onthe skin, changed to a receptor potential.
In one example, a type ofreceptor called the mechaner receptor, as
shown in figure, possesses specialized membranesthat respond to pressure. Disturbance of these
dendrites by compressing them or bending themopens gated ion channels in the plasma membrane
of the sensory neuron changing its electricalpotential. Recall that in the nervous system,
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a positive change of a neuron's electricalpotential, also called the membrane potential,
depolarizes the neuron. Receptor potentials aregraded potentials. The magnitude of these
graded receptor potentials varies with the strengthof the stimulus. If the magnitude of
depolarization is sufficient, that is,if membrane potential reaches a threshold, the
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neuron will fire an action potential.In all cases, the appropriate stimulus WI
will cause a change in the membranepotential of the sensory cell. The exact
mechanism for changing the membrane potential willbe different for different sensory calls. Illustration
A shows a closed gated ion channelembedded in the plasma membrane. A hair
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like tether connects the channel to theextracellular matrix outside the cell, and another
tether connects the channel to the innercytoskeleton. When the extracellular matrix is deflected,
the tether tugs on the gated ionchannel, pulling it open. Ions
may now enter or exit the cell. Illustration beach shows stereocilia hair like projections
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on outer hair cells that attach tothe tectorial membrane of the inner ear.
The outer hair cells are connected tothe cochlear nerve. A Mechanosensitive ion channels
are gated ion channels that respond tomechanical deformation of the plasma membrane. A
mechanosensitive channel is connected to the plasmamembrane and the cytoskeleton by hair like tethers.
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When pressure causes the extracellular matrix tomove, the channel opens, allowing
ions to enter or exit the cell. B Stereocilia in the human ear are
connected to mechano sensitive ion channels.When a sound causes the stereocilia to move,
mechano sensitive ion channels transduce the signalto the cochlear nerve Sensory receptors for
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different senses are very different from eachother, and they are specialized according to
the type of stimulus they sense.They have receptor specificity. For example,
touch receptors, light receptors, andsound receptors are each activated by different stimuli.
Touch receptors are not sensitive to lightor sound. They are sensitive only
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to touch or pressure. However,stimuli may be combined at higher levels in
the brain, as happens with allfaction contributing to our sense of taste perception.
Perception is an individual's interpretation of asensation. Although perception relies on the
activation of sensory receptor, perception happensnot at the level of the sensory receptor,
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but at higher levels in the nervoussystem in the brain. The brain
distinguishes sensory stimuli through a sensory pathway. Action potentials from sensory receptors travel along
neurons that are dedicated to a particularstimulus. These neurons are dedicated to that
particular stimulus and synapse with particular neuronsin the brain or spinal cord. All
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sensory signals, except those from theolfactory system, are transmitted though the central
nervous system and are routed to thethalamus and to the appropriate region of the
cortex. Recall that the thalamus isa structure in the forebrain that serves as
a clearinghouse and relay station for sensoryas well as motor signals. When the
sensory signal exits the thalamus, itis conducted to the specific area of the
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cortex figure dedicated to processing that particularsense. How are neural signals interpreted?
Interpretation of sensory signals between individuals ofthe same species is largely similar, owing
to the inherited similarity of their nervoussystems. However, barrisome individual differences.
A good example of this is individualtolerances to a painful stimulus such as dental
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pain, which certainly differ. Interestingly, studies have shown that the allele that
results in red hair in humans homozygousfor that allele, known as MC one
R, is a member of thefamily of sensory receptors that detect pain,
and redheads are more sensitive to painand require about twenty percent more anesthetic during
surgery or dental work. So benice to your red headed friends. Illustration
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A shows side view of a humanbrain. The thalamus is in the inner
middle part. Illustration beach shows thelocation of sensory processing regions in the brain.
The visual processing region is at theback of the brain, the auditory
processing region is in the middle ofthe brain, and the somatosensory processing region
is located in a sliverlike region inthe upper part of the brain and extending
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halfway down. In humans, withthe exception of all faction, all sensory
signals are routed from the athalamus tobe final processing regions in the cortex of
the brain. Credit be modification ofwork by Paulina Tishina. Taste and smell.
Taste also called gustation and smell alsocalled all faction, are the most
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interconnected senses in that both involve moleculesof the stimulus entering the body and bonding
to receptors. Smell lets an animalsense the presence of food or other animals,
whether potential mates, predators, orprey, or other chemicals in the
environment that can impact their survival.Similarly, the sense of taste allows animals
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to discriminate between types of foods.While the value of a sense of smell
is obvious, what is the valueof a sense of taste? Different tasting
foods have different attributes, both helpfuland harmful. For example, sweet tasting
substances tend to highly chloric, whichcould be necessary for survival in lean times.
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Bitterness is associated with toxicity, andsourness is associated with spoiled food Salty
foods are valuable in maintaining homeostasis byhelping the body retain water and by providing
irons necessary for cells to function.Tastes and odors, both taste and odor
stimuli, are molecules taken in fromthe environment. The primary tastes detected by
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humans are sweet, sour, bitter, salty, and numami. The first
four tastes need little explanation. Theidentification of amammy as a fundamental taste occurred
fairly recently. It was identified innineteen oh eight by Japanese scientist Kukuni Acada
while he worked with seaweed broth,but it was not widely accepted as a
taste that could be physiologically distinguished untilmany years later. The taste of umammy,
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also known as savoriness, is attributableto the taste of the amino acid
ilglutamate, in fact, monosodia glutamate, where MSG is often used in cooking
to enhance the savory taste of certainfoods. What is the adaptive value of
being able to distinguish humami? Savorysubstances tend to be high in protein.
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All odors that we perceive are volublechemicals in the air we breathe. If
a substance does not release molecules intothe air from its surface, it has
no smell, and if a humanor other animal does not have a receptor
that recognizes a specific molecule, thenthat molecule has no smell. Humans have
about three hundred and fifty olfactory receptorsubtypes that work in various combinations to allow
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us to sense about ten thousand differentodors. Compare that to mice, for
example, which have about one thousand, three hundred olfactory receptor types and therefore
probably sends more odors. Both odorsand tastes involve molecules that stimulate specific chemoreceptors.
Although humans commonly distinguish taste as onesense and smell as another, they
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work together to create the perception offlavor. A person's perception of flavor is
reduced if he or she has congestednasal passages. Reception and transduction taste.
Detecting a taste gustation is fairly similarto detecting an odor of faction, given
that both taste and smell rely onchemical receptors being stimulated by certain molecules.
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The primary organ of taste is thetaste bud. A taste bud is a
cluster of gustatory receptors taste cells thatare located within the bumps on the tongue,
called papilli singular papilla illustrated in figure. An illustration shows small philiform papilli
scattered across the front two thirds ofthe tongue. Larger circumvallate popilli form an
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inverted v at the back of thetongue. Medium sized fungiform papili are shown
scattered across the back two thirds ofthe tongue. Foliate popilli form ridges on
the back edges of the tongue.A micrograph shows a cross section of a
tongue in which the foliate papilli canbe seen as square protrusions about two hundred
microns across and deep i. Foliatecircumvallate and fungiform papili are located on different
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regions of the tongue. B Foliatepapili are prominent protrusions on the slight micrograph.
Credit a modification of work by NCIscale bar data from Matt Russell.
Each taste bud's taste cells are replacedevery ten to fourteen days. These are
elongated cells with hair like processes calledmicrovilli at the tips that extend into the
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taste bud pore illustrate in figure foodmolecules. Tastints are dissolved in saliva and
they bind with and stimulate the receptorson the microvilli. The receptors for tastins
are located across the outer portion andfront of the tongue, outside of the
middle area where the filiform papili aremost prominent. A taste bud is shaped
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like a garlic bulb and is embeddedin the epidermis of the tongue. Together,
the two types of cells that makethe taste bud, taste cells and
supporting cells, resemble cloves. Hairlike microvilli extend from the tips of the
taste cells into a taste porre onthe surface of the tongue. Nerve endings
extend into the bottom of the tastebud from the dermis. Pores in the
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tunnel allow tastings to enter taste poresin the tongue credit modification of work by
Vincenzo Rizzo. In humans, thereare five primary tastes, and each taste
has only one corresponding type of receptor. Thus, like all faction, each
receptor is specific to its stimulus tastened. Both tasting abilities and sense of smell
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change with age. In humans,the senses decline dramatically by age fifty and
continue to decline. A child mayfind a food to be too spicy,
whereas an elderly person may find thesame food to be bland and unappetizing.
Hearing and equilibrium audition or hearing isimportant to humans and to other animals for
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many different interactions. It enables anorganism to detect and receive information about danger,
such as an approaching predator, andto participate in communal exchanges like those
concerning territories or mating. On theother hand, although it is physically linked
to the auditory system, the vestibularsystem is not involved in hearing. Instead,
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an animal's vestibular system detects its ownmovement, both linear and angular acceleration
and deceleration, and balance. Soundauditory stimuli are sound waves, which are
mechanical pressure waves that move through amedium such as air or water. There
are no sound waves in a vacuum, since there are no air molecules to
move in waves. The speed ofsound waves differs based on altitude, temperature,
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and medium, but at sea leveland a temperature of twenty degree C
sixty eight degrees a sound waves travelin the air at about three hundred and
forty three meters per second, Asis true for all waves. There are
four main characteristics of a sound wave, frequency, wavelength, period, and
amplitude. Frequency is the number ofwaves per unit of time, and in
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sound is heard as pitch. Highfrequency greater than are equal to fifteen point
zero zero zero herts. Sounds arehigher pitched short wavelength than low frequency long
wavelengths less than are equal to onehundred herts. Sounds frequency is measured in
cycles per second, and for sound, the most commonly used unit is herts
hc or cycles per second. Mosthumans can perceive sounds with frequencies between thirty
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and twenty thousand herts. Women aretypically better at hearing high frequencies, but
everyone's ability to hear high frequencies decreaseswith age. Dogs detect up to about
forty thousand herts, cats sixty thousandherts, bats one hundred thousand herts,
and dolphins one hundred and fifty thousandherts. And American shad a loosasapidissima of
fish can hear one hundred and eightythousand herts. Those frequencies above the human
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range are called ultrasound. Reception ofsound in mammals, sound waves are collected
by the external cartilachinus part of theear called the pinna, then travel through
the auditory canal and cause vibration ofthe thin diaphragm called the tympanum or ear
drum. The innermost part of theouter ear illustrated in figure. Interior to
the tympanum is the middle ear.The middle ear holds three small bones called
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the oscles, which transfer energy fromthe moving tympanum to the inner ear.
The three osscles are the malleus alsoknown as the hammer, the incus,
the anvil, and stapies the stirrup. The aptly named stapies looks very much
like a stirrup. The three ossclesare unique to mammals and each plays a
role in hearing. The malleus attachesat three points to the interior surface of
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the tympanic membrane. The incus attachesthe malleus to the stapies. In humans,
the stepies is not long enough toreach the tympanum. If we did
not have the malleus and the incus, then the vibrations of the tympanum would
never reach the inner ear. Thesebones also function to collect force and amplify
sounds. The ear osicles are homologousto bones in a fish mouth. The
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bones that support gills and fish arethought to have been adapted for use in
the vertebrate ear over evolutionary time.The illustration shows the parts of the human
ear. The visible part of theexterior ear is called the pinna. The
ear canal extends inward from the pinnato a circular membrane called the tympanum.
On the other side of the tympanumis the Eustachian tube. Inside the Eustacean
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tube, the malleus, which touchesthe inside of the tympanum, is attached
to the incas, which is inturn attached to the horseshoe shaped stepies.
The steepies is attached to the roundwindow, a membrane. In the snail
shell shaped cocchlea. Another window,called the round window, is located in
the wide part of the cocchlea.Ring like semicircular canals extend from the cocchlea.
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The cochlear nerve and vestibular nerve ofboth connect to the cocchlea. Sound
travels through the outer ear to themiddle ear, which is bounded on its
exterior by the tympanic membrane. Themiddle ear contains three bones called aussicles that
transfer the sound wave to the ovalwindow. The exterior boundary of the inner
ear. The organ of cordy,which is the organ of sound transduction,
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lies inside the COCCHLEA credit modification ofwork by Lars Chica Axle Brochman vestibular information.
The stimuli associated with the vestibular systemare linear acceleration, gravity, and
angular acceleration and deceleration. Gravity accelerationand deceleration are detected by evaluating the inertia
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on receptive cells in the vestibular system. Gravity is detected through head position.
Angular acceleration and deceleration are expressed throughturning or tilting of the head. The
vestibular system has some similarities with theauditory system. It utilizes as hair cells
just like the auditory system, butit excites them in different ways. There
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are five vestibular receptor organs in theinner ear, the utricle, the saccule,
and three semicircular canals. Together theymake up what's known as the vestibular
labyrinth that is shown in figure.The utricle and saccule respond to acceleration in
a straight line, such as gravity. The roughly thirty thousand hair cells and
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the utricle and sixteen thousand hair cellsand the saccule lie below a gelatinous layer
with their stereocilia projecting into the gelatin. Embedded in this gelatin are calcium carbonate
crystals, like tiny rocks. Whenthe head is tilted, the crystals continue
to be pulled straight down by gravity, but the new ankle of the head
causes the gelatin to shift, therebybending the stereocilia. The bending of the
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stereocilia stimulates the neurons and they signalto the brain that the head is tilted,
allowing the maintenance of balance. Itis the vestibular branch of the vestibulo
cochlear cranial nerve that deals with balans. This illustration shows the snail shell shaped
cochlea which widens into the vestibule.Two circular organs, the utricle and the
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saccule, are located in the vestibule. Three ring like canals, the horizontal
canal, the posterior canal, andthe superior canal, extend from the top
of the vestibule. Each canal projectsin a different direction. The structure of
the vestibular labyrinth is shown credit modificationof work by nih. The fluid filled
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semicircular canals are tubular loops set atoblique angles. They are arranged in three
spatial plains. The base of eachcanal has a swelling that contains a cluster
of hair cells. The hair's projectinto a gelatinous cap called the cupula,
and monitor angular acceleration and deceleration fromrotation. They would be stimulated by driving
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your car around a corner, turningyour head, or falling forward. One
canal lies horizontally, while the othertwo lie at about forty five degree angles
to the horizontal axis, as illustratedin figure. When the brain processes input
from all three canals together, itcan detect angular acceleration or deceleration in three
dimensions. When the head turns,the fluid in the canal shifts, thereby
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bending stereocilia and sending signals to thebrain. Upon cessation, accelerating or decelerating,
or just moving, the movement ofthe fluid within the canal slows or
stops. For example, imagine holdinga glass of water. When moving forward,
water may splash backwards onto the hand, and when motion has stopped,
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water may splash forward onto the fingers. While in motion, the water settles
in the glass and does not splash. Note that the canals are not sensitive
to velocity itself, but to changesin velocity. So moving forward at sixty
miles per hour with your eyes closedwould not give the sensation of movement,
but suddenly accelerating or breaking with stimulatethe receptors. Vision. Vision is the
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ability to detect light patterns from theoutside environment and interpret them into images.
Animals are bombarded with sensory information,and the sheer volume of visual information can
be problematic. Fortunately, the visualsystems of species have evolved to attend to
the most important stimuli. The importanceof vision to humans is further substantiated by
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the fact that about one third ofthe human cerebral cortex is dedicated to analyzing
and perceiving visual information light. Aswith auditory stimuli, light travels in waves.
The compression waves that composed sound musttravel in a medium a gas,
a liquid, or a solid.In contrast, light is composed of electromagnetic
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waves and needs no medium. Lightcan travel in a vacuum figure. The
behavior of light can be discussed interms of the behavior of waves, and
also in terms of the behavior ofthe fundamental unit of light, a packet
of a tromagnetic radiation called a photon. A glance at the electromagnetic spectrum shows
that visible light for humans is justa small slice of the entire spectrum,
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which includes radiation that we cannot seeas light because it is below the frequency
of visible red light and above thefrequency of visible violet light. Certain variables
are important when discussing perception of light. Wavelength, which varies inversely with frequency,
manifests itself as hue. Light atthe red end of the visible spectrum
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has longer wavelengths and is lower frequency, while light at the violet end has
shorter wavelengths and is higher frequency.The wavelength of light is expressed in nanimeters,
and one nanometer is one billionth ofa meter. Humans perceive light that
ranges between approximately three hundred and eightynanimeters and seven hundred and forty nanimeters.
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Some other animals, though, candetect wavelengths outside of the human range.
For example, b s naraltraviolet lightin order to locate nectar guides on flowers,
and some non avian reptiles sense infraredlight heat that prey gives off.
The illustration shows the electromagnetic spectrum,which consists of different wavelengths of electromagnetic radiation.
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Radio waves have the longest wavelength aboutone hundred and three meters. Wavelength
gets increasingly shorter for microwave, infrared, visible ultraviolet, X rays, and
gamma rays. Gamma rays have awavelength of about ten to twelve meters.
Frequency is inversely proportional to wavelength.In the electromagnetic spectrum, visible light lies
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between three hundred and eighty nanimeters andseven hundred and forty nanimeters credit modification of
work by NASA. Wave amplitude isperceived as luminous intensity or brightness. The
standard unit of intensity of light isthe candela, which is approximately the luminous
intensity of a one common candle.Light waves travel two hundred and ninety nine
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thousand, seven hundred and ninety twokilometers per second in a vacuum and somewhat
slower in various medias such as airand water, and those waves arrive at
the eye as long red, mediumgreen, and short blue waves. What
is termed white light is light thatis perceived as white by the human eye.
This effect is produced by light thatstimulates equally the color receptors in the
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human eye. The apparent color ofan object is the color or colors that
the object reflects Thus a red objectreflects the red wavelengths in mixed white light
and absorbs all other wavelengths of light. Anatomy of the eye. The photoreceptive
cells of the eye where a transductionof light to nervous impulses occurs, are
located in the retina shown in figureon the inner surface of the back of
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the eye. But light does notimpinge on the retina unaltered. It passes
through other layers that process it sothat it can be interpreted by the retina
Figure EBB. The cornea, thefrat transparent layer of the eye, and
the crystalline lens, a transparent convexstructure behind the cornea, both for fract
bend light to focus the image onthe retina. The iris, which is
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conspicuous as the colored part of theeye, is a circular muscular ring lying
between the lens and cornia that regulatesthe amount of light entering the eye.
In conditions of high ambient light,the iris contracts, reducing the size of
the pupil at its center. Inconditions of low light, the iris relaxes
and the pupil enlarges. The leftillustration shows a human eye which is round
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and filled with vitreous humor. Theoptic nerve and retinal blood vessels exit the
back of the eye. At thefront of the eye is the lens with
a pupil in the middle. Thelens is covered by the iris, which
in turn is covered by the cornea. The aqueous humor is a jelic substance
between the cornea and iris. Theretina is the lining of the inner eye.
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A second illustration is a blow upwhich shows that the optic nerve is
at the surface of the retina.Beneath the optic nerve is a layer of
ganglian cells, and beneath this isa layer of bipolar cells. Both ganglia
and bipolar cells are nerve cells withroot like appendages. Beneath the bipolar cell
layer are the rods and cones.Rods and cones are similar in structure and
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column like a The human eye isshown in cross section B. A blow
up shows the layers of the retina. The main function of the lens is
to focus light on the retina andphobous entrallis. The lens is dynamic,
focusing and refocusing light as the eyerests on near and far objects. In
the visual field. The lens isoperated by muscles that stretch it flat or
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allow it to thicken, changing thefocal length of light coming through it to
focus it sharply on the retina.With age comes the loss of the flexibility
of the lens, and a formof far sidedness called presbiopia results. Presbiopia
occurs because the image focuses behind theretina. Presbiopia is a deficit similar to
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a different type of far sightedness calledhyperopia, caused by an eyeball that is
too short. For both defects,images in the distance are clear, but
images nearby are blurry. Myopia nearsightedness occurs when an eyeball is elongated and
the image focus falls in front ofthe ratina. In this case, images
in the distance are blurry but imagesnearby are clear. There are two types
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of photoreceptors in the ratina, rodsand cones, named for their general appearance
as illustrated in figure. Rods arestrongly photosensitive and are located in the outer
edges of the ratina. They detectdim light and are used primarily for peripheral
and night time vision. Cones areweakly photosensitive and are located near the center
of the ratina. They respond tobright light and their primary role is in
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daytime color vision. This illustration showsthat rods and cones are both long column
like cells, with the nucleus locatedin the bottom portion. The rod is
longer than the cone. The outersegment of the rod contains rhodopsin the outer
segment of the rod contains other photopigments. An oil droplet is located beneath the
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outer segment. Rods and cones arephotoreceptors in the retina. Rods respond in
low light and can detect only shadesof gray. Cones respond in intense light
and are responsible for color vision.Credit modification of work by Pyotr Sliwa.
The phovia is the region in thecenter back of the eye that is responsible
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for acute vision. The phovia hasa high density of cones. When you
bring your gaze to an object toexamine it intently in bright light, the
eyes orients so that the object's imagefalls on the phovia. However, when
looking at a star in the nightsky or other object in dim light,
the object can be better viewed bythe peripheral vision because it is the rods
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at the edges of the retina ratherthan the cones at the center that operate
better in low light. In humans, cones far outnumber rods in the phobia
transduction of light. The rods andcones are the site of transduction of light
to a neural signal. Both rodsand cones contain photopigments. Invertebrates, the
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main photopigment, rhodopsin, has twomain parts, an opsin, which is
a membrane protein in the form ofa cluster of alpha helicies that span the
membrane, and retinal, a moleculethat absorbs light. When light hits a
photoreceptor, it causes a shape changein the retinal, altering its structure from
a bent cis form of the moleculeto its linear trans isomer. This isomerization
(31:40):
of retinal activates the rhodopsin, startinga cascade of events that ends with a
change in the membrane potential of therod or cone cell trichromatic coding. There
are three types of cones with differentphotopsins, and they differ in the wavelength
to which they are most responsive,as shown in figure. Some cones are
maximally responsive to shore light waves offour hundred and twenty nanimeters, so they
(32:01):
are called s cones S for short. Others respond maximally to waves of five
hundred and thirty nanimeters m cones formedium. A third group responds maximally to
light of longer wavelengths at five hundredand sixty nanimeters. Al or long cones.
With only one type of cone,color vision would not be possible,
and a two cone dichromatic system haslimitations. Primates use a three cone trichromatic
(32:28):
system, resulting in full color vision. The color we perceive is a result
of the ratio of activity of ourthree types of cones. The colors of
the visual spectrum, running from longwavelength light to short are red seven hundred
nanimeters, orange six hundred nanimeters,yellow five hundred and sixty five nanimeters,
green four hundred and ninety seven nanimeters, blue four hundred and seventy nanimeters,
(32:52):
indigo four hundred and fifty nanimeters,and violet four hundred and twenty five nanimeters.
Humans have very sensitive perception of colorand can distinguish about five hundred levels
of brightness, two hundred different huesand twenty steps of saturation or about two
million distinct colors. Graph plots normalizedabsorbents for rods and s, M and
(33:13):
L cons against wavelength For all fourcell types. The trend is an approximately
bell shaped curve with a steeper decreasethan increase. For AS cons, the
peak absorbents is four hundred and twentyn animators. For rods, the peak
absorbents is four hundred and ninety eightnnimators. For M cons, the peak
absorbents is five hundred and thirty fournanimators. For L cones, the peak
(33:37):
absorbents is five hundred and sixty fournanimators. Human rod cells and the different
types of cone cells each have anoptimal wavelength. However, there is considerable
overlap in the wavelengths of light detected. This podcast will be released episodically and
follow the sections of the textbook inthe description for a deeper understanding. We
(34:00):
encourage you review the text version ofthis work voice by voicemaker Dotayne. This
was produced by Brandon Casturo as acreative Common Sense production.
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