35 sensors
DESCRIPTION
Concept 35.1 Sensory Systems Convert Stimuli into Action Potentials Sensory receptor cells, or sensors or receptors, transduce physical and chemical stimuli into a change in membrane potential. The change in membrane potential may generate an action potential that conveys the sensory information to the CNS for processing. Sensory transduction—begins with a receptor protein that can detect a specific stimulus. The receptor protein opens or closes ion channels in the membrane, changing the resting potential. See Concept 34.2TRANSCRIPT
35 Sensors Concept 35.1 Sensory Systems Convert Stimuli into
Action Potentials
Sensory receptor cells, or sensors or receptors,transduce physical
and chemical stimuli into achange in membrane potential. The change
in membrane potential may generatean action potential that conveys
the sensoryinformation to the CNS for processing. Sensory
transductionbegins with a receptorprotein that can detect a
specific stimulus. The receptor protein opens or closes ion
channelsin the membrane, changing the resting potential. See
Concept 34.2 Concept 35.1 Sensory Systems Convert Stimuli into
Action Potentials
Receptor potentialsgraded membranepotentials that travel a short
distance. Receptor potentials can generate actionpotentials in two
ways: Can generate action potentials in thereceptor cell Can
trigger release of neurotransmitter sothat a postsynaptic neuron
generates anaction potential LINK Review the mechanics of graded
membrane potentials and action potentials in Concept 34.2 and of
synaptic transmission in Concept 34.3 Concept 35.1 Sensory Systems
Convert Stimuli into Action Potentials
Stretch receptors in crayfish cause receptorpotentials when the
attached muscle isstretched. Receptor potentials spread to the base
ofthe axon and generate action potentials. The rate of firing
depends on the magnitudeof the receptor potential, which dependson
the amount of stretching. Figure 35.1 Stimulating a Sensory Cell
Produces a Receptor Potential Concept 35.1 Sensory Systems Convert
Stimuli into Action Potentials
Different sensory receptors respond toparticular stimuli:
Mechanoreceptors detect physical forcessuch as pressure (touch) and
variations inpressure (sound waves). Thermoreceptors respond to
temperature. Electrosensors are sensitive to changes inmembrane
potential. Concept 35.1 Sensory Systems Convert Stimuli into Action
Potentials
Chemoreceptors respond to the presenceor absence of certain
chemicals. Photoreceptors detect light. Some sensory receptor cells
are organizedwith other cells in sensory organs, such aseyes and
ears. Sensory systems include sensory cells,associated structures,
and neural networksthat process the information. Figure 35.2Sensory
Receptor Proteins Respond to Stimuli by Opening or Closing Ion
Channels Concept 35.1 Sensory Systems Convert Stimuli into Action
Potentials
Sensation depends on which part of theCNS receives the sensory
messages. Intensity of sensation is coded as thefrequency of action
potentials. Some sensory cells transmit information tothe brain
about internal conditions, withoutconscious sensation. Concept 35.1
Sensory Systems Convert Stimuli into Action Potentials
Adaptationdiminishing response torepeated stimulation. Enables
animals to ignore backgroundconditions but remain sensitive
tochanging or new stimuli. Some sensory cells dont adapt
(e.g.,mechanoreceptors for balance). Concept 35.2 Chemoreceptors
Detect Specific Molecules or Ions
Chemoreceptorsreceptor proteins thatbind to various molecules,
responsible fortaste and smell. Also monitor internal environment,
such asCO2 levels in blood. Olfactionsense of smell; depends
onchemoreceptive neurons embedded inepithelial tissue at top of
nasal cavity (invertebrates). Figure 35.3Olfactory Receptors
Communicate Directly with the Brain (Part 1) Figure 35.3Olfactory
Receptors Communicate Directly with the Brain (Part 2) Concept 35.2
Chemoreceptors Detect Specific Molecules or Ions
Axons from olfactory sensors extend to theolfactory bulb in the
braindendrites endin olfactory hairs on the nasal epithelium.
Odoranta molecule that activates anolfactory receptor protein
Odorants bind to receptor proteins on theolfactory cilia. Olfactory
receptor proteins are specific forparticular odorants. Concept 35.2
Chemoreceptors Detect Specific Molecules or Ions
When an odorant binds to a receptorprotein, it activates a G
protein, whichactivates a second messenger (cAMP). The second
messenger causes an influx ofNa+ and depolarizes the olfactory
neuron. Many more odorants can be discriminatedthan there are
olfactory receptors. In the olfactory bulb, axons from neuronswith
the same receptors converge onglomeruli. Concept 35.2
Chemoreceptors Detect Specific Molecules or Ions
Pheromoneschemical signals used byinsects to attract mates.
Example: Female silkworm moth releasesbombykol. Male has receptors
forbombykol on the antennae. One molecule of bombykol is enough
togenerate action potentials. Figure 35.4 Some Scents Travel Great
Distances (Part 1) Figure 35.4 Some Scents Travel Great Distances
(Part 2) Concept 35.2 Chemoreceptors Detect Specific Molecules or
Ions
Vomeronasal organ (VNO) is found inmany vertebratesspecialized
forpheromones It is a paired tubular structure embedded inthe nasal
epithelium. When animal sniffs, the VNO draws asample of fluid over
chemoreceptors inwalls. Information goes to an accessory
olfactorybulb and on to other brain regions. APPLY THE CONCEPT
Chemoreceptors detect specific molecules or ions Concept 35.2
Chemoreceptors Detect Specific Molecules or Ions
Gustation is the sense of taste. Taste budsclusters of
chemoreceptors. Some fish have taste buds on the skin;
theduck-billed platypus has taste buds on itsbill. Human taste buds
are embedded in thetongue epithelium, on papillae. Thesensory cells
generate action potentialswhen they detect certain chemicals.
Figure 35.5 Taste Buds Are Clusters of Sensory Cells (Part 1)
Figure 35.5 Taste Buds Are Clusters of Sensory Cells (Part 2)
Concept 35.2 Chemoreceptors Detect Specific Molecules or Ions
Humans taste salty, sour, sweet, bitter, andumamia savory, meaty
taste. Salty receptors respond to Na+depolarizing the cell. Sour
receptors detect acidity as H+, andsweet receptors bind different
sugars. Umami receptors detect the presence ofamino acids, as in
MSG. Bitterness is more complicated and involvesat least 30
different receptors. Concept 35.3 Mechanoreceptors Detect Physical
Forces
Mechanoreceptors are cells that detect physical forces. Distortion
of their membrane causes ion channels to open and a receptor
potential to occur. This may lead to the release of a
neurotransmitter. Concept 35.3 Mechanoreceptors Detect Physical
Forces
The skin has diverse mechanoreceptors: Free nerve endings detect
heat, cold, andpain. Merkels discs: Adapt slowly, givecontinuous
information. Meissners corpuscles: Adapt quickly, giveinformation
about change. INTERACTIVE TUTORIAL 35.1 Sensory Receptors Concept
35.3 Mechanoreceptors Detect Physical Forces
Ruffini endings: Deep, adapt slowly, reactto vibrating stimuli of
low frequencies. Pacinian corpuscles: Deep, adapt rapidly,react to
vibrating stimuli at highfrequencies. Figure 35.6 The Skin Feels
Many Sensations Concept 35.3 Mechanoreceptors Detect Physical
Forces
Muscle spindles: Mechanoreceptors inmuscle cells, called stretch
receptors. When muscle is stretched, action potentialsare generated
in neurons. CNS adjusts strength of contraction tomatch load on
muscle. Concept 35.3 Mechanoreceptors Detect Physical Forces
Golgi tendon organ: Anothermechanoreceptor, in tendons
andligaments. Provides information about the forcegenerated by
muscle; prevents muscletearing. Figure 35.7 Stretch Receptors (Part
1) Figure 35.7 Stretch Receptors (Part 2) Concept 35.3
Mechanoreceptors Detect Physical Forces
Hair cellsmechanoreceptors in organs ofhearing and equilibrium.
Hair cells have projections calledstereocilia that bend in response
topressure. Bending of stereocilia can depolarize orhyperpolarize
the membrane. Figure 35.8Hair Cells Have Mechanosensors on Their
Stereocilia (Part 1) Figure 35.8Hair Cells Have Mechanosensors on
Their Stereocilia (Part 2) Concept 35.3 Mechanoreceptors Detect
Physical Forces
Auditory systems use hair cells to convertpressure waves to
receptor potentials. Outer ear: Pinnae collect sound waves and
direct themto the auditory canal. The tympanic membrane covers the
end ofthe auditory canal and vibrates inresponse to pressure waves.
Figure 35.9 Structures of the Human Ear (Part 1) Concept 35.3
Mechanoreceptors Detect Physical Forces
Middle earair filled cavity: Open to the throat via the eustachian
tube.Eustachian tubes equilibrate air pressurebetween the middle
ear and the outside. Ossiclesmalleus, incus, stapestransmit
vibrations of tympanic membraneto the oval window. VIDEO 35.1 Human
ear drums and bones Figure 35.9 Structures of the Human Ear (Part
2) Concept 35.3 Mechanoreceptors Detect Physical Forces
Inner ear has two sets of canalsthevestibular system for balance
and thecochlea for hearing. The cochlea is a tapered and
coiledchamber composed of three parallelcanals separated by
Reissnersmembrane and the basilar membrane. Figure 35.9 Structures
of the Human Ear (Part 3) Concept 35.3 Mechanoreceptors Detect
Physical Forces
The organ of Corti sits on the basilarmembranetransduces pressure
wavesinto action potentials. Contains hair cells with
stereociliatips areembedded in the tectorial membrane. Hair cells
bend and create a gradedpotential that can alter
neurotransmitterrelease. VIDEO 35.2 Hair cells of the cochlea
responding to music Concept 35.3 Mechanoreceptors Detect Physical
Forces
Upper and lower canals of the cochlea arejoined at distal end. The
round window is a flexible membraneat the end of the canal.
Traveling pressure waves of differentfrequencies will produce
flexion of thebasilar membrane. Concept 35.3 Mechanoreceptors
Detect Physical Forces
Different pitches, or frequency of vibration,flex the basilar
membrane at differentlocations. Action potentials stimulated
bymechanoreceptors at different positionsalong the organ of Corti
are transmitted toregions of the auditory cortex via theauditory
nerve. ANIMATED TUTORIAL 35.1 Sound Transduction in the Human Ear
Figure 35.10 Sensing Pressure Waves in the Inner Ear Concept 35.3
Mechanoreceptors Detect Physical Forces
Conduction deafness: Loss of function oftympanic membrane or
ossicles. Nerve deafness: Damage to inner ear orauditory nerve
pathways. Hair cells in the organ of Corti can bedamaged by loud
sounds. This damage iscumulative and irreversible. Concept 35.3
Mechanoreceptors Detect Physical Forces
The vestibular system in the mammalianinner ear has three
semicircular canalsat angles to each other, and twochambersthe
saccule and the utricle. Hair cells sense position and orientation
ofhead by shifting of endolymph. Cupulae in canals contain hair
cellstereociliaotoliths in membrane exertpressure and bend
stereocilia. Figure 35.11 Organs of Equilibrium (Part 1) Figure
35.11 Organs of Equilibrium (Part 2) Figure 35.11 Organs of
Equilibrium (Part 3) Concept 35.4 Photoreceptors Detect Light
Photosensitivitysensitivity to light A range of animal species from
simple tocomplex can sense and respond to light. All use same
pigmentsrhodopsins. ANIMATED TUTORIAL 35.2 Photosensitivity Concept
35.4 Photoreceptors Detect Light
Rhodopsin molecule consists of opsin (aprotein) and a
light-absorbing group, 11- cis-retinal. Rhodopsin molecule sits in
plasmamembrane of a photoreceptor cell. 11-cis-retinal absorbs
photons of light andchanges to the isomer all-trans-retinal changes
the conformation of opsin. Concept 35.4 Photoreceptors Detect
Light
In vertebrate eyes, the retinal and opsineventually separate,
called bleaching. A series of enzymatic reactions is requiredto
return all-trans-retinal back to 11-cis- retinal, which recombines
with opsin tobecome photosensitive rhodopsin again. Figure 35.12
Light Changes the Conformation of Rhodopsin Concept 35.4
Photoreceptors Detect Light
Rod cells are modified neurons with: An outer segment with discs of
plasmamembrane containing rhodopsin to capturephotons An inner
segment that contains thenucleus and organelles A synaptic terminal
where the rod cellcommunicates with other neurons Figure 35.13 A
Rod Cell Responds to Light (Part 1) Figure 35.13 A Rod Cell
Responds to Light (Part 2) Concept 35.4 Photoreceptors Detect
Light
Stimulation of rod cells by light makes themembrane potential more
negative(hyperpolarized)the opposite of othersensory cells
responding to their stimuli. The dark current is a flow of Na+ ions
thatcontinually enters the rod cell in the dark. Rod cell is
depolarized and releasesneurotransmitter continually.
Hyperpolarizing effect of light decreasesneurotransmitter release.
Concept 35.4 Photoreceptors Detect Light
When rhodopsin absorbs a photon of light, acascade of events
begins, starting with theactivation of a G protein, transducin.
Transducin activates PDE which convertscGMP to GMPthe Na+ channels
close,and the membrane is hyperpolarized. Figure 35.14 Light
Absorption Closes Sodium Channels Concept 35.4 Photoreceptors
Detect Light
Rhodopsin in a variety of visual systems: Flatwormsphotoreceptor
cells in pairedeye cups. Arthropodscompound eyes. Each eyeconsists
of units called ommatidia. Each ommatidium has a lens to focus
lightonto photoreceptor cells. INTERACTIVE TUTORIAL 35.2 Visual
Receptive Fields Figure 35.15 Ommatidia: The Functional Units of
Insect Eyes (Part 1) Figure 35.15 Ommatidia: The Functional Units
of Insect Eyes (Part 2) Concept 35.4 Photoreceptors Detect
Light
Vertebrates have image-forming eyes bounded by sclera, connective
tissue thatbecomes transparent cornea on front ofeye. Iris
(pigmented)controls amount of lightreaching photoreceptors;
openingpupil. Lenscrystalline protein, focuses image,allows
accommodation, can changeshape. Retinaphotosensitive layer, back of
eye. VIDEO 35.3 Human iris responding to changes in light Figure
35.16 The Human Eye (Part 1) Concept 35.4 Photoreceptors Detect
Light
The retina has five layers of neuronsincluding photoreceptors (rods
and cones)at the back. Photoreceptors send information to
bipolarcells, which send information to theganglion cell layer.
Axons from ganglion cells conductinformation to the brain. VIDEO
35.4 Human retina Figure 35.16 The Human Eye (Part 2) Concept 35.4
Photoreceptors Detect Light
Two other cell types communicate laterallyacross the retina:
Horizontal cells form synapses with bipolarcells and
photoreceptors. Amacrine cells form local synapses withbipolar
cells and ganglion cells. Ultimately, all information converges
onganglion cells. Concept 35.4 Photoreceptors Detect Light
A receptive fielda group ofphotoreceptors that receive
informationfrom a small area of the visual field andactivate one
ganglion cell. The receptive field of a ganglion cell resultsfrom a
pattern of synapses betweenphotoreceptors, bipolar cells and
lateralconnections. Concept 35.4 Photoreceptors Detect Light
Receptive fields have two concentricregions, a center and a
surround. A field can be either on- or off-center. Light falling on
an on-center receptive fieldexcites the ganglion cell, while light
fallingon an off-center receptive field inhibits theganglion cell.
The surround area has the opposite effectso ganglion cell activity
depends on whichpart of the field is stimulated. Concept 35.4
Photoreceptors Detect Light
Neurons of the visual cortex, like retinalganglion cells, have
receptive fields. Cortical neurons are stimulated by bars oflight
in a particular orientation,corresponding to rows of circular
receptivefields of ganglion cells. The brain assembles a mental
image of theworld by analyzing the edges in patterns oflight and
dark. Concept 35.4 Photoreceptors Detect Light
Vertebrate photoreceptors consist of rodcells and cone cells. Rod
cells are responsible for night vision;cone cells are responsible
for color vision. Foveaarea where cone cell density ishighest.
Figure 35.17 Rods and Cones (Part 1) Concept 35.4 Photoreceptors
Detect Light
Humans have three types of cone cells withslightly different opsin
moleculestheyabsorb different wavelengths of light. This allows the
brain to interpret input fromthe different cones as a full range of
color. Color blindness is the loss of function of atype of cone
cellthe result of anonfunctional gene. Figure 35.17 Rods and Cones
(Part 2) Answer to Opening Question
All of these animals make use of other sensesbesides vision to
perceive their surroundings inthe dark. Information is also
conveyed through tactilestimuli, olfaction, heat-detection, and
auditoryinput.