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.