ASIM'S WEBWORLD View: Main School Main AP Biology Chapter Guides RELATED LINKS Review Questions and Answers		AP BIOLOGY: Chapter Forty-Eight Outline INTRODUCTION Human Brain Requires Sensory Input for Proper Functioning Input to Central Nervous System (CNS) Via Afferent Sensory Neurons Information based on frequency of impulses Information based on identity of transmitting neurons THE NATURE OF NEUROSENSORY COMMUNICATION Path of Sensory Information to the CNS fig 48.1 Stimulation: physical stimulus on sensory receptor Transduction: sensory receptor initiates opening/closing of ion channel in sensory neuron Transmission: sensory neuron conducts action potential along afferent pathway to CNS Comparison of Sensory Receptors All initiate nerve impulses in sensory neuron membranes Differ as to the nature of the stimulus that initiates this event Four primary senses use different classes of receptors tbl 48.1 Mechanical receptors: mechanoreceptors (hearing) Chemical receptors: chemoreceptors (taste and smell) Photoreceptors (vision) Free nerve endings Simplest sensory receptors Respond to bending or stretching of sensory neuron membrane Respond to changes in temperature or chemicals in extracellular fluid More complex receptors involve association with epithelial cells Sensing the Exterior World Defined as exteroception fig 48.2a Information depends on receptor, medium in which stimulus travels Most sensory systems evolved in water, later adapted to air Many senses operate better in air than water, need no alteration Other senses required changes to work well in air: hearing Few that work in water do not work in air: electrical charges Other senses evolved in the air that cannot work in the sea: infrared vision Sensory systems provide several levels of information Determine only that an object is present, call attention to object Location and direction of object, can move in relation to it Compose three-dimensional image of object and surroundings Sensing the Internal Environment of the Body Defined as interoception, inner perception Receptors detect changes related to muscle length and tension, limb position, pain, blood chemistry, blood pressure, body temperature Internal receptors are generally simpler than exterior receptors Comparison of interoceptors and exteroceptors tbl 48.2 THE MECHANISMS OF SENSORY TRANSDUCTION Receptor Potentials Cells possess stimulus-gated ion channels in their membranes fig 48.2 Cause cells to respond to stimuli Application of stimulus opens or closes channels Resulting change in membrane permeability produces shift in membrane potential Generator potentials = receptor potentials fig 48.3 Most stimulus-gated ion channels pass Na+ and K+ Photoreceptors are the exception Resting potential (-70 mV) closer to K+ equilibrium potential (-90 mV) than Na+ equilibrium potential(+60 mV) More Na+ enters cell than K+ leaves cell Results in depolarization of sensory cell One or more axon action potentials initiated if depolarization reaches threshold SENSING TEMPERATURE Skin Contains Two Populations of Thermoreceptors Cold receptors stimulated by lowering temperature Heat receptors stimulated by increasing temperature Thermoreceptors in Hypothalamus Monitor temperature of blood Provide information about body's internal, core temperature SENSING PAIN Stimulus that Causes Tissue Damage Is Sensed as Pain Cause changes in heartbeat and blood pressure Cause reflexive withdrawal of body segments if from external source Receptors Called Nociceptors Mostly free nerve endings throughout body, especially near surface May respond to various stimuli Extremes in temperature Intense mechanical stimulation Specific chemicals in extracellular fluid, including ones released by injured cells Receptor thresholds vary Some respond only to actual tissue damage Others respond before damage has occurred SENSING FORCES Mechanoreceptors Sense Changes in Mechanical Force on Membrane Ion channels open in response to mechanical distortion Initiate depolarizing receptor potential Afferent nerve fires a series of action potentials Touch and Pressure Receptors in epidermis, dermis and subcutaneous tissue fig 48.4 Fine touch receptors located on fingertips and face Precisely localize cutaneous stimuli Phasic: hair follicle receptors, Meissner`s corpuscles on hairless body surfaces Tonic: Ruffini endings, touch dome endings (Merkel cells) on surface of skin Receptors measure duration of touch and extent to which it is applied Pacinian corpuscles are phasic pressure-sensitive receptors End of afferent axon surrounded by capsule of layers of cells and extracellular fluid Elastic capsule absorbs sustained pressure, axon ceases to produce impulses Monitor onset and removal of pressure, as in vibrations Muscle Length and Tension Special muscle spindles are buried in muscles, parallel with fibers fig 48.5 Stretch-sensitive axon of sensory neuron wrapped around each spindle Spindle functions as stretch receptor, a type of proprioceptor Muscle spindle elongates when muscle is stretched Associated sensory neurons conduct action potentials to spinal cord Synapse with somatic motor neurons that innervate same muscle Cause motor neurons to produce action potentials, cause muscle to contract Pathways is basis for muscle stretch reflex and knee-jerk reflex Functions as muscle length detector If muscle stretched, length detectors stimulated, muscle contracts With contraction tension removed, reduces activity of sensory neurons Golgi tendon organs Monitor tension at tendon-muscle boundary of origin and insertion If too high, causes reflex to inhibit motor neuron innervating muscle Ensures that muscles do not contract too strongly, damaging their tendons Blood Pressure Receptors in carotid sinus (in wall of carotid arteries) and in aortic arch Baroreceptors are highly branched network of afferent neurons Detect tension in blood artery walls Rate of firing decreases with decrease in blood pressure CNS responds by stimulating sympathetic division of autonomic system Increases heart rate and vasoconstriction Rate of firing increases with increase in blood pressure Reduces sympathetic activity, increases parasympathetic activity Slows heart, lowers blood pressure Gravity Statocysts help brain determine orientation of body with respect to gravity In vertebrates, receptors are in hollow chambers in inner ear Composed of saccule and utricle fig 48.6 Walls lined by sensory cells with projecting cilia, called hair cells Each contains gelatinous matrix containing calcium carbonate otoliths Cilia of hair cells beneath otolith bend with weight of otolith Bent cilia exerts pressure on membrane of hair cell, pressure depolarizes hair cell Increases frequency of action potentials in afferent axons from statocysts to brain Movement causes different set of hair cells to depolarize Brain continually apprised of orientation of statocysts Angular Motion Process similar to orientation with respect to gravity Three fluid-filled semicircular canals located within the inner ear fig 48.7 Canals oriented in different planes to detect motion in any direction Sensory cells protrude into canals in ampulla Tips of cilia embedded in gelatin-filled cupula Rotation of head causes movement of fluid, pushes against cupula Deformation of cupula bends cilia Bending of cilia depolarizes or hyperpolarizes hair cells Converted into a decrease or increase in frequency of nerve firing Movement in any direction sensed by at least one canal Brain analyzes complex movements Vestibular apparatus: saccule, utricle and semicircular canals Saccule and utricle sense linear acceleration Semicircular canals sense angular acceleration Information from all help maintain body's position in space, balance, equilibrium Lateral Line Organs Fish also have hair cells with cilia embedded in cupulae Cupulae extend into lateral line organs, grooves along sides of fish fig 48.8 Water moving past lateral line exerts pressure on cupula, bends cilia Cilia oriented so some sense movement of water in either direction Receptors also indicate rate of movement of water Enable fish to detect motionless objects by sensing deflection of pressure waves Analogous to a sense of hearing, similar cellular mechanism Terrestrial vertebrate hearing hair cells may have evolved from these organs SENSING CHEMICALS Some Sensory Cell Membranes Contain Special Proteins Bind to specific chemicals in environment or extracellular fluid With binding, membrane depolarizes Taste Mediated by taste buds, collection of chemosensitive receptors In fish, taste buds are located all over body, used to locate food Most sensitive vertebrate chemoreceptors In terrestrial vertebrates, taste buds concentrated on papillae in mouth fig 48.9 Humans respond to salt, sweet, sour and bitter tastes Perception of taste is a combination of impulses from these axons Smell In terrestrial vertebrates, located in upper portion of nasal passage fig 48.10 Cell bodies in nasal epithelium, dendrites extend into mucus layer Sense of taste used like a fish`s sense of taste Sense chemical environment around itself Specialized to detect airborne particles Extremely acute sense Sense thousand's of different smells May be a thousand different genes to code for different smell receptor proteins Particular set of olfactory neurons respond to a given odor That set serves as an odor fingerprint for identification Blood Chemistry Peripheral chemoreceptors, carotid bodies embedded within walls of certain arteries Central chemoreceptors in medulla of brain Sensitive to oxygen and carbon dioxide concentration in blood and to blood pH With low breathing rate O2 levels decrease slowly pH decreases rapidly CO2 levels increases rapidly Receptors more sensitive to changes in pH and CO2 concentration Sensitivity to O2 only important at high altitudes HEARING Terrestrial Vertebrates Detect Vibration in Air Via Mechanical Receptors in the Ear Analogous to and evolved from lateral line organs in fish Sense more accurate in water than in air Provides more information about direction than chemoreceptors Provide little information about distance Structure of the Ear Terrestrial vertebrates evolved ears for hearing fig 48.11 Sound waves are weaker in air than in water Terrestrial animals need to amplify sound to use the same receptor Sound waves beat against tympanic membrane or eardrum Membrane separates outer ear from middle ear Causes vibrations of three small bones, ossicles: hammer, anvil and stirrup Connected to oval window, membrane that leads to cochlea Cochlea is coiled, fluid-filled chamber in inner ear Stirrup pushes on oval window causes it to vibrate Vibrations set up pressure waves in fluid of cochlea, actual site of hearing System amplifies sound waves Ossicles act as lever system, increase force of vibration from tympanum to oval window Oval window smaller than tympanum, vibrations produce more force per unit area Middle ear connected to throat by Eustachian tube Equalizes pressure in middle and outer ear Ear pressure changes with rapid change in altitude causes ear popping Transduction in the Cochlea Cochlea divided into upper and lower chamber by cochlear duct fig 48.11 Both chambers and duct are filled with fluid Stirrup vibrations on oval window produce pressure waves in upper chamber Transmitted to lower chamber Cause vibrations in basilar membrane, separates cochlear duct from lower chamber Sensory hair cells located on top of basilar membrane Cilia project into overhanging gelatinous structure called the tectorial membrane Organ of Corti: basilar and tectoral membranes plus hair cells Basilar membrane vibration bends hair cell cilia as it moves relative to the tectorial membrane Bending depolarizes the hair cells Hair cells cause afferent neurons to transmit impulses to brain Impulses interpreted as sound Frequency Localization in the Cochlea Analysis of sound frequency based on resonance Vibrating tuning fork or strings exhibit characteristic resonant frequency String length and taughtness determines resonant frequency in stringed instrument Basilar membrane composed of elastic fibers of varying length and stiffness Short and stiff at base of cochlea (near oval window) = high resonant frequency Long and flexible at apex (far end) = low resonant frequency Sound wave energy moves basilar membrane up and down Energy imparted to region with most similar resonant frequency fig 48.12 Causes maximum deflection at that point Depolarization of hair cells greatest at that point Action potentials arriving in brain interpreted as sound of that frequency or pitch Flexibility of basilar membrane limits human hearing Frequency range of 20-20,000 cycles per second (Hz) in children Hearing high-pitch sounds declines with age Other vertebrates sense sounds lower than 20 Hz, higher than 20,000 Hz Hair cells are innervated by efferent axons from brain Impulses can make hair cells less sensitive Increase individual's ability to concentrate on one signal Other sounds effectively tuned-out by efferent axons Sonar Two ears of terrestrial vertebrates enable localization of sound Can be used to determine direction Not highly accurate to provide measure of distance Sonar circumvents limitations of living in darkness Bat can avoid a wire less than 1 millimeter in diameter fig 48.13 Examples: shrew, whale, dolphin Emit sounds, determine time for sound to reach object and return Allows for three-dimensional imaging Allows bats to occupy birds environment, but in darkness VISION Visual Stimulus Is Electromagnetic Energy Travels in straight line, arrives almost instantaneously Provides information to determine direction and distance of objects The Evolution of the Eye Less advanced animals perceive light with eyespots, but cannot construct visual image Eyes evolved independently in many different groups All use same visual pigment Structure of the Vertebrate Eye Vertebrate eyes are lens-focused fig 48.14 Light passes through transparent cornea, begins to focus it Light continues through lens completes focusing process Lens is a fat disk, attached by ligaments to ciliary muscles Contraction of muscles changes shape of lens fig 48.15 Fish and amphibian lenses have a constant shape Focusing achieved by moving lens in and out Alters point of focus on retina at back of eye Photoreceptors located on retina Amount of light entering eye controlled by iris Sphincter muscle that lies between cornea and lens Light passes through pupil, zone in iris Bright light reduces size of opening Enlarges in dim light to allow more light to enter eye Lenses limited by chromatic aberration Short wavelengths refracted or bent more than longer wavelengths Short wavelengths focus at different point than long wavelengths Vertebrate eye thus filters out short-wavelength ultraviolet light Insects do not focus light and can perceive ultraviolet light Vertebrate Photoreceptors Vertebrate retina contains rods and cones fig 48.16 Rods used for black-and-white vision when illumination is dim Cones are used for color vision, are shorter than rods Humans have 100 million rods and 3 million cones in each retina Most cones found in fovea Location where eye forms its sharpest image Almost no rods found here Cellular structure of rods and cones very similar Inner segment Rich in mitochondria Contains numerous vesicles filled with neurotransmitter molecules Outer segment: connected to inner segment by narrow stalk Packed with hundreds of flattened disks, stacked on one another Light-capturing photopigment molecules on membranes of these disks Rhodopsin is rod cell photopigment Opsin protein coupled to molecule of cis-retinal fig 48.17 Cis-retinal produced from carotene Photopsin is rod cell photopigment Three kinds of cones, each has cis-retinal plus opsin with slightly different amino acid sequence Sequence shifts absorption maximum from 500 nanometers of rhodopsin fig 48.18 455 nm is blue-absorbing 530 nm is green-absorbing 625 nm is red absorbing Different light-absorbing properties account for different cone color sensitivities Sensory Transduction in Photoreceptors Rod or cone contains many Na+ channels in plasma membrane of outer segment In dark many channels are open Na+ ions continually diffuse into outer segment, across stalk to inner segment Small flow in absence of light called the dark current Causes membrane to be somewhat depolarized in the dark In the light, Na+ channels in outer segment close rapidly Reduces dark current Causes photoreceptor to hyperpolarize Only know receptor to respond by hyperpolarizing rather than depolarizing Light causes Na+ channels to close Cis-retinal is converted to trans-retinal when the photopigment absorbs light Isomerization causes retinal to dissociate from opsin: bleaching reaction Opsin protein changes shape Shape change activates G protein In turn activates hundreds of phosphodiester molecules This breaks down intracellular messenger cyclic guanosine monophosphate (cGMP) Photopigments, G proteins and phosphodiesterase embedded in outer segment disks cGMP found in cytoplasm between disks and plasma membrane cGMP serves as link between events in disk membrane and Na+ channels in plasma membrane cGMP is required to keep channels open When light is absorbed by photopigment, cGMP is broken down Channels close at rate of 1000 per second Each photopigment coupled to many G proteins each to many phosphodiesterases Absorption of one photon cascades to block entry of over a million Na+molecules Photoreceptor thus hyperpolarizes Visual Processing in the Vertebrate Retina Retina composed of three layers of cells fig 48.19 Rods and cones in layer closest to external surface of eyeball Next layer contains bipolar cells Layer closest to inside of eye composed of ganglion cells Light must pass through ganglion and bipolar cells to reach retina Rods and cones synapse with bipolar cells Bipolar cells synapse with ganglion cells Flow of sensory information is opposite the path of light Ganglion cells are stimulated to fire action potentials When light is absorbed by particular area of retina Interpreted by brain as light in specific areas of receptive field Pattern of activity encodes point-to point map Retina and brain image objects in visual space Frequency of impulses indicates light intensity at each point Relative activity of ganglia cells attached to three types of cones provides information about color Relationship between receptors, bipolar cells and ganglion cells differs within retina In fovea Each cone connects to one bipolar cell, each to one ganglion cell Provides high visual acuity in fovea Outside fovea Transmission modified by other cells in middle layer fig 48.19 Horizontal cells channel output of many rods to single bipolar cells Each bipolar cell converges on a single ganglion cell In periphery one ganglion cell can get information from more than 125 rods Amacrine cells connect many ganglion cells outside the fovea Carry out extensive processing of visual patterns Peripheral vision is less acute, more sensitive to low levels of light Effects of many rods summated on ganglion cells Fovea serves as inspector, periphery serves as detector Binocular Vision Visual images of vertebrate eyes Eyes on opposite sides of head, each sees object at different angle Parallax permits sensitive depth perception, stereoscopic vision Predators have eyes set in front of head to increase stereoscopic vision fig 48.20 Prey have eyes set on sides of head to enlarge total receptive field Must learn to perceive distance, not inborn OTHER ENVIRONMENTAL SENSES IN VERTEBRATES Heat Electromagnetic radiation with wavelengths longer than visible light Infrared radiation (longer than red) detected as radiant heat Not possessed by aquatic animals as water absorbs heat Sensed by pit vipers (including rattlesnakes) Heat-detecting pit organs located on either side of the head fig 48.21 Perceive heat emanating from motionless animals in complete darkness Two pit organs provide stereoscopic information Electricity Not possessed by terrestrial animals, air does not conduct electricity Some fishes use weak electrical charges to locate prey animals Electrical discharges produced by special organs of modified muscle Forms columns of disk-shaped electroplates One surface has nerve endings, the other does not When axons generate action potentials, release excitatory neurotransmitter Causes electroplate to produce own action potential on surface where they synapse Transient voltage difference of 150 millivolts on one electroplate Electroplates line up in series, voltages add up Series arrangement of disks can produce charges of 500 volts Electric fishes produce weaker charges to survey their surroundings fig 48.22 Sense an object as it distorts the electrical field Receptors include ampullae of Lorenzini Magnetism Navigational, used by many birds, eels, sharks and even bacteria fig 30.16 Birds in blind cages orient to the earth`s magnetic fields Orientation does not occur in cages shielded by steel Orientation improper with artificially altered magnetic field Nature of magnetic receptor poorly understood AN OVERVIEW OF SENSORY SYSTEMS Sensory Systems Utilize a Broad Variety of Cues Individual Vertebrate Systems Differ from One Another