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PowerLecture: Chapter 14 Sensory Systems Learning Objectives Describe the characteristics of a receptor and list the various types of receptors. Contrast mechanisms by which the chemical and the somatic senses work. Understand how the senses of balance and hearing function. Describe how the sense of vision has evolved through time. Learning Objectives (cont’d) Draw a medial section of the human eyeball through the optic nerve, identify each structure, and tell the function of each. Identify some common disorders of the eye. Impacts/Issues Private Eyes Private Eyes Iris scanning is one of the newest security techniques. First, each person’s unique arrangement of smooth muscle fibers in the iris of the eye must be recorded in an electronic database. Each time the person passes through a check point, a small camera looks at the iris and compares it with the database. Usually, we use our eyes to see, but in this new technology, our eyes are seen. How Would You Vote? To conduct an instant in-class survey using a classroom response system, access “JoinIn Clicker Content” from the PowerLecture main menu. In which situations should individuals be required to submit to iris scanning and registration? a. For any reason; it's not any different than other forms of identification. b. In place of or to enhance government identification, such as a driver's license or passport. c. For employment at any company that chooses to require it. d. It should never be required, it should only be used as a voluntary convenience, and then, strictly regulated. Section 1 Sensory Receptors and Pathways Sensory Receptors and Pathways In a sensory system, a stimulus activates a receptor, which transduces (converts) it to an action potential that travels to the brain where it triggers sensation or perception. A stimulus is any form of energy that activates receptor endings of a sensory neuron. Sensations are conscious responses to the stimuli. Perception is an understanding of what sensations mean. stimulus energy received stimulus energy converted to action potential brain response (sensation or perception) In-text Fig., p. 250 c Stretched muscle stimulates d Message travels from stimulated a stretch receptor (the ending of a sensory neuron to motor neuron sensory neuron) that is adjacent to it. and interneuron in spinal cord. sensory neuron interneuron in spinal cord motor neuron in spinal cord axon endings of motor neuron terminating on the same muscle b e Message is sent back to the muscle, also to other interneurons in the brain. muscle spindle Fig. 14.1, p. 251 Sensory Receptors and Pathways Six major categories of sensory receptors. Mechanoreceptors detect changes in pressure, position, or acceleration. Thermoreceptors detect heat or cold. Nociceptors (pain receptors) detect tissue damage. Chemoreceptors detect ions or molecules. Osmoreceptors detect changes in solute concentration in surrounding fluid. Photoreceptors detect the energy of visible light. Animation: Mechanoreceptors CLICK TO PLAY Sensory Receptors and Pathways All action potentials are the same; the brain determines the nature of a given stimulus based on which nerves are signaling, the frequency of the action potentials generated, and the number of axons responding. Specific sensory areas interpret action potentials in specific ways. Strong signals make receptors fire action potentials more often and longer. Sensory Receptors and Pathways Stronger stimuli recruit more sensory receptors. Sensory adaptation is the diminishing response to an ongoing stimulus. Figure 14.1 Section 2 Somatic Sensations Somatic Sensations Somatic sensations occur when receptor signals from body surfaces reach the somatosensory cortex in the cerebrum. Figure 14.3 Animation: Somatosensory Cortex CLICK TO PLAY Somatic Sensations Receptors near the body surface sense touch, pressure, and more. Sensations of touch, pressure, cold, warmth, and pain are discerned near the body surface by receptors whose numbers vary by body region. Free nerve endings are the simplest receptors. • • These are thinly myelinated or unmyelinated dendrites of sensory neurons. One type coils around hair follicles to detect movement; another detects chemicals. Somatic Sensations Encapsulated receptors are surrounded by a capsule of epithelial or connective tissue. • • • • Merkel’s discs adapt slowly and are important for steady touch. Meissner’s corpuscles respond to light touching. Ruffini endings are sensitive to steady touch and pressure. The Pacinian corpuscles are sensitive to deep pressure and vibrations. Figure 14.4 Animation: Sensory Receptors CLICK TO PLAY free nerve endings hair (pain) Meissner’s corpuscle (light touch) dermis Merkel’s discs (steady touch) Meissner’s corpuscle Ruffini endings (pressure, touch) Merkel’s discs hair follicle receptor (hair displacement) Pacinian corpuscle (deep pressure, vibrations) Ruffini endings Pacinian corpuscle Fig. 14.4, p. 253 Somatic Sensations Mechanoreceptors in skeletal muscle, joints, tendons, ligaments, and skin are responsible for awareness of the body’s position and of its limb movements. Pain is the perception of bodily injury. Pain is the perception of injury to some region of the body. Somatic Sensations Nociceptors are subpopulations of free nerve endings distributed throughout the skin (somatic pain) and internal tissues (visceral pain). • • • When cells are damaged, they release chemicals (bradykinins, histamine, and prostaglandins) to activate neighboring pain receptors. Pain receptors signal interneurons, which release substance P. Substance P allows for natural opiates called endorphins and enkephalins to be released to reduce pain perception. Somatic Sensations Referred pain is a matter of perception. Much visceral pain is referred pain; that is, it is felt at some distance from the real stimulation point. Phantom pain is the sensation that amputees feel when they sense the missing part as if it were still there. lungs,diaphragm heart stomach liver, gallbladder pancreas small intestine ovaries colon appendix urinary bladder kidney ureter © 2007 Thomson Higher Education Fig. 14.5, p. 253 Animation: Referred Pain CLICK TO PLAY Section 3 Taste and Smell: Chemical Senses Taste and Smell: Chemical Senses Taste and smell are chemical senses; they begin at chemoreceptors, the signals traveling to the brain where they are perceived, transmitted to the limbic system, and remembered. Taste and Smell: Chemical Senses Gustation is the sense of taste. Sensory organs called taste buds hold the taste receptors. • • Receptors are located on the tongue, roof of the mouth, and throat. The five general taste categories are sweet, sour, salty, bitter, and umami. The flavors of most foods are a combination of the five basic tastes plus sensory input from olfactory receptors in the nose. Animation: Taste Receptors CLICK TO PLAY a tonsil taste bud hairlike ending of taste receptor bitter sour salty sweet b c d sensory nerve Fig. 14.6, p. 254 Taste and Smell: Chemical Senses Olfaction is the sense of smell. Olfactory receptors in the olfactory epithelium of the nose detect water-soluble or volatile substances—odors. • • The interpretation of smell is done by the olfactory bulbs located in the brain. Olfaction is one of the most ancient senses, useful in survival as the receptors respond to molecules from food, mates, and predators. Humans also have a vomeronasal organ whose receptors can detect pheromones, which are signaling molecules with roles in sexual attraction. Animation: Olfactory Pathway CLICK TO PLAY olfactory nerve tract olfactory bulb olfactory receptor cell body © 2007 Thomson Higher Education Fig. 14.7, p. 255 Video: Tongue Tied CLICK TO PLAY From ABC News, Human Biology in the Headlines, 2006 DVD. Section 4 A Tasty Morsel of Sensory Science A Tasty Morsel of Sensory Science Receptors in taste buds associate the five main taste categories with particular “tastant” molecules that the brain interprets depending on the action potentials that come its way. Each taste bud has receptors that can respond to tastants of at least two, if not all five, of the taste classes. Not all taste receptors, however, are equally sensitive; bitter receptors tend to be the most sensitive. A Tasty Morsel of Sensory Science Various tastants commingle together with odors into what we perceive as flavors. Section 5 Hearing: Detecting Sound Waves Hearing: Detecting Sound Waves Sounds are waves of compressed air; the amplitude (loudness) and frequency (pitch) of sounds are detected by vibrationsensitive mechanoreceptors deep in the ear. Amplitude one cycle Frequency per unit time Soft Low note Loud High note Same frequency, different amplitude Same loudness, different pitch Figure 14.8 Animation: Wavelike Properties of Sound CLICK TO PLAY Hearing: Detecting Sound Waves The ear gathers and sends “sound signals” to the brain. The outer ear collects sound waves and turns them into vibrations, which are amplified in the middle ear; vibrations are distinguished in the inner ear. Inner ear structures include semicircular canals for balance and the cochlea where hearing takes place. Hearing: Detecting Sound Waves Sensory hair cells are the key to hearing. Vibrations are passed from the tympanic membrane to the middle ear bones (malleus, incus, stapes) and on to the oval window, stretched across the entrance to the cochlea. • • Sound is amplified because the oval window is smaller than the tympanic membrane. The cochlea has two compartments in its outer chamber (the scala vestibuli and scala tympani), which curl around an inner cochlear duct; all are fluid filled. Fig. 14.9a, p. 256 INNER EAR vestibular apparatus, cochlea OUTER EAR pinna, auditory canal MIDDLE EAR eardrum, ear bones Fig. 14.9a, p. 256 MIDDLE EAR BONES OVAL WINDOW (behind stirrup) stirrup auditory nerve anvil hammer COCHLEA auditory EARDRUM canal round window Fig. 14.9b, p. 256 oval window (behind stirrup) waves of air pressure scala vestibuli eardrum © 2007 Thomson Higher Education waves of fluid pressure scala tympani round window cochlear duct Fig. 14.9c, p. 257 Hearing: Detecting Sound Waves • • • Vibrations of the oval window send pressure waves through the fluid to the basilar membrane on the floor of the cochlear duct; resting on the membrane is the organ of Corti, which includes sensory hair cells. The tips of the hair cells rest against the jellylike tectorial membrane; vibrations cause the hair cells to bend. Bending causes the release of neurotransmitters, triggering action potentials that travel to the brain. scala vestibuli cochlear duct organ of Corti sensory neurons (to the auditory nerve) scala tympani Fig. 14.9d, p. 257 Hearing: Detecting Sound Waves Loudness is determined by the total number of cells that become stimulated; tone or “pitch” depends on the frequency of vibration. The round window at the far end of the cochlea serves as a release valve for the pressure waves in the middle ear. The eustachian tube extending from the middle ear to the throat permits equalization of pressures. Animation: Ear Structure and Function CLICK TO PLAY Video: Sound Detection CLICK TO PLAY Section 6 Balance: Sensing the Body’s Natural Position Balance: Sensing the Body’s Natural Position The sense of balance depends on messages from receptors in the eyes, skin, and joints, as well as organs of equilibrium in the inner ear. The vestibular apparatus is a closed system of fluid-filled sacs and semicircular canals inside the ear; the canals are arranged to represent the three planes of space. Figure 14.10 Animation: Vestibular Apparatus and Equilibrium CLICK TO PLAY Balance: Sensing the Body’s Natural Position • • • Rotational receptors are located at the base of each semicircular canal; sensory hair cells project into a jellylike cupula. Movement of the head causes the hairs to bend within the jelly, generating action potentials. Rotation of the head determines dynamic equilibrium. Animation: Dynamic Equilibrium CLICK TO PLAY Balance: Sensing the Body’s Natural Position Static equilibrium, the head’s position in space, is monitored by two sacs in the vestibular apparatus, the utricle and saccule. • • • The sacs contain the otolith organs (hair cells) and otoliths (ear stones), which detect changes in orientation as well as acceleration and deceleration. Action potentials from different parts of the vestibular apparatus travel to reflex centers in the brainstem. As signals are integrated, the brain orders compensatory movements necessary to maintain postural balance. vestibular apparatus, a system of fluid-filled sacs and canals inside the ear posterior canal horizontal canal superior canal utricle saccule A vestibular apparatus (part of each inner ear) consists of a utricle, a saccule, and the three canals labeled here. nerve fluid pressure Fig. 14.10, p. 258 stereocilium otolith cupula hair cell sensory neuron Fig. 14.11, p. 258 Balance: Sensing the Body’s Natural Position Extreme motion or continuous overstimulation of the hair cells of the vestibular apparatus can result in motion sickness. Section 7 Disorders of the Ear Disorders of the Ear The hearing apparatus of the ears is sturdy, but it can be damaged by various illnesses and injuries. Otitis media, painful inflammation of the middle ear, often occurs in children following spread of a respiratory infection; pus and/or fluid buildup as a result can cause the eardrum to rupture. Tinnitus, or ringing or buzzing in the ears, can be triggered by infection, aspirin consumption, or other, unknown causes. Disorders of the Ear Deafness is the partial or complete loss of hearing; deafness may be congenital or due to aging, disease, or environmental causation. The loudness of sounds is measured in decibels. Quiet conversation occurs at about 50 decibels. Damage begins when exposed to sounds between 75-85 decibels over extended periods. Rock concerts easily reach 130 decibels. Outer Hair Cells scars Fig. 14.13, p. 259 Section 8 Vision: An Overview Vision: An Overview Vision is an awareness of the position, shape, brightness, distance, and movement of visual stimuli as detected by the sensory organs, the eyes. The eye is built for photoreception. The eye has three layers, sometimes called “tunics.” • • • The outer layer consists of the sclera and transparent cornea. The middle layer consists of a choroid, ciliary body, and iris. The inner layer is the retina. Vision: An Overview The sclera (“white” of the eye) protects the eye; the dark-pigmented choroid underlies the sclera and prevents light from scattering. Most of the blood vessels lie in the choroid. Behind the cornea is the pigmented iris; the hole at the center of the iris is the pupil, the entrance for light which can be adjusted depending on the level of light present. • • The lens is found behind the iris; the lens is attached to the ciliary body, a muscle functioning in the focusing of light. The lens focuses light onto a layer of photoreceptor cells in the retina. Vision: An Overview • A clear fluid (aqueous humor) bathes both sides of the lens; vitreous humor fills the chamber behind the lens. The retina is a thin layer of neural tissue at the back of the eyeball; axons from some of the neurons converge to form the optic nerve, which sends signals to the visual cortex in the thalamus. Animation: Eye Structure CLICK TO PLAY Parts of the Eye Vision: An Overview The curved surface of the cornea bends incoming light so that light rays converge at the back of the eyeball; images appear “upside down and backwards” on the retina but are corrected in the brain. Eye muscle movements fine-tune the focus. Because of the bending of the light rays by the cornea, accommodation must be made by the lens so that the image is in focus on the retina. Accommodation is performed by the ciliary muscles attached to the lens. Vision: An Overview Eye muscle movements fine-tune the focus. Because of the bending of the light rays by the cornea, accommodation must be made by the lens so that the image is in focus on the retina. Accommodation is performed by the ciliary muscles attached to the lens. Fig. 14.15a, p. 261 muscle contracted close object slack fibers Accommodation for close objects (lens bulges) muscle relaxed distant object taut fibers Accommodation for distant objects (lens flattens) Fig. 14.16, p. 261 Animation: Visual Accomodation CLICK TO PLAY Video: To See Again CLICK TO PLAY From ABC News, Biology in the Headlines, 2005 DVD. Section 9 From Visual Signals to “Sight” From Visual Signals to “Sight” Rods and cones are the photoreceptors. The retina’s basement layer is pigmented and is covered by photoreceptors called rod cells and cone cells. Rod cells are sensitive to dim light and detect changes in light intensity; cone cells respond to high-intensity light and contribute to sharp daytime vision. Fig. 14.17a, p. 262 rod cell stacked, pigmented membranes cone cell Fig. 14.17b, p. 262 From Visual Signals to “Sight” Visual pigments in rods and cones intercept light energy. From Visual Signals to “Sight” Each rod contains more than a billion molecules of rhodopsin; this pigment can detect and respond to even a few photons of light, allowing us to see in dim light. • • • Rhodopsin consists of a protein (opsin) and a signal molecule (cis-retinal) that is derived from vitamin A. Photons of blue-green light stimulate rhodopsin to change shape; shape changes alter the distribution of ions across the rod cell membrane and slow down the release of an inhibitory neurotransmitter. Without the inhibitor, neurons send visual signals to the brain. From Visual Signals to “Sight” Cone cells have different visual pigments (red, green, or blue); absorption of photons also prevents release of neurotransmitters, thus allowing signaling to the brain. Visual acuity is greatest in the fovea, a depression located at the center of the retina that is densely packed with photoreceptors. Figure 14.18 From Visual Signals to “Sight” The retina processes signals from rods and cones. Signals flow from rods and cones to bipolar interneurons, and then to ganglion cells, the axons of which form the optic nerves. Before leaving the retina, signals are dampened or enhanced by horizontal cells and amacrine cells. horizontal cells amacrine cells rods cones incoming rays of light ganglion cells (axons get bundled into one of two optic nerves) bipolar cells Fig. 14.19, p. 263 Animation: Organization of Cells in the Retina CLICK TO PLAY From Visual Signals to “Sight” Receptive fields in the retina. • • The retina’s surface is organized into “receptive fields,” areas that influence the activity of individual sensory neurons. Some fields respond to differences in light, others to motion, color, or rapid changes in light intensity. Signals move on to the visual cortex. • • The visual field represents the part of the outside world a person actually sees. The right side of each retina gathers light from the left half of the visual field and the left side gathers light from the right half of the field. From Visual Signals to “Sight” • The optic nerve from each eye sends signals from the left visual field to the right cerebral hemisphere, and signals from the right visual field to the left hemisphere. Axons of the optic nerves end in the lateral geniculate nucleus, from which they proceed to the brain’s visual cortex, which has several visual fields sensitive to direction, movement, color, and so on; here is where final interpretation of the signals is made to produce an organized sense of sight. to optic nerve optic nerve lateral geniculate nucleus visual cortex retina Fig. 14.20, p. 263 Animation: Pathway to the Visual Cortex CLICK TO PLAY Section 10 Disorders of the Eye Disorders of the Eye Normal eye function can be disrupted by disease, injury, inherited abnormalities, and aging. Missing cone cells cause color blindness. Total color blindness results when an individual has only one of the three kinds of cones. Disorders of the Eye Red-green color blindness is the inability to distinguish red and green colors in dim light (and sometimes bright light) due to a lack of red and green cone cells. Malformed eye parts cause common focusing problems. In astigmatism, one or both corneas have uneven curvature and cannot bend light to the same focal point. Figure 14.23 Disorders of the Eye Nearsightedness (myopia) results when the image is focused in front of the retina. Farsightedness (hyperopia) is due to an image focused behind the retina. Figure 14.21 (focal point) (focal point) distant object close object Fig. 14.21 (top), p. 264 Fig. 14.21 (bottom), p. 264 Animation: Focusing Problems CLICK TO PLAY Disorders of the Eye The eyes are also vulnerable to infections and cancer. Conjunctivitis, inflammation of the membrane lining the inside of the eyelids and covering the sclera, is among the most common reasons for doctor visits in the U.S. Figure 14.22 Disorders of the Eye Trachoma, caused by the bacterium responsible for the sexually transmitted disease chlamydia, damages both the eyeball and the conjunctiva, possibly leading to blindness. Herpes infection of the cornea results from infection with various herpes simplex viruses and can also lead to blindness. Malignant melanoma is eye cancer that develops in the choroid; retinoblastoma is cancer of the retina that occurs in infants. Disorders of the Eye Aging increases the risk of cataracts and some other eye disorders. Cataracts, the gradual clouding of the lens associated with aging and diabetes, can completely block light from entering the eye. Macular degeneration is an age-related degeneration of the retina. Glaucoma results from excess of fluid in the eyeball, causing pressure on the retina. Disorders of the Eye Medical technologies can remedy some vision problems and treat eye injuries. Corneal transplant surgery can replace defective corneas with artificial plastic corneas or donor corneas; cataracts may be corrected in a similar fashion by replacing the lens. “Lasik” (laser-assisted in situ keratomilieusis) or “lasek” (laser-assisted subepithelial keratectomy) surgeries can be used to correct severe nearsightedness. Disorders of the Eye Conductive keratoplasty (CK) uses radio waves to reshape the cornea. Retinal detachment can result from a physical blow to the head; laser coagulation can be used to “reattach” the retina to the underlying choroid.