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Chapter 44 Lecture Outline See separate PowerPoint slides for all figures and tables pre-inserted into PowerPoint without notes and animations. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Sensory Systems Chapter 44 Senses • Sensory receptors give an organism the senses of: – Vision – Hearing – Taste – Smell – Touch • Senses provide information about the environment 3 Overview of Sensory Receptors • Sensory receptors provide information from our internal and external environments that is crucial for survival and success • Exteroceptors sense external stimuli – Some function well on land but not in water, and vice versa • Interoceptors sense internal stimuli – Usually simpler than exteroceptors 4 Overview of Sensory Receptors • Receptors can be grouped into three classes 1. Mechanoreceptors are stimulated by mechanical forces such as pressure 2. Chemoreceptors detect chemicals or chemical changes 3. Electromagnetic receptors react to heat and light energy 5 Overview of Sensory Receptors • Sensory information is conveyed to the CNS and perceived in a four-step process 1. Stimulation 2. Transduction 3. Transmission 4. Interpretation 6 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Stimulus Transduction of stimulus into receptor potential in sensory receptor Transmission of action potential in sensory neuron Interpretation of stimulus in central nervous system 7 Overview of Sensory Receptors • Sensory cells respond to stimuli via stimulusgated ion channels in their membranes – Open or close depending on the sensory system involved • In most cases, a depolarization of the receptor cell occurs – Analogous to the excitatory postsynaptic potential (EPSP) • Referred to as receptor potential 8 Overview of Sensory Receptors • Receptor potential like a graded potential – The larger the sensory stimulus, the greater the degree of depolarization • The greater the sensory stimulus, the greater the depolarization of the receptor potential and the higher the frequency of action potentials 9 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Voltage (mV) 20 –10 Stimulus Threshold –40 Na+ Na+ Stimulus-gated channels –70 Time Stimulus Receptor potential applied a. Voltage (mV) 20 –10 –40 Voltage-gated channels –70 Stimulus applied b. Time Train of action potentials To central nervous system 10 Mechanoreceptors • Cutaneous receptors – Receptors in the skin – Classified as interoreceptors – Respond to stimuli at the border between internal and external environments – Receptors for pain, heat, cold, touch, and pressure 11 Mechanoreceptors • Nociceptors – Transmit impulses perceived as pain – Sensitive to noxious substances and tissue damage – Most consist of free nerve endings located throughout the body, especially near surfaces – Transient receptor potential (TRP) ion channel • One responds to capsaicin – sensation of heat and pain 12 Mechanoreceptors • Thermoreceptors – Naked dendritic endings of sensory neurons that are sensitive to changes in temperature – Contain TRP ion channels that are responsive to hot and cold – Cold receptors are stimulated by a fall in temperature and are inhibited by warming • Reverse for warm receptors – Cold receptors are located higher in the skin – Cold receptors are more numerous than warm receptors 13 Mechanoreceptors • Several types of mechanoreceptors in the skin detect the sense of touch – Contain sensory cells with ion channels that open in response to membrane distortions – 2 types • Phasic – intermittently activated – Hair follicle receptors, Meissner corpuscles, Pacinian corpuscles • Tonic – continuously activated – Ruffini corpuscles, Merkel’s disks 14 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1. Merkle cell Free nerve ending 2. Meissner corpuscle 3. Ruffini corpuscle 4. Pacinian corpuscle 1. Merkle Cell Tonic receptors located near the surface of the skin that are sensitive to touch pressure and duration. 2. Meissner Corpuscle Phasic receptors sensitive to fine touch, concentrated in hairless skin. 15 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 3. Ruffini Corpuscle Tonic receptors located near the surface of the skin that are sensitive to touch pressure and duration. 4. Pacinian Corpuscle Pressure-sensitive phasic receptors deep below the skin in the subcutaneous tissue. 16 Mechanoreceptors • Proprioceptors – Monitor muscle length and tension – Provide information about the relative position or movement of animal’s body parts – Examples • Muscle spindles – monitor stretch on muscle – receptors that lie in parallel with muscle fibers – knee jerk reflex • Golgi tendon organs – monitor tension on tendons – reflex inhibits motor neurons – prevents damage to tendons 17 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Biceps extension causes it to stretch Nerve Spindle sheath Specialized muscle fibers (spindle fibers) Motor neurons Sensory neurons Skeletal muscle 18 Mechanoreceptors • Baroreceptors – Monitor blood pressure – Located at carotid sinus and aortic arch – Detect tension or stretch in the walls of these blood vessels – When blood pressure decreases, the frequency of impulses produced by baroreceptors decreases • Results in increased heart rate and vasoconstriction 19 Hearing • Detection of sound waves • Sound is the result of vibration, or waves, traveling through a medium • Detection of sound waves is possible through the action of specialized mechanoreceptors that first evolved in aquatic organisms 20 Lateral Line System in Fish • Sense objects that reflect pressure waves and low-frequency vibrations – Supplements hearing • Consists of hair cells within a longitudinal canal in the fish’s skin that extends along each side of the body and within several canals in the head • Hair cells’ surface processes project into a gelatinous membrane called a cupula • Hair cells are innervated by sensory neurons that transmit impulses to the brain 21 Lateral Line System in Fish • Bending of stereocilia in the direction of the kinocilium has a stimulatory effect • Bending in opposite direction is inhibitory Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Canal Lateral line scale Nerve Opening Inhibition Excitation Kinocilium Stereocilia Cupula Hair cell Cilia Hair cell Afferent axons Lateral line organ Stimulation of sensory neuron Sensory nerves Lateral line a. 22 b. Hearing Structures in Fish • Hearing structures in fish – Called otoliths – Composed of calcium carbonate crystals – Contained in the otolith organs of the membranous labyrinth – Otoliths vibrate against stereocilia projecting from hair cells – Produces action potentials 23 Ear Structure of Land Vertebrates • Air vibrations are channeled through the ear canal of the outer ear • Vibrations reach the tympanic membrane causing movement of three small bones (ossicles) in the middle ear – Malleus (hammer), incus (anvil), and stapes (stirrup) • The stapes vibrates against the oval window, which leads into the inner ear 24 Ear Structure of Land Vertebrates • The inner ear consists of the cochlea – Bony structure containing part of the cochlear duct • The vestibular canal lies above this duct, while the tympanic canal lies below it • All three chambers are filled with fluid • Pressure waves travel down the tympanic canal to the round window, which is another flexible membrane • Transmits pressure back to middle ear 25 Ear Structure of Land Vertebrates Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Outer ear Middle ear Inner ear Semicircular canals Skull Auditory nerve to brain Oval window Malleus Pinna Stapes Incus Auditory canal Cochlea Tympanic membrane Round window Eustachian tube Eustachian tube a. b. 26 Ear Structure of Land Vertebrates • As pressure waves are transmitted through the cochlea to the round window, they cause the cochlear duct to vibrate • Organ of Corti – Basilar membrane contains sensory hair cells – Stereocilia from hair cells project into tectorial membrane • Bending of stereocilia depolarizes hair cells • Hair cells send action potentials to the brain 27 Evolution of the mammalian inner ear Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. During inner ear embryology in modern mammals, ear bones develop in association with the lower jaw bone before moving inward to the inner ear Inner Side of Jaw Outer Side of Jaw Dog Early mammal Ma Ma s Morganucodon Q Ar Ar Cynodont Q Ar Ar Q – quadrate Ar- articular bones S - stapes M - malleus Q Synapsid Ar Ar 28 Ear Structure of Land Vertebrates Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Organ of Corti Tectorial membrane Vestibular canal Hair cells Cochlear duct Bone Tympanic canal Basilar membrane Auditory nerve c. Sensory neurons To auditory nerve d. 29 Ear Structure of Land Vertebrates • Basilar membrane of the cochlea consists of elastic fibers that respond to different frequencies, or pitch, of sound • Hair cell depolarization is greatest in region that responds to a particular frequency • Afferent axons from that region stimulated more • Brain interprets that as representing sound of a particular frequency or pitch 30 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. High Frequency (20,000 Hz) Malleus Incus Stapes Oval window Vestibular canal Tympanic canal Cochlear duct Basilar membrane Apex Tympanic Round Base membrane window a. Medium Frequency (2000 Hz) b. Low Frequency (500 Hz) c. 31 Navigation by Sound • A few mammals have the ability to perceive presence and distance of objects by sound – Bats, shrews, whales, dolphins • They emit sounds and then determine the time it takes these sounds to return • This process is called echolocation • The invention of sonar and radar are based on the same principles 32 Detection of Body Position • Most invertebrates can orient themselves with respect to gravity using a statocyst – Consists of ciliated hair cells embedded in a membrane with calcium carbonate stones (statoliths) • In vertebrates, the gravity receptors consist of two chambers in the membranous labyrinth – Utricle and saccule 33 Detection of Body Position • Within the utricle and saccule are hair cells with stereocilia and a kinocilium • Processes embedded in the calcium carbonaterich otolith membrane • Utricle more sensitive to horizontal acceleration • Saccule more sensitive to vertical acceleration • Both types of accelerations cause cilia to bend, thus producing an action potential in an associated sensory neuron 34 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Semicircular canals Vestibular nerve Sensory neuron Cochlea Kinocilium Stereocilia Hair cell Utricle (horizontal acceleration) Saccule (vertical acceleration) Gravitational force Gelatinous matrix Otoliths Signal Hair cells Supporting cells Stationary a. Movement b. 35 Detection of Body Position • The utricle and saccule are continuous with three semicircular canals that detect angular acceleration in any direction • At the ends of the canals are swollen chambers called ampullae • Groups of cilia protrude into them • Tips of cilia are embedded within a gelatinous cupula that protrudes into the endolymph fluid of each canal 36 Detection of Body Position • When the head rotates, the semicircular canal fluid pushes against the cupula, causing the cilia to bend • Bending in the direction of the kinocilium causes a receptor potential • Stimulates an action potential in the associated sensory neuron • Saccule, utricle, and semicircular canals are collectively called the vestibular apparatus 37 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Semicircular canals Vestibular nerve Vestibule Ampullae Direction of body movement Flow of endolymph Stationary Movement Cupula Endolymph Cilia of hair cells Hair cells Supporting cell Vestibular nerve a. Stimulation b. 38 Chemoreceptors • Bind to particular chemicals in the extracellular fluid • Membrane of sensory neuron becomes depolarized and produces action potentials • Chemoreceptors are used in the senses of taste and smell • Also important in monitoring the chemical composition of blood 39 Taste (gustation) • Mixture of physical and psychological factors • Broken down into five categories – Sweet, sour, salty, bitter, and umami (hearty) • Taste buds are collections of chemosensitive cells associated with afferent neurons 40 Taste • In fish, taste buds are scattered all over the body surface • In land vertebrates, taste buds are located in the epithelium of the tongue and oral cavity within raised areas called papillae • Salty and sour tastes act directly through ion channels • Other tastes act indirectly by binding to specific G protein–coupled receptors 41 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Taste bud Circumvallate papilla Foliate papilla Fungiform papilla a. b. Support cell Nerve fiber c. d: © Dr. John D. Cunningham/Visuals Unlimited Receptor cell with microvilli Taste bud Taste pore 544 µm d. 42 Taste • Many arthropods have taste chemoreceptors • Flies have them in sensory hairs located on their feet Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Signals to brain Different chemoreceptors Proboscis Sensory hair on foot Pore 43 Smell (olfaction) • In land vertebrates, involves neurons located in the upper portion of the nasal passages • Receptors project into the nasal mucosa, and their axons project directly into the cerebral cortex • Particles must first dissolve in extracellular fluid before they can activate the olfactory receptors • Humans can detect thousands of different smells 44 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Olfactory bulb Nasal passage Olfactory bulb Cribiform plate Axon of olfactory nerve Basal cell Olfactory receptor Support cell Olfactory hairs Odor 45 pH • Peripheral chemoreceptors – Found in the aortic and carotid bodies – Sensitive primarily to the pH of plasma • Central chemoreceptors – Found in the medulla oblongata of the brain – Sensitive to the pH of cerebrospinal fluid • Increased CO2 in blood lowers pH – Stimulates respiratory control center 46 Vision • Begins with the capture of light energy by photoreceptors • Can be used to determine both the direction and distance of an object • Invertebrates have simple visual systems with photoreceptors clustered in an eyespot • Flatworms can perceive the direction of light but cannot construct a visual image 47 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Photoreceptors Eyespot Light Pigment layer Flatworm will turn away from light 48 Vision • Members of four phyla have evolved welldeveloped, image-forming eyes – Annelids, mollusks, arthropods, and chordates • Although these eyes are similar in structure, they have evolved independently – Example of convergent evolution 49 Vision Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Insect Mollusk Chordate Lenses Retinular cell Optic nerve Retina Optic nerve Retina Lens Lens Optic nerve 50 Structure of the Vertebrate Eye • Sclera – White portion of the eye, formed of tough connective tissue • Cornea – Transparent portion through which light enters; begins to focus light • Iris – Colored portion of the eye – Contraction of iris muscles in bright light decreases the size of its opening, the pupil • Lens – Transparent structure that completes focusing of light 51 onto the retina Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Optic nerve Suspensory ligament Iris Pupil Lens Cornea Vein Artery Fovea Ciliary muscle Retina Sclera Ciliary muscle Lens Suspensory ligament 52 Structure of the Vertebrate Eye • The lens is attached to the ciliary muscles by the suspensory ligament – Changes shape of lens • In near vision, ciliary muscles contract – Lens becomes more rounded and bends light more strongly • In distance vision, ciliary muscles relax – Lens becomes more flattened and bends light less 53 Structure of the Vertebrate Eye Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Normal Distant Vision Nearsighted Farsighted Nearsighted, Corrected Farsighted, Corrected Lens Iris Retina Suspensory ligaments Normal Near Vision a. b. c. • People who are nearsighted or farsighted do not properly focus the image on the retina 54 Structure of the Vertebrate Eye • Vertebrate retina contains two types of photoreceptors – Rods • Responsible for black-and-white vision when illumination is dim – Cones • Responsible for color vision and high visual acuity (sharpness) • Most are located in the central region of the retina known as the fovea • Sharpest image is formed 55 Structure of the Vertebrate Eye • Rods and cones have same basic structure • Both have inner segment rich in mitochondria and vesicles filled with neurotransmitter molecules • Connected by narrow stalk to the outer segment • Packed with hundreds of flattened disks which contain photopigments 56 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Rod Connecting cilium Inner segment Outer segment Synaptic terminal Nucleus Mitochondria Pigment discs Cone 57 Structure of the Vertebrate Eye • Photopigment in rods is rhodopsin • Photopigments of cones are photopsins – Humans have three kinds of cones – Each possesses a photopsin consisting of a cis-retinal bound to a protein with a slightly different amino acid sequence – These shift the absorption maximum, the region of the electromagnetic spectrum that the pigment best absorbs 58 Light absorption (percent of maximum) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Blue cones Rods 420 nm 500 nm Green Red cones cones 530 nm 560 nm 100 80 60 40 20 400 500 600 Wavelength (nm) 700 59 Structure of the Vertebrate Eye • The retina consists of three layers of cells 1. External layer contains the rods and cones 2. Middle layer contain bipolar cells 3. Layer closest to eye cavity contains ganglion cells • Once photoreceptors are activated, they stimulate bipolar cells, which in turn stimulate ganglion cells • Ganglion cells transmit impulses to brain via optic nerve 60 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Axons to optic nerve Amacrine cell Horizontal cell Light Ganglion Bipolar cell cell Cone Rod Choroid • Blind spot – optic nerve exits 61 Sensory Transduction • In the dark – Photoreceptor cells release an inhibitory neurotransmitter that hyperpolarizes the bipolar neurons – Prevents the bipolar neurons from releasing excitatory neurotransmitter to the ganglion cells that signal to the brain • In the presence of light – Photoreceptor cells stop releasing their inhibitory neurotransmitter, in effect, stimulating bipolar cells – Bipolar cells in turn stimulate the ganglion cells, which transmit action potentials to the brain 62 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Dark Fluid inside disk Light 11-cis-retinal Fluid inside disk Phosphodiesterase All-transretinal Phosphodiesterase Rhodopsin G protein K+ Photoreceptor cell In the dark cGMP levels are high and keep chemically cGMP gated Na+ channels open. The Na+ influx depolarizes Na+ the membrane causing an influx of Ca+, which leads to a release of inhibitory neurotransmitter. This prevents signaling from the bipolar cell. Ca2+ cGMP K+ Photoreceptor cell GMP When Rhodopsin absorbs light, 11cis-retinal is converted to allNa+ trans-retinal. This causes rhodopsin to activate a G protein that stimulates phosphodiesterase, which converts cGMP to GMP. The reduced levels of cGMP close the Na+ channels 2+ Ca hyperpolarizing the membrane. This prevents the release of inhibitory neurotransmitter allowing bipolar cells to fire. ( – ) Inhibitory neurotransmitter Bipolar cell Bipolar cell 63 Visual Processing • Action potentials in the optic nerves are relayed from the retina to the lateral geniculate nuclei of the thalamus • They are then projected to the occipital lobes of the cerebral cortex • Each hemisphere of the cerebrum receives input from both eyes 64 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Right eye Left eye Optic nerves Optic chiasm Optic tracts Lateral geniculate nuclei Occipital lobes of cerebrum (visual cortex) 65 Visual Acuity • Relationship between receptors, bipolar cells, and ganglion cells varies in different parts of the retina – Fovea – one-to-one connections = high acuity – Outside of fovea, many rods converge on a single bipolar cell and many bipolar cells converge on a single ganglion cell • More sensitive to dim light • At the expense of acuity and color vision 66 Visual Processing • Color blindness is due to an inherited lack of one or more types of cones • People with normal vision are trichromats – Have all three cones • Color blind individuals are dichromats • Sex-linked recessive trait – More common in men 67 Binocular vision • Primates and most predators have two eyes, one located on each side of the face • 2 fields of vision overlap • Parallax permits binocular vision – Ability to perceive 3-D images and depth • In contrast, prey animals generally have eyes located to the sides of the head – Prevents binocular vision, but enlarges the overall receptive field 68 Diversity of Sensory Experiences • Sensing infrared radiation – The only vertebrates that can sense infrared radiation are several types of snakes – Pit vipers • Have a pair of heat-detecting pit organs on either side of the head between the eye and nostril • Locate heat sources in the environment, including prey in darkness • Paired pits appear to be stereoscopic • To some extent can form a thermal image 69 Diversity of Sensory Experiences Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Pit Outer chamber Inner chamber Membrane © Leonard Lee Rue III 70 Diversity of Sensory Experiences • Detect electrical currents – Elasmobranchs (sharks, rays, and skates) have electroreceptors called the ampullae of Lorenzini – Can sense electrical currents generated by the muscle contractions of their prey • Detect magnetic fields – Eels, sharks, bees, and many birds appear to navigate along the magnetic field lines of the Earth 71