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Chapter 6B The Peripheral Nervous System: Special Senses Review (6A) • Pathways, perceptions, sensations • Receptor Physiology – Receptors have differential sensitivities to various stimuli. – graded receptor potentials. – Receptor potentials may initiate action potentials – Receptors adaptation (slow/fast) – Each somatosensory pathway is “labeled” – Acuity is influenced by receptive field size and lateral inhibition. – PAIN -The brain has a built-in analgesic system. What are your special senses? • • • • • • Vision Hearing Balance and equilibrium Taste Smell Touch Vision outline • Anatomy • Muscles and light control • Refraction and refractive structures – Refractive problems • Retina, photoreceptors, transduction • Visual cortical processing Anatomy Eye protection Eyelids Act like shutters to protect eye from environmental hazards Eyelashes Trap fine, airborne debris such as dust before it can fall into eye Tears Continuously produced by lacrimal glands Lubricate, cleanse, bactericidal Eyesocket Iris regulates amount of light Eye middle layer underneath sclera which contains blood vessels that nourish retina tough outer layer of connective tissue; forms visible white part of the eye Circular and radial muscle controlling the amt. of light entering eye opening anterior, transparent outer layer through which light rays pass into interior of eye •Maintains eye shape Aqueous humor is formed by capillary network in ciliary body, then drains into the canal of Schlemm, and eventually enters the blood. Nutrients for cornea and lense Eye • Interior consists of two fluid-filled cavities separated by the lens – Posterior cavity • Larger cavity between lens and retina • Contains vitreous humor – Important in maintaining the spherical shape of eyeball – Anterior cavity • Anterior cavity between cornea and lens • Contains aqueous humor – Carries nutrients for cornea and lens – Produced by capillary network within ciliary body • • • Fovea – Pinhead-sized depression in exact center of retina – Point of most distinct vision – Has only cones Macula lutea – Area immediately surrounding fovea – Fairly high acuity Macular degeneration – Leading cause of blindness in western hemisphere Vision outline • Anatomy • Light and muscle control • Refraction and refractive structures – Refractive problems • Retina, photoreceptors, transduction • Visual cortical processing Eye • Convex structures of eye produce convergence of diverging light rays that reach eye Eye Focusing on Distant and Near Light Sources What happens to light rays when they leave the light source? Eye • Two structures most important in eye’s refractive ability are – Cornea • Contributes most extensively to eye’s total refractive ability • Refractive ability remains constant because curvature never changes – Lens • Refractive ability can be adjusted by changing curvature as needed for near or far vision • Accommodation – Change in strength and shape of lens – Accomplished by action of ciliary muscle and suspensory ligaments – Age-related reduction in accommodation ability - presbyopia Mechanics of Accommodation Far vision * Light moves towards thick part of lens Near vision Fig. 6-11, p. 193 Vision outline • Anatomy • Muscles and light control • Refraction and refractive structures – Refractive problems • Retina, photoreceptors, transduction • Visual cortical processing Emmetropia, Myopia, and Hyperopia Vision outline • Anatomy • Muscles and light control • Refraction and refractive structures – Refractive problems • Retina, photoreceptors, transduction • Visual cortical processing Retinal Layers • Retina – receptor containing portion is actually an extension of the CNS • Neural portion of retina consists of three layers of excitable cells – Outermost layer containing rods and cones – Middle layer of bipolar cells – Inner layer of ganglion cells • Axons of ganglion cells join to form optic nerve – Point on retina at which optic nerve leaves is the optic disc » Region often called the blind spot because no image can be detected here because of lack of rods and cones • • Rod and cone cells Consist of three parts – Outer segment Photoreceptors • Detects light stimulus – Inner segment • Contains metabolic machinery of cell – Synaptic terminal • Transmits signal generated in photoreceptor on light stimulation to next cells in visual pathway Photopigments • • Undergo chemical alterations when activated by light Consists of two components – Opsin • Protein that is integral part of disc membrane – Retinene • Derivative of vitamin A • Light-absorbing part of photopigment • Four different photopigments – Rod pigment • Provide vision only in shades of gray • Rhodopsin – Absorbs all visible wavelengths – Cone pigments • Respond selectively to various wavelengths of light • Make color vision possible – Red cones – Green cones – Blue cones Inside disc Dark Light Rhodopsin (Absorption of light) Retinal in 11-cis form Retinal changes to all-trans form, activating photopigment 11-cis-retinal all-trans-retinal Activates transducin High concentration of cGMP Takes place in outer segment cGMP Activates phosphodiesterase Decreases concentration of cGMP Keeps Na+ channels in outer segment open Phototransduction Light Takes place in outer segment Na+ channels in outer segment close Rod Takes place in retina Hyperpolarization of photoreceptor (the receptor potential) Hyperpolarization Depolarization of photoreceptor (Spreads to synaptic terminal) (Spreads to synaptic terminal) Takes place in synaptic terminal Opens Ca2+ channels in synaptic terminal Takes place in synaptic terminal Neurotransmitter Release of neurotransmitter Closes Ca2+ channels in synaptic terminal Release of neurotransmitter Direction of retinal processing Direction of light Depolarization of onHyperpolarization center bipolar (and (–) of off-center subsequently) ganglion bipolar (and cells subsequently) ganglion cells No action potentials in off-center ganglion cells On-center bipolar cell Further retinal processing in bipolar and ganglion cells Neurotransmitter Further retinal processing in bipolar and ganglion cells Depolarization of on-center bipolar (and subsequently) ganglion cells Action potentials in on-center ganglion cells Action potentials in on-center ganglion cells Depolarization and action potentials Hyperpolarization (–) of off-center bipolar (and subsequently) ganglion cells No action potentials in off-center ganglion cells On-center ganglion cell Propagation to visual cortex Propagation to visual cortex To visual cortex Receptive field of photoreceptor perceived as darkness Light Illuminated receptive field of photoreceptor perceived as part of visual image Figure 6-26 p208 Fig. 6-25, p. 202 Table 6-3 p210 The sensitivity of the eyes varies through dark and light adaptation. •Dark adaptation •Can gradually distinguish objects as you enter a dark area. •Due to the regeneration of rod photopigments that had been broken down by previous light exposure. •Light adaptation •Can gradually distinguish objects as you enter an area with more light. •Due to the rapid breakdown of cone photopigments. Vision outline • Anatomy • Muscles and light control • Refraction and refractive structures – Refractive problems • Retina, photoreceptors, transduction • Visual fields • Visual cortical processing Visual Processing • Blending color – 3 cone types – blue, green, red – Stimulated in a ratio to produce blends % max • Distinguishing contours – On center and off center ganglion cells • Images on the retina are upside down and backwards. • Depth perception Hearing outline • Anatomy • • • • • • – Outer, middle, inner Hearing Transmission of sound waves Hair cells and transduction Cochlea and canals/ducts Pitch and loudness Auditory cortical processing Ear • Consists of three parts – External ear • Consists of pinna, external auditory meatus, and tympanum • Transmits airborne sound waves to fluid-filled inner ear • Amplifies sound energy – Middle ear • Transmits airborne sound waves to fluid-filled inner ear • Amplifies sound energy – Inner ear • Houses two different sensory systems – Cochlea » Contains receptors for conversion of sound waves into nerve impulses which makes hearing possible – Vestibular apparatus » Necessary for sense of equilibrium Ear Hearing outline • Anatomy – Outer, middle, inner • Hearing • • • • • Transmission of sound waves Pitch and loudness Hair cells and transduction Cochlea and canals/ducts Auditory cortical processing • Neural perception of sound energy • Involves two aspects – Identification of the sounds (“what”) – Localization of the sounds (“where”) • Sound waves – Traveling vibrations of air – Consist of alternate regions of compression and rarefaction of air molecules Hearing Hearing • Pitch (tone) of sound – Depends on frequency of air waves 20-20,000 cps, 1000-4000 • Intensity (loudness) – Depends on amplitude of air waves • Timbre (quality) – Determined by overtones Hearing outline • Anatomy – Outer, middle, inner • hearing • Transmission of sound waves • • • • Pitch and loudness Hair cells and transduction Cochlea and canals/ducts auditory cortical processing Transmission of Sound Waves • • • • • Tympanic membrane vibrates when struck by sound waves Middle ear transfers vibrations through ossicles (malleus, incus, stapes) to oval window (entrance into fluid-filled cochlea) Waves in cochlear fluid set basilar membrane in motion Receptive hair cells are bent as basilar membrane is deflected up and down Mechanical deformation of specific hair cells is transduced into neural signals that are transmitted to auditory cortex in temporal lobe of brain for sound perception Fig. 6-33, p. 213 Hearing outline • Anatomy – Outer, middle, inner • Hearing • Transmission of sound waves • Pitch and loudness • Hair cells and transduction • Cochlea and canals/ducts • Auditory cortical processing • • Inner – Deformation and rubbing on the tectoral membrane hyper or depolarizes the cells resulting in a signal. Outer – Do not signal the brain – Fine tuning – Accentuates movement of basilar membrane • (lengthening and shortening) Transduction to Auditory nerve amplification Fig. 6-33c, p. 213 Fig. 6-35, p. 215 Sound waves Bending of ha hair cells of or as basilar mem ment displace in relation to o tectorial memb the hairs and e Vibration of tympanic membrane Vibration of middle ear bones Graded pote changes (receptor po receptor cel Vibration of oval window Fluid movement within cochlea Vibration of round window Changes in action potentials g auditory ner In ear Vibration of basilar membrane Dissipation of energy (no sound perception) Propagation o potentials to a in temporal lo sound percep F Hearing outline • Anatomy – Outer, middle, inner • hearing • Transmission of sound waves • Pitch and loudness • Hair cells and transduction • Cochlea and canals/ducts • auditory cortical processing Auditory Cortical Processing – Primary auditory cortex is tonotopically organized – Locations on basilar membrane map to locations in the cortex – Pathway • Hair cells-afferent auditory nerve- synapses in brainstem and thalamus (LGN)-higher auditory cortex • Cortex – Higher processing • Basal nuclei – Control of movement, inhibitory, negative • Thalamus – Relay and processing of sensory information – Awareness, a positive screening center for information • Hypothalamus – Hormone secretion, regulation of the internal environment • Cerebellum – Important in balance and in planning and executing voluntary movement • Brain Stem – Relay station (posture and equilibrium), cranial nerves, control centers, reticular integration, sleep control Equilibrium outline • Anatomy – Semicircular canals • otoliths Equilibrium • Vestibular apparatus – In inner ear – Consists of • Semicircular canals – Detect rotational acceleration or deceleration in any direction • Utricle and saccule – Detect changes in rate of linear movement in any direction – Provide information important for determining head position in relation to gravity Fig. 6-38a, p. 219 Equilibrium • Neural signals generated in response to mechanical deformation of hair cells by specific movement of fluid and related structures • Vestibular input goes to vestibular nuclei in brain stem and to cerebellum for use in maintaining balance and posture, controlling eye movement, perceiving motion and orientation • Cortex – Higher processing • Basal nuclei – Control of movement, inhibitory, negative • Thalamus – Relay and processing of sensory information – Awareness, a positive screening center for information • Hypothalamus – Hormone secretion, regulation of the internal environment • Cerebellum – Important in balance and in planning and executing voluntary movement • Brain Stem – Relay station (posture and equilibrium), cranial nerves, control centers, reticular integration, sleep control Semicircular canals Detect rotational acceleration or deceleration in any direction Utricle and saccule Detect changes in rate of linear movement in any direction Provide information important for determining head position in relation to gravity Equilibrium Receptive hair cells Ampulla Cupula – moves in the Direction of movement Inertia! XYZ Kino – Stero-ionchannels Semicircular canals Detect rotational acceleration or deceleration in any direction Utricle and saccule Detect changes in rate of linear movement in any direction Provide information important for determining head position in relation to gravity Hair cells-afferent neurons-vestibular nerve-vestibulocochlear nerve- Fig. 6-38, p. 219 Fig. 6-38b, p. 219 Fig. 6-38c, p. 219 Otoliths Semicircular canals Detect rotational acceleration or deceleration in any direction Utricle and saccule Detect changes in rate of linear movement in any direction Provide information important for determining head position in relation to gravity Body moves Head moves Chemical Senses Taste and smell • Receptors are chemoreceptors • In association with food intake, influence flow of digestive juices and affect appetite • Stimulation of receptors induces pleasurable or objectionable sensations and signals presence of something to seek or to avoid Taste (Gustation) • • • • Chemoreceptors housed in taste buds Present in oral cavity and throat Taste receptors have life span of about 10 days Taste bud consists of – Taste pore • Opening through which fluids in mouth come into contact with surface of receptor cells – Taste receptor cells • Modified epithelial cells with surface folds called microvilli • Plasma membrane of microvilli contain receptor sites that bind selectively with chemical molecules Location and Structure of Taste Buds Taste • Tastant (taste-provoking chemical) • Binding of tastant with receptor cell alters cell’s ionic channels to produce depolarizing receptor potential • Receptor potential initiates action potentials within terminal endings of afferent nerve fibers with which receptor cell synapses • Terminal afferent endings of several cranial nerves synapse with taste buds in various regions of mouth • Signals conveyed via synaptic stops in brain stem and thalamus to cortical gustatory area Receptor cell-afferent nerve-cranial nerves- brain stem- thalamus- cortical gustatory area Taste • Five primary tastes – Salty • Stimulated by chemical salts, especially NaCl – Direct entry of sodium ions thru sodium channels – Sour • Caused by acids which contain a free hydrogen ion, H+ – hydrogen ions block potassium channels (depolarization) – Sweet • Evoked by configuration of glucose – G protein - cAMP pathway blockage of potassium channels (depolarization) – Bitter • Brought about by more chemically diverse group of tastants • Examples – alkaloids, toxic plant derivatives, poisonous substances – G protein linked – Umani • Meaty or savory taste – G protein linked Taste Perception • Influenced by information derived from other receptors, especially odor • Temperature and texture of food influence taste • Psychological experiences associated with past experiences with food influence taste • How cortex accomplishes perceptual processing of taste sensation is currently unknown Smell (Olfaction) • Olfactory receptors in nose are specialized endings of renewable afferent neurons • Olfactory mucosa – 3cm2 of mucosa in ceiling of nasal cavity – Contains three cell types • Olfactory receptor cell – Afferent neuron whose receptor portion is in olfactory mucosa in nose and afferent axon traverses into brain – Axons of olfactory receptor cells collectively form olfactory nerve • Supporting cells – Secrete mucus • Basal cells – Precursors of new olfactory receptor cells (replaced about every two months) Smell (Olfaction) • Odorants – Molecules that can be smelled • To be smelled, substance must be – Sufficiently volatile that some of its molecules can enter nose in inspired air – Sufficiently water soluble that it can dissolve in mucus coating the olfactory mucosa Smell (Olfaction) • 1000 different types of olfactory receptors • Odorants act through second-messenger systems to trigger action potentials • Afferent signals are sorted according to scent component by glomeruli within olfactory bulb Fig. 6-43, p. 225 Olfactory receptor cells • Enlarged knob bearing several cilia • Have olfactory receptors • Odorants – Must be volatile – Water soluble Processing of Scents in Olfactory Bulb Olfactory processing • Odors dissected into components • Each part of an odor detected by one of a thousand receptor • G protein, cAMP, Na channel transduction • Olfactory bulb – Above bone layer – Glomeruli and mitral cells together • Limbic system in the primary olfactory cortex of the temporal lobe • Through the thalamus to the cortex Processing • Each odorant molecule activates multiple receptors and glomeruli • Odor discrimination based on “patterns” of glomerular excitation Vomeronasal Organ (VNO) • Common in mammals but until recently was thought to nonexistent in humans • Located about half an inch inside human nose next to vomer bone • Detects pheromones – Nonvolatile chemical signals passed subconsciously from one individual to another • Role in human behavior has not been validated