Sensory Physiology PDF

SENSORY PHYSIOLOGY Afferent Nervous System Receptor/Sensor Visceral Afferents – Subconscious input – Blood pressure – CO2 concentration – Etc. Sensory Afferents – Conscious awareness – Somatic (body) sensations Somesthetic Proprioception – Special Senses Eyes Ears Tongue Nose Sensory Cells Sensory c...

SENSORY PHYSIOLOGY Afferent Nervous System Receptor/Sensor Visceral Afferents – Subconscious input – Blood pressure – CO2 concentration – Etc. Sensory Afferents – Conscious awareness – Somatic (body) sensations Somesthetic Proprioception – Special Senses Eyes Ears Tongue Nose Sensory Cells Sensory cells have ion channels and receptor proteins with specific modalities. Transduction – Conversion of one form of energy (external stimulus) into electrical energy – Opening and closing of ion channels often due to actions of receptor proteins Types of Gated Channels – Mechanically-gated – Chemically-gated – Voltage-gated – Thermally-gated Classification of Sensors or Receptors Classification based on modalities or the kind of stimulus they react to; 1. Mechanoreceptors 2. Chemoreceptors 3. Thermoreceptors 4. Photoreceptors 5. Electroreceptors 6. Magnetoreceptors 7. Nocireceptors Roles of Sensors or Receptors Animal senses can be classified into three roles. 1. Interoreceptors Information regarding the body fluids 2. Proprioceptors Information regarding movement and position 3. Exteroreceptors Information regarding the “classic” external sensations Receptor Cell Physiology Receptors may be – Specialized ending of an afferent neuron – Separate cell closely associated with peripheral ending of a neuron Stimulus alters receptor’s permeability which leads to graded receptor potential Usually causes nonselective opening of all small ion channels This change in membrane permeability can lead to the influx of sodium ions. The magnitude of the receptor potential represents the intensity of the stimulus. A receptor potential of sufficient magnitude can produce an action potential. This action potential is propagated along an afferent fiber to the CNS. May adapt slowly or rapidly to sustained stimulation Types of receptors according to their speed of adaptation – Tonic receptors Do not adapt at all or adapt slowly Muscle stretch receptors, joint proprioceptors – Phasic receptors Rapidly adapting receptors Tactile receptors in skin Uses of Information from Receptors Afferent input is essential for the control of efferent output, both for regulating motor behavior in accordance with external circumstances and for coordinating internal activities directed at maintaining homeostasis. Processing of sensory input by the reticular activating system in the brain stem is critical for cortical arousal and consciousness. Central processing of sensory information gives rise to our perceptions of the world around us. Selected information delivered to the CNS may be stored for future reference. Sensory stimuli can have a profound impact on our emotions. Mechanoreception Detection of physical forces Mechanoreceptors – Integument – Muscles, tendons, joints – Statocysts – Lateral line system – Vestibular apparatus – Cochlea Integument Unencapsulated 1. Merkel Cells Epithelial tactile cells for light touch sensation and sensation of texture 2. Free Nerve Endings Respond to high and low temperatures, pain, itching, and tactile functions 3. Root Hair Plexuses Detect movement of hairs Encapsulated 1. Meissner Corpuscles Initiate impulses when deformed due to light touch or low-frequency stimuli 2. Pacinian Corpuscles For sensing coarse touch, pressure, and vibrations 3. Krause End Bulbs Sense low frequency vibrations in penis and clitoris 4. Ruffini Corpuscles Stimulated by stretch (tension) or twisting (torque) in skin Proprioception – Detection of motion and position Statocyst Gravity receptors – Statoliths Grains in lobsters CaCO3 Lateral Line System Somesthetic and proprioceptive mechanoreceptor system Neuromast organ Hair cell (stereocilia, kinocilium) Tip links (CAMs) Ca2+ channels (fish Vestibular Apparatus Proprioceptive structure essential for equilibrium and posture Semicircular Canals Detect rotational or angular acceleration or deceleration of the head (equilibrium). Otolith Organs Provide position of the head relative to gravity and changes in rate of linear motion. Utricle – detect horizontally directed linear acceleration and deceleration Saccule – vertically directed linear acceleration and deceleration Otoliths (CaCO3) Chemoreception Chemoreceptors Detecting chemicals to generate neural signals. – Exteroreceptors and interoreceptors Gustation (taste) Olfaction (smell) Functions of taste and smell – in association with food intake in some species, it can influence the flow of digestive juices and affect appetite – permit social recognition of kin – induce pleasurable or objectionable sensations – have a major role in finding direction, finding shelter, seeking prey, avoiding predators, and sexual attraction to a mate Gustation Sensilla – Specialized projections from the cuticle Taste buds – Chemoreceptors found in different regions (tongue, digestive tract, lungs, etc.) Tastant – Substance that alters the cell’s ionic channels to produce a depolarizing receptor potential Transduction Pathways Salt Taste – Stimulated by chemical salts – Na+ enters via specialized ion channels (regulated by ADH and aldosterone) – Depolarization Sour Taste – Stimulated by acids – H+ direct entry – H+ blocks K+ channels – Depolarization Sweet Taste – Stimulated by glucose – G-protein coupled mechanism (cAMP pathway or IP3 pathway) – Phosphorylation – blockage of K+ channels – Depolarization (Na+ entry) Bitter Taste – Stimulated by alkaloids – Employ various signaling pathways to bring about a receptor potential – G-protein (gustducin) coupled mechanism Umami Taste – Stimulated by amino acid – – – Glutamate G-protein coupled mechanism Details of pathway unknown Olfaction Olfactory receptors are specialized endings of renewable afferent neurons. Olfactory Epithelium Odorant – 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. Olfactory Transduction and Processing Odors are dissected into components. Each part of an odor is detected by one of a thousand receptor. Spatial summation G-protein coupled mechanism Depolarization Olfactory bulb Glomeruli “Smell files” Mitral cells Olfactory tract Subcortical route Regions of the limbic system (primary olfactory cortex of the temporal lobes) Behavioral reactions associated with feeding, mating, and direction orienting Thalamic-cortical route With hypothalamic involvement for conscious perception and fine discrimination of smell Olfactory Adaptation Reduced sensitivity or habituation – Intrinsic receptor adaptation – Extrinsic adaptation in the CNS Lingering of smell – “Odor-eating” enzymes – Similar property to detoxifying liver enzymes Vomeronasal Organ Vomeronasal Organ (VNO) – In mammals including humans Jacobson’s Organ – In nonavian reptiles “Sexual nose” – Governs reproductive and social behaviors Pheromones – Odorless substances affecting reproductive and social behaviors – Female: (z)-7-dodecenyl acetate, copulins – – – Male: darcin, androstenone Flehman position (horse’s laugh) Lordosis Photoreception Vision Light detection uses universal photopigments. Chromatophores – Pigment containing skin cells Two types of photoreceptors – Ciliary Large single cilium with a highly folded membrane (often internal) that contains photopigments Rod cells – Rhabdomeric With numerous parallel microvilli to increase surface area Light-sensing Organs Eyespots – Consist of small number of photoreceptors lining an open cup or pit – Eg. Planarian flatworms, jellies, sea stars Pinhole eye – Reduced size of the cup aperture – Eg. Nautilus Camera eye – Eyes with clear focusing orb known as lens – Eg. Vertebrates Compound eye – With densely packed units known as ommatidia, each having its own lens and pigment shielded photoreceptors – Eg. Arthropods Light Light: electromagnetic radiation Intensity: amplitude or height Light ray: forward movement of a light wave in a particular direction Refraction: bending of light ray Vertebrate Eyes Sensory organ for vision Mechanisms that help protect eyes from injury Eyeball is sheltered by bony socket in which it is positioned. 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 Spherical, fluid-filled structure enclosed by three tissue layers Sclera/cornea Sclera – tough outer layer of connective tissue; forms visible white part of the eye Cornea – anterior, transparent outer layer through which light rays pass into interior of eye Choroid/ciliary body/iris Choroid - middle layer underneath sclera which contains blood vessels that nourish retina Choroid layer is specialized anteriorly to form ciliary body and iris Retina Innermost coat under choroid Consists of outer pigmented layer and inner nervous-tissue layer Rods and cones 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 Iris and Pupil Iris – Controls amount of light entering eye – Contains two sets of smooth muscle networks Circular (or constrictor) muscle Radial (or dilator) muscle – Pigment in iris is responsible for eye color – Unique for each individual Basis for latest identification technology Pupil – Round opening through which light enters the eye Cornea and Lens Two structures most important in eye’s refractive ability – 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 Lens Convex structures of eye produce convergence of diverging light rays that reach eye Degree of refraction – The greater the difference in density, the greater the degree of bending. – The greater the angle, the greater the refraction. Accommodation – The ability to adjust the lens strength in order to focus both near and distant objects – Change in strength and shape of lens – Accomplished by action of ciliary muscle and suspensory ligaments – Age-related reduction in accommodation ability - presbyopia – Emmetropia – Refractive Disorders Myopia Nearsightedness Hyperopia Farsightedness Astigmatism Multiple focal points Retina 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 Photoreceptors Rod and cone cells Consist of three parts Outer segment – 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 Dark and Light Adaptation Dark Adaptation – Photopigments broken down during light exposure are gradually regenerated. Light Adaptation – Breakdown of photopigments due to light – Shift from rod system to cone system Night Blindness – Dietary deficiencies of Vit. A – Rods are nonfunctional due to lack of photopigments Rods Provide indistinct gray vision at night In humans, there are more rods than cones Greater sensitivity of rods vs. cones – Outer segment are longer thus containing more photopigments – Connection of rods with other neurons in their processing pathway Respond to dim light of night Nocturnal animals Cones Long (L) – 560 nm – red Middle (M) – 530 nm – green Short (S) – 420 nm – blue Ultraviolet (UV) – 360-380 nm White and black Cones Greater acuity of cones vs. rods – Connection to other retinal neuronal layers – Sharpness: ability to distinguish between two nearby points Diurnal animals Color discrimination 1. Fishes – Shallow-water: trichromatic (L, M, S cones) – Deep-water: dichromatic (M, S cones) – Other fishes: with UV cones 2. Non-avian Reptiles – Diurnal snakes and lizards: with cones but no rods, many can see UV – Geckos: UV, L, M, no rods – Birds – Tetrachromatic or pentachromatic – Pigeon: L, M, S, UV, yellow cones 3. Mammals – Dogs, cats, mice: M or LM cones, S or UV cones – Primates: trichromatic (L, M, S cones) 4. Arthropods – Various – UV and polarized light – Dichromatic – Trichromatic: UV, S, yellow (flies, bees) – Tetrachromatic (Japanese swallowtail butterfly) – Dodedachromatic Blind Spot Optic disc – Point in the retina at which the optic nerve leaves and through which the blood vessels pass – No image formed due to lack of rods and cones Thermoreception Thermoreceptors Ability to detect environmental temperature Three kinds – Mammalian skin and tongue Cold sensors: respond to decrease in temperature – Cold-gated channels: TRPA1 (ice-cold temperature) and TRPM8 (8oC and 28oC) Warmth sensors: respond to increase in temperature – Heat-gated channels: TRPV1 (42 oC), TRPV2 (52oC), TRPV3 (33oC) – Pit vipers, some phytons, and boas Infrared sensors: respond to infrared radiation – TRPA1 Electroreception Electrolocation – Directional detection of external electric fields via electroreception Navigation, prey detection, communication Types of electroreception – Passive: detection of extraneous electric fields primarily for ecolocation Ampullary electroreceptors Ampullae of Lorenzini – Active: assessing the environment by actively emitting signals and receiving feedback signals Electric organs Tuberous electroreceptors Electric Organs Production of electric organ discharges (EODs) Electrocytes (+150 mV discharge) Magnetoreception Detection of magnetic fields for long-range navigation Three mechanisms 1. Magnetic induction Production of electric currents by passing through a magnetic field line Ampullae of Lorenzini 2. Magnetic minerals Magnetic crystals, Fe3O4 Magnetosome 3. Magnetochemical reactions Chemical reactions involving free-radical formation that can be affected by magnetic fields Cryptochromes Nocireception Pain A protective mechanism meant to bring to awareness the fact that tissue damage is occurring or is about to occur. Three types of nocireceptors 1. Mechanical Mechanical damage such as cutting, crushing, pinching 2. Thermal Temperature extremes, especially heat 3. Polymodal Respond equally to all kinds of damaging stimuli Two Phases and Two Signaling Pathways 1. Fast pain pathway Quick response to direct damage 2. Slow pain pathway Prolonged chemical response to inflammation and cells contents released by damage Referred Pain Pain in a part of the body that is fairly remote from the tissue causing pain. Abnormalities of Pain and Other Somatic Sensations Hyperalgesia – Excessively excitable pain nervous pathway – Sunburn skin Herpes zoster (shingles) – Infection of dorsal root ganglion – Segmental type of pain Headache – Pain referred to the surface of the head from deep head structures Headaches Intracranial Headaches – Meningitis – Low CSF Pressure – Migraine – Alcohol – Constipation Extracranial Headaches – Muscle spasms – Irritation of nasal structures – Eye problems References: Sherwood, L. (2012). Fundamentals of Human Physiology (4 th ed.) CA, USA: Brooks/Cole Cengage Learning Sherwood. L., Klandorf, H. & Yancey, P. H. (2013). Animal Physiology From Genes to Organisms (2nd ed.). CA, USA: Brooks/Cole, Cengage Learning