Sensory Organs (BIOL 221-001) PDF

Andrews University BIOL 221-001 Chapter 16 Sensory Organs Part 1...

Andrews University BIOL 221-001 Chapter 16 Sensory Organs Part 1 Umami Professor: Brian Y.Y. Wong, Ph.D. Gustation (taste)— the sensation that begins with V X action of chemical stimulants (tastants) on taste buds – 4,000 taste buds found mainly on the tongue Bitter Some found (especially in children) inside G IX cheeks, and on the soft palate, pharynx, and epiglottis Sour Lingual papillae--bumps F VII – Filiform: no taste buds Salty Help sense food texture – Foliate: weakly developed Taste buds degenerate by age 3 Sweet – Fungiform: a few taste buds At tips and sides of the tongue Taste – Vallate (circumvallate) Activate second-messenger systems At the rear of the tongue in a “V” Depolarize cells directly Contains up to one-half of all taste buds  Lingual papillae( & taste buds) No taste buds von Ebner’s glands serous secretion smooth small red dots 10-15 large taste buds deep Occasional taste buds trench  Introduction Sensory input is vital to the integrity of personality and intellectual function –Sensory deprivation can cause hallucinations Some information communicated by sense organs never comes to our conscious attention – Blood pressure, body temperature, and muscle tension – These sense organs initiate somatic and visceral reflexes that are indispensable to homeostasis and our survival 16-3  Properties and Types of Sensory Receptors Sensory receptor—a structure specialized to detect a stimulus – Some receptors are bare nerve endings – Others are true sense organs: nerve tissue surrounded by other tissues that enhance response to a certain type of stimulus Accessory tissues may include added epithelium, muscle, or connective tissue 16-4  General Properties of Receptors Transduction— the conversion of one form of energy to another – The fundamental purpose of any sensory receptor is the conversion of stimulus energy (light, heat, touch, sound, etc.) into nerve signals – Transducers can also be nonbiological devices (e.g., a lightbulb) Receptor potential— small local electrical change on a receptor cell brought about by a stimulus – Results in release of neurotransmitter or a volley of action potentials that generates nerve signals to the CNS Sensation— a subjective awareness of the stimulus – Most sensory signals delivered to the CNS produce no conscious sensation Filtered out in the brainstem, thus preventing information overload Some signals do not require conscious awareness like pH and body temperature 16-5  General Properties of Receptors Sensory receptors transmit four kinds of information: modality, intensity, location, duration (MILD) 1. Modality— the type of stimulus or sensation it produces – Vision, hearing, taste – Labeled line code: all action potentials are identical. Each nerve pathway from sensory cells to the brain is labeled to identify its origin, and the brain uses these labels to interpret what modality the signal represents 2. Intensity— encoded in three ways - Brain can distinguish stimulus intensity by: Which fibers are sending signals How many fibers are doing so How fast these fibers are firing 16-6  General Properties of Receptors 3. Location—encoded by which nerve fibers are firing - Receptive field: area within which a sensory neuron detects stimuli Receptive fields vary in size – Neurons in fingertips have small, receptive fields allowing for fine two-point touch discrimination Sensory projection: The brain identifies the site of stimulation Projection pathways: pathways followed by sensory signals to their ultimate destinations in the CNS  General Properties of Receptors 4. Duration— how long the stimulus lasts – Changes in firing frequency over time – Sensory adaptation: if a stimulus is prolonged, the firing of the neuron gets slower over time Phasic receptors— adapt rapidly: generate a burst of action potentials when first stimulated, then quickly reduce or stop signaling even though the stimulus continues – Smell, hair movement, and cutaneous pressure Tonic receptors— adapt slowly: generate nerve signals more steadily throughout the presence of a stimulus – Proprioceptors— body position, muscle tension, and joint motion 16-8  Classification of Receptors 1. By modality – Thermoreceptors, photoreceptors, nociceptors, chemoreceptors, and mechanoreceptors 2. By origin of stimuli – Exteroceptors: detect external stimuli – Interoceptors: detect internal stimuli – Proprioceptors: sense body position and movements 3. By distribution – General (somesthetic) senses: widely distributed – Special senses: limited to head Vision, hearing, equilibrium, taste, and smell 16-9  The General Senses Receptors for the general senses are relatively simple in structure and physiology Consist of one or a few sensory nerve fibers and a spare amount of connective tissue 16-10  A. Unencapsulated nerve endings lack connective tissue wrappings For pain and temperature For light touch and texture Coils around a hair follicle Skin and mucous Associated with Merkel cells Monitor movement of hair membrane at base of epidermis Tactileccell Tactile ell Nerve Nerveending ending Free nerve endings Tactile disc Hair receptor Tactile (Meissner) corpuscles Bulbous (Ruffini) corpuscles—tonic Light touch and texture Heavy touch, pressure, joint movements, Dermal papillae of hairless skin and skin stretching Wrapping Krause end bulbs enhances Tactile; in mucous membranes sensitivity or Bulbous Tactile corpuscle End bulb selectivity of corpuscle response Lamellar (pacinian) corpuscles— phasic Deep pressure, stretch,tickle and vibration Periosteum of bone, and deep dermis of skin Lamellar corpuscle Muscle spindle Tendon organ B. Encapsulated nerve endings are wrapped by glial cells or connective tissue  Unencapsulated Nerve Endings Unencapsulated nerve endings lack connective tissue wrappings Free nerve endings – For pain and temperature – Skin and mucous membrane Tactile discs – For light touch and texture – Associated with Merkel cells at the base of the epidermis Hair receptors – Coil around a hair follicle – Monitor movement of hair  Encapsulated Nerve Endings Tactile (Meissner) corpuscles – Light touch and texture – Dermal papillae of hairless skin Krause end bulbs – Tactile; in mucous membranes Lamellar (pacinian) corpuscles—phasic – Deep pressure, stretch, tickle, and vibration – Periosteum of bone, and deep dermis of the skin Bulbous (Ruffini) corpuscles—tonic – Heavy touch, pressure, joint movements, and skin stretching 16-13  Somatosensory Projection Pathways From receptor to final destination in the brain, most somesthetic signals travel by way of three neurons First-order neuron – From the body, enters the posterior horn of the spinal cord via spinal nerves – From the head, enters pons or medulla via cranial nerve – Touch, pressure, and proprioception fibers are large, fast, myelinated axons – Heat and cold fibers are small and unmyelinated Second-order neuron Decussate to the opposite side in the spinal cord, medulla, or pons – End in the thalamus, except for proprioception, which ends in cerebellum Third-order neuron – Thalamus to the primary somesthetic cortex of the cerebrum 16-14  Pain Pain— discomfort caused by tissue injury or noxious stimulation, and typically leading to evasive action – Important since it helps protect us – Lost in diabetes mellitus— diabetic neuropathy Nociceptors— two types providing different pain sensations – Fast pain travels myelinated fibers at 12 to 30 m/s Sharp, localized, stabbing pain perceived with injury – Slow pain travels unmyelinated fibers at 0.5 to 2 m/s Longer-lasting, dull, diffuse feeling 16-15  Pain Somatic pain— from skin, muscles, and joints Visceral pain— from the viscera – Stretch, chemical irritants, or ischemia of viscera (poorly localized) Injured tissues release chemicals that stimulate pain fibers – Bradykinin: the most potent pain stimulus known – Makes us aware of injury and activates cascade or reactions that promote healing – Histamine, prostaglandin, and serotonin also stimulate nociceptors 16-16  Projection Pathways for Pain Two main pain pathways to the brain, and multiple subroutes Pain signals from the head – First-order neurons travel in cranial nerves V, VII, IX, and X and end in the medulla – Second-order neurons start in the medulla and ascend to the thalamus – Third-order neurons from the thalamus, reach the postcentral gyrus of the cerebrum 16-17  Projection Pathways for Pain (SSG) Pain signals from neck down – Travel by way of three ascending tracts Spinothalamic tract— most significant pain pathway – Carries most somatic pain signals Spinoreticular tract— carries pain signals to reticular formation – Activate visceral, emotional, and behavioral reactions to pain Gracile fasciculus— carries signals to the thalamus for visceral pain 16-18 Projection Pathways for Pain  Projection Pathways for Pain Referred pain— pain in viscera often mistakenly thought to come from the skin or other superficial site – Results from the convergence of neural pathways in CNS – Brain “assumes” visceral pain is coming from the skin Brain cannot distinguish the source – Heart pain felt in shoulder or arm because both send pain input to spinal cord segments T1 to T5 16-20 Referred Pain T1 to T5  CNS Modulation of Pain Analgesic (pain-relieving) mechanisms of CNS just beginning to be understood Endogenous opioids: internally produced opium-like substances – Enkephalins: two analgesic oligopeptides with 200 times the potency of morphine – Endorphins and dynorphins— larger analgesic neuropeptides discovered later Secreted by the CNS, pituitary gland, digestive tract, and other organs Act as neuromodulators that block pain and give pleasure 16-22  CNS Modulation of Pain Spinal gating— stops pain signals at the posterior horn of the spinal cord – Analgesic fibers arise in the brainstem, descend in the reticulospinal tract, and block pain signals in the spinal cord Review of normal pain pathway – Nociceptor stimulates second-order nerve fiber with Substance P neurotransmitter – Second-order fiber sends a signal up the spinothalamic tract to the thalamus – Thalamus relays signal to the cerebral cortex where awareness of pain occurs 16-23  CNS Modulation of Pain Pathway for pain blocking (modulation) – Signals from the hypothalamus and cerebral cortex feed into the central gray matter of the midbrain Allows both autonomic and conscious influences on pain perception – Midbrain relays signals to certain nuclei in the reticular formation of the medulla oblongata – Medulla issues descending, serotonin-secreting analgesic fibers to the spinal cord via the reticulospinal tract The fibers terminate in the posterior horn at all levels of the spinal cord 16-24 CNS Modulation of Pain S E P  CNS Modulation of Pain Pathway for pain blocking (modulation) In the posterior horn, descending analgesic fibers synapse on short spinal interneurons – The interneurons secrete enkephalins and inhibit the second-order neuron (postsynaptically) – Some fibers from the medulla also exert presynaptic inhibition by synapsing on the axons of nociceptors and blocking the release of substance P 16-26  CNS Modulation of Pain Another pathway of spinal gating— rubbing or massaging injury – Pain-inhibiting neurons of the posterior horn receive input from mechanoreceptors in the skin and deeper tissues Rubbing stimulates mechanoreceptors, which stimulate spinal interneurons to secrete enkephalins that inhibit second-order pain neurons 16-27  Taste All taste buds look alike Lemon-shaped groups of 40 to 60 taste cells, supporting cells, and basal cells Taste cells – Have tuft of apical microvilli (taste hairs) that serve as receptor surface for taste molecules – Taste pores: pit into which the taste hairs project – Taste hairs are epithelial cells, not neurons – Synapse with and release neurotransmitters onto sensory neurons at their base  Epiglottis Lingual tonsil Fig. 16.6 Vagus nerve Palatine tonsil Vallate Vallate papillae papillae Glossopharyngeal Filiform Foliate papillae papillae Facial nerve Taste Fungiform buds papillae (a) Tongue (b) Vallate papillae Foliate Supporting Synaptic cell papilla vesicles Taste Sensory Cell (40-60) nerve Taste pore fibers Taste pore Taste bud Basal Taste Hairs cell Tongue epithelium 100 µm (c) Foliate papillae (d) Taste bud  Taste Basal cells – Stem cells that replace taste cells every 7 to 10 days Supporting cells – Resemble taste cells without taste hairs, synaptic vesicles, or sensory role To be tasted, molecules must dissolve in saliva and flood the taste pore Five primary sensations – Salty: produced by metal ions (sodium and potassium) – Sweet: associated with carbohydrates and other foods of high caloric value – Sour: acids such as in citrus fruits – Bitter: associated with spoiled foods and alkaloids such as nicotine, caffeine, quinine, and morphine – Umami: “meaty” taste of amino acids in chicken or beef broth  Taste Taste is influenced by food texture, aroma, temperature, and appearance – Mouthfeel: detected by branches of lingual nerve in papillae Hot pepper stimulates free nerve endings (pain), not taste buds Regional differences in taste sensations on the tongue – Tip is most sensitive to sweet, edges to salt and sour, and rear to bitter 16-31  Taste Two mechanisms of action – Activate second-messenger systems Sugars, alkaloids, and glutamate bind to receptors which activates G proteins and second-messenger systems within the cell – Depolarize cells directly Sodium and acids penetrate cells and depolarize them directly Either mechanism results in the release of neurotransmitters that stimulate dendrites at the base of taste cells 16-32  Taste receptors & signaling mechanism Neuroepithelial cells H+ → Na+ → selectively express only one class Amiloride-sensitive Amiloride-sensitive Na+ channels Na+ channels H+ block PKD1L3 & PKD2L1 depolarization of membrane exclusively expressed in cells involved in sour taste transduction PLC, phospholipase C IP2, inositol-1,4-diphosphate IP3, inositol 1,4,5- trisphosphate.  Taste The facial nerve collects sensory information from taste buds over anterior two-thirds of the tongue Glossopharyngeal nerve from posterior one-third of the tongue Vagus nerve from taste buds of palate, pharynx, and epiglottis All fibers reach the solitary nucleus in the medulla oblongata From there, signals are sent to two destinations – The hypothalamus and amygdala control autonomic reflexes: salivation, gagging, and vomiting – Thalamus relays signals to the postcentral gyrus of the cerebrum for a conscious sense of taste Sent on to orbitofrontal cortex to be integrated with signals from nose and eyes; form impression of flavor and palatability of food 16-34  Smell Olfaction— sense of smell – Response to odorants (chemicals) Olfactory mucosa – Contains 10 to 20 million olfactory cells (neurons), epithelial supporting cells, and basal stem cells – Mucosa of the superior concha, nasal septum, and roof of nasal cavity covering about 5 cm2 – On average 2,000 to 4,000 odors distinguished  Smell Olfactory cells – Are neurons – Shaped like bowling pins – Head bears 10 to 20 cilia called olfactory hairs – Have binding sites for odorant molecules and are nonmotile – Lie in a tangled mass in a thin layer of mucus  Olfactory tract Olfactory bulb Olfactory nerve fascicle Olfactory mucosa (reflected) (a) Olfactory bulb Granule cell Olfactory tract Axon Each Mitral cell glomerulus is Tufted cell dedicated to a Glomerulus Neurosoma single odor Olfactory 350 kinds nerve fascicle Dendrite receptors Cribriform plate of ethmoid bone (c) Olfactory Basal cell hairs Supporting cells Olfactory cell Olfactory gland Hydrophilic odorants diffuse Olfactory hairs Mucus through mucus Hydrophobic ones are Odor molecules transported by odorant- (b) Airflow binding protein in mucus  Smell Olfactory cells (continued) – The basal end of each cell becomes the axon – Axons collect into small fascicles and leave the cranial cavity through the cribriform foramina in the ethmoid bone – Fascicles are collectively regarded as cranial nerve I Only neurons in the body are directly exposed to the external environment – Have a lifespan of only 60 days – Basal cells continually divide and differentiate into new olfactory cells Supporting cells Basal cells – Divide and differentiate to replace olfactory cells  Smell Humans have a poorer sense of smell than most other mammals – Still, more sensitive than our sense of taste – Women are more sensitive to odors than men; especially to certain odors at the time they are ovulating – Humans have only about 350 kinds of olfactory receptors Odorant molecules bind to membrane receptors on olfactory hair – Hydrophilic odorants diffuse through mucus – Hydrophobic ones transported by odorant-binding protein in mucus 16-39  Smell Odorant activates G protein and cAMP system in olfactory cell Opens ion channels for Na+ or Ca2+ – Depolarizes membrane and creates receptor potential Triggers action potential that travels to the brain Olfactory receptors adapt quickly – Due to synaptic inhibition in olfactory bulbs Some odorants act on nociceptors of the trigeminal nerve (CN V) – Ammonia, menthol, chlorine, and capsaicin of hot peppers 16-40  Olfactory transduction pathway Different odorant molecules bind with different affinity to the olfactory receptors Olfactory receptors activate the enzyme adenylyl cyclase and initiate the cAMP cascade of events leading to the opening of specific Na+ and Ca2+ channels. The influx of Na+ and Ca2+ is responsible for cell depolarization.  Smell Olfactory projection pathways: – Olfactory cells synapse in the olfactory bulb on dendrites of mitral and tufted cells Dendrites meet in spherical clusters called glomeruli – Each glomerulus dedicated to a single odor – Tufted and mitral cell axons form olfactory tracts Reach the primary olfactory cortex in the inferior surface of the temporal lobe Secondary destinations: hippocampus, amygdala, hypothalamus, insula, and orbitofrontal cortex – Identify odors, integrate with taste, evoke memories, emotions, and visceral reactions Fibers reach back to olfactory bulbs where granule cells inhibit the mitral and tufted cells – Odors change under different conditions – Food smells more appetizing when hungry 16-42 Olfactory Projection Pathways  Human Pheromones Human body odors may affect sexual behavior A person’s sweat and vaginal secretions affect other people’s sexual physiology – Dormitory effect The presence of men seems to influence female ovulation Ovulating women’s vaginal secretions contain pheromones called copulins, that have been shown to raise men’s testosterone level 16-44