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Chemical Senses Chemical senses – gustation (taste) and olfaction (smell) Their chemoreceptors respond to chemicals in aqueous solution Taste – to substances dissolved in saliva Smell – to substances dissolved in fluids of the nasal membranes Sense of Smell The organ of smell is the olfactory epithelium, which covers the superior nasal concha Olfactory receptor cells are bipolar neurons with radiating olfactory cilia Olfactory receptors are surrounded and cushioned by supporting cells Basal cells lie at the base of the epithelium Sense of Smell Figure 15.3 Olfactory Mucous Membrane -Olfactory receptor cells -Area of 5cm2 in roof of nasal cavity near the septum -10 to 20 million receptor cells -Each olfactory receptor is a neuron -Olfactory mucous membrane is the place in body where NS is closest to external world Composition of Olfactory epithelium -Each neuron has a thick dendrite with an expanded end called olfactory rod -From rods cilia project to the mucous surface -Each receptor neuron has 1020 cilia -Axons of olfactory receptor neurons pierce cribriform plate of ethmoid bone and enter olfactory bulbs -Olfactory neurons have halftime of few weeks. Mucus producing Glands -Olfactory mucous membrane is constantly covered by mucus -Mucus is produced by Bowman’s glands, placed just under the basal lamina of the menbrane Physiology of Smell Olfactory receptors respond to several different odor-causing chemicals When bound to ligand these proteins initiate a G protein mechanism, which uses cAMP as a second messenger cAMP opens Na+ and Ca2+ channels, causing depolarization of the receptor membrane triggers an action potential Odorant binding protein Odorant chemical Na+ Olfactory Transduction Process Inactive Active Na+ influx causes depolarization ATP Adenylate cyclase cAMP Cytoplasm Depolarization of olfactory receptor cell membrane triggers action potentials in axon of receptor Figure 15.4 Olfactory Bulbs -Axons of receptors contact the primary dendrites of mitral cells and tufted cells. -Forming complex globular synapses called olfactory glomeruli. -Periglomerular cells are inhibitory neurons connecting one glomerulus to another Transmission of smell signals to CNS Olfactory thresholds and discrimination Olfactory receptors respond only to substances in contact with olfactory epithelium and need to be dissolved in mucus Methyl marcaptan one of the substances in garlic can be smelled at very low concentration showing the remarkable sensitivity of olfactory receptors Humans can recognize more than 10.000 different odors However determination of intensity of odor is poor Substance mg/L of Air Ethyl ether 5.83 Chloroform 3.30 Pyridine 0.03 Oil of peppermint 0.02 Iodoform 0.02 Butyric acid 0.009 Propyl mercaptan 0.006 Artificial musk 0.00004 Methyl mercaptan 0.0000004 Role of Pain Fibers in the Nose Many trigeminal pain fibers are found in olfactory membrane They are stimulated by irritating substances Are responsible for initiating sneezing, lacrimation and other reflex responses. Abnormalities Anosmia – absence of sense of smell Hyposmia – diminished olfactory sensitivity Dysosmia – distorted sense of smell More than 75% of humans over the age of 80 have an impaired ability to identify smells Taste Buds Most of the 10,000 or so taste buds are found on the tongue Taste buds are found in papillae of the tongue mucosa Papillae come in three types: filiform, fungiform, and circumvallate Fungiform and circumvallate papillae contain taste buds Taste Buds Figure 15.1 Anatomy of a Taste Bud Each taste bud consists of three major cell types Supporting cells – insulate the receptor Basal cells – dynamic stem cells Gustatory cells – taste cells Taste Sensations There are five basic taste sensations Sweet – sugars, saccharin, alcohol, and some amino acids Salt – metal ions Sour – hydrogen ions Bitter – alkaloids such as quinine and nicotine Umami – elicited by the amino acid glutamate Physiology of Taste In order to be tasted, a chemical: Must be dissolved in saliva Must contact gustatory hairs Binding of the food chemical: Depolarizes the taste cell membrane, releasing neurotransmitter Initiates a generator potential that elicits an action potential Taste Transduction The stimulus energy of taste is converted into a nerve impulse by: Na+ influx in salty tastes H+ in sour tastes (by directly entering the cell, by opening cation channels, or by blockade of K+ channels) Gustducin in sweet (cAMP) and bitter (Ca) tastes Gustatory Pathway Cranial Nerves VII , IX and X carry impulses from taste buds to the solitary nucleus of the medulla These impulses then travel to the thalamus, and from there fibers branch to the: Gustatory cortex (taste) Hypothalamus and limbic system (appreciation of taste) Gustatory Pathway Figure 15.2 Influence of Other Sensations on Taste Taste is 80% smell Thermoreceptors, mechanoreceptors, nociceptors also influence tastes Temperature and texture enhance or detract from taste Abnormalities Ageusia: Absence of taste sensation Dysgeusia: Disturbed taste sensation Hypogeusia: Diminished taste sensation Gustatory hallucination: false sensation of certain substance.
