Introduction to Biological Psychology Part IV Chapter 11 Chemical Senses Taste and Smell

CHEMICAL SENSES 451 11 . CHEMICAL SENSES TASTE AND SMELL Paloma and Learning objectives By the end of this chapter , you will be able to identify the stimuli and sensory structures involved in taste and smell sensations understand the transduction mechanisms in place to transform chemical information into action potentials within each sense describe the neural pathways supporting gustatory and olfactory perception .

452 SENSES Sensing chemical compounds in the environment is the most archaic sensory mechanism in living organisms . Very early on in the history of life on Earth , unicellular organisms developed chemical detection to distinguish food from toxins , to mates and avoid danger . All the way to the present day , motivated and emotional behaviours in human and human animals are greatly influenced by detection of environmental chemical signals . In this chapter , we will revise the current knowledge of chemical senses in humans , paying special attention to the signals they can detect , how the transduction from chemical to neural code takes place , and what brain regions are involved in each case . Humans can detect chemical compounds in the environment via the olfactory ( smells ) and gustatory ( tastes ) systems . Flavours are also a product of chemical sensation , but they result from the combined perception of smells and tastes . Through these specialised senses , chemical information in the surrounding environment is captured by receptors , located mainly in the mouth and nasal cavity . Information regarding quality and quantity of chemicals is converted into the language of the nervous system , action potentials , through sensory transduction , and then transmitted to the central nervous system . In the brain , this information is integrated to produce olfactory and gustatory perception that will ultimately and behavioural selection and action .

SENSES 453 Even though somewhat less studied than senses such as vision and audition , the chemical senses can be organised in two systems gustation and olfaction . The gustatory sense , or sense of taste , picks up on soluble chemical compounds , present in the mouth . The olfactory sense , or sense of smell , reacts to airborne molecules that reach the nasal cavity . These sensory systems provide animals with key environmental information for producing adaptive behaviours . Smells serve as and signals , whereas tastes only act in the , after we ingest food or drinks . This information may be crucial for , selecting and consuming food , a potential mate or regulating social interactions with others . Even though the chemical senses are composed by systems , their coordinated action can produce even more complex sensory capacities such as detecting , which involves the activation of common sensory neurons within the cortex the part of the brain that first processes olfactory information ( Fu , Yoshimura , 2004 ) Sense of taste Anatomical overview Our sense of taste starts with molecules reaching our mouth through ingestion .

454 CHEMICAL SENSES The lumps that you see on your tongue are mistakenly called taste buds , but they are actually epithelia tissues called papillae . are or chemical substances , present in food or drinks , that create the sensation of taste when detected by taste receptor cells ( within the mouth . We are quite familiar with the many little raised bumps on our tongue epithelium that can be seen by looking at the tongue with a naked eye . These lumps are called papillae , and it is within the walls and of the papillae that we the taste buds that contain the taste receptor cells . There are thousands of taste buds in each papillae , which are divided into three categories , depending on their location the foliate papillae located on the sides of the posterior section of the tongue the circumvallate papillae located at the back of the tongue and the fungiform papillae located in the anterior part of the tongue ( Figure )

CHEMICAL SENSES 455 Fig . The tongue . Tam ma buds Baum Female call cell Each taste bud contains groups of between 50 and 150 taste receptor cells , and presents an upper aperture called the taste pore . project extensions , or , out of taste pores into the buccal cavity , where they encounter the . In humans , there are three main types of taste receptor cells , according to their function . Type I cells have primarily housekeeping functions . Type II cells are sensitive to sweet , bitter , and umami tastes . Type III cells appear to mediate sour taste perception . Detection of compounds by receptor cells leads to neurotransmitter release ( usually ) and generation of action potentials in neurons at the base of these receptor cells . The axons of these neurons form the afferent nerves that transmit the information to the brain via three different cranial nerves , IX and . Different papillae areas

456 I CHEMICAL SENSES are by different branches of the cranial nerves , IX , and . The anterior of the tongue , with the fungiform papillae , is supplied by branches of cranial nerve . The posterior third of the tongue is by branches of nerve IX , the glossopharyngeal nerve . The posterior regions of the oesophagus and the soft palate are by branches of cranial nerve Nerves , IX and project into the brainstem where they synapse with the rostral part of the nucleus of the solitary tract ( which relays information to the ventral posterior medial nucleus of the thalamus . The thalamus projects to the anterior insular cortex and to a region called the primary gustatory cortex or insular taste cortex . Neural signals from the insular taste cortex travel to the secondary gustatory cortex , within the medial and lateral orbitofrontal cortex , and project also to structures like the amygdala , hippocampus , striatum and hypothalamus , where this sensory information can affect different stages of and behavioural output ( Figure )

SENSES 457 Ventral nucleus of me Nucleus of Nucleus we solitary mu Fig . Taste information from the tongue travels through cranial nerves , IX , and to the nucleus of the solitary tract in the medulla . Neurons in the brainstem project to the ventral posterior medial nucleus of the thalamus and then on to the gustatory cortex .

458 CHEMICAL SENSES Insular and of frontal taste cortices Hypothalamus i Amygdala nerve nerve IX , Cranial nerve Fig . Neuronal circuitry involved in taste perception and processing Did you know ?

Taste buds have a life span of about two weeks , allowing them to grow back even when they are CHEMICAL SENSES 459 destroyed , for example when we burn our tongues . This makes them akin to skin cells , but they also share characteristics akin to neurons . For example , they have excitable membranes and release neurotransmitters . Sensory transduction How is the chemical information contained in the quality and quantity of transformed into neural signals that the brain can interpret ?

enter the papillae through the taste pore and induce different mechanisms in taste receptor cells . Each receptor cell has distinct mechanisms for the chemical information into neural activity . are divided into salty , sour , sweet , bitter , and , umami derived from the Japanese word meaning deliciousness . The umami taste is produced by monosodium glutamate , and probably other related amino acids .

460 SENSES selective Fig . Taste receptor cells As we have heard , components of salty chemicals are key for survival in several animals . ions contained in the saltiest of all salts , sodium chloride ( is key for maintaining muscle and neuronal functioning . A of Type II taste receptor cells are specialised for salt detection . These cells express receptors that detect and react to the presence of salty substances containing . Similar Type 11 receptors express channels that allow other free such as released by acid compounds into the cell ( Figure ) Receptor cells expressing ion channels for or allow these into the intracellular space , the membrane , leading to release of neurotransmitter , typically , and action potential in the neurons that make up the cranial nerves . Recent research has the ion channel responsible for detection in mice . Deletion of the gene that produces

SENSES I 461 the epithelial sodium channel ( in mice affected a of Type II taste receptor cells . Mice lacking showed complete loss of salt attraction and sodium taste responses compared to control animals ( et , 2010 ) This was the evidence that salt is detected by a protein expressed in a distinctive type of categories of molecules , those perceived as sweet , bitter , and umami , activate receptors ( Figure ) I or cou pled receptors small sweet molecules As we have heard in earlier chapters , are receptors associated on their cytoplasmic side with . They use a key to lock mechanism for the transduction of the chemical information into neural activity . When a particular molecule is recognised by a ,

462 I SENSES the associated is activated , into on and . These can activate further intracellular signalling cascades , leading to an increase in intracellular calcium concentration that ultimately results in the release of neurotransmitters , usually . In mammals , sweet and umami receptors are heteromeric named and , respectively . These receptors are a combination of proteins from families , or , and can detect sweet and umami taste compounds . Like the knockout mice , animals without fail to detect umami compounds , whereas animals lacking fail to detect sweet tastes ( Zhao et , 2003 ) Exercise Domestic cats , lions or tigers do not have the genes that codify for receptors . This means they can not taste sweet tastes and are unable to experience sweetness . How do you think this fact their strictly carnivore diet ?

SENSES I 463 distribution of taste receptors Historically , scientists had a rigid view of the topography or distribution of taste receptors , but this concept is slowly being abandoned . Nowadays , we recognise that taste zones across the surface of the tongue are not absolute , and that all zones can detect all tastes , albeit with different detection capacities . Taste sensitivity thresholds , rather than receptor distribution , vary across the surface of the tongue , with all areas showing higher or lower sensitivity to all . For instance , receptors with higher sensitivity For bitter tend to be distributed posteriorly in the tongue . Salty and sweet tastes are more easily detected in the tip of the tongue and are conveyed primarily by cranial nerve . Bitter sensations are mainly relayed by cranial nerve IX , which provides innervation to the posterior third of the tongue .

464 CHEMICAL SENSES Fig . Although all tastes can be perceived across the entire tongue , levels vary for each taste . The front ofthe tongue has the lowest threshold for sweet , salt , and umami tastes the side ofthe tongue has the lowest threshold for sour tastes , and the back of the tongue has the

SENSES i 465 lowest Coding of information in th , the gustatory system tastes . There is generally a proportional relationship between the concentration of the and the rate of order axons that enter the brain stem , so coding of taste intensity is based , at least in part , on frequency of action potentials . Coding of gustatory information is also based on the topographical distribution of the taste receptor cells sensitivity . This distribution provides the foundation for coding ( Squire et , 2012 ) meaning that information about the nature of the taste is provided by which cell has been activated . In other words , an axon that receives information from a sweet receptor is labeled as sweetness . Hence , whenever this axon an action potential and conveys that signal into the brainstem , the received input is interpreted as sweetness . This is similar to the principles of encoding we encounter in the somatic sensory system , where the identity of the activated neuron , rather than the rate , indicates the quality of the signal carried by it ( for example activation of a neuron the is perceived as coming from that area , and the type of neuron activated what sensation is perceived ) In the case of gustation , we might recognise activity of axons as signals of the presence of sour , bitter , salty , sweet , and umami . Other evidence , however , suggests that the

466 CHEMICAL SENSES pattern of activity across neurons that preferentially respond to different taste characteristics is used to code for tastes ( pattern or ensemble coding ) Within the central nervous system , identity is preserved in the relays from the nucleus of the solitary tract , into the ventral posterior medial complex of the thalamus , the gustatory cortex or insula ( Doty , 2015 ) For some time , it was assumed that the insula would represent taste categories in a map , but the empirical evidence has been elusive . Recent studies , using genetic tracing of taste receptor cells into the gustatory cortex , suggest that there are distinctive spatial patterns within the cortex , but no region is assigned to a single ( et , 2007 ) Finally , the information from reaches the orbitofrontal cortex , where it is integrated with sensory information from different , suggesting that this area integrates tastes with other information to create more complex perceptual ' Did you ?

There is some evidence of coding in all of the sensory systems .

SENSES I 467 The concept refers to the idea that the line , or the pathway , from peripheral receptor into the brain , is labelled based on the presence of particular receptors that accomplish the sensory transduction process . Sense of smell Anatomical overview In humans , olfaction , or the sense of smell , detects airborne molecules or that enter the nasal cavity . interact with olfactory sensory neurons ( located in the olfactory epithelium that covers the dorsal and medial aspect of the nasal passageway ( Figure )

468 I SENSES ( ram . um . man if cells on neurons Olfactory bulb Nasal Pain 00 inhaled an nerve Mucus Pam oi inhaled air molecules la ) Nasal ( Fig . Details of the olfaction system . a ) The olfactory system begins in the peripheral structures of the nasal cavity . The olfactory receptor neurons are within the olfactory epithelium . are in charge of the chemical information of , encoding information about quality and quantity of smells , into action potentials that can be interpreted by the brain . Olfactory receptor cells extend their axons through the ethmoid bone , also called the cribriform plate , These axons make synaptic contact with the mitral cells within structures known as within the olfactory bulb . Axons from the mitral cells bundle together and join the first cranial nerve , conveying olfactory information to various brain regions . Information about the presence and quantity of smells

SENSES I 469 leaves the olfactory bulb via the lateral olfactory tract . On the olfactory pathway , the lateral olfactory tract connects back to the inferior and posterior parts of the frontal lobe , near the junction of the frontal lobe and the temporal lobe , which constitutes the beginnings of the olfactory cortex ( Figure 451 ) bulb tract cortex ol temporal lobe ( Hypothalamus perception ol smell ) Hippocampus ( memory ) Amygdala ( emotional responses ) formation ( visceral responses to smell ) Unlike other primary sensory cortices , primary olfactory cortex comprises a number of different structures . These include subcortical structures such as the olfactory tubercle , in the ventral part of the striatum , and part of the amygdala as well as cortical regions in the medial part of the temporal lobe ( cortex ) and its junction with the frontal lobe

470 SENSES ( cortex ) The divisions of the olfactory cortex are interconnected , and even though there is most emphasis on the cortex , the entire extended network of these regions constitute the olfactory cortex . Furthermore , these divisions of the olfactory cortex also project to other brain areas , including the thalamus , hypothalamus , hippocampus and , especially importantly , the orbital and frontal parts of the prefrontal cortex . Unlike other sensory systems , in olfaction , there is not a thalamic relay between the peripheral sensory structures , the olfactory bulb , and the cortex ( 2019 ) In the olfactory system the connection with the thalamus is downstream from the cerebral cortex . Sensory transduction and odour representation Several types of cells are present at the olfactory epithelium . Supporting cells provide metabolic and physical support for the epithelium , but smell detection and transduction relies on mature cells called olfactory sensory neurons ( The nasal cavity is a challenging environment for living cells due to changes in environmental conditions such as humidity and temperature , which result in a short lifespan of . Constant mitotic divisions and maturation of basal cells replenishes the pool of , maintaining their number . In addition to the sensory and supporting cells , the epithelium

CHEMICAL SENSES 471 is composed of glandular cells that produce and secrete the thick mucus that covers and protects its more exposed cellular structures ( Figure ) Odorant molecules that access the nasal cavity and diffuse through the mucus interact with olfactory cilia , extensions projecting from the end of the dendrite . Embedded in the membrane of the olfactory cilia are the receptor proteins that bind with the . Humans have around one thousand different odour receptor ( OR ) genes but can perceive more than a trillion different odours ( et , 2014 ) In a characteristic arrangement of one , each express only one type of OR gene , and all expressing the same OR protein project their axons to the same within the olfactory bulb ( Figure ) Hence , activation OR activation , reproducing a combinatorial code of activity unique to each odour . The current understanding of how odours are recognise at the neural level is explained by the theory , which proposes that each scent activates unique arrays of olfactory receptors in the epithelium . The molecular attributes of odours will determine how many OR can bind to them . Hence , one odour will activate a series of OR with more or less intensity , and this pattern of OR activation is what the brain recognises as a label for that particular odour molecule . Different odours will trigger different OR activation patterns , but familiar odours ( sharing some molecular properties

472 I SENSES like compounds belonging to the alcohol molecules family ) will trigger more similar patterns since they may be recognised by overlapping but slightly differing OR combinations . Note that scents are usually a combination of more than one odour molecule , and scent perception is associated with a yet more complex pattern of OR activation and representation . A graphical representation of this mechanism is presented in Figure . PATTERN OF PERIPHERAL ACTIVATION II I I IRE III Ell I I III HUI HIE III Fig . A representation of scent perception

SENSES 473 Odours are represented as geometrical shapes , and OR as a structure . Odours will more or less well within structures , with better being associated with higher activation . combinations of odours ( scents ) will produce distinctive OR activation patterns , which will be univocally by the olfactory sensory brain areas . When bind the OR on a given , a series of intracellular events take place , the chemical information into action potentials . like rhodopsin , glutamate receptors and some taste receptors , are . When odours bind to their OR , the associated is activated and the at and by dissociate , and a second messenger pathway is activated . In this case this second messenger pathway is the activation of and production of adenosine , cAMP ) from . This increase in intracellular cAMP levels opens cation selective channels , allowing calcium and sodium to enter the , it and making the action potentials ( if the signal is strong enough ) These action potentials are transmitted along the axons out of the nasal epithelium through the olfactory nerve ( cranial nerve I ) At the , make synaptic contact and activate mitral cells , which convey the information to the brain ( 1998 ) In contrast with other senses , the olfactory system lacks a topographic map of the sensory environment in the olfactory

474 SENSES cortex . Instead odours are associated with unique activation patterns of primary regions within the olfactory cortex , which correspond with associated activity patterns at the and levels . Expression of OR varies from individual to individual . In humans , only a third of all OR genes present in the genome are expressed into receptor proteins , but this number is highly variable between individuals . Olfactory experience depends on which OR genes are expressed , and how many copies of a receptor each individual has . Two people , expressing 358 and 388 different OR , respectively , will both be normal , but the sensory experience associated with a given odour molecule for each one of them may be different . For instance , in a recent study , examined the perception of coriander smell and taste by different volunteers . They found people are lovers and haters of coriander in roughly equal parts . While lovers are attracted by coriander fantastically savoury smell , haters smell soap . This difference is apparently linked to the ability to detect some of the compounds present in coriander , the unsaturated , that make haters smell something like soap . Lovers , on the contrary , are insensitive to the unsaturated , so do not detect a soap smell , leaving only the more pleasant characteristics of coriander to be detected by these individuals .

CHEMICAL SENSES 475 Key Takeaways Taste and smell are two senses specialised in detecting chemical compounds that reach the mouth or nose , respectively Taste sensory experience is the result of detection in a small number , mainly salty , sour , sweet , bitter , and umami Each sensory dimension is indexed by a specific type of taste receptor distributed along the tongue surface Smell detection is supported by a large number of odour receptor neurons , which are activated in a combinatorial fashion to give rise to activation patterns within the olfactory cortex Normal smell sensation is highly variable between individuals and depends on the quality and quantity of odour receptors expressed between subjects .

476 CHEMICAL SENSES References , A . 2007 ) Differential spatial representation of taste in the rat gustatory cortex . The journal of , 27 ( 2020 ) Trigeminal mechanisms of . In ( in headache management , rationale and clinical data ( Springer International Publishing . Hargreaves , Hu , 2008 ) Trigeminal mechanisms of Peripheral and brainstem organization . In ( The A comprehensive reference . Pain ( Academic Press . 2019 ) Chemical 5671565 , and Or , are you going to eat that ?

Cambridge University Press . 9781108644372 , Keller , A . 2014 ) Humans can discriminate more than trillion olfactory stimuli . Science , 343 ( 6177 ) Oka , SENSES I 477 , 2010 ) The cells and peripheral representation of sodium taste in mice . Nature 464 ( 7286 ) Doty , 2015 ) Handbook . John Wiley Sons . 9781118971758 Fu , Yoshimura , 2004 ) Convergence of olfactory and gustatory connections onto the nucleus in the rat . 126 ( Doty , 2009 ) The neurology of . Cambridge University Press . Hummel , Boyle , 2009 ) Central processing of trigeminal activation in humans . the New Academy of Sciences , 1170 ( Carr , 2021 ) Olfactory Inhibits nociceptive signal processing at the input stage of the central trigeminal system . 479 , Price , Daly , 2021 ) care 30606

478 I SENSES , 1998 ) Transduction mechanisms in vertebrate olfactory receptor cells . Review 78 ( Sell , 2014 ) Chemistry and the of smell . John Wiley 9781118522981 Squire , Berg , Bloom , Du Lac , 2012 ) Fundamental ( Academic Press . 2011 ) properties of the trigeminal system . Zhao , Zhang , Hoon , 2003 ) The mammalian and umami taste . About the Authors Paloma UNIVERSITY or Paloma is a Research Fellow in the School of Psychology at the University of .

SENSES I 479 UNIVERSITY OF obtained a in biology at the University of Buenos Aires , investigating the neurobiology of memory in crabs . He then moved to the University of Cambridge as a Newton International Fellow of The Royal Society and specialised in behavioural neuroscience , focusing on the effect of retrieval on memory persistence . recently became a lecturer in the School of Psychology at the University of , where he convenes a module on the Science of Memory , and lectures on sensory and motor systems , and motivated behaviour in several undergraduate and graduate modules .