PHYS#04 - Chemical Senses (Taste and Olfaction) - PDF

Pag. 1 a 11 PHYS#04 – 09/11/21​ THE CHEMICAL SENSES: OLFACTION AND TASTE 1.​ The chemical senses The chemosensory system is involved in detection of chemicals in the environment. It’s related to the chemosensory triad: -​ olfactory system - involved in odorant perception and representation...

Pag. 1 a 11 PHYS#04 – 09/11/21​ THE CHEMICAL SENSES: OLFACTION AND TASTE 1.​ The chemical senses The chemosensory system is involved in detection of chemicals in the environment. It’s related to the chemosensory triad: -​ olfactory system - involved in odorant perception and representation at the level of the brain -​ gustatory (taste) system - involved in tastants and taste perception -​ trigeminal chemosensory - involved in the perception of irritating molecules, part of the nociceptive system. These rely on receptors in the nasal, oral cavities and the face, mainly close to the eyes. The senses of taste and smell allow us to: -​ separate undesirable or even lethal foods from those that are pleasant to eat and nutritious -​ elicit physiological responses involved in the digestion and utilization of foods -​ the sense of smell allows animals to recognize the proximity of other animals or even individual animals -​ Both senses are strongly tied to primitive emotional and behavioral functions of our nervous systems. THE OLFACTORY SYSTEM The receptors are located in the olfactory epithelium or membrane (in the nasal cavity), they send information through the olfactory nerve to olfactory bulb (lies over the cribiform plate), where they terminate in multiple globular structures called glomeruli; each glomerulus also is the terminus for dendrites from about 25 large mitral cells and about 60 smaller tufted cells (the cell bodies of which lie in the olfactory bulb superior to the glomeruli): these dendrites receive synapses from the olfactory cell neurons and the mitral and tufted cells send axons through the olfactory tract (or CN I) to reach the target areas: -​ the piriform cortex, in the anteromedial part of the temporal lobe  Pag. 2 a 11 -​ the entorhinal cortex -​ the olfactory tubercle -​ the amygdala These structures all belong to the limbic system. Information is sent to different brain structures located at cortical and subcortical level: -​ the piriform cortex and amygdala integrate their information at the level of the orbitofrontal cortex, the thalamus and the hypothalamus -​ the entorhinal cortex sends its information to the hippocampus (structure fundamental for memory retention and retrieval) The olfactory tract enters the brain at the anterior junction between the mesencephalon and cerebrum; there, the tract divides into two pathways: one passing medially into the medial olfactory area of the brain stem and the other passing laterally into the lateral olfactory area. The medial olfactory area represents a very primitive olfactory system, whereas the lateral olfactory area is the input to a less old olfactory system and a newer system. The medial olfactory area consists of a group of nuclei located in the midbasal portions of the brain immediately anterior to the hypothalamus. Most conspicuous are the septal nuclei, midline nuclei that feed into the hypothalamus and other primitive portions of the brain’s limbic system. This is the brain area most concerned with basic behavior. The lateral olfactory area is divided in: a.​ Less old olfactory system Composed mainly of the prepyriform and pyriform cortex plus the cortical portion of the amygdaloid nuclei. From these areas, signal pathways pass into almost all portions of the limbic system, especially into less primitive portions such as the hippocampus, which seem to be most important for learning to like or dislike certain foods depending on one’s experiences with them. An important feature of the lateral olfactory area is that many signal pathways from this area also feed directly into an older part of the cerebral cortex called the paleocortex in the anteromedial portion of the temporal lobe (this area is the only area of the entire cerebral cortex where sensory signals pass directly to the cortex without passing first through the thalamus). b.​ Newer pathway A newer olfactory pathway that passes through the thalamus, passing to the dorsomedial thalamic nucleus and then to the lateroposterior quadrant of the orbitofrontale cortex, has been found. On the basis of studies in monkeys, this newer system probably helps in the conscious analysis of odor. These systems vary widely among mammals, the human olfactory epithelium is approximately 10 cm2, while in the cat the epithelium has an area of 20 cm2. The ratio between volume of their body and space dedicated to this epithelium reflects how important it is in animals for survival and biological functions.  Pag. 3 a 11 The olfactory system is unique among the sensory system: it does not include a thalamic relay from primary receptors to the neocortical region (the thalamus is only involved later in the pathway). This different anatomical-functional organization reflects how phylogenetically old this system is: the piriform cortex is, in fact, just 3 layers thick (unlike the neocortex that has 6 layers). Neurons in this structure respond to different odors and some processing occurs even with just 3 layers. The lateral olfactory tract sends information to the most superficial layer, unlike other sensory systems that send their information to the fourth layer mainly. The second layer is divided into 2a (more superficial) and 2b, while the third is the deepest: -​ in the 2a layer there are semilunar cells that send projections to the amygdala and entorhinal cortex -​ the 2b layer, getting info from the superficial pyramidal cells, sends projections to the olfactory bulb and info to the prefrontal and orbitofrontal cortex.  Pag. 4 a 11 1.​ Physiological responses to olfactory perception Odors can elicit specific physiological and behavioral responses: -​ bad odors increase our heart rate frequency to let us escape from danger: the amygdala has the central nucleus specifically able to influence the heat rate -​ good smell increase salivation, related to reflexive visceromotor responses triggered by odorants (phylogenetically very old). In addition to olfactory perception, odorants can elicit other physiological responses such as endocrine and reproductive functions e.g., -​ the menstrual function - explains why women that live together often have synchronized menstrual cycles -​ mother-child interaction - newborns cannot see well, since the visual system is not yet fully developed, so they perceive the mother through the olfactory system, which is much better developed; this drives their behavior, important for food intake and survival, because it allows them to recognize their caregiver. Another physiological response is the perception of the pheromones, molecules perceived by the vomeronasal organ, that sends info to specific nuclei in the olfactory bulb, and then to the hypothalamus, which regulate our internal state (influencing motivation and behavior, social and affiliative). This organ is present in 8% of human adults. The function of pheromone perception is unknown in humans, but it is probably a vestigial function.  Pag. 5 a 11 2.​ Systems associated to the structures of the olfactory system Different systems can be associated with the different functional clusters of the olfactory system: -​ anterior olfactory nucleus (AON) -​ olfactory tubercle (TUB) -​ pyriform frontal (PirF) -​ pyriform temporal (PirT) They suggest dissociable whole-brain networks formed by the sub-regions of primary olfactory cortex. Studies that analyze these structures use the RSFC (resting state functional connectivity) in the brain, where the bold signal is mediated by the amount of metabolism in different parts of the brain. These signals can oscillate at different frequencies: regions that show oscillations with high coherence are regions that belong to the same functional system. These experiments highlighted the functional networks present in the structures of the olfactory system. (Indirect measure of cortical activity, BOLD, means blood oxygenation dependent response. When a neuron fires, blood is needed in the surrounding area, to allow metabolism and neuron to discharge, neurovascular coupling allows to measure the neuronal activity based on the oxygen level in blood reaching this region.) a.​ Frontal Pyriform network (PirF) This network is involved in the motor programming of the mouth movements, connected to some areas like: -​ caudate and putamen basal ganglia -​ regions in the precentral gyrus that control mouth and jaw movements -​ left supramarginal gyrus -​ thalamus -​ anterior cingulate cortex -​ frontal operculum, in the gustatory region. It seems that these regions might trigger behavioral responses meaningful for specific odors, for example removing things from the mouth, and coordinating our behavior. b.​ Temporal Pyriform network (PirT) This network is connected to the brainstem, in particular with: -​ raphe magnus -​ posterior insula -​ anterior part of the temporal lobe. This region can trigger responses linked to object recognition based on specific odors; in addition, the connection with brainstem and posterior insula, can be important (in the context of olfaction) for respiratory modulation during breathing: this reflex is called protective fast respiratory reduction - if a noxious stimulus enters our nose, we can use respiration to blow it out as a reflex.  Pag. 6 a 11 c.​ Olfactory Tuberculous (TUB) This network is connected to the left fusiform gyrus (fusiform face area - the middle region of the temporal lobe), involved in triggering face and identity recognition based on odors. This is a brain region that is highly responsive to human faces and also responsive for motor behavior. d.​ Anterior olfactory nucleus (AON) This nucleus has connections with orbitofrontal cortex, considered the secondary gustatory area, which might be involved in the odor-object representation, and anterior insular cortex. 🡨 There are areas in common among all these structures: -​ orbitofrontal cortex -​ hippocampus -​ anterior cingulate cortex 3.​ Olfaction and social behaviour Other experiments have been done related to the olfactory system in social behavior. In the experiment on the left, people in FMRI have to perceive specific odors, while their brain activity is recorded. The same odor is then shown as visual info and perceived by others, (from the face only, the subject can associate the type of odor and the chemical information).  Pag. 7 a 11 At the brain level, when we perceive odors, we activate the structures mentioned above such as the amygdala or the left parahippocampal cortex disgusting (red) and pleasant (green), with a large degree of overlap (orange). Pleasant and disgusting odors also activate the anterior insula: -​ Disgusting odorants bilateral and anterior activity -​ Pleasant odorants posterior location of R insula No wide overlap. In some experiments the overlapping of these activities is studied, by seeing others perceive odorants. In a specific region of the anterior insula vision and olfaction of disgust significantly overlap, and mirror neurons specific for disgust are found in the region. The main finding of the present study is that the observation of disgust (blue) automatically activates neural substrates that are selectively activated during the feeling of disgust (red); this suggests that the understanding of the facial expressions of disgust as displayed by others involves the activation of neural substrates normally activated during the own experience of the same emotion. Understanding facial expressions of disgust can be used for example to encode others' experience without direct personal experience. THE TASTE SYSTEM The taste system acts in concert with the olfactory and trigeminal systems, all these should indicate whether a food should be ingested, and detect the taste as a general role. Taste is mainly a function of the taste buds in the mouth, but it is common experience that one’s sense of smell also contributes strongly to taste perception. In addition, the texture of food, as detected by tactual senses of the mouth, and the presence of substances in the food that stimulate pain endings, such as pepper, greatly alter the taste experience. The importance of taste lies in the fact that it allows a person to select food in accord with desires and often in accord with the body tissues’ metabolic need for specific substances.  Pag. 8 a 11 Receptors for this system are located in the face, larynx and tongue. Taste impulses from: -​ Anterior 2/3 of tongue pass first into the lingual nerve, through the chorda tympani into the facial nerve, and finally into the rostral part of the solitary tract nucleus in the brain stem -​ Posterior 1/3 of the tongue and from posterior regions of inner mouth and throat transmitted through the glossopharyngeal nerve also into the middle part of the solitary tract nucleus, but at a slightly more posterior level -​ Larynx through the laryngeal branch of the vagus nerve to the the ventral posterior (caudal) part of the solitary tract nucleus. All taste fibers synapse in the posterior brain stem in the nuclei of the tractus solitarius. These nuclei send second-order neurons to a small area of the ventral posterior medial nucleus of the thalamus (located slightly medial to the thalamic terminations of the facial regions of the dorsal column–medial lemniscal system). From the thalamus, third-order neurons are transmitted to the lower tip of the postcentral gyrus in the parietal cerebral cortex, where it curls deep into the sylvian fissure, and into the adjacent frontal opercular insular area (anteriorly) and in the insular cortex (posteriorly) – these 2 regions constitute the primary gustatory cortex. This area lies slightly lateral, ventral, and rostral to the area for tongue tactile signals in cerebral somatic area I. The vagus nerve is also important for our visceromotor activity, in fact, the caudal most portion of the solitary nucleus conveys information also from the visceral system and subdiaphragmatic branches of the vagus nerve, to control gastric motility. At this level there is a first integration between the gustatory and visceral systems.  Pag. 9 a 11 Taste organization at the level of the insular cortex and opercular region: -​ bitter taste perception activates the middle insula -​ sweet taste activates the posterior portion -​ salty, sour and umami tastes activate anterior portion. In a particular study on monkeys shown on left, the main gustatory region of the insular cortex is stimulated, to see the elicited behavior in nonhuman primates. Stimulation of the regions indicated with the red dots induced an ingestive behavior, the animals tended to bring the food to the mouth (eat): this shows how visceromotor and gustative information allows to produce a behavior in response, based on the sensory perception only. When the blue areas were stimulated, a vomiting and repulsive effect was produced. Also in this case, the sensory perception could trigger a visceromotor effect. The insular cortex is a very strange area activated by many stimuli, such as (in addition to the ones mentioned before) nociception and pleasant touch information or information from lamina 1 of the spinothalamic cortical pathway: it is indeed a polimodal area. In the monkey, this can be indicated as the primary gustatory region, but in humans this is not the only function  Pag. 10 a 11 (the animal’s insula is characterized by less gyri and is smaller, lacking the most anterior part). In the image below, a summary of FMRI studies on activation of the insular cortex by error awareness, “free won’t” movement of recognition, time perception, and other stimuli. Among these, also awareness of our heartbeat rate and self recognition seem connected to this area. Tumors that develop in the insula also bring problems in autonomic regulation resulting in: -​ hypertension -​ problems in coordination between breath and speech -​ … It seems also that neurons in this region, like in the amygdala, can mediate production of molecules related to stress responses. Animals are very different from humans in this type of response, since the stress lasts only the time of the identification of the danger, whereas in the human it takes longer to go back to the relaxed state, since this emotion representation is maintained and elicits production of catecholamines, linked with autonomous nervous system activation and production of psychosomatic impairments. The role of this structure is still a hot topic in neuroscience and is being currently studied. Many postmortem studies show a huge presence of Von Economo’s neurons in the insular cortex. These neurons are associated with consciousness and aren't present in all mammals. The orbitofrontal cortex The orbitofrontal cortex is considered the secondary gustatory cortex, a multimodal region, in which single neurons can receive a combination of visual, somatic sensory, olfactory and gustatory stimuli. When a given food is consumed to the point of satiety, specific orbitofrontal neurons diminish their activity: these neurons are involved in the motivation to eat (or not to eat) particular foods. The reciprocal projection between these stimuli, connect the nucleus of the solitary tract, via the pons to the hypothalamus and amygdala. They influence appetite, satiety and other homeostatic responses associated with eating so they are important to regulate the feeding behavior. Studies by Edmund Rolls investigated the gustatory cortex, by recording activity in this region, where there are: -​ unimodal neurons for o​ olfactory info o​ taste info o​ visual info -​ a class of neurons that integrate all these. Close to the anterior portion of the primary gustatory region, there are many unimodal gustatory neurons, and moving more medially, more integration with more visual and olfactory information can occur.  Pag. 11 a 11 Antonio Damasio has investigated this region in stroke patients and in the case of Phineas Gage, who, due to an accident, had a lesion at the level of his ventromedial prefrontal cortex and survived with psychopathic behavioral problems. Damasio reconstructed his lesion and associated his behavior to it. IOFC = orbitofrontal cortex. vmPFC = ventromedial prefrontal cortex The trigeminal chemoreception The trigeminal system is related to nociception: it gets information from the different branches of the trigeminal nerve (ophthalmic, mandibular and maxillary). As the gustatory system, information is sent to the VPM complex and then to nociceptive regions, in the primary somatosensory cortex and others implicated in translating nociception into its related psychological translation, which is pain (pain-related areas).