Taste and Smell Prof. K. Sivapalan

Introduction. Taste and Smell are classified as visceral senses because of relationship to gastro-intestinal function. Sensations are inter-related: without smell taste becomes different. They help to identify foods, proximity of other animals and individuals among animals. They are tied with primitive emotional and behavioral functions of the nervous system Olfactory portions of the brain are among the first structures evolved in the brain. These structures have later got involved in emotions and behavior through limbic system. 1/1/2019 Taste and Smell

Taste. Taste receptor cells are found in the taste buds. They are ovoid bodies found in epiglottis, palate, and pharynx and in the walls of fungiform and vallate papillae of the tongue. About10,000 buds found in one person. 1/1/2019 Taste and Smell

Receptors. Receptor has a microvillus that projects into taste pore. Each taste bud receives about 50 nerve fibers and each fiber is connected to five receptors. Half life of receptor is 10 days. New receptors develop from epithelial cells. De-nervation results in atrophy of taste bud Regeneration of the nerve results in development of new taste bud. 1/1/2019 Taste and Smell

Afferents. Corda tympani- anterior two thirds of the tongue Glossopharyngeal- posterior third of the tongue. Rest by vagus. 1/1/2019 Taste and Smell

Central Connection. Gustatory portion of the nucleus tractus solitarius in Medulla. Taste signals transmitted to salivary nuclei mediate taste reflexes Second order neurons through medial leminiscus to thalamus [no crossing]. Third order neuron to face area in primary sensory cortex and insular cortex [ipsilateral]. 1/1/2019 Taste and Smell

Taste Modalities Sweet Sour Bitter Salt Umami ? Hot 1/1/2019 Taste and Smell

Sweet This sensation is given by a wide variety of substances. They include sugars, glycol, alcohols, aldehydes, ketons, amides, esters, amino acids, small proteins, sulfonic acids, halogenated acids, and inorganic salts of led and beryllium. Slight alteration in structure can change sweet to bitter. Receptor molecules in the membranes and their specificity are poorly understood. 1/1/2019 Taste and Smell

Bitter. This sensation is produced by a variety of unrelated compounds. Long chain organic substances containing nitrogen and alkaloids [quinine, caffeine, strychnine, nicotine]. Many are poisons. Bitter is a warning to avoid poisons. There may be as many as 24 receptor molecules in taste buds. 1/1/2019 Taste and Smell

Other Taste Modalities Sour- triggered by protons. Salt- by Sodium [cations]. Umami [Japanese word meaning delicious]. Monosodium glutamate is thought to be responsible. Pleasant sensation but different from sweet. Found in meat extract, aging cheese. Hot- Pain sensation transmitted by somatosensory nerves, not in taste buds. 1/1/2019 Taste and Smell

Flavor. Almost infinite variety of tastes are synthesized from the basic modalities. Pain, texture and temperature add to flavor. Smell is plays an important role in the overall sensation produced by food. Miraculin- when applied to tongue, acid tastes sweet- taste modifier 1/1/2019 Taste and Smell

General facts about taste. All modalities are sensed from all parts of mouth. Adaptation- within one minute but mainly central mechanism. Taste preferences and needs of the body- Salt depleted animals select salty water in preference to pure water for drinking. Animals select sweetest food after insulin injection for hypoglycaemia. Calcium depleted animals select water with high calcium content. Effects of artificial flavors. 1/1/2019 Taste and Smell

Smell. Olfactory mucous membrane- yellowish, 2.5x2 cm2 in the roof of nasal cavity, near the septum. 10-20 million receptors may be found in nose. The receptor is a part of the neuron. Constantly replaced with half life of few weeks. The receptors are rapidly adapting. 1/1/2019 Taste and Smell

Receptors. Each neuron has a short, thick dendrite with expanded end – olfactory rod. Cilia [10-20 per cell] project from the rods to the surface of the mucus. Axons pierce the cribriform plate and enter the olfactory bulb. Axons from one type of odorant receptors end in one glomerulus in olfactory bulb. 1/1/2019 Taste and Smell

Sniffing. Area of olfactory epithelium is poorly ventilated. It protects from irritants. Air moves gently over the turbinates in quite breathing. Some air moves near the olfactory epithelium due to eddy currents. Sniffing increases air reaching the epithelium by directing the air stream up ward. It is a semi-reflex response to a new odor. 1/1/2019 Taste and Smell

Central Connection. Olfactory bulbs are in the under surface of the frontal lobe. Axons of the receptors synapse with dendrites of mitral cells and tufted cells in olfactory glomeruli. Axons of both cells go to olfactory cortex. 1/1/2019 Taste and Smell

Olfactory Cortex. Axons reach olfactory cortex in intermediate and lateral olfactory stria. 1/1/2019 Taste and Smell

Cortical Mechanisms. Medial olfactory area is very old olfactory system that subserves basic olfactory reflexes – licking, salivation. Lateral olfactory area comprises less old and new olfactory system. Less old system subserves automatic but partially learned control of food intake and aversion to toxic and unhealthy foods through connections with lymbic system. Newer system passes through thalamus and is used for conscious perception and analysis of olfaction. 1/1/2019 Taste and Smell

Receptor Mechanism. Odor-producing substances – 3-20 carbon atoms. Different configuration of molecule- different odor. Water and lipid solubility are characteristic of strong odorants. 1/1/2019 Taste and Smell

Odorants. Olfactory threshold varies with substances.. Humans can recognize more than 10,000 different odors. Determination of intensity is poor. 1/1/2019 Taste and Smell

Pheromones These are sensed by vomeronasal organ in mammals and a specialized area in a pit in the anterior third of the nasal septum in humans. Pheromones are related to sexual function. Olfaction is more acute in females and most during ovulation. Smell can trigger long-term memories. 1/1/2019 Taste and Smell

Irritants. Pain fibers in the nasal cavity are stimulated by irritants. They cause sneezing, lacrimation, respiratory inhibition 1/1/2019 Taste and Smell