Olfactory and gustatory receptors

Chemoreceptors
Chemoreceptors or organs of chemical sense consist of olfactory organs and organs of taste. Both these organs are stimulated only by chemical substances or odours in air (nostrils) and in solution (tongue).
The medium for dissolving substances for taste is water for aquatic animals and mucus for land animals.
The olfactory organs can respond to a low concentration of the dissolved substance, whereas organs of taste need a higher concentration of the dissolved substance for a response.
Olfactory Organs in Vertebrates:
Odours bind to and activate olfactory receptors located on the dendrites of sensory neurons in the nose. Olfactory organs (olfactory-receptors) are a pair of invaginations of the ectodermal cells of the skin forming olfactory sacs on the anterior end of head.
Their external openings are called nostrils or nares.
In most fishes the olfactory organs consist of a pair of pits lined with folds or ridges of sensory epithelium.
The cyclostomes have a single median olfactory organ. This is a blind pit in the lampreys, but in hagfishes it opens into the pharynx.
Dipnoans resemble higher vertebrates in possessing paired nasal passages that open by means of choanae into pharynx. The nasal passages, therefore, have both internal and external openings. The olfactory epithelium within canals appears in the form of folds.
Sensory systems consist of peripheral receptor cells and integrating neurons in the brain.
Impulses are transmitted from receptors by sensory fibres to the central nervous system where they are interpreted as sensations or messages, which are sent to effector organs through efferent or motor nerve fibres, for responding in an appropriate manner.
A vertebrate has receptors or sense organs for touch, smell, taste, sight, and hearing, which are stimulated by the environment. These sense organs are termed external receptors or exteroceptors.
There are other sense organs found in the body, which detect temperature, pain, hunger, thirst, fatigue, and muscle position. They are spoken of as internal receptors or interoceptors.
Besides these two, third is proprioceptors, which are stretch receptors found in the muscles, joints, tendons, connective tissue and skeletons. All receptors are closely associated with the nervous system and respond to external or internal stimuli.
List of Common Senses:
1. Touch.- It includes contact, pressure, heat and cold, etc.
2. Taste. -Receive stimulus by chemicals in solution.
3. Smell.- Receive volatile chemicals and gases in air.
4. Hearing.- Receive sound vibrations.
5. Sight. -Receive light waves.

1. Chemoreceptors (Taste and Olfaction in Vertebrates) Dr. P.B.Reddy M.Sc,M.Phil,Ph.D, FIMRF,FICER,FSLSc,FISZS,FISQEM PG DEPARTMENT OF ZOOLOGY GOVERTNAMENT PG COLLEGE, RATLAM.M.P [email protected]

2. TYPES OF RECEPTORS Detecting a taste ( gustation ) is fairly similar to detecting an odor (olfaction), given that both taste and smell rely on chemical receptors being stimulated by certain molecules. Receptors or Sense organs are present in the body to detect the environmental changes and internal changes. All animals have sense organs for touch, smell, taste ,sight and hearing. The senses of taste and smell are related because they use the same types of receptors and are stimulated by molecules in solutions or air. Key Points Humans can taste sweet, sour, bitter, salty, and umami; umami is the savoriness of certain foods that are commonly high in protein. Odors come from molecules in the air that stimulate receptors in the nose; if an organism does not have a receptor for that particular odor molecule, for that organism, the odor has no smell. The senses of smell and taste are directly related because they both use the same types of receptors. If one’s sense of smell is not functional, then the sense of taste will also not function because of the relationship of the receptors. Key Terms umami : one of the five basic tastes, the savory taste of foods such as seaweed, cured fish, aged cheeses and meats. olfactory : concerning the sense of smell receptor : a protein on a cell wall that binds with specific molecules so that they can be absorbed into the cell in order to control certain functions

3. Key points Sensory systems consist of peripheral receptor cells and integrating neurons in the brain. Impulses are transmitted from receptors by sensory fibres to the central nervous system where they are interpreted as sensations or messages, which are sent to effector organs through efferent or motor nerve fibres, for responding in an appropriate manner. A vertebrate has receptors or sense organs for touch, smell, taste, sight, and hearing, which are stimulated by the environment. These sense organs are termed external receptors or exteroceptors. There are other sense organs found in the body, which detect temperature, pain, hunger, thirst, fatigue, and muscle position. They are spoken of as internal receptors or interoceptors. Besides these two, third is proprioceptors, which are stretch receptors found in the muscles, joints, tendons, connective tissue and skeletons. All receptors are closely associated with the nervous system and respond to external or internal stimuli. List of Common Senses: 1. Touch.- It includes contact, pressure, heat and cold, etc. 2. Taste. -Receive stimulus by chemicals in solution. 3. Smell.- Receive volatile chemicals and gases in air. 4. Hearing.- Receive sound vibrations. 5. Sight. -Receive light waves.

4. Chemoreceptors Chemoreceptors or organs of chemical sense consist of olfactory organs and organs of taste. Both these organs are stimulated only by chemical substances or odours in air (nostrils) and in solution (tongue). The medium for dissolving substances for taste is water for aquatic animals and mucus for land animals. The olfactory organs can respond to a low concentration of the dissolved substance, whereas organs of taste need a higher concentration of the dissolved substance for a response. Olfactory Organs in Vertebrates: Odours bind to and activate olfactory receptors located on the dendrites of sensory neurons in the nose . Olfactory organs (olfactory-receptors) are a pair of invaginations of the ectodermal cells of the skin forming olfactory sacs on the anterior end of head. Their external openings are called nostrils or nares. In most fishes the olfactory organs consist of a pair of pits lined with folds or ridges of sensory epithelium. The cyclostomes have a single median olfactory organ. This is a blind pit in the lampreys, but in hagfishes it opens into the pharynx. Dipnoans resemble higher vertebrates in possessing paired nasal passages that open by means of choanae into pharynx. The nasal passages, therefore, have both internal and external openings. The olfactory epithelium within canals appears in the form of folds.

5. The olfactory epithelium is located at the top of the nasal cavity. Unlike the receptors of the other special senses, the olfactory receptor membrane is found on the primary afferent neurons (i.e., receptor=primary afferent) . Olfactory receptor cells are the only neurons that are regularly replaced throughout life (via development of basal cells) . Axons of olfactory receptor neurons (ORNs) enter the CNS through the cribriform plate to synapse in the olfactory bulb. Odourants are dissolved in the mucus layer and are bound to olfactory binding proteins . Olfactory epithelium

6. Olfactory neurons are bipolar neurons (neurons with two processes from the cell body ). Each neuron has a single dendrite buried in the olfactory epithelium. E xtending from this dendrite are 5 to 20 receptor-laden, hair-like cilia that trap odorant molecules. The sensory receptors on the cilia are proteins. Each olfactory sensory neuron has only one type of receptor on its cilia. The receptors are specialized to detect specific odorants, so the bipolar neurons themselves are specialized. When an odorant binds with a receptor that recognizes it, the sensory neuron associated with the receptor is stimulated. Olfactory stimulation is the only sensory information that directly reaches the cerebral cortex, whereas other sensations are relayed through the thalamus .

7. The olfactory epithelium in amphibians is generally smooth and is restricted to the upper part of the nasal passages . Within each of the nasal passages, which are elongate in higher reptiles because of the development of a secondary palate, there is a shelf or concha which serves to increase the surface for the olfactory epithelium. In birds, the nasal passages have three shelves or conchae. In most birds there are external nostrils leading into the nasal passages, but in certain members of Pelecaniformes these are closed. Olfactory epithelium in most birds is restricted to the surface of the upper most or superior concha. This is correlated with the small size of olfactory lobes and poor sense of smell. But kiwi has relatively more developed sense organs. In mammals, the sense of smell is highly developed in many mammals. It is due to the development of the nasal conchae into elaborate scroll-like structures, which greatly increase the surface available for olfactory epithelium. In whales, the olfactory organs are essentially non-functional.

8. Organs of Jacobson or Vomero-nasal Organ: In many tetrapoda there is a pair of vomeronasal organs which are sac-like chambers lying below the nasal cavities but above the buccal cavity. They have a pigmented epithelial lining like that of the olfactory organs. Each opens by a short duct into the olfactory organ in amphibians, but in others the duct opens into the buccal cavity. The organ of Jacobson receives nerves from the nervus terminalis, a branch from the olfactory nerve, and a branch from the trigeminal nerve. The organ is believed to aid by smelling the recognition of food held in the mouth, and in lizards and snakes it appreciates the scent introduced into it by the tip of the tongue. The organ of Jacobson first appears in amphibians as evaginations from the nasal passage and is lined with olfactory epithelium. It is believed to be an aid in tasting food. They may also be important in reproductive behaviour in case of Ensatina ( lungless plethodontid salamander) since the first act of male in courtship to nose the females’ head and neck. It is best developed in Sphenodon, lizards and snakes and is connected with the roof of mouth rather the nasal canal. It is also well formed in monotremes, marsupials, insectivores, and rodents. But in turtles, crocodiles, birds, and many mammals such as Primates and Cetacea, it is found only in the embryo and is absent in the adult.

9. Comparative anatomy In the lancelet there is a ciliated pit (which later divides into two) above the anterior end of the central nervous system, which is probably a rudiment of an unpaired olfactory organ. In fishes there are also two lateral pits, the nostrils of which open sometimes, as in the sharks and rays, onto the ventral surface of the snout and sometimes, as in the higher fishes, onto the dorsal surface. Up to this stage, the olfactory organs are mere pits, but in mudfish an opening is established from them into the front of the roof of the mouth, and so they serve as respiratory passages and organs for the sense of smell. In the higher amphibians the nasal organ becomes included in the skull, and respiratory and olfactory parts are distinguished. In this class, too, turbinal ingrowths are found, and the nasolachrymal duct appears. In lizards the olfactory and respiratory parts are very distinct, the latter being lined only by stratified epithelium unconnected with the olfactory nerves. There is one true turbinal bone growing from the outer wall, and close to this is a large nasal gland. In crocodiles the hard palate is formed, and there is henceforward a considerable distance between the openings of the external and internal nares. In crocodiles, also, air sinuses are first found extending from the olfactory cavities into the skull bones. The birds’ arrangement is very like that of the reptiles; olfactory and respiratory chambers are present, and into the latter projects the true turbinal, though there is a pseudoturbinal in the upper or olfactory chamber. In mammals the olfactory chamber of the nose is variously developed: most of them are macrosmatic and have a large area of olfactory mucous membrane; some, like seals, baleen whales, monkeys, and humans, are microsmatic, while the toothed whales have the olfactory region practically suppressed in the adult and are said to be anosmatic. There are generally five turbinal bones in macrosmatic mammals, although humans have a reduced number.

10. Gustatory or Organs of Taste in Vertebrates: Organs of taste (gustatoreceptors) consist of taste buds which perceive the sense of taste from the dissolved substances. Each taste bud consists of cluster of neurosensory cells and supporting cells arranged to form a barrel-like structure embedded in stratified epithelium . Each neurosensory cell is long and narrow with a thin taste hair at its free tip and a sensory nerve fibre at its base. Taste hairs project into a depression or taste pore in mammals, in others they project above the surface, there being no taste pore. In man, there are four fundamental sensations of taste- sweet, salty, bitter, and sour. Taste buds are widely distributed in fishes of the mouth, pharynx and outer surface of the head. In some fishes they occur on the entire body surface including even the fins. The taste buds are innervated by the V, VII, IX and X cranial nerves. In amphibians, the taste buds are restricted to the roof of the mouth, the tongue and mucosa that lines the jaws. In most reptiles, the taste buds are restricted largely to the pharyngeal region. Taste buds are lacking on the tongue of most birds, although they are found on the lining of the mouth and pharynx. In mammals, there are various kinds of papillae on the tongue which possess taste buds except filiform papillae. Some taste buds of vertebrates are not for tasting but for testing substances in the pharynx to cause reflexes which prevent solid particles from entering air passages.

11. The senses of taste In terrestrial vertebrates , including humans, taste receptors are confined to the oral cavity . They are most abundant on the tongue but also occur on the palate and epiglottis and in the upper part of the esophagus . The taste receptor cells, with which incoming chemicals interact to produce electrical signals, occur in groups of 50–150. Each of these groups forms a taste bud . On the tongue , taste buds are grouped together into taste papillae. On average, the human tongue has 2,000–8,000 taste buds, implying that there are hundreds of thousands of receptor cells. However, the number of taste buds varies widely; some humans have only 500, whereas others have as many as 20,000. Healthy humans may have anywhere from three to several thousand taste buds per square centimetre on the tip of the tongue, and this variability contributes to differences in the taste sensations experienced by different people. The taste buds are embedded in the epithelium of the tongue and make contact with the outside environment through a taste pore. Slender processes ( microvilli ) extend from the outer ends of the receptor cells through the taste pore, where the processes are covered by the mucus that lines the oral cavity. At their inner ends the taste receptor cells synapse , or connect, with afferent sensory neurons , nerve cells that conduct information to the brain . Each receptor cell synapses with several afferent sensory neurons, and each afferent neuron branches to several taste papillae, where each branch makes contact with many receptor cells. Unlike the olfactory system , in which input to the brain involves a single nerve, the afferent sensory neurons occur in three different nerves running to the brain—the facial nerve , the glossopharyngeal nerve, and the vagus nerve . Taste receptor cells of vertebrates are continually renewed throughout the life of the organism.

12. The taste receptor system of terrestrial vertebrates is concerned with the detection of chemicals that are taken into the oral cavity and are present at relatively high concentrations. In humans, five different classes, or modalities , of taste are usually recognized: sweet , salt , sour , bitter , and umami . In general, animals are unable to taste proteins, but they do taste amino acids (from which proteins are made). Some of the amino acids taste sweet to humans, whereas others taste sour, and umami taste, which is meat like, is a response to glutamic acid and its derivatives, such as monosodium glutamate (MSG). Sweet taste comes mainly from sugars (carbohydrates), and bitter taste derives from potentially harmful chemicals present in food. The constituents of inorganic salts , such as sodium chloride, potassium chloride, and calcium chloride, are essential nutrients, but the quantities required to fulfill animal nutrient requirements are relatively small. It is possible that the salt taste reflects an animal’s need to avoid ingesting too much salt, which would increase the osmotic pressure in body tissues, producing adverse effects on cell metabolism . Minor essential nutrients, such as sterols and vitamins , are not known to be tasted by animals. Except for bitter-tasting substances, the chemicals that stimulate taste receptors are generally water soluble. There is evidence that all taste buds exhibit sensitivity to all taste sensations. However, in humans and some other mammals , there are certain taste papillae with receptor cells highly sensitive to sweet taste, as well as receptors preferentially tasting salt and receptors preferentially tasting bitter substances. The taste receptor cells of other animals can often be characterized in similar ways to those of humans, because all animals have the same basic needs in selecting food. In addition, some organisms have other types of receptors that permit them to distinguish between classes of chemicals not directly related to diet and that enable them to make further distinctions within the modalities.

13. Tastes and Odors Both taste and odor stimuli are molecules taken in from the environment. The primary tastes detected by humans are sweet, sour, bitter, salty, and umami. The identification of umami as a fundamental taste occurred fairly recently. It was identified in 1908 by Japanese scientist Kikunae Ikeda while he worked with seaweed broth, but it was not widely accepted as a taste that could be physiologically distinguished until many years later. The taste of umami, also known as savoriness (pleasant), is attributable to the taste of the amino acid L-glutamate. In fact, monosodium glutamate, or MSG, is often used in cooking to enhance the savory taste of certain foods. The adaptive value of being able to distinguish umami is that savory substances tend to be high in protein . This chemoreception in regards to taste, occurs via the presence of specialized taste receptors within the mouth that are referred to as taste cells and are bundled together to form taste buds. These taste buds, located in papillae which are found across the tongue, are specific for the five modalities: salt, sweet, sour, bitter and umami. These receptors are activated when their specific stimulus (i.e. sweet or salt molecules) is present and signals to the brain. In addition to the activation of the taste receptors, there are similar receptors within the nose that coordinates with activation of the taste receptors. When you eat something, you can tell the difference between sweet and bitter. It is the sense of smell that is used to distinguish the difference. Although humans commonly distinguish taste as one sense and smell as another, they work together to create the perception of flavor. A person’s perception of flavor is reduced if he or she has congested nasal passages.

14. Uniform distribution of taste receptors (the myth of the tongue map) : Humans detect taste using receptors called taste buds. Each of these receptors is specially adapted to determine one type of taste sensation. Recent evidence suggests that taste receptors are uniformly distributed across the tongue; thus, this traditional tongue map is no longer valid.

15. Taste The taste receptors are located around the small structures known as papillae found on the upper surface of the tongue, soft palate, upper esophagus, the cheek, and epiglottis. These structures are involved in detecting the five elements of taste perception: salty, sour, bitter, sweet and umami . The primary organ of taste is the taste bud . A taste bud is a cluster of gustatory receptors (taste cells) that are located within the bumps on the tongue called papillae (singular: papilla). There are several structurally-distinct papillae. Filiform papillae, which are located across the tongue, are tactile, providing friction that helps the tongue move substances; they contain no taste cells. In contrast, fungiform papillae, which are located mainly on the anterior two-thirds of the tongue, each contain one to eight taste buds; they also have receptors for pressure and temperature. The large circumvallate papillae contain up to 100 taste buds and form a V near the posterior margin of the tongue.

16. In humans, there are five primary tastes; each taste has only one corresponding type of receptor. Thus, like olfaction, each receptor is specific to its stimulus ( tastant ). Transduction of the five tastes happens through different mechanisms that reflect the molecular composition of the tastant . A salty tastant (containing NaCl) provides the sodium ions (Na + ) that enter the taste neurons, exciting them directly. Sour tastants are acids which belong to the thermoreceptor protein family. Binding of an acid or other sour-tasting molecule triggers a change in the ion channel which increases hydrogen ion (H + ) concentrations in the taste neurons; thus, depolarizing them. Sweet, bitter, and umami tastants require a G-protein-coupled receptor. These tastants bind to their respective receptors, thereby exciting the specialized neurons associated with them. Both tasting abilities and sense of smell change with age. In humans, the senses decline dramatically by age 50 and continue to decline. A child may find a food to be too spicy, whereas an elderly person may find the same food to be bland and unappetizing .