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Jacobson's Organ


Charlottesville

July 5, 2022


Jacobson’s organ, also called vomeronasal organ, an organ of chemoreception that is part of the olfactory system of amphibians, reptiles, and mammals, although it does not occur in all tetrapod (four-legged vertebrates) groups. It is a patch of sensory cells within the main nasal chamber that detects heavy moisture-borne odour particles. Airborne odours, in contrast, are detected by the olfactory sensory cells located in the main nasal chambers. Some groups of mammals also initiate a behaviour known as the flehmen response, in which the animal facilitates the exposure of the vomeronasal organ to a scent or pheromone by opening the mouth and curling the upper lip during inhalation.

This organ was named for its discoverer, Danish anatomist Ludvig Levin Jacobson, in 1811. It is a paired structure; in the embryo stages of all tetrapods, each half arises as an evagination of the floor of a nasal sac. In fully developed crocodilians, turtles, birds, cetaceans, and many advanced primates, this structure is absent or substantially underdeveloped. For most tetrapods that possess a Jacobson’s organ, ducts connect the organ directly to the nasal cavity; however, in squamates (lizards and snakes), each organ opens on the roof of the buccal cavity (mouth). The tongue carries odour particles from the outside into the mouth. It is possible that some particles attached to the top of the tongue may enter the vomeronasal openings on the roof of the mouth. It is also possible that particles attached to several parts of the tongue mix with fluids already present in the mouth before some of this particle-laden fluid is pushed into the vomeronasal openings by hydraulic pressure caused by the tongue’s pistonlike movements. After these particles reach the organ, some of the chemical compounds they contain bind to receptor molecules, and sensory messages are sent to the brain.


The Jacobson’s organ is useful in the process of communicating chemical messages, such as readiness for sexual activity, between members of the same species. The organ helps snakes hunt and track their prey. Much evidence suggests that this organ may also be involved in the detection of chemical signals related to aggression and territoriality. See also chemoreception.


Chemoreception, the process by which organisms respond to chemical stimuli in their environments that depends primarily on the senses of taste and smell. Chemoreception relies on chemicals that act as signals to regulate cell function, without the chemical necessarily being taken into the cell for metabolic purposes. While many chemicals, such as hormones and neurotransmitters, occur within organisms and serve to regulate specific physiological activities, chemicals in the external environment are also perceived by and elicit responses from whole organisms. All animals and microorganisms such as bacteria exhibit this latter type of chemoreception, but the two commonly recognized chemosensory systems are the senses of taste, or gustation, and smell, or olfaction. The senses of taste and smell 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. Circumvallate papillae, located on the surface of the back part of the tongue, contain taste buds (indicated by asterisks). Specialized hairlike structures (microvilli) located at the surface of taste buds in minute openings called taste pores (indicated by arrows) detect dissolved chemicals ingested in food, leading to the activation of receptor cells in the taste buds and the sensation of taste. Uniformed Services University of the Health Sciences (USUHS) 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. 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. But this is an anthropocentric view of a system that has evolved to give animals information about the nutrient content and the potential dangers of the foods they eat. The major nutrient requirements of all animals are carbohydrates, which act principally as a source of energy. Many lipids can be synthesized from carbohydrates, and animals use proteins derived from carbohydrates to assemble their own body proteins. 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 meatlike, 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, especially plants, which produce thousands of chemicals that offer the plants some protection from herbivores. 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. Animals experiencing a salt deficit actively seek out and eat sodium chloride, but the sensory basis for this salt appetite is not understood. Minor essential nutrients, such as sterols and vitamins, are not known to be tasted by animals. They are probably of such widespread occurrence that an animal’s normal food contains sufficient quantities, which is true for inorganic salts. However, associative learning may also have an important role in ensuring that appropriate levels of these compounds are obtained (see below Behaviour and chemoreception: Associative learning). Except for bitter-tasting substances, the chemicals that stimulate taste receptors are generally water soluble. Humans do not make further distinctions within the five modalities. For example, different sugars may have different degrees of sweetness, but they do not have distinct tastes. Similarly, bitter-tasting substances, such as quinine or caffeine, taste bitter but do not induce separate tastes, despite great differences in their molecular structures. However, the umami receptor does give the ability to distinguish between naturally occurring amino acids and is sensitive to MSG. Natural foods contain many different chemicals; for example, the taste of an apple may stimulate all the different types of receptors to different degrees. 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. Smell

The olfactory system is concerned with the detection of airborne or waterborne (in aquatic animals) chemicals that may be present in very low concentrations. Olfactory receptor cells are present in very large numbers (millions), forming an olfactory epithelium within the nasal cavity. Each receptor cell has a single external process that extends to the surface of the epithelium and gives rise to a number of long, slender extensions called cilia. The cilia are covered by the mucus of the nasal cavity. Unlike taste receptor cells, olfactory receptor cells have axons that connect directly to the brain. Olfactory receptor cells are continually replaced, with new cells developing from basal cells in the olfactory epithelium. In humans the receptor cells are replaced about every 60 days. The relative size of the olfactory epithelium reflects the importance of olfaction in the lives of different animals. In some dogs the olfactory epithelium has an area of about 170 cm2, with a total of about 1 billion olfactory receptor cells; in oxen the olfactory epithelial area is only about 1–4 cm2, and the number of cells is less than 30 million. By comparison the human olfactory epithelium covers about 5–10 cm2 and has about 10–40 million olfactory receptor cells. Another major difference between the olfactory system and the taste system is that the axons of olfactory receptor cells extend directly into a highly organized olfactory bulb, where olfactory information is processed. Within the olfactory bulb are discrete spheres of nerve tissue called glomeruli. They are formed from the branching ends of axons of receptor cells and from the outer (dendritic) branches of interneurons, known in vertebrates as mitral cells, that pass information to other parts of the brain. Tufted cells, which are similar to but smaller than mitral cells, and periglomerular cells, another type of interneuron cell, also contribute to the formation of glomeruli. The axons of all the receptor cells that exhibit a response to a specific chemical or a range of chemicals with similar structures converge on a single glomerulus, where they connect via synapses with the interneurons. In this way, information from large numbers of receptor cells with similar properties is brought together. Thus, even if only a few receptors are stimulated because of very low concentrations of the stimulating chemical, the effects of signals from these cells are maximized. In mice there are about 1,800 glomeruli on each side of the brain, in rabbits there are about 2,000, and in dogs there are as many as 5,000. Since there are millions of olfactory receptor cells, the degree of convergence of axons, and therefore of information about a particular odour, is enormous. For example, in a rabbit, axons from about 25,000 receptor cells converge on each glomerulus. The olfactory system enables an animal to perceive chemicals originating outside itself that are important in the animal’s behaviour and ecology. These signals do not fall into such relatively clear categories as the taste receptor system, and most organisms have the ability to distinguish between hundreds or even thousands of odours, including some odours that have very similar chemical structures. An example of the human ability to discriminate between odours is the difference in smell between caraway seed and spearmint. Yet the chemicals producing these odours, the S- and R-forms of carvone, are stereoisomers (having the same three-dimensional chemical structure, but one being a mirror image of the other). This ability to distinguish between different compounds depends on the possession of olfactory receptor cells with specific, limited ranges of sensitivity. Many of the compounds that stimulate the olfactory system of terrestrial animals are not water soluble. In terrestrial vertebrates the olfactory epithelium is in the nasal cavity. Because air passes through this cavity to the lungs, the epithelium is continually bathed with a fresh supply of air as the animal breathes. The airflow can be enhanced so that the volume of air sampled is increased by sniffing, a technique commonly used by cats, dogs, and many other animals. When bird dogs are searching for a scent on the ground, they may sniff very rapidly, perhaps creating turbulence of the air in the nasal cavity and enhancing the likelihood that odour molecules will reach the olfactory epithelium. When these dogs run into the wind with their heads held high, attempting to pick up the scent of prey, a continuous flow of inhalant air is maintained through the nostrils and thus over the olfactory epithelium.

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