Chemoreception taste and smell relationship

Chemoreception |

chemoreception taste and smell relationship

The chemoreceptors of taste and smell are closely related; in fact one The constituents of the AH,B relationship are not limited to hydrogen and oxygen. CHEMORECEPTION CONCEPT Chemoreception is a physiological process whereby We tend to associate taste and smell, and indeed there is some relation. In the gustatory and olfactory systems, as well as in the vomeronasal organ, information is mediated by chemoreceptors. In the somatosensory.

The contact chemoreceptor is specific to one type of chemical. In terrestrial vertebrates, olfaction occurs in the nose. Volatile chemical stimuli enter the nose and eventually reach the olfactory epithelium which houses the chemoreceptor cells known as olfactory sensory neurons often referred to as OSNs. Embedded in the olfactory epithelium are three types of cells: While all three types of cells are integral to normal function of the epithelium, only OSN serve as receptor cells, i.

chemoreception taste and smell relationship

For example, antennae on moths are made up of long feathery hairs that increase sensory surface area. Each long hair from the main antenna also has smaller sensilla that are used for volatile olfaction. In many terrestrial vertebrates, the tongue serves as the primary gustatory sensory organ. As a muscle located in the mouth, it acts to manipulate and discern the composition of food in the initial stages of digestion.

The tongue is rich in vasculature, allowing the chemoreceptors located on the top surface of the organ to transmit sensory information to the brain. Salivary glands in the mouth allow for molecules to reach chemoreceptors in an aqueous solution. The chemoreceptors of the tongue fall into two distinct superfamilies of G protein-coupled receptors.

GPCR's are intramembrane proteins than bind to an extracellular ligand- in this case chemicals from food- and begin a diverse array of signaling cascades that can result in an action potential registering as input in an organism's brain.

Large quantities of chemoreceptors with discrete ligand-binding domains provide for the five basic tastes: The salty and sour tastes work directly through the ion channels, the sweet and bitter taste work through G protein-coupled receptorsand the savory sensation is activated by glutamate.

Contact chemoreception is dependent on the physical contact of the receptor with the stimulus. The receptors are short hairs or cones that have a single pore at, or close to the tip of the projection. They are known as uniporous receptors.


Some receptors are flexible, while others are rigid and do not bend with contact. They are mostly found in the mouthparts, but can also occur on the antennae or legs of some insects. There is a collection of dendrites located near the pores of the receptors, yet the distribution of these dendrites changes depending on the organism being examined. The method of transduction of the signal from the dendrites differs depending on the organism and the chemical it is responding to.

Within the biological and medical disciplines, recent discoveries have noted that primary cilia in many types of cells within eukaryotes serve as cellular antennae. These cilia play important roles in chemosensation.

The current scientific understanding of primary cilia organelles views them as "sensory cellular antennae that coordinate a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation.

As all life processes are ultimately based on chemistry it is natural that detection and passing on of the external input will involve chemical events. The chemistry of the environment is, of course, relevant to survival, and detection of chemical input from the outside may well articulate directly with cell chemicals. For example, the emissions of a predator's food source, such as odors or pheromones, may be in the air or on a surface where the food source has been.

Cells in the head, usually the air passages or mouth, have chemical receptors on their surface that change when in contact with the emissions. It passes in either chemical or electrochemical form to the central processor, the brain or spinal cord. The resulting output from the CNS central nervous system makes body actions that will engage the food and enhance survival.

They also sense increases in CO2 partial pressure and decreases in arterial pHbut to a lesser degree than for O2. In this case, although the taste receptor system is completely unimpaired, access to the olfactory epithelium is blocked.

It is clear that the taste and smell systems are distinct in both their anatomy and their neural processing of inputs. The term flavour is an alternative to taste in the context of food, with flavour referring to the overall perception that results from both taste and smell. Use of this term avoids the confusion otherwise produced by using taste to refer specifically to the sensations produced by stimulation of taste receptors, as well as to the combined sensations of taste and smell.

Although the same arguments apply to other terrestrial vertebrates, there is little knowledge of the extent to which flavour, as opposed to taste, is important in other organisms. This entails processes that are initiated at the taste or smell receptor cells.

chemoreception taste and smell relationship

First, the molecule must be captured in and traverse a layer of mucus, in which the endings of the receptor cell are bathed; these are known as perireceptor events. Second, the molecule must interact with the surface of the receptor cell in a specific way to produce reactions within the cell. These reactions lead to a change in cellular electrical charge, which generates a nerve impulse. Transformation of an external stimulus into a cellular response is known as signal transduction.

The electrical signal produced by a particular nerve cell is the same regardless of the nature of the stimulus. If chemicals are to be distinguished from one another, they must stimulate separate cells. Thus, different cells are responsible for the reception of sweet, salt, sour, and bitter tastes and for distinguishing the different odours detected by the olfactory system. Perireceptor events Water-soluble compoundssuch as sugars and amino acidscan move freely in the mucus covering the taste and olfactory receptor cells.

However, most bitter-tasting and many volatile compounds are not water soluble and must be made soluble if they are to reach the receptors. This is achieved by binding them to soluble proteinswhich can move freely through the mucus. Such proteins have been isolated both from saliva and from the mucus in the nasal epitheliumalthough the precise role of soluble proteins in transporting chemicals to receptor cells has yet to be clearly demonstrated in mammals.

In insectstaste and olfactory neurons are contained within cuticular structures, but the sensitive nerve endings are bathed in a fluid called sensillar lymph that is analogous to the mucus of vertebrates. In the olfactory system this fluid contacts odour-binding receptors that presumably function in the same way as those of vertebrates but that are produced by different families of genes.

Three families of these receptor proteins have been identified.

Chemoreception - Interaction between taste and smell |

One family, consisting of pheromone -binding proteins, is restricted to receptors known to be sensitive to pheromones. The remaining two families contain general odorant receptors that respond to other odours not pheromones.

These proteins, to differing extents, govern which chemicals reach the membrane of the receptor cell and can be regarded as filters. Differences in their binding capacity could account for some of the differences in sensitivity of different receptor cells. It is important that taste and odour molecules be removed from the immediate environment of the receptor cell; otherwise the cell, and thus the animal, continues to respond to something that is no longer relevant.

Removal of the unwanted molecules is thought to be achieved, at least in part, by odorant-degrading enzymes that are also present in the mucus or other fluid surrounding the sensitive endings of the receptor cells. Signal transduction Information is conveyed along neurons by electrical signals called action potentials that are initiated by electrical changes in receptor cells. In the case of chemoreceptorsthese electrical changes are induced by chemicals.

The initial changes are called receptor potentialsand they are produced by the movement of positively charged ions e.

Chemoreceptor - Wikipedia

Thus, in order to stimulate a receptor cell, a chemical must cause particular ion channels to be opened. This is achieved in various ways, but it most commonly involves specific proteins called receptors that are embedded in the cell membrane.

Within the cell membrane, receptor proteins are oriented in such a way that one end projects outside the cell and the other end projects inside the cell.

This makes it possible for a chemical outside the cell, such as a molecule of an odorant or a tastant compoundto communicate with and produce changes in the cellular machinery without entering the cell.

The outer and inner ends of receptor proteins involved in taste and smell are connected by a chain of amino acids. Because the chain loops seven times through the thickness of the cell membrane, it is said to have seven transmembrane domains. The sequence of amino acids forming these proteins is critically important. A change in a single amino acid can change the form of the pocket, thus altering the chemicals that fit into the pocket. For example, one olfactory receptor protein in rats produces a greater response in the receptor cell when it interacts with an alcohol called octanol eight carbon atoms rather than with an alcohol known as heptanol seven carbon atoms.

Changing one amino acid from valine to isoleucine in the fifth transmembrane domain, which is thought to contribute to the shape of the pocket, alters the receptor protein in such a way that heptanol, instead of octanol, produces the greatest effect.

In mice the equivalent receptor is normally in this form, producing a greater response to heptanol than to octanol. This illustrates the importance of amino acid molecules in determining the specificity of receptor cells. When a receptor protein binds with an appropriate chemical known as a ligandthe protein undergoes a conformational change, which in turn leads to a sequence of chemical events within the cell involving molecules called second messengers.

Second-messenger signaling makes it possible for a single odour molecule, binding with a single receptor protein, to effect changes in the degree of opening of a large number of ion channels. In mammalsfive families of genes encoding chemoreceptor proteins have been identified.

Genes are considered to belong to the same family if they produce proteins in which high proportions of the amino acids are arranged in similar sequences. Two families of genes are associated with taste, one with smell, and two with the vomeronasal system see below Chemoreception in different organisms: There are about 1, genes in the olfactory gene family, the largest known family of genes.

Since each gene produces a different odour receptor protein, this contributes to the ability of animals to smell many different compounds.