Mechanism of Transduction of Receptors of Special Senses - Essay Example

Your Prof Mechanism of transduction of receptors of special senses Introduction In simple terms senses are the physiological capacities of organisms that provide data for awareness of sensation. There are two types of senses - General senses and Special Senses. The human body consists of various kinds of stimuli. Some of these stimuli are received by sensory organs distributed in whole body. As they respond quickly to immediate contact they are shorter in range. They are known as General senses. The various general senses are pain, temperature, touch, proprioception, pressure, and vibration. Certain other stimuli like Hearing (Audition), Sight (Vision), Taste (Gustation), Smell (Olfaction), Balance (Equilibrium) are received by receptor organs like Cochlea and Membranous labrinyth (ear), Bulbus oculi (Eye), taste buds in mouth, and olfactory hair cells in nose. These senses are known as Special Senses. The specialized cells with nerve endings that respond to internal or external environment of body are called sensory receptors. The sensory information received by these receptors reaches the Central Nervous System and the impression produced by the brain using this information is called sensation Description of Senses Below are the descriptions of three of the special senses – smell, taste, and hearing. Olfaction: The Sense of Smell There are lots of molecules of the various materials present in the air we breathe. Smell depends on sensory receptors that react towards airborne chemicals. These sensory receptors are called chemoreceptors. These chemoreceptors are located in olfactory epithelium which is present in the upper interval of nasal chambers in the brain. Three different types of cells are present in the olfactory epithelium: Sensory neurons: Primary cilium is present with each of them. Supporting cells: They are present between sensory neurons. Basal cells: They divide in regular interval and produce new sensory neurons which replace the old or died sensory neurons. The olfactory epithelium contains 10-20 million olfactory receptors within 5 sq cm. This (olfaction) is the only sensation that reaches directly to cerebral cortex without synapsing in the thalamus Mechanism of Transduction of the Receptors for Olfaction Source: (J. W. Kimball) Sensory neurons contain primary cilium. These primary cilia are submerged in a mucous layer. Odorant molecules which one can smell in air get dissolved in mucous layer. In mucous layer they bind to the cilia. When these odorant molecules bind to cilia it activates G protein which is attached to olfactory receptors in cytoplasmic side. The olfactory receptors are distinguished as one of the largest groups of G protein linked receptors. In short these G-protein linked receptors help in the biosynthesis of neurotransmitters, cAMP and ionsitol tri phosphate which helps in further release of cations resulting into action potential and signaling. When we look in detail, the coupling of G protein to receptors leads to activation of adenyl cyclase. Now adenyl cyclase, an enzyme, catalyzes the conversion of ATP to cAMP , the second messenger in cystol. In fact, there is one more second messenger found in some vertebrate species, that is IP3 (Ionistol triphosphate). IP3 as well as cAMP are described to have the same function but chemically they are quiet different and they have two separate signal transduction pathways in olfactory neurons (Schild and Restrepo, 1998). The transduction pathway of cAMP is that, it opens up the ligand gated channels of sodium into the cell for facilitated diffusion of Na+. Due to the influx of Na+ there is a reduction of potential across plasma membrane. When this depolarization reaches threshold, it lead to generation of action potential. Action potential performs back along olfactory nerve to the brain. The brain judges this and olfactory signals reach it as particular odor. Gustation: The Sense of taste Taste is related to chemical sensing system. It is a complicated process and occurs when a molecule is released by substances stimulates special cells in mouth or throat. Then those sensory cells transmit messages from nerve cells to brain when a particular taste is identified. There are following five basic taste sensation:-Salty, Sweet, Sour, Bitter and Umami. Various properties of taste system There are around 50 to 100 taste cells in a single taste bud which responds to the five taste sensations. Every taste cell has receptors located on apical surface. They are known as transmembrane proteins. They allow ions to come which give salty or sour taste sensation. Some of them are attached to molecules which give sensation of bitter or sweet. These taste receptor cells are associated to sensory neurons through an ATP releasing synapse which leads back to brain. Salty and Sour Source: (Purves D) Salty: The mechanism for salt sensation is very simple. The receptor for table salt (Sodium chloride - Nacl) is an ion channel which allows sodium ions Na+ to directly enter into the taste cell. There occurs depolarization of cell membrane and opening of voltage dependent Ca++. Ca++ releases the ATP at synapse to join with sensory neurons which results into generation of action potential. Sour: Food which has sour sensation is generally acidic in nature with low pH. Acid (HCl), when dissolved in water releases H+ ions. Like Na+ it also passes through the ion channels. The distinction between salty and sour taste is made as H+ blocks a K+ channel, which results into depolarization of the cell due to a net movement + ions. Because of this depolarisation, Ca++ opens the channel and a result there is release of ATP. Sweetness: On the cell surface G protein coupled receptors are present. Sweet substance like sucrose or fructose generally binds to these GPCRs. Each receptor has two subunits T1R2 and T1R3, which are coupled with G protein. They are known as G protein because they bind to guanine nucleotides GDP and GTP which are linked with inner surface of plasma membrane and hormone’s transmembrane receptors which are known as G protein linked receptors (GPCRs). The name of complex of this G protein is gustducin because it has similar function as transducin that plays important part in rod vision. More details about the nature and the associated pathways of gustducin is still unknown as it has been recently discovered. But it plays an important role in transduction of all the taste sensation. As soon as gustducin activates, various interacellular reactions take place. First of all G protein activates adenyl cyclase. There is a formation of cylic AMP ( cAMP) , a second messenger which causes k+ channel to be blocked and causes depolarization of the cell. Ca++ channels open and there is release of ATP. Bitter Humans have special types of gene which encodes 25 different types of bitter receptors. Many of these genes are expressed by a taste cells which is responsive to bitter taste which makes it different from olfactory system as there only single odor cell expresses only single type of odor receptor cells. But still it is unexplained a single taste cells some types response to some bitter taste first then to others. As in sweetness, here also binding of quinine (bitter taste substance) takes place at G protein coupled receptors that are linked to gustducin. But here cAMP releases Ca++ ions from endoplasmic reticulum which helps to release neurotransmitter at synapse to sensory neurons. There are two systems for it. In system I, K+ channel is directly blocked by bitter tastants. Due to this depolarization of cell takes place and Ca++ channels get opened and the Ca++ moves inside. And increasing level of Ca++ causes release of neurotransmitter. In system II, bitter tastant gets attached to bitter receptors. Then G protein activates an enzyme phospholipase C which causes release of Ca++ from intracellular storage. Increase in level of Ca++ causes neurotransmitter release. As a result, action potential occurs in both the system after Neurotransmitter release in gustatory afferent axon. Source: (Purves D). Unami Unami taste sensation is the sensory response to putting salts containing glutamic acid (like monosodium glutamate) on one’s tongue. Monosodium glutamate is a non-essential amino acid which is used mainly to enhance the flavor of food. It is used in preparing many Asian cuisines and also found in processed cheese and meat. On G protein linked receptors binding of the glutamic acid and various other amino acids take place. They are attached to hetrodimer of T1R1 and T1R3 the protein subunits. Neural Pathway of Taste: Three different cranial nerves (VII, IX, and X) stimulates the taste buds with nerve impulse and carry taste information from epiglottis, palate, tongue, and esophagus. Hearing: The Auditory System (Ear) There are highly specialized set of structures for hearing mechanism. These set of structures are so arranged that it allows the sound to travel from environment to brain via them (structures of ear). These set of structures are following External: Ear and external auditory Middle ear: It is central to tympanic membrane which is consists of auditory ossicles Inner ear: consists of the sensory organs for hearing. The essential structure in auditory system is hair cells. In higher vertebrates the hair cells get immersed in internal fluid of inner ear. But still these hair cells are able to sense the movements in surrounding cells. Due to the specialization of ear structure it manages to respond to many forms of mechanical stimulation. The organ of Corti contains hair cells in cochlea of ear are highly responsive towards sound. The cristae ampullars in the semicircular ducts contains hair cells which respond to angular acceleration. Hair cells present in maculae of the saccule and utricle respond to gravity. Hair cells are surrounded by the fluid known as Endolymph is rich in potassium. Endolymph controls the ionic imbalance which gives an energy store, which pushes neural action potentials when hair cells are moved. A barrier is formed due to tight junctions between hair cells and supporting cells, between endolymph and perilymph which maintains ionic imbalance. The mechanical transduction of ear takes place at the tip of the cilia which emerges from the apical surface of hair cells. The length of them gradually increases along the consistent axis. There are small threads like connection between one cilium to tip of the potassium of taller neighboring cilium. The potassium channels are opened once a cilia bends towards the taller one. Due to the opening of potassium channel, there is a influx of potassium which further leads to opening of calcium channels and as a result initiation of receptor potential occurs. Such mechanism leads to change of mechanical energy into neural impulses. The action potential takes place in the dendrites of VIII cranial nerve. And then auditory signal is transmitted to brain via these auditory nerves. When the cilia bend in opposite direction and not towards the taller one, it leads to closing the channels and reduction in the afferent activity. Long time exposure of ear to loud sounds damages the hair of organ of Corti. The hearing loss may also occur due to damage in any portion of primary auditory cortex or damage in any portion of neural pathway. Conclusion The mechanism of transduction of the receptors for olfactory recptors, gustatory receptors, and auditory receptors have clarified that how the brain detects and how these senses work in such a dynamic way. The enzymes involved in transduction pathway of olfactory receptor cells and gustatory receptor cells are quiet similar whereas in auditory system there is a transduction of auditory receptor cells which help to move the process by direct gating of mechanosensory cells( the cells functioning or related to mechanical stimuli ) to get fast transduction. The entrance of Ca++ is important and common in mechanism of transduction for all of them. Each of them requires metabolic energy at some point and there is a change in this energy consumption when single transduction occurs. The special senses are key to human survival. They consist of the eyes, ears, nose, throat and skin. Each of these organs have specialized functions that make it possible for us to experience our environment and to make that experience more pleasant. An understanding of the mechanism of their functioning helps not only to understand how to deal with problems in these organs, but also to make every experience in life better. References Fesenko E.E., Nonoselov V.I., Krapivinskaya L.D. Molecular mechanisms of olfactory reception, Biochim. Biophys. Acta, 587:424. (1979). Gray, lincoln. Neurosciece online. 1997. 27 september 2011 . Kimball, John W. Biology Pages: The Sense of Smell. 2011. 28 September 2011 . Kimball, John W. Kimball's Biology Pages: The Sense of Taste. 1994. 27 september 2011 . Potter, Robert A. Seeing, Hearing, and Smelling the World. Chevy Chase, Maryland: Howard Hughes Medical Institute, 1997. Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Taste receptors and transduction of taste signals. Sunderland (MA): Sinauer Associates, 2001. Schild, D and Restrepo, D. Transduction mechanisms in vertebrate olfactory receptor cells, Physiol. . 1998. Rev.78(2). 429-466. Sherwood, Lauralee. "The peripheral nervous system: Afferent divisions and special senses." Fundamentals of Human Physiology, 4th edition. Cengage Learning, 2010. Ch.6. 141. Read More