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special senses.docx

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Krista Howarth

Chapter 15- The Special Senses Olfaction - Sense of smell occurs in response to odours that stimulate receptors located In the extreme superior region of the nasal cavity. - Only a small superior part of the nasal cavity is dedicated to olfaction. - Olfactory epithelium is the nasal epithelium in the olfactory region. Olfactory Epithelium and Bulb - 10 million olfactory neurons are present within the olfactory epithelium. - Axons of these bipolar neurons project to the olfactory bulbs where they synapse with secondary neurons. - Olfactory tracts project from the bulbs to the cerebral cortex. - Dendrites of olfactory neurons extend to the epithelial surface of the nasal cavity, and their ends are modified into bulbous enlargements called olfactory vesicles. - These vesicles possess cilia called olfactory hairs . - Airborne molecules enter the nasal cavity and are dissolved in the fluid covering the olfactory epithelium. - Some of these odorants bind to transmembrane odorant receptor molecules of the olfactory hair membranes. - A G protein, associate with each odorant receptor, is activated by the binding of the odorant. - The a subunit of the activated G protein binds to and activates adenylate cyclase, which in turn catalyzes the formation of cyclic AMP from ATP, Camp in these cells causes na+ and ca2+ channels to open. - The influx of ions into the olfactory hairs results in depolarization ad the production of action potentials in the olfactory neurons. - Odor receptors are composed of seven transmembrane polypeptide subunits produced by a large family of genes. - Combinations of subunits from approximately 1000 different odorant receptors can be produced. - These receptors can react to odorants of different sizes, shapes and functional groups. - These capabilities, together with multiple intracellular pathways involving G proteins, adenylate cyclase and ion channels allow for a wide variety of detectable smells, which is about 4000 for the avg. person. - Seven primary classes have been proposed: camphoraceous(eg. Moth balls), musky, floral, pepperminty, ethereal (eg. Fresh pears), pungent and putrid. - Threshold for the detection of odors is very low, so very few odorant molecules are required to trigger the response. - Low specificity- a given receptor may react to more than one type of odorant. - Odorant receptors may become saturated with odorants and no longer respond to odorant molecules. - Makes people less sensitive to as odor after beong exposed to it for a short time. - The primary olfactory neurons have the most exposed nerve ending of any neurons, and they are constantly being replaced. - The entire olfactory epithelium, including the neurosensory cells, is lost about every 2 months as the olfactory epithelium degenerates and is lost from the surface. - Basal cells in the olfactory epithelium replace lost olfactory cells. - Unique among neurons. Neuronal Pathways for Olfaction - Axons from the olfactory neurons form the olfactory nerves, which enter the olfactory bulbs where they synapse with mitral cells or tufted cells. - These cells relay olfactory info to the brain through the olfactory tracts and synapse with granule cells in the olfactory bulb. - Olfactory bulb neurons also receive input from nerve cell processes entering the olfactory bulb from the brain. - As a result of input from both mitral cells and the brain, olfactory bulb neurons can modify olfactory info before it leaves the olfactory bulb. - This enhances the accommodation occurring in the odorant receptors. - Olfaction is the only major sensation that is relayed directly to the cerebral cortex without first passing through the thalamus. - Each olfactory tract terminates in an area of the brain called the olfactory cortex. - The olfactory cortex is in the frontal and temporal lobes, within the lateral fissure of the cerebrum, and it can be divied structurally and functionally into three areas: lateral, medial and intermediate. - The lateral area is involved in the conscious perception of smell. - The medial olfactory area is responsible for visceral and emotional reactions to odors and has connections to the limbic system, through which it connects to the hypothalamus. - Axons extend from the intermediate olfactory area along the olfactory tract to the bulb and synapse with the olfactory bulb neurons, thus constituting a major mechanism by which sensory information is modulated within the olfactory bulb. - The feedback from the intermediate olfactory is mostly inhibitory, further enhancing the rapid accommodation of the olfactory system. - Accommodation is what makes the olfactory system insensitive to an odorant after a brief exposure. Taste - Sensory system that detect gustatory or taste stimuli are the taste buds. - Associated with specialized portions of the tongue called papillae. - Also located on other areas of the tongue, the palate and even the lips and throat, especially in children. - The four major types of papillae according to their shape are: vallate, fungiform, foliate, and filiform. - Taste buds are associated with vallate, fungiform and foliate papillae. - Filiform are the most numerous but have no taste buds. - They provide a rough surface on the tongue. - Vallate are the largest but least numerous. - 8-12 of these form a v-shaped row along the border between the anterior and posterior parts of the tongue. - Fungiform are scattered irregularly over the entire superior surface of the tongue, appearing as small, red dots interspersed among the far more numerous filiform papillae. - Foliate papillae are distributed in folds on the sides of the tongue and contain the most sensitive of the taste buds. - They are most numerous in young children and decrease with age. - They are located mostly posteriorly in adults. Histology of Taste Byds - Taste buds are oval structures embedded in the epithelium of the tongue and mouth. - Each of the 10 000 taste buds on a person’s tongue consists of three major types of specialized epithelial cells. - The sensory cells of each taste bud consist of about 50 taste or gustatory cells. - The remaining two cell types which are nonsensory, are basal cells and supporting cells. - The cells are replaced continuously. - Having a lifespan of about 10 days. - Each taste cell has several microvilli called gustatory hairs, extending from its apex into a tiny opening in the epithelium called the taste or gustatory, pore. Function of Taste - Substances called tastants dissolved in saliva, enter the taste pores and by various mechanisms, cause the taste cells to depolarize. - These cells do not have classic axons but have short connections with secondary sensory neurons. - These connections have some characteristics of chemical synapses. - Neurotransmitters( including ATP) are released from the taste cells and stimulate action potentials in the axons of sensory neurons associated with them. - The taste of salt results when na+ diffuse through na+ channels of the gustatory hairs or other cell surfaces of taste cells, resulting in depolarization of the cells. - Hydrogen ions of acids, which result in sour taste can cause depolarization of taste cells by one of three mechanisms: they can enter the cell through h+ channels, they can bind to ligand-gated k+ channels and block the exit of k+ from the cell, or they can open ligand-gated channels for other positive ions and allow them to diffuse into the cell. - Sweet and bitter tastants bind to receptors on the gustatory hairs of taste cells and cause depolarization through a G protein mechanism. - A taste called umami results when amino acids such as glutamate bind the receptors on gustatory hairs of taste cells and cause depolarization through a G protein mechanism. - Texture of food also affects the perception of taste - Hot or cold food may interfere with the ability of the taste buds to function in tasting food. - If a cold fluid is held in the mouth, the body warms the fluid and the taste becomes enhanced. - Adaptation is very rapid for taste. - Adaptation occurs at the level of the taste bud and within the CNS. - Adaptation may occur 1 or 2 sec after a taste sensation is perceived, and complete adaptation may occur within 5 minutes. - Humans can perceive a fairly large number of different tastes - Many of the sensations thought of as being taste are strongly influenced by olfactory sensations. - Thresholds vary for the five primary tastes. - Sensitivity for bitter substances is the highest. - Sweet and salty is the lowest. - Many alkaloids are poisonous, thus the high sensitivity for bitter tastes may be protective. - On the other hand, humans tend to crave sweet, salty and umami tastes, perhaps in response the the body’s need for sugars, carbs, proteins and minerals. Neuronal Pathways for Taste - Taste from the anterior two thirds of the tongue except from the circumvallate papillae is carried by the means of the facial nerve called the chorda tympani. - It crosses over the surface of the tympanic membrane of the middle ear. - Taste from the posterior one thi
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