Smell - PDF

CHEMICAL SENSES: SMELL Sense of Smell  Smell  Taste (gustation) and smell (olfaction) are classified as visceral senses  Physiologically, they are related to each other  Smell receptors are also chemoreceptors  Smell receptors are also ext...

CHEMICAL SENSES: SMELL Sense of Smell  Smell  Taste (gustation) and smell (olfaction) are classified as visceral senses  Physiologically, they are related to each other  Smell receptors are also chemoreceptors  Smell receptors are also extereceptors  Dissolved chemical molecules in mucus stimulate receptors  Humans have approximately 350 different odorant receptors  Odorant receptors belong to the G protein-coupled receptor superfamily  Smell  Taste and smell sensetions allow individuals to distinguish between estimates of up to 30 million compounds  Stimulation of the taste or smell receptors induces pleasurable or objectionable sensations:  to seek: a nutritionally useful, good–tasting food  to avoid: a potentially toxic, bad-tasting substance  In lower animals, smell also plays a major role in finding direction, in seeking prey, or avoiding predators, and in sexual attraction to a mate  Smell  The sense of smell is less sensitive in human and less important in influencing our behaviour  Men and women spent annually more money (millions of dollars):  to smell better  to be more socially attractive Olfactory membrane and cells-1  The olfactory membrane lies in the superior part of nasal cavity  In each nostril, olfactory membrane has a surface area of about 2.4 -5 cm2.  There are 10 million-100 million bipolar olfactory neurones.  Their life span is about 30-60 days  Are continuously replaced from a layer of basal stem cells in the epithelium  Olfactory membrane and cells-2  Each cell has 4-25 cilia (0.1-0.3 µm in diameter and up to 200 µm in lenght)  Olfactory cilia projections reach into the mucus that coats the inner surface of the nasal cavity  Resting membrane potantial is about -55 mV  The olfactory receptors adapt about 50% within first second after stimulation  Humans can discriminate more than 10 000 different odors The Structure of the olfactory system Bowman glands secrete mucus onto the surface of the olfactory membrane  The stimulation of olfactory neurones Odorant (Chemical mechanism) Receptor alfa subunit breaks away from G protein Adenil cyclase is activated ATP cAMP Na+/Ca++ channel is activated And so depolarization occurs  Physiologic Factors for the stimulation of olfactory cell  Substances must be volatile  Substances must be (at least) slightly water soluble  Substance must be slightly lipid soluble  Olfactory System At each glomerulus: 25 Mitral cells 60 Tufted cells and 25 000 olfactory axons OG: Olfaktör Glomerulus, M: Mitral hücre, PG: Periglomerüler hücre, Püsküllü hücre, Gr: Granüler hücre, CP: Kribriform plak). Neuronal Connections of Olfactory System  The Medial Olfactory area consist of a group of nuclei located in the midbasal portions of the brain immediately anterior to the hypothalamus (especially septal nuclei). This area is associated with the more primitive responses to olfaction such as licking the lips, salivation, and other feeding responses caused by smell of food or by primitive emotional drives The Lateral Olfactory Area composed mainly of the Pyriform cortex, prepyriform cortex + the cortical portion of the amygdaloid nuclei. This area is the most important for learning to like or dislike certain foods depending on one’s experiences with them (that have caused nausea and vomiting).  Abnormalities in Odor Detection-1 Anosmia (inability to smell) and hyposmia or hypesthesia (diminished olfactory sensitivity) can result from simple nasal congestion or nasal polyps.  Abnormalities in Odor Detection-2 Anosmia may also be a sign of a more serious problem such as: 1- damage to the olfactory nerves due to fractures of the cribriform plate or head trauma, 2- tumors such as neuroblastomas or meningiomas, 3- respiratory tract infections (such as abscesses) 4- Prolonged use of nasal decongestants can lead to anosmia 5- Olfactory nerve damage in patients with Alzheimer 6- Aging  Abnormalities in Odor Detection-3 Congenital anosmia is a rare disorder in which an individual is born without the ability to smell.  Abnormalities in Odor Detection-4 Hyperosmia(enhanced olfactory sensitivity) is less common than loss of smell, but pregnant women commonly become oversensitive to smell.  Abnormalities in Odor Detection-4 Dysosmia (distorted sense of smell) can be caused by several disorders including: 1- sinus infections, 2- partial damage to the olfactory nerves, and 3- poor dental hygiene.