2. Mastication (Chewing) The incisors provide a strong cutting action and the molars cause grinding action. Function: 1- increase total surface area exposed to the digestive secretions. 2- Prevents excoriation of the gastrointestinal tract. 3- Break the indigestible cellulose in most fruits and raw vegetables around their nutrient portions.
3. Chewing mechanism (chewing reflex): The presence of food in the mouth reflex inhibition of the muscles of mastication the lower jaw drop stretch reflex of the jaw muscles contraction and the jaw raise again to cause closure of the teeth, compresses the bolus again , which inhibits the jaw muscles once again, allowing the jaw to drop and rebound ; this is repeated.
4. Sense of Smell (Olfaction) Function of smell: Initiate conditional reflexes concerned with food intake. Animals heavily on olfaction for survival searching for food , defense, and reproduction Together with taste add flavor for food.
5. Sense of Smell Olfactory receptors: Olfactory cells which are bipolar nerve cells found in the olfactory epithelium their ends contains olfactory cilia that project into the nasal mucosa. These cilia react to odors in the air and stimulate the olfactory cells.
6. Stimulation of the Olfactory Cells (Receptors): The odorant substance, on coming in contact with the olfactory membrane it diffuses into the mucus that covers the cilia. Then: It binds with receptor in the cilium. And then activation of the receptor protein by the odorant with activation of the G-protein . This, in turn, activates adenylylcyclase inside the olfactory cell membrane with formation of cAMP. The cAMP opens sodium ion channels and allows large numbers of sodium ions to enter the olfactory cell. Thus exciting the olfactory neuron and transmitting action potentials to the CNS.
7. Most odorants cause depolarization of the olfactory cell membrane, decreasing the negative potential in the cell from the normal level of –55 millivolts to –30 millivolts or less. Physical factors of the odorant sustances that affect the degree of stimulation: Only volatile substances that can be sniffed into the nostrils can be smelled. Water soluble substances to pass through the mucus to reach the olfactory cilia. Lipid soluble to pass through cilia membrane.
8. Transmission of Smell Signals intothe Central Nervous System - Axons from the olfactory cells pass through perforations in the cribriform plate and terminate in the olfactory bulb that send its axons through the olfactory tract (cranial nerve I ) to transmit olfactory signals to olfactory cortex in the limbic system which is a part of the brain intimately associated with emotions, food intake, and sexual behavior. Olfactory Pathways into the Central Nervous System: Olfactory tract divides into two pathways the medial olfactory area (very old olfactory system), and the lateral olfactory area input to less old olfactory system, a newer system:
10. 1) A very old olfactory system: subserves the basic olfactory reflexes, such as licking the lips, salivation, and other feeding responses caused by the smell of food. 2) less old system that provides partially learned control of food intake and aversion to toxic and unhealthy foods. 3) A newer system that is used for conscious perception and analysis of olfaction.
11. Olfactory sensitivity: - Smell has low threshold, a minute quantity of odor in air can elicit a smell sensation. - A substance as methylmercaptan can be smelled in very small concentration. Because of this very low threshold, this substance is mixed with natural gas to give the gas an odor can be detected when gas leak occurs. - Sensitivity is greater in hungry subjects; in women more than men; smoking decrease smell sensitivity, also smell sensitivity decrease with age and when the mucosa is congested, as in a common cold.
12. Olfactory Adaptation: The olfactory receptors adapt about 50% in the first second after stimulation. Thereafter, they adapt very slowly. smell sensations adapt almost to disappearance within a minute. Most of the adaptation occurs within the CNS. Olfactory discrimination: At least 100 primary sensations of smell are found, in contrast to only three primary sensations of color discriminated by the eyes and only five primary sensations of taste detected by the tongue. Some people have odor blindness for certain substances due to lack of the receptor protein in olfactory cells for that substance.
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14. Taste Bud and Its Function: Detects chemicals that are dissolved in the saliva. The taste buds are found on three types of papillae of the tongue: surround the circumvallate papillae, on the fungiform papillae and on the foliate papillae. The receptor cells are arranged in the taste buds like the segments of an orange arround a taste pore. From the tip of each taste cell microvilliprotrude outward into the taste pore. Each taste bud responds to one of the five primary taste stimuli (sour, salty, sweet, bitterer and umami taste).
15. Mechanism of Stimulation of Taste Buds: The inner membrane of the taste cell in the taste buds has a negative potential. Application of a substance to the taste microvilli causes loss of this negative potential, and the taste cell becomes depolarized and stimulate taste nerve fibers.
16. The neuronal pathways for transmission of taste signals to the CNS: Taste impulses from the anterior two thirds of the tongue pass in the facial nerve. From the posterior regions of the tongue and throat are transmitted through the glossopharyngeal nerve. From the base of the tongue by the vagus nerve. All taste fibers ascend and synapse in the brain stem in the nuclei of the tractussolitarius. These nuclei send second-order neurons to the ventral posterior medial nucleus of the thalamus. From the thalamus, third-order neurons are transmitted to the postcentralgyrus in the parietal cerebral cortex, taste pathways, is closely parallel the somatosensory pathways from the tongue.
