1. Special senses by
Proph Israa F Jaafar

2. Sense of smell and taste
Objectives
Explain types of taste sensation
how the mechanisms for sour and salty tastes are similar to each other, and how these differ from the mechanisms
responsible for sweet and bitter tastes.
3. Explain how odorant molecules stimulate the olfactory receptors.

3. Taste and Smell
The receptors for taste and smell respond to molecules that are dissolved in fluid; hence, they are classified as chemoreceptors.
Although there are only four basic modalities of taste, they combine in various ways and are influenced by the sense of smell, thus permitting a wide variety of different sensory experiences.
interoceptors; Chemoreceptors that respond to chemical changes in the internal environment .
Exteroceptors: those that respond to chemical changes in the external environment.

4. Exteroceptors
are taste (gustatory) receptors, which respond to chemicals dissolved in food or drink, and smell (olfactory) receptors, which respond to gaseous molecules in the air. This distinction is somewhat arbitrary, however, because odorant molecules in air must first dissolve in fluid within the olfactory mucosa before the sense of smell can be stimulated. Also, the sense of olfaction strongly influences the sense of taste, as can easily be verified by eating an onion (or almost anything else) with the nostrils pinched together.

5. Taste
Gustation, the sense of taste, is evoked by receptors that consist of barrel-shaped taste buds Located primarily on the dorsal surface of the tongue.
each taste bud consists of 50 to 100 specialized epithelial cells with long microvilli that extend through a pore in the taste bud to the external environment, where they are bathed in saliva. Although these sensory epithelial cells are not neurons, they behave like neurons; they become depolarized when stimulated appropriately, produce action potentials, and release neurotransmitters that stimulate sensory neurons associated with the taste buds.

6. Nerve supply:
Taste buds in the anterior two-thirds of the tongue are innervated by the facial nerve (VII), and those in the posterior third of the tongue by the glossopharyngeal nerve (IX). Dendritic endings of the facial nerve (VII) are located around the taste buds and relay sensations of touch and temperature. Taste sensations are passed to the medulla oblongata, where the neurons synapse with second-order neurons that project to the thalamus. From here, third-order neurons project to the area of the postcentral gyrus of the cerebral cortex that is devoted to sensations from the tongue.

7. taste cells:
The specialized epithelial cells of the taste bud are known as taste cell.
The different categories of taste are produced by different chemicals that come into contact with the microvilli of these cells .
Four different categories of taste are traditionally recognized: salty, sour, sweet, and bitter. There may also be a fifth category of taste, termed umami (a Japanese term related to a meaty flavor), for the amino acid glutamate (and stimulated by the flavor-enhancer monosodium glutamate)

8. Previously was believed that different regions of the tongue were specialized for different tastes.
Now it seems that each taste bud contains taste cells responsive to each of the different taste categories! It also appears that a given sensory neuron may be stimulated by more than one taste cell in a number of different taste buds, and so one sensory fiber may not transmit information specific for only one category of taste. The brain interprets the pattern of stimulation of these sensory neurons, together with the nuances provided by the sense of smell, as the complex tastes that we are capable of perceiving.

9. The salty taste of food is due to the presence of sodium ions (Na+), or some other cations: which activate specific receptor cells for the salty taste.
Different substances taste salty to the degree that they activate these particular receptor cells. The Na+ passes into the sensitive receptor cells through channels in the apical membranes. This depolarizes the cells, causing them to release their transmitter. The anion associated with the Na+, however, modifies the perceived saltiness to a surprising degree:
NaCl tastes much saltier than other sodium salts (such as sodium acetate).
There is evidence to suggest that the anions can pass through the tight junctions between the receptor cells, and that the Cl– anion passes through this barrier more readily than the other anions. This is presumably related to the ability of Cl–to impart a saltier taste to the Na+ than do the other anions.

10. Sour taste: like salty taste, is produced by ion movement
through membrane channels. Sour taste, however, is due to the presence of hydrogen ions (H+); all acids therefore taste sour.
In contrast to the salty and sour tastes, the sweet and bitter tastes are produced by interaction of taste molecules with specific membrane receptor proteins.
Most organic molecules, particularly sugars, taste sweet to
varying degrees. Bitter taste is evoked by quinine and seemingly unrelated molecules. It is the most acute taste sensation and is generally associated with toxic molecules (although not all toxins taste bitter). Both sweet and bitter sensations are mediated by receptors that are coupled to G-proteins .
The particular type of G-protein involved in taste has recently been identified and termed gustducin..

11. Dissociation of the gustducin G-protein subunit activates second-messenger systems, leading to depolarization of the receptor cell. The stimulated receptor cell, in turn, activates an associated sensory neuron that transmits impulses to the brain, where they are interpreted as the corresponding taste perception.
Although all sweet and bitter taste receptors act via G-proteins, the second-messenger systems activated by the G-proteins depend on the molecule tasted.

12. the sweet taste of sugars, for example, the G-proteins activate adenylate cyclase, producing cyclic AMP (cAMP;. The cAMP, in turn, produces depolarization by closing K+ channels that were previously open.
the sweet taste of the amino acids phenylalanine and tryptophan, as well as of the artificial sweeteners saccharin and cyclamate, may enlist different second-messenger systems. These involve the activation of a membrane enzyme that produces the second messengers inositol triphosphate (IP3) and diacylglycerol (DAG). respectively, by means of G-protein-coupled receptors and the actions of second messengers. 

13. A taste bud. Chemicals dissolved in the fluid at the pore bind to receptor proteins in the microvilli of the sensory cells. This ultimately leads to the release of neurotransmitter, which activates the associated sensory neuron 

14. The four major categories of taste
The four major categories of taste. Each category of taste activates specific taste cells by different means. Notice that taste cells for salty and sour are depolarized by ions (Na+ and H+, respectively) in the food, whereas taste cells for sweet and bitter are depolarized by sugars and quinine, respectively, by means of G-protein-coupled receptors and the actions of second messengers

15. The adenylate cyclase-cyclic AMP second-messenger system
The adenylate cyclase-cyclic AMP second-messenger system. The hormone causes the production of cAMP within the target cell cytoplasm, and cAMP activates protein kinase. The activated protein kinase then causes the activation or inactivation of a number of specific enzymes. These changes lead to the characteristic effects of the hormone on the target cell.

16. Adenylate Cyclase–Cyclic AMP Second-Messenger System
the hormone—acting through an increase in cAMP production—causes an increase in protein kinase enzyme activity within its target cells.
Active protein kinase catalyzes the phosphorylation of (attachment of phosphate groups to) different proteins in the target cells. This causes some enzymes to become activated and others to become inactivated. Cyclic AMP, acting through protein kinase, thus modulates the activity of enzymes that are already present in the target cell. This alters the metabolism of the target tissue in a manner characteristic of the actions of that specific hormone.

17. Smell
The receptors responsible for olfaction are located in the olfactory epithelium.
The olfactory apparatus consists of:
receptor cells (bipolar neurons).
supporting (sustentacular) cells.
basal (stem) cells. The basal cells generate new receptor cells every 1 to 2 months to replace the neurons damaged by exposure to the environment. The supporting cells are epithelial epithelial cells rich in enzymes that oxidize hydrophobic, volatile odorants, making these molecules less lipid-soluble and less able to penetrate membranes and enter the brain.

18. Neural pathway of smell
Each bipolar sensory neuron has one dendrite projects into the nasal cavity, where it terminates in a knob containing cilia The bipolar sensory neuron also has a single unmyelinated axon that projects through holes in the cribriform plate of the ethmoid bone into the olfactory bulb of the cerebrum, where it synapses with second-order neurons. unlike other sensory modalities that are relayed to the cerebrum from the thalamus, the sense of smell is transmitted directly to the cerebral cortex. The processing of olfactory information begins in the olfactory bulb, where the bipolar sensory neurons synapse with neurons located in spherically shaped arrangements called glomeruli

19. Smell perception
each glomerulus receives input from one type of olfactory receptor. The smell of a flower, which releases many different molecular odorants, may be identified by the pattern of excitation it produces in the glomeruli of the olfactory bulb. Identification of an odor is improved by lateral inhibition in the olfactory bulb, which appears to involve dendrodendritic synapses between neurons of adjacent glomeruli.
Neurons in the olfactory bulb project to the olfactory cortex in the medial temporal lobes, and to the associated hippocampus and amygdaloid nuclei. These structures are part of the limbic system, The human amygdala, has been implicated in the emotional responses to olfactory stimulation.

20. Perhaps this explains why the smell of a particular odor can so powerfully evoke emotionally charged memories.
The molecular basis of olfaction is complex. odorant molecules bind to receptors and act through G-proteins to increase the cyclic AMP within the cell. This, in turn, opens membrane channels and causes the depolarization of the generator potential. This stimulates the production of action potentials, Up to fifty G-proteins may be associated with a single receptor protein.

21. Dissociation of these G-proteins releases many G-protein subunits, thereby amplifying the effect many times.
This amplification could account for the extreme sensitivity of the sense of smell: the human nose can detect a billionth of an ounce of perfume in air. Even at that, our sense of smell is not nearly as keen as that of many other mammals.
A family of genes that codes for the olfactory receptor proteins has been discovered. This is a large family that may include as many as a thousand genes. The large number may reflect the importance of the sense of smell to mammals in general. Even a thousand different genes coding for a thousand different receptor proteins, however, cannot account for the fact that humans can distinguish up to 10,000 different odors. Clearly, the brain must integrate the signals from several sensory neurons that have different olfactory receptor proteins and then interpret the pattern as a characteristic “fingerprint” for a particular odor. 

22. Figure The neural pathway for olfaction
Figure The neural pathway for olfaction. The olfactory epithelium contains receptor neurons that synapse with neurons in the olfactory bulb of the cerebral cortex. The synapses occur in rounded structures called glomeruli. Secondary neurons, known as tufted cells and mitral cells, transmit impulses from the olfactory bulb to the olfactory cortex in the medial temporal lobes. Notice that each glomerulus receives input from only one type of olfactory receptor, regardless of where those receptors are located in the olfactory epithelium.