The Chemical Senses Taste & Smell

Taste Sensation Taste Sensation

Objectives 1.Describe the location, structure & afferent pathway of the taste receptors. 2. Name the basic taste sensations. 3. Describe the cell of taste bud. 4.Identify the cranial nerves that transmit taste information to the cerebral cortex.

Taste sensation is mainly a function of the taste buds in the mouth. There are four primary sensations of taste 1. Sour. 2. Sweet. 3. Salty. 4. Bitter.

The primary sensations of taste 1- The sour taste It is caused by the acids & the intensity of the taste sensation is proportional to the logarithm of the hydrogen ion concentration (more acidic- stronger sensation). 2- The salty taste It is caused by the ionized salt, the cations mainly responsible for salty taste, & also the anions to the lesser extent.

3- The sweet taste a list of some of the types of chemicals that cause this taste includes: Sugars, aldehyds, esters, haloginated acid Glycols, ketons, aminase, inorganic salts of lead & beryllium, Alcohols, amides,sulfonic acids

4. Bitter taste Substances that cause bitter taste are almost entirely organic substances: 1.Long chain organic substances containing nitrogen. 2.Alkaloids (quinine, caffeine, strychnine & nicotine). The bitter when it occurs in high intensity usually causes the person or animal to reject the food, & this is an important function of the bitter taste sensation because many of the deadly toxins found in poisonous plants are alkaloids.

The taste bud & its function It is composed of about modified epithelial cells: 1. Some of which are supporting (sustentacular) cells. 2. The others are taste cells which are continually being replaced by mitotic division from the surrounding epithelial cells.

The outer tips of the taste cells are arranged around minute taste pore. From each of taste cells tip several microvillus or taste hairs (2-3 microns) protrude outward into the taste pore to reach the cavity of the mouth, these microvillus is believed to provide receptor surface for taste. Among taste cells is a branching terminal network of several taste nerve fibers stimulated by taste cells.

Adults have approximately ( )taste buds.

Location of taste buds Large number of taste buds are on the wall of the troughs that surround the circumvallate papillae which form V-line towards the posterior of the tongue. 1. Moderate number of taste buds are on the fungiform papillae over the front surface of the tongue.

3.Moderate numbers are on the foliate papillae located in folds along the lateral surfaces of the tongue. 4.Additional taste buds are located on the palate, & few on the tonsillar pillars & other points around nasophanynx.

Location of taste sensation 1. Sweet & salty sensation: are located principally on the tip of the tongue. 2. Sour taste: on two lateral sides of tongue. 3. Bitter taste: on the circumvallate papillae an posterior tongue and soft plate.

Specificity of taste buds for primary taste stimuli: a- Each taste bud usually responds to only one of the four primary taste stimuli when the taste substances is in low concentration. b- But at high concentration, most buds can be excited by 2, 3 even 4 of the primary taste stimuli as well as by a few other taste stimuli that do not fit into the “primary” categories.

Mechanism of stimulation of taste buds The microvilli on the tip taste believed to provide the receptor surface for taste. The membrane of the taste cell, like that of other sensory receptors cells, is negatively charged on the inside with respect to the outside. So application of a taste substance to the taste hairs causes partial loss of this negative potential, means the taste cell depolarized getting receptor potential.

Receptor potential is initiated by: 1.Binding of the taste chemicals to protein receptor molecules that protrudes through the villus membrane. 2.This open ion channels (sodium ions) which allow sodium ions to enter and depolarize the cell.

Adaptation of Taste This adaptation is about one half by taste buds & the extreme degree of adaptation occurs in the C.N.S.

Transmission of taste signals into the central nervous system: 1- Taste impulses from anterior two thirds of the tongue pass into the chorda tympani into the facial nerve. 2- Taste sensations from the circumvallate papillae on the back of the tongue & from other posterior regions of the mouth are transmitted through the glosso- pharyngeal nerve. 3- Finally few taste signals are transmitted from the base of the tongue & other parts of the pharyngeal region by vagus.

On each side, the taste nerve fibers in these three nerves unite in the medulla oblongata to form the tractus solitorius. Second order neurons are located in the nucleus of this tract. Their axons cross midline & join medial lamniscus ending in specific nuclei in thalamus. Impulses then relayed from there to the taste projection area in the cerebral cortex at the lower tip of post central gyrus in the parietal cortex where it pass deep in the Sylvain fissure.

Smell Sensation

Objectives 1. Describe the locations, structures, & afferent pathways of smell receptors. 2.Explain how olfactory receptors are activated, & the olfactory mechanism. 3.Describe the structures & functions of central olfactory centers.

The olfactory membrane lies in: 1.Superior part of each nostril. 2.Medially it folds down over the surface of the superior septum. 3. Laterally it folds over the superior turbinate. 4.Even over the small portion of the upper surface of the middle turbinate.

The surface area of olfactory membrane in each nostril is 2.4 seq.cm & it is always wet by the secretion of alveolar glands (Bowman’s glands) which are present just below the olfactory mucosa. These secretions dissolve the odorous material inorder to make the stimulation of the olfactory receptors possible.

The olfactory cells The receptor cells for the smell sensation are the olfactory cells which are actually bipolar nerve cells derived originally from the central nervous system itself. There are about 100 million of these cells in the olfactory epithelium interspersed among sustentacular cells.

Olfactory hairs The mucosal end of the olfactory cell forms a knob from which (6-12) olfactory hairs or cilia project into mucus that coats the inner surface of the nasal cavity. These olfactory cilia form a dense mat in the mucus & these cilia react to odors in the air & then stimulate the olfactory cells.

Stimulation of olfactory cells The portion of olfactory cell respond to olfactory chemical stimuli is the cilia. So olfactory chemical substances combine with (receptor portion) on the cilia & protrude inside ciliray membrane. This odorous or olfactory chemical causes excitation of the receptor & this cause an alpha subunit of a G-protein to be separated & the G- protein is attaching to receptor protein.

This alpha subunit activate adenyl cyclase that produce cyclic AMP from ATP (adinosin monophosphate from adenosine triphosphate). This cyclic AMP activate another membrane protein (a gated sodium channel that allow large number of sodium ions to pour into receptor cell cytoplasm. Sodium ions contribute to the inside positivity of receptor to get excitation.

Physical factors affect degree of stimulation: 1.Only volatile substances that can be sniffed into nostril can be smelled. 2.Stimulating substance must be slightly water soluble to pass through mucus to reach receptors. 3.At least slightly fat soluble because of lipid constituent of cilia.

Adaptation: Smell sensation adapt within a minute when one enters a strong odorous atmosphere. 1.Fifty percent (50%) of this adaptation in the first second by the receptors. 2.The other 50% adaptation by the central nervous system by strong inhibitory signals on transmission in the olfactory bulb.

Olfactory cells send axons into the olfactory bulb in the brain to end on dendrites from mitral cells in a structure called glomerulus. Approximately axons enter each glomerulus & synapse with the mitral cells that in turn send signals into the brain.

From the mitral cells… The major pathway of transmission from mitral cells the olfactory signals pass to the brain. The fibers from the cells travel through the olfactory tract & terminate either primarily or after really neurons in three principal areas of the brain: 1.The medial olfactory area. 2.The lateral olfactory area respectively. 3.Through thalamus to orbitofrontal cortex.

1- The medial olfactory area: very old olfactory area. Consists of group of nuclei located in mid-basal portion of the brain anterior to the hypothalamus. It serve the basic olfactory reflexes. 2- The lateral olfactory area: less old olfactory system. Composed mainly of prepyriform & pyriform cortex plus cortical portion of amygdaloid nuclei. From these areas signals pathways pass into almost all portion of limbic system specially to hippocampus. This system provides automatic but partially learned control of food intake & rejection of toxic & unhealthy foods. 3- The newer pathway: Passes through the thalamus & then to the orbito frontal cortex. This newer system used for conscious perception of olfaction.