The Chemical Senses

 Primitive senses to alert us to savor or avoid substances  Chemoreceptors of gustation and olfaction respond to chemicals in aqueous solution  Taste receptors excited by food dissolved in saliva  Smell receptors excited by airborne chemicals dissolved in fluids coating the nasal membrane  80% of taste is actually smell!!!  Both senses are heightened in pregnant women (due to increased estrogen)

 Pseudostratified epithelium at roof of nasal cavity  Millions of bowling pin shaped olfactory receptor cells  Bipolar neurons  Olfactory cilia covered by thin mucus  Mucus traps and dissolves airborne odorants  Unmyelinated axons project through cribiform plates and synapse in olfactory bulbs  Lifespan days (rare neurons that are replaced)  Humans distinguish about 10,000 odors!  Some smells are actually stimulating pain receptors (ammonia, menthol, chili peppers)  Surrounded by columnar cells

 10,000 Taste buds!  Few on soft palate, inner cheeks, pharynx, epiglottis of larnyx  Most on papillae (peg-like projections) of tongue  Fungiform papillae Mushroom shaped over entire tongue  Foliate papillae Epithelium of side walls  Circumvallate papillae Large round, form an inverted “V” at back of tongue

 epithelial cells  Gustatory (taste) cells  Microvilli “hairs” extend thru a taste pore and bathed in salvia at surface  Replaced every 7-10 days  Most respond to 2 or more taste qualities  “taste maps” are misrepresentations.. All types of taste can be elicited from all areas of mouth w/taste buds  Basal cells  Stem cells differentiate into new gustatory cells

 Sweet  organic substances including sugars, saccharin, alcohols, some amino acids, and lead salts  Sour  acids (H + ) in solution  Salty  inorganic salts (metal ions)  Bitter  alkoloids (quanine, caffiene, nicotine, morphine, and strychnine) and some nonalkoloids such as aspirin  Umami  amino acids glutamate & aspartate (beef taste, MSG, aged cheese)  Homeostatic Value  “likes” of sugar and salt satisfy body's need for carbs and minerals  Acidic foods sources of essential Vitamin C  Natural poisons and spoiled foods are bitter and disliked

 Chemical must be dissolved in saliva, diffuse into taste pore, and contact gustatory ‘hair’  Each taste has its own mechanism for inducing release of nuerotransmitters  Induces graded potential eliciting action potentials in nerve fibers  Facial nerve, glossopharyngeal nerve, and vagus nerve conduct impulses to thalmus, gustatory cortex, hypothalmus, and limbic system  Triggers digestive reflexes  increase salivation  Gastric juice secretion  Gagging or vomiting  Different taste receptors have varying thresholds  Bitter – detect in minute amounts  Partial adaptation in 3-5 seconds complete in 1-5 minutes