Physio chapter 15

Chemoreceptors

Are used by Olfaction (smell) and Gustation(taste) systems which are complementary senses that let us know whether a substance should be savored or avoided. Smell receptors are excited by chemicals dissolved in nasal fluids. Taste receptors respond to chemicals dissolved in saliva.

Physiology of taste: activation of taste receptors

Binding of food chemical (tastant) depolarizes cell membrane of gustatory epithelial cell membrane, causing release of neurotransmitter. Different gustatory cells have different thresholds for activation

Gustatory pathway through the VII cranial nerve.

Facial nerve (VII) carries impulses from anterior two-thirds of tongue

Olfactory pathway.

Filaments of olfactory nerves synapse with mitral cells located in overlying olfactory bulb Mitral cells are second-order neurons that form olfactory tract Synapse occurs in structures called glomeruli .

How taste is triggered (e.g., saliva dissolves food, liquid).>May initiate protective reactions, such as:

Gagging Reflexive vomiting

Gustatory pathway through the IX cranial nerve.

Glossopharyngeal (X) carries impulses from posterior one-third and pharynx

Lipids

Growing evidence humans can taste long-chain fatty acids from lipids Perhaps explain liking of fatty foods.

Physiology of taste: activation of taste transduction.

Gustatory epithelial cell depolarization

Physiology of taste: Sour taste is due ___________

H+ acting intracellularly by opening channels that allow other cations to enter

How taste is triggered (e.g., saliva dissolves food, liquid).>Triggering reflexes involved in digestion, such as:

Increased secretion of saliva into mouth Increased secretion of gastric juice into stomach

Mouth also contains what receptors

Mouth also contains thermoreceptors. mechanoreceptors. nociceptors.

Physiology of taste: Salty taste is due to __________

Na+ influx that directly causes depolarization

Physiology of taste: activation of taste receptors

Neurotransmitter binds to dendrite of sensory neuron and initiates a generator potential that lead to action potentials. All adapt in 3-5 seconds, with complete adaptation in 1-5 minutes

hot chili peppers

Spicy hot foods can excite pain receptors in mouth, which some people experience as pleasure

Influence of Other Sensations on Taste

Taste is 80% smell If nose is blocked, foods taste bland

Gustatory pathway through the X cranial nerve.

Vagus nerve transmits from epiglottis and lower pharynx

Bitter

alkaloids such as quinine and nicotine,caffeine, and nonalkaloids such as aspirin

Umami

amino acids glutamate and aspartate; example: beef (meat) or cheese taste, and monosodium glutamate

Physiology of taste: Unique receptors for sweet, bitter, and umami

but all are coupled to G protein gustducin

basal epithelial cells

dynamic stem cells that divide every 7-10 days

Sour

hydrogen ions in solution

Salty

metal ions (inorganic salts); sodium chloride tastes saltiest

Taste buds>structure>Foliate papillae

on side walls of tongue

Structure of olfactory receptors and cells: olfactory epithelium

organ of smell Located in in roof of nasal cavity Covers superior nasal conchae. Contains olfactory sensory neurons Supporting cells surround and cushion olfactory receptor cells Olfactory stem cells lie at base of epithelium.

Physiology of taste: Activation causes

release of stored Ca2+ that opens cation channels, causing depolarization and release of neurotransmitter ATP

Taste buds

sensory organs for taste Most of 10,000 taste buds are located on tongue in papillae, peglike projections of tongue mucosa.

Sweet

sugars, saccharin, alcohol, some  amino acids, some lead salts

gustatory epithelial cells

taste receptor cells have microvilli called gustatory hairs that project into taste pores, bathed in saliva.

Taste buds>structure>Fungiform papillae

tops of these mushroom-shaped structures house most taste buds; scattered across tongue