The Chemical Senses - Taste and Smell

what are the five basic tastes and what is the chemical that causes each one?

• sweet: glucose, fructose, sucrose

• bitter: Mg²⁺ and K⁺

  • instinctively rejected

• sour: HCl (acid)

• salty: NaCl

• umami (savory): amino acids; glutamate, monosodium, monosodium glutamate (MSG)

the tongue, taste bud, and taste cells

receptor potential for taste

• when the appropriate chemical activates the receptor cell, the membrane potential changes (usually depolarizes)

• depolarization causes the voltage gated Ca+ channels to open and Ca+ enters the cell

• this moves the message from the receptor cell to the postsynaptic sensory nerve —> creating action potentials to the brain stem

what is a tastant? how can it affect a receptor cell?

• tastant = taste stimuli

• taste transduction: taking the tastant and converting it to a nerve impulse

• pass directly through the ion channels (salty and sour)

• bind to and block ion channels (sour)

• bind to G-protein coupled receptors (bitter, sweet, and umami)

describe the taste pathway

• CN7 (facial), CN9 (glossopharyngeal), and CN10 (vagus_ all carry primary gustatory axons (nerve directly enters the brain)

• all eventually bundle together and enter the gustatory nucleus of the medulla

• neurons of the gustatory nucleus synapse at the ventral posterior medial nucleus (VPM) of the thalamus

• neurons of the VPM synapse at the primary gustatory cortex on the insula

smell

pheromones: chemicals released by the body that may be used to mark territories, identify other individuals, and indicate aggression/stress

basic organs of smell

• odorant: chemical of smell

• olfactory epithelium holds the olfactory receptor cells (neurons)

• these neurons are replaced regularly

• neurons go through the cribriform plate and join in the olfactory bulb

olfactory transduction

• odorants —> bind to receptor proteins —> stimulate G-protein —>

• open Ca²⁺ activated Cl^- channels after depolarization —> cause current flow _ membrane depolarization (receptor potential)

olfactory pathway

• receptor neuron axons terminate in the glomeruli

  • olfactory information can be modified by inhibitory and excitatory interactions within and between the glomeruli

• the glomeruli house the dendritic receptors of the 2nd order neurons whose axons make up the olfactory tract

• Olfactory tract can progress directly to the olfactory cortex of the temporal lobe