12.3 The Chemical Senses

Three short movements: the case, the anatomyThe study of the structure of the human body. and physiologyThe study of how the body functions., then the tie back to Stina. Each links to the scene where you’ll see it move.

I · Taste

Taste is thoroughly conventional, which is why it goes first. The tongue is covered in <strong>stratified squamous epithelium</strong> raised into bumps called <strong>papillae</strong>, four kinds by shape — <em>filiform</em> (the rough ones, with no taste buds; they grip food), <em>fungiform</em> (mushroom-shaped, scattered over the front and sides), <em>foliate</em> (leaf-like folds at the back edges) and <em>circumvallate</em> (the row of large ones across the back in a V). <strong>Taste buds</strong> sit in the walls of the last three. Each bud is a flask of fifty to a hundred cells (standard anatomy) with a <strong>taste pore</strong> at the top, and the cells that matter are <strong>gustatory receptor cells</strong> — specialized <em>epithelial</em> cells, not neurons, with microvilliTiny projections on the surface of epithelial cells that increase surface area for absorption. poking through the pore into the saliva, and a working life of about ten days (standard anatomy) before basal cells replace them. (That turnover is why a burned tongue tastes again within a week, and why some chemotherapy drugs that hit dividing cells make food taste of metal.) A dissolved molecule binds or enters a gustatory cell, the cell depolarizes in proportion to how much landed — a graded <strong>receptor potential</strong> — and it releases transmitter onto the dendritesBranch-like extensions from a neuron that receive signals from other neurons. of a sensory neuron from cranial nerve <strong>VII, IX or X</strong>. The cell does not fire; the fiber does. Five qualities, and two mechanisms. <strong>Salty</strong> is Na⁺ itself, diffusing into the cell down its gradient and depolarizing it directly; <strong>sour</strong> is H⁺, the more acidA substance that releases hydrogen ions (H⁺) in solution. the saliva the bigger the potential — the worked example is orange juice, at a pHA measure of hydrogen ion concentration in a solution. near 3. <strong>Sweet, bitter and umami</strong> work through <strong>G protein–coupled receptors</strong>: glucoseA simple sugar that is the main source of energy for cells. and the sweeteners for sweet, the amino acidThe building blocks of proteins, consisting of an amino group, carboxyl group, and side chain. <strong>L-glutamate</strong> for umami (the savory of meat and broth), and for bitter an unruly crowd of molecules dominated by plant <strong>alkaloids</strong> — coffee, hops, tannins, tea, aspirin — with the highest density of bitter receptorsProteins located on the surface or inside cells that bind specific molecules (e.g., neurotransmitter at the <em>back</em> of the tongue, where a gag can still spit out a poison. That is the only honest version of the “tongue map”: every quality is sensed everywhere there are buds, with a modest bias toward bitter at the back. There is no sweet tip and no sour edge, however many lab manuals printed one.

See it move: Scene 1 · Buds and Papillae · Scene 2 · Five Qualities, Two Mechanisms

II · Three nerves, one insula

Three cranial nerves divide the tongue between them. <strong>VII, the facial</strong>, takes taste from the <strong>anteriorThe front of the body or toward the front when standing in the anatomical position. two thirds</strong> (by way of a branch with the best name in the head, the <em>chorda tympani</em>, which crosses the middle ear on its way); <strong>IX, the glossopharyngeal</strong>, takes the <strong>posterior third</strong>; <strong>X, the vagus</strong>, takes the extreme back of the tongue, the epiglottisA flap of cartilage that covers the trachea during swallowing to prevent food from entering the airw and the pharynxThe muscular passageway connecting the mouth to the esophagus and larynx., and carries the fibers of the gag reflex with it. Touch and temperature from the front of the tongue are someone else’s — the trigeminal’s V3 — which is why a V3 lesion leaves a tongue that can taste but cannot feel hot coffee, and a VII lesion the reverse. All three taste nerves relay in the <strong>solitary nucleus</strong> of the medulla, then the <strong>ventralRelating to the front or belly side of the body. posterior nucleusThe control center of the cell that contains DNA and directs cellular activities. of the thalamus</strong>, then the <strong>gustatory cortex</strong>, which sits tucked inside the lateralAway from the midline of the body. fissureA deep groove in the brain’s surface. in the <strong>insula</strong> — the fifth lobe, the one you can only see by prying the temporal lobeThe brain region responsible for processing sound and memory. aside. The bedside test (the neurological exam) is salt or sugar on one side of the tongue, with a caution worth keeping: the stimulus dissolves into saliva and spreads, so a one-sided deficit is easy to miss. Taste also has a reflex half, and it is the autonomic nervous system’s. You cannot taste dry food: a molecule has to dissolve in saliva to reach a pore, and saliva — 99.5 percent waterThe universal solvent essential for life., with amylase and lipase starting on starch and fat before you swallow — is made on order. Food in the mouthThe opening of the digestive tract where food enters and mastication begins. (and the smell or sight of it, by way of the hypothalamusA small but vital brain region controlling hormones, temperature, and autonomic functions.) drives the <em>salivatory nuclei</em> of the brainstemThe lower part of the brain that connects to the spinal cord and controls vital functions. — one in the ponsA part of the brainstem that connects the cerebrum to the cerebellum and helps regulate breathing. for VII, one in the medulla for IX — and the <strong>parasympathetic</strong> answer rides VII, through the submandibular ganglionA cluster of neuron cell bodies located in the peripheral nervous system (PNS)., to the submandibular and sublingual glands, and IX, through the otic ganglion, to the parotid — acetylcholinealso know as ACh A neurotransmitter that stimulates muscle contraction. on muscarinic receptorsCholinergic receptors found on the effector organs of the parasympathetic nervous system; they media, and a gland that pours out a watery, enzyme-rich flood. The <strong>sympathetic</strong> division reaches the same glands by a different road and does the opposite: norepinephrineA neurotransmitter involved in attention, arousal, and the fight-or-flight response. on α and β receptors squeezes out a little thick, mucous saliva and clamps the gland’s blood vessels, so the mouth goes dry. That is the same cranial parasympathetic outflow and the same receptor table you built last module, and it explains two things on her chart at once: the mouth so dry at her desk on fever nights that the words stuck (Autonomic Nervous System, 11.1 — sympathetic tone winning), and the dry mouth a patient gets from an antihistamine or a bladderA muscular organ that stores urine before excretion. drug that blocks muscarinic receptors (the site’s own Xing-mae Chang case, on tolterodine). Block the parasympathetic lock and food goes flat, because taste has nothing to dissolve in — which is why a dry mouth is on the side-effect list of half the drug shelf, and why the Digestive module will start with saliva.

See it move: Scene 3 · Three Nerves, One Insula

III · Smell, and the sense that skips the thalamus

Smell is different in almost every respect. It begins in a patch of <strong>olfactory epithelium</strong> the size of a postage stamp in the roof of each nasal cavityThe internal space behind the nose that filters, warms, and humidifies incoming air., and its receptor cells are true <strong>bipolar neurons</strong> — the rare shape from Nervous Tissue, doing the job it was named for. Each sends a dendrite up into the mucus, tipped with ciliaHair-like projections on the surface of some cells that move fluids or particles. carrying <strong>G protein–coupled odorant receptors</strong>; an odorant dissolves, is carried to the cilia by binding proteinsLarge molecules made of amino acids with various functions in the body., docks, and the neuron depolarizes and fires on its own axon — the one chemical-sense receptor that owns its action potentialA rapid, temporary electrical charge that travels along neurons, allowing signal transmission.. Those axons bundle into threads, pass up through the tiny holes of the <strong>cribriform plate of the ethmoid</strong>, and synapseThe junction between two neurons where communication occurs. in the <strong>olfactory bulb</strong> on the underside of the frontal lobeThe brain region responsible for decision-making, personality, and voluntary movements.; the <strong>olfactory tract</strong> then carries the signal back. (the textbook §14.1 has that order backwards in one sentence — bulb first, then tract; §14.2 says it right.) The tract goes to the <strong>primary olfactory cortex</strong> on the medialToward the midline of the body surface of the temporal lobe — anatomists call it the <em>piriform cortex</em>, and the whole region has an older and better name, the <em>rhinencephalon</em>, the nose brain — and straight into the <strong>limbic system and hypothalamus</strong>. <strong>Smell is the one sense that does not synapse in the thalamus first</strong>, which is the anatomical reason a smell can drop you into a childhood kitchen faster than a photograph can. Two more things nobody expects of neurons: olfactory receptor cells are <strong>replaced throughout life</strong>, new axons growing along the old ones into the bulb, one of the few examples of regrowth in the mature nervous systemThe organ system that controls body functions using electrical and chemical signals.; and the whole system can be torn in one blow, because a hard enough hit to the head slides the frontalForehead bone; forms the front part of the skull and roof of the orbits. Smooth and curved. lobe across the plate and shears the threads — permanent <strong>anosmia</strong>, common in boxers. Here is the head-cold truth. Odor molecules reach that epithelium two ways: through the nostrils, and up behind the soft palate from the mouth while you chew. Five qualities on the tongue, thousands of odors behind the nose, and the cortex quietly assembling the difference into <em>flavor</em>. Block the nose and food goes bland while the tongue works perfectly; and in the elderly, when the epithelium stops replacing itself, the usual answer is more salt on the plate — which raises blood volume and blood pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange., which is where a sense organ turns into a cardiovascular risk. In Stina all of this is intact: coffee and mint, one nostril at a time, identified. Write it down.

See it move: Scene 4 · Through the Plate · Scene 5 · The Sense That Skips the Thalamus

5 scenes. Each one has a short animation or a slide, something to explore, a true/false spot-check of your thinking, and a quick check. Every scene gives you an evidence card for your board.

Every scene gave you an evidence card, and the chart adds more. Sort each card into the column it supports: Two chemical senses intact — and written down, Neither on its own, or Nothing to record — normal means nothing to say. Some cards can honestly go in more than one place — that’s allowed, and the feedback tells you which cards decide the case.

Time to decide. Look back at your bet, then build your verdict from statements that hold up — no writing needed.