taste buds (sense organs for taste) see 1826 (2-4) p.
total ≈ 10.000 taste buds.
each taste bud is innervated by ≈ 50 nerve fibers; each nerve fiber receives input from ≈ 5 taste buds.
if sensory nerve is cut, taste buds it innervates degenerate and eventually disappear;
if nerve regenerates, epithelial cells in neighborhood become organized into new taste buds.
anterior two-thirds → chorda tympani branch of facial nerve.
posterior third → glossopharyngeal nerve.
Areas other than tongue → vagus nerve.
all taste fibers (myelinated but slowly conducting) unite in rostral third of nucleus tractus solitarii (in medulla oblongata); further way:
axons cross midline and join medial lemniscus → specific sensory relay nuclei of thalamus.
gustatory lemniscus - uncrossed ascending fibers; some switch in parabrachial nuclei (rostral pons) and continue to thalamus, hypothalamus, amygdala.
some axons connect to adjacent RF and dorsal motor nucleus n. vagi – salivatory and lingual reflexes.
taste projection area is in foot of postcentral gyrus (parietal operculum).
N.B. taste does not have separate cortical projection area and is represented in face area.
Basic Taste Modalities
gustatory chemoreceptors (located on microvilli of taste cells) respond to sapid (taste-producing) substances dissolved in oral fluids bathing them.
concentrating & transporting protein (delivers taste-producing molecules to receptors) is produced by Ebner glands.
sweet - at tongue tip.
most sweet substances are organic: sucrose, maltose, lactose, glucose, polysaccharides, glycerol, some alcohols and ketones, chloroform, beryllium salts, various amides of aspartic acid.
artificial sweeteners (saccharin and aspartame) produce satisfactory sweetening without calorie burden.
thaumatin and monellin (proteins isolated from African berries) are 100,000 times as sweet as sucrose; structures of these two proteins are very different, yet antibodies to one cross-react with antibodies to other (some sort of common 3D structure).
receptor activation: sweet substances act via GS protein → cAMP↑ → reduced K+ conductance → depolarization.
sour - along posterior half of tongue side (also on palate).
ageusia (absence of taste sense); e.g. drugs which contain sulfhydryl groups (e.g. captopril, penicillamine) cause temporary ageusia.
hypogeusia (diminished taste sensitivity) - many different diseases.
dysgeusia (disturbed taste sense).
Bibliography for ch. “Taste” → follow this link >>
Ganong “Review of Medical Physiology”, 2002
NMS Neuroanatomy 1998, Physiology 2001
Viktor’s Notes℠ for the Neurosurgery Resident
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