Gustatory+System

=**Gustatory System**=


 * 1. Describe the major pathways of the gustatory system.**

Primary pathways of the gustatory system travel along CN VII, CN IX, and CN X. The chorda tympani branch of CN VII has cell bodies in the geniculate ganglion and innervates the rostral 2/3 of the tongue, mostly through the fungiform papillae. Chorda tympani fibres project ipsilaterally to the rostral part of the solitary nucleus. The CN IX has cell bodies in the petrosal ganglion and innervate the caudal third of the tongue, mostly through the circumvallate and foliate papillae. CN IX projects ipsilaterally to the solitary nucleus, just caudal to where the chorda tympani fibers terminate. The superior laryngeal branch of CN X has cell bodies in the nodose ganglion and innervate taste buds in the throat regions. Superior laryngeal branch projects ipsilaterally to the solitary nucleus, caudal to where CN IX fibers terminate. Somatosensory sensations from filiform papillae of the tongue are mediated by the trigeminal nerve which projects ipsilaterally to the spinal trigeminal nucleus for the rostral 2/3 of the tongue, and by the CN IX for the caudal 1/3 of the tongue.

All the primary taste pathways terminate on secondary neurons that reside in the solitary nucleus. Axons project mainly through ipsilateral central tegmental tract to ipsilateral ventral posteromedial nucleus (VPM) of the thalamus.

Secondary neurons from the VPM project to ipsilateral cortical taste regions, presumably in the insula and adjacent frontal cortex (frontal operculum). Some projections from the solitary nucleus go to the pontine taste area in the parabrachial nucleus of the pons. Some of these projections continue to the substantia innominata, near the hypothalamus, where test information can be combined with olfactory information.


 * 2. State the process and time course of taste receptor cell regeneration.**

Receptor cells continuously degenerate and are replaced by new ones every 10 days. Basal cells develop into supporting cells. Supporting cells then develop into receptor cells.


 * 3. Explain how information about tastes is converted to electrical activity in taste buds.**

Taste buds are chemoreceptors that are stimulated by substances in fluids on the tongue, resulting in depolarization. Depolarization can be caused directly by the tastant effect on ion channels or indirectly by second messenger systems. Constant taste stimulation results in adaptation.

Salty tastes result in direct Na+ entry into the receptor cell; Sour tastes result in decreased permeability to K+.

Bitter tastes result from binding to G-protein (gustducin) coupled receptors, leading to increased IP3 and intracellular Ca2+. Sweet tastes result from binding to G-protein coupled receptors, leading to increased cAMP and decreased permeability to K+. Depolarization causes Ca2+ entry and release of neurotransmitters on to primary sensory neurons. Action potentials propagate to terminals in the solitary nucleus. Nerve fibers are small and mostly unmyelinated. Each nerve fiber receives input from about 5 taste buds (convergence) but each taste bud synapses with about 50 nerve fibers (divergence), resulting in overlapping receptive fields.


 * 4. State the 4 primary taste modalities, where they are mainly sensed on the tongue, and what types of substances have each taste.**


 * **Taste Modality** || **Location** || **Substances** || **Threshold** ||
 * Sweet || Tip of tongue || Variety of compounds: sugars, alcohols, ketones, amino acids || Sucrose @ 10mM ||
 * Salty || Front and sides of tongue || Inorganic salts: NaCl, NaF, NH4Cl, MgCl2 || NaCl @ 10mM ||
 * Sour || Middle part of each side || Acids || HCl @ 0.1mM ||
 * Bitter || Back, at circumvallate papillae || Variety of compounds: organic alkaloids, toxic compounds || Quinine @ 0.01mM ||
 * Umami || Tip of tongue || Glutamate || Who cares? I don’t ||


 * 5. Describe what is known about the coding of taste quality and intensity in the primary neurons of the gustatory system.**

Stimulus strength is coded by the number of active nerve fibers and by the frequency of action potentials in each fiber.


 * 6. State factors that can affect the tastes of foods.**

Most actual food tastes are combinations of taste bud stimulation, somatosensory receptor stimulation, and olfactory stimulation. Additionally, taste of a particular substance can be influenced by the internal chemical environment of the body. There are also genetic differences in tastes (e.g. phenylthiourea is bitter tasting to some people at a lower threshold than others). Lastly, tastes of particular foods can change after bad experiences and aging (lose taste buds especially after 45 years of age).


 * 7. State 3 factors that can lead to loss of taste sensation.**

Lost of taste is ageusia. Decreased sensation of taste is hypogeusia. Taste disorders can results form lesion of crania nerves (CN VII, and CN IX). Lesions of facial nerve often occur during middle ear disease or surgery because chorda tympani crosses the middle ear. Head trauma and aging can also result in taste disorders.