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Central Neural Mechanisms of Taste

Full Article on Wiley Online Library



Abstract

The sections in this article are:

1 Behavioral and Physiological Responses to Gustatory Stimuli
2 Anatomy and Physiology of Central Gustatory System
2.1 Central Terminations of Peripheral Gustatory Neurons in Medulla
2.2 Coding Gustatory Sensation in Brain
2.3 Thalamic Gustatory Area
2.4 Gustatory Neocortex
2.5 Dorsal Pontine Gustatory Area
3 Conclusion
Figure 1. Figure 1.

Coronal sections through rostral medulla of rat depicting relationship of rostral end of nucleus of solitary tract (NST) to other anatomical landmarks. A: normal brain, cresyl Lecht violet stain. The NST boundaries are difficult to distinguish, particularly against subjacent reticular formation (RF). Cells of rostral NST are smaller than in descending vestibular (DVN) and spinal trigeminal (SV) nuclei. DCN, dorsal cochlear nucleus; MVN, medial vestibular nucleus; M VII, facial motor nucleus; RB, restiform body; T V, spinal trigeminal tract. Scale, 1.00 mm. B: autoradiograph in dark‐field illumination at similar level of medulla after injection of tritiated amino acid into root of facial nerve including geniculate ganglion. Largest white area, dense silver grains reduced by tritiated amino acid in synaptic terminals and preterminal ramifications of intermediate nerve within rostral NST. Two smaller white areas result from labeled intermediate nerve axons in solitary tract dorsal to NST and in spinal trigeminal tract lateral to it. Photomicrograph has been purposely overexposed so that some of the anatomical landmarks are visible. Abbreviations as in A. Scale in A represents 0.5 mm in B.

Figure 2. Figure 2.

Semidiagrammatic drawing of horizontal section through nucleus of solitary tract (NST) shows terminal distribution of intermediate facial (seventh), trigeminal (fifth), and vagoglossopharyngeal (ninth and tenth) nerves determined by techniques of Nauta and of Glees for differential staining of degenerating axons. Terminations illustrated by filled triangles, dots, and open circles. CN, cuneate nucleus; G VII, genu of facial root; M V, motor trigeminal nucleus; l., lateral division of NST; m., medial division of NST; P V, principal sensory trigeminal nucleus; S V‐, spinal trigeminal nucleus (c, pars caudalis; ip, pars interpolaris; o, pars oralis); T V, spinal trigeminal tract.

Adapted from Torvik
Figure 3. Figure 3.

Distribution of responses within nucleus of solitary tract (STn) of rats elicited by electrical stimulation of nerves innervating the tongue: chorda tympani branch of facial nerve; lingual‐tonsillar branch of glossopharyngeal (ninth) nerve; and lingual branch of trigeminal nerve. Com n, commissural nucleus of Cajal; SVn, spinal trigeminal nucleus; XII n, hypoglossal nucleus.

From Blomquist and Antem
Figure 4. Figure 4.

Relative magnitude of summated neural responses to 0.01 M quinine hydrochloride (QHCl) recorded in 29 rats in tongue areas of nucleus of solitary tract. Magnitudes of summated neural responses in arbitrary units, adjusted to 100 U for response to 0.1 M NaCl on same region of tongue.

From Halpern and Nelson
Figure 5. Figure 5.

Response profiles of 14 single neurons isolated in nucleus of solitary tract of rats. Response elicited by chemical stimuli flowing over anterior tongue. Unless otherwise noted, stimuli and their concentrations are listed above unit 8. Data represent number of impulses during 1st s of stimulus flow. Arrowhead, resting rate of unit after water rinse.

From Pfaffmann et al. , reprinted from Sensory Communication, edited by W. A. Rosenblith, by permission of The MIT Press, Cambridge, Massachusetts. Copyright © 1961 by the Massachusetts Institute of Technology
Figure 6. Figure 6.

Average concentration response functions for gustatory neurons to variety of chemical stimuli flowed on anterior tongue. A: chorda tympani (CT). B: nucleus of solitary tract (NTS). Number of neurons used in each determination ranged from 19 to 40. Data derived from number of impulses during first 3 s of stimulus flow. Spontaneous rate equals average impulse rate during the second prior to stimulus onset. Arrow in B indicates average rate of firing with distilled water flowing on tongue. Ordinate response scale for NTS is 4 × scale for CT to facilitate comparison of data from 2 levels. NaSac, sodium saccharin; QHCl, quinine hydrochloride; Suc, sucrose.

From Ganchrow and Erickson
Figure 7. Figure 7.

Coronal section through thalamus of rat demonstrates relationship of lingual‐gustatory area nucleus ventralis posteromedialis parvicellularis (VPMpc) to other anatomical landmarks (cresyl violet stain). Dorsal and ventral boundaries of VPMpc are clearly delineated by cell‐poor zones. In rats, lateral and medial limits are often indistinct. Dorsolateral border appears only as transition from darker to lighter gray extending laterally from base of parafascicular nucleus (PFN). Medial edge occurs at border of two small, darkly staining clumps of cells. Cytoarchitectural distinct area includes neurons that respond to all lingual modalities; touch, temperature, and taste. In monkeys a pure gustatory zone is more often designated VPMpc (cf. Fig. ). Hb, habenula; HIT, habenula‐interpeduncular tract; IC, internal capsule; ML, medial lemniscus; VPM, nucleus ventralis posteromedialis. Scale, 1.0 mm.

Figure 8. Figure 8.

Comparison of distribution of thalamic multiunit responses generated by electrical stimulation of chorda tympani (CT), glossopharyngeal (IX), and lingual nerves in rat and squirrel monkey. Vertical lines, electrode tracks; thickened lines, responsive areas. Data represent results of several preparations for each nerve replotted onto standard coronal sections through appropriate levels of thalamus. Although fewer ipsilateral penetrations occur in the rat, areas responding to CT and IXth nerve stimulation appear to be roughly equivalent; lingual area is exclusively contralateral. In the monkey CT and IX representations are largely ipsilateral; lingual is bilateral. CM, centre medianum.

Adapted from Blomquist and Emmers and Benjamin
Figure 9. Figure 9.

Diagrammatic comparison of thalamic areas responsive to tongue‐nerve stimulation in rat and squirrel monkey represented on a horizontal plane. Stippled areas, chorda tympani responses; hatched areas, glossopharyngeal; dashed lines, lingual; solid lines, lingual subnucleus (VPMpc) for rat, and nucleus ventralis posteromedialis (VPM) and VPMpc for squirrel monkey.

From Blomquist and Emmers and Benjamin
Figure 10. Figure 10.

Response profiles to sapid stimuli for 27 neurons isolated in nucleus ventralis posteromedialis parvicellularis of squirrel monkey. Each profile indicates presence or absence of response to a particular chemical at a given concentration, but not magnitude of response. Threshold for response is indicated on logarithmic concentration scale, with highest concentration 100 of each chemical listed in upper right. The higher the bar, the weaker the concentration of that chemical required to alter activity of unit. Numbers within some profiles indicate that more than one unit exhibited this pattern of sensitivity. Open bars, unit did not respond to stimulus (HCl in all cases), but was activated by subsequent water rinse. Letter F over bar also indicates a response to water rinse. Symbol for plus, unit was a positive potential. Note that more than one‐half of units (16/27) responded to only 1 or 2 chemicals. Most common sensitivity was to sucrose or NaCl.

From Benjamin , reproduced with permission from Pergamon Press, Ltd
Figure 11. Figure 11.

Comparison of distribution on exposed cortex of evoked potentials elicited by electrical stimulation of chorda tympani, glossopharyngeal (IXth), or lingual nerves in squirrel monkey. Only cortex rostral to Sylvian fissure is included in diagrams.

From Benjamin et al.
Figure 12. Figure 12.

Relative location of tongue‐nerve projection zones of squirrel monkey. Stippled area, lateral convexity, includes both ipsi‐ and contralateral evoked potentials resulting from stimulation of chorda tympani (CT), glossopharyngeal (ninth), and lingual nerves. Solid area, buried opercular cortex, includes only ipsilateral potentials from CT and ninth nerve.

From Benjamin and Burton
Figure 13. Figure 13.

Loci of cortical units responding to sapid stimuli (solid stars) or mechanical stimulation of oral cavity (open stars) in awake, behaving squirrel monkeys. Taste units are concentrated in opercular evoked potential zone. Two other points are worth noting: First, although potentials evoked by lingual nerve are not recorded from opercular cortex, oral mechanosensory neurons occur in operculum intermixed with taste units. Second, both gustatory and oral mechanosensory responses occur caudal to buried gustatory nerve zones mapped by Benjamin and Burton .

From Sudakov et al.
Figure 14. Figure 14.

Diagrammatic representation of gustatory cortex (G) in relation to the first (SI) and second (SII) somatosensory areas in rat. Visual (V) and auditory (A) cortex are also outlined. Gustatory area is based on degeneration arising from a lesion confined to caudal end of nucleus ventralis posteromedialis parvicellularis in thalamus.

From Donaldson et al.
Figure 15. Figure 15.

Distribution of degenerating axons charted onto tracings of projected coronal sections through rostral medulla and caudal pons. Large aligned dots, degenerating fascicles of fibers. Smaller dots, single fibers or preterminal arborization. Top panel, lesion (L) was placed in electrophysiologically identified gustatory area in rostral nucleus of solitary tract. Degenerating fibers do not appear in either medial lemniscus or thalamus but rather ascend a short distance in ipsilateral reticular formation and terminate in caudal parabrachial nuclei. BC, brachium conjunctivum; CST, cerebello‐spinal tract; Mes V, mesencephalic trigeminal tract and nucleus; VII, facial motor nucleus.

From Norgren and Leonard
Figure 16. Figure 16.

Pontine gustatory area in caudal parabrachial nuclei of rat. A: low‐power photomicrograph of coronal section through pons illustrates relationship of parabrachial nuclei to other anatomical landmarks (cresyl Lecht violet stain). Scale, 1.0 mm. BC, brachium conjunctivum; DTN, dorsal tegmental nucleus; lc, locus ceruleus; MCP, middle cerebellar peduncle; Mes V, mesencephalic trigeminal tract and nucleus; MV or MoV, trigeminal motor nucleus; PBN, parabrachial nuclei; PV, principal sensory trigeminal nucleus; STA, supratrigeminal area; TrV, trigeminal sensory tract. B: location of single units responding to gustatory and other intra‐ and perioral stimuli plotted on tracings of projected sections through dorsal pons. Sections separated by 200 μm. Filled circles, neurons that respond best to sapid stimuli applied on anterior tongue (fungiform papilla innervated by chorda typmani nerve). Open circles, units that respond best to sapid stimuli applied in posterior oral cavity and that presumably activate receptors in circumvallate and foliate papillae (innervated by glossopharyngeal nerve), and on palate (innervated by greater superficial petrosal nerve). Open squares, response to tongue thermal or tactile stimuli. Filled squares, response to jaw stretch or tooth top. Cross, units that did not respond to any stimulus tested. Sapid stimuli consisted of 0.25 M NaCl, 0.5 M sucrose, 0.003 M quinine HCl, and 0.003 N HCl in distilled water at room temperature. LC, locus ceruleus. Other abbreviations as for Part A.

B: from Norgren and Pfaffman
Figure 17. Figure 17.

Filmed oscilloscope traces of activity of single unit isolated in parabrachial nuclei. Dots above traces indicate spikes counted as data. NaCl (0.25 M) applied to anterior tongue (Na, A) elicited some background activity, but inhibited counted unit. When applied to posterior oral cavity (Na, P), same stimulus elicited a sustained response. Remaining gustatory stimuli produced less distinct inhibition when applied to anterior tongue, and less activation when applied to posterior oral cavity. Response to 0.003 M quinine HCl (Q, A and Q, P) is illustrated. Lines beneath traces indicate onset and duration of water rinse (unlabeled) and sapid stimuli. Calibrations, 2.0 s and 100 μV; negative, toward top.

From Norgren and Pfaffmann
Figure 18. Figure 18.

Charts of rostral distribution of [3H]proline on tracings of projected sections of rat brain processed for autoradiographic demonstration of axonal ramifications. Labeled proline was iontopho‐retically applied to electrophysiologically identified gustatory zone in dorsal pons. (a, b): Hatched area, densest label at injection site. Large dots within hatched area, zona in which somata concentrated the labeled proline. BC, brachium conjunctivum; LC, locus ceruleus; Mes V, mesencephalic trigeminal nucleus; MLF, medial longitudinal fasciculus; MRV, motor root of trigeminal nerve; M V, trigeminal motor nucleus; P V, principal trigeminal sensory nucleus; ST V, spinal trigeminal tract. (c‐i): Short lines, orientation of labeled fascicles of fibers. Dots, relative density of label distributions not obviously oriented along axons. Number beside each section, distance in millimeters rostral to injection site. AC, anterior commissure; AHA, anterior hypothalamic area; Amyg, amygdala; BLA, basolateral complex of amygdala; CG, central gray; C‐P, caudate putamen; F, fornix; Fb, fimbria; Hb, habenula; HIT, habenulo‐interpeduncular tract; HPC, hippocampus; IC, internal capsule; IP, inter‐peduncular nucleus; ITR, inferior thalamic radiations; LOT, lateral olfactory tract; MFB, medial forebrain bundle; MG, medial geniculate; ML, medial lemniscus; MIT, mammillothalamic tract; OT, optic tract; PLAC, posterior limb of the anterior commissure; RN, red nucleus; S, septum; SC, superior colliculus; SM, stria medullaris; SN, substantia nigra; SOC, supraoptic commissure; ST, stria terminalis, STN, subthalamic nucleus; V, ventricle; Vb, ventrobasal complex; VMH, ventromedial nucleus of the hypothalamus; ZI, zona incerta.

From Norgren


Figure 1.

Coronal sections through rostral medulla of rat depicting relationship of rostral end of nucleus of solitary tract (NST) to other anatomical landmarks. A: normal brain, cresyl Lecht violet stain. The NST boundaries are difficult to distinguish, particularly against subjacent reticular formation (RF). Cells of rostral NST are smaller than in descending vestibular (DVN) and spinal trigeminal (SV) nuclei. DCN, dorsal cochlear nucleus; MVN, medial vestibular nucleus; M VII, facial motor nucleus; RB, restiform body; T V, spinal trigeminal tract. Scale, 1.00 mm. B: autoradiograph in dark‐field illumination at similar level of medulla after injection of tritiated amino acid into root of facial nerve including geniculate ganglion. Largest white area, dense silver grains reduced by tritiated amino acid in synaptic terminals and preterminal ramifications of intermediate nerve within rostral NST. Two smaller white areas result from labeled intermediate nerve axons in solitary tract dorsal to NST and in spinal trigeminal tract lateral to it. Photomicrograph has been purposely overexposed so that some of the anatomical landmarks are visible. Abbreviations as in A. Scale in A represents 0.5 mm in B.



Figure 2.

Semidiagrammatic drawing of horizontal section through nucleus of solitary tract (NST) shows terminal distribution of intermediate facial (seventh), trigeminal (fifth), and vagoglossopharyngeal (ninth and tenth) nerves determined by techniques of Nauta and of Glees for differential staining of degenerating axons. Terminations illustrated by filled triangles, dots, and open circles. CN, cuneate nucleus; G VII, genu of facial root; M V, motor trigeminal nucleus; l., lateral division of NST; m., medial division of NST; P V, principal sensory trigeminal nucleus; S V‐, spinal trigeminal nucleus (c, pars caudalis; ip, pars interpolaris; o, pars oralis); T V, spinal trigeminal tract.

Adapted from Torvik


Figure 3.

Distribution of responses within nucleus of solitary tract (STn) of rats elicited by electrical stimulation of nerves innervating the tongue: chorda tympani branch of facial nerve; lingual‐tonsillar branch of glossopharyngeal (ninth) nerve; and lingual branch of trigeminal nerve. Com n, commissural nucleus of Cajal; SVn, spinal trigeminal nucleus; XII n, hypoglossal nucleus.

From Blomquist and Antem


Figure 4.

Relative magnitude of summated neural responses to 0.01 M quinine hydrochloride (QHCl) recorded in 29 rats in tongue areas of nucleus of solitary tract. Magnitudes of summated neural responses in arbitrary units, adjusted to 100 U for response to 0.1 M NaCl on same region of tongue.

From Halpern and Nelson


Figure 5.

Response profiles of 14 single neurons isolated in nucleus of solitary tract of rats. Response elicited by chemical stimuli flowing over anterior tongue. Unless otherwise noted, stimuli and their concentrations are listed above unit 8. Data represent number of impulses during 1st s of stimulus flow. Arrowhead, resting rate of unit after water rinse.

From Pfaffmann et al. , reprinted from Sensory Communication, edited by W. A. Rosenblith, by permission of The MIT Press, Cambridge, Massachusetts. Copyright © 1961 by the Massachusetts Institute of Technology


Figure 6.

Average concentration response functions for gustatory neurons to variety of chemical stimuli flowed on anterior tongue. A: chorda tympani (CT). B: nucleus of solitary tract (NTS). Number of neurons used in each determination ranged from 19 to 40. Data derived from number of impulses during first 3 s of stimulus flow. Spontaneous rate equals average impulse rate during the second prior to stimulus onset. Arrow in B indicates average rate of firing with distilled water flowing on tongue. Ordinate response scale for NTS is 4 × scale for CT to facilitate comparison of data from 2 levels. NaSac, sodium saccharin; QHCl, quinine hydrochloride; Suc, sucrose.

From Ganchrow and Erickson


Figure 7.

Coronal section through thalamus of rat demonstrates relationship of lingual‐gustatory area nucleus ventralis posteromedialis parvicellularis (VPMpc) to other anatomical landmarks (cresyl violet stain). Dorsal and ventral boundaries of VPMpc are clearly delineated by cell‐poor zones. In rats, lateral and medial limits are often indistinct. Dorsolateral border appears only as transition from darker to lighter gray extending laterally from base of parafascicular nucleus (PFN). Medial edge occurs at border of two small, darkly staining clumps of cells. Cytoarchitectural distinct area includes neurons that respond to all lingual modalities; touch, temperature, and taste. In monkeys a pure gustatory zone is more often designated VPMpc (cf. Fig. ). Hb, habenula; HIT, habenula‐interpeduncular tract; IC, internal capsule; ML, medial lemniscus; VPM, nucleus ventralis posteromedialis. Scale, 1.0 mm.



Figure 8.

Comparison of distribution of thalamic multiunit responses generated by electrical stimulation of chorda tympani (CT), glossopharyngeal (IX), and lingual nerves in rat and squirrel monkey. Vertical lines, electrode tracks; thickened lines, responsive areas. Data represent results of several preparations for each nerve replotted onto standard coronal sections through appropriate levels of thalamus. Although fewer ipsilateral penetrations occur in the rat, areas responding to CT and IXth nerve stimulation appear to be roughly equivalent; lingual area is exclusively contralateral. In the monkey CT and IX representations are largely ipsilateral; lingual is bilateral. CM, centre medianum.

Adapted from Blomquist and Emmers and Benjamin


Figure 9.

Diagrammatic comparison of thalamic areas responsive to tongue‐nerve stimulation in rat and squirrel monkey represented on a horizontal plane. Stippled areas, chorda tympani responses; hatched areas, glossopharyngeal; dashed lines, lingual; solid lines, lingual subnucleus (VPMpc) for rat, and nucleus ventralis posteromedialis (VPM) and VPMpc for squirrel monkey.

From Blomquist and Emmers and Benjamin


Figure 10.

Response profiles to sapid stimuli for 27 neurons isolated in nucleus ventralis posteromedialis parvicellularis of squirrel monkey. Each profile indicates presence or absence of response to a particular chemical at a given concentration, but not magnitude of response. Threshold for response is indicated on logarithmic concentration scale, with highest concentration 100 of each chemical listed in upper right. The higher the bar, the weaker the concentration of that chemical required to alter activity of unit. Numbers within some profiles indicate that more than one unit exhibited this pattern of sensitivity. Open bars, unit did not respond to stimulus (HCl in all cases), but was activated by subsequent water rinse. Letter F over bar also indicates a response to water rinse. Symbol for plus, unit was a positive potential. Note that more than one‐half of units (16/27) responded to only 1 or 2 chemicals. Most common sensitivity was to sucrose or NaCl.

From Benjamin , reproduced with permission from Pergamon Press, Ltd


Figure 11.

Comparison of distribution on exposed cortex of evoked potentials elicited by electrical stimulation of chorda tympani, glossopharyngeal (IXth), or lingual nerves in squirrel monkey. Only cortex rostral to Sylvian fissure is included in diagrams.

From Benjamin et al.


Figure 12.

Relative location of tongue‐nerve projection zones of squirrel monkey. Stippled area, lateral convexity, includes both ipsi‐ and contralateral evoked potentials resulting from stimulation of chorda tympani (CT), glossopharyngeal (ninth), and lingual nerves. Solid area, buried opercular cortex, includes only ipsilateral potentials from CT and ninth nerve.

From Benjamin and Burton


Figure 13.

Loci of cortical units responding to sapid stimuli (solid stars) or mechanical stimulation of oral cavity (open stars) in awake, behaving squirrel monkeys. Taste units are concentrated in opercular evoked potential zone. Two other points are worth noting: First, although potentials evoked by lingual nerve are not recorded from opercular cortex, oral mechanosensory neurons occur in operculum intermixed with taste units. Second, both gustatory and oral mechanosensory responses occur caudal to buried gustatory nerve zones mapped by Benjamin and Burton .

From Sudakov et al.


Figure 14.

Diagrammatic representation of gustatory cortex (G) in relation to the first (SI) and second (SII) somatosensory areas in rat. Visual (V) and auditory (A) cortex are also outlined. Gustatory area is based on degeneration arising from a lesion confined to caudal end of nucleus ventralis posteromedialis parvicellularis in thalamus.

From Donaldson et al.


Figure 15.

Distribution of degenerating axons charted onto tracings of projected coronal sections through rostral medulla and caudal pons. Large aligned dots, degenerating fascicles of fibers. Smaller dots, single fibers or preterminal arborization. Top panel, lesion (L) was placed in electrophysiologically identified gustatory area in rostral nucleus of solitary tract. Degenerating fibers do not appear in either medial lemniscus or thalamus but rather ascend a short distance in ipsilateral reticular formation and terminate in caudal parabrachial nuclei. BC, brachium conjunctivum; CST, cerebello‐spinal tract; Mes V, mesencephalic trigeminal tract and nucleus; VII, facial motor nucleus.

From Norgren and Leonard


Figure 16.

Pontine gustatory area in caudal parabrachial nuclei of rat. A: low‐power photomicrograph of coronal section through pons illustrates relationship of parabrachial nuclei to other anatomical landmarks (cresyl Lecht violet stain). Scale, 1.0 mm. BC, brachium conjunctivum; DTN, dorsal tegmental nucleus; lc, locus ceruleus; MCP, middle cerebellar peduncle; Mes V, mesencephalic trigeminal tract and nucleus; MV or MoV, trigeminal motor nucleus; PBN, parabrachial nuclei; PV, principal sensory trigeminal nucleus; STA, supratrigeminal area; TrV, trigeminal sensory tract. B: location of single units responding to gustatory and other intra‐ and perioral stimuli plotted on tracings of projected sections through dorsal pons. Sections separated by 200 μm. Filled circles, neurons that respond best to sapid stimuli applied on anterior tongue (fungiform papilla innervated by chorda typmani nerve). Open circles, units that respond best to sapid stimuli applied in posterior oral cavity and that presumably activate receptors in circumvallate and foliate papillae (innervated by glossopharyngeal nerve), and on palate (innervated by greater superficial petrosal nerve). Open squares, response to tongue thermal or tactile stimuli. Filled squares, response to jaw stretch or tooth top. Cross, units that did not respond to any stimulus tested. Sapid stimuli consisted of 0.25 M NaCl, 0.5 M sucrose, 0.003 M quinine HCl, and 0.003 N HCl in distilled water at room temperature. LC, locus ceruleus. Other abbreviations as for Part A.

B: from Norgren and Pfaffman


Figure 17.

Filmed oscilloscope traces of activity of single unit isolated in parabrachial nuclei. Dots above traces indicate spikes counted as data. NaCl (0.25 M) applied to anterior tongue (Na, A) elicited some background activity, but inhibited counted unit. When applied to posterior oral cavity (Na, P), same stimulus elicited a sustained response. Remaining gustatory stimuli produced less distinct inhibition when applied to anterior tongue, and less activation when applied to posterior oral cavity. Response to 0.003 M quinine HCl (Q, A and Q, P) is illustrated. Lines beneath traces indicate onset and duration of water rinse (unlabeled) and sapid stimuli. Calibrations, 2.0 s and 100 μV; negative, toward top.

From Norgren and Pfaffmann


Figure 18.

Charts of rostral distribution of [3H]proline on tracings of projected sections of rat brain processed for autoradiographic demonstration of axonal ramifications. Labeled proline was iontopho‐retically applied to electrophysiologically identified gustatory zone in dorsal pons. (a, b): Hatched area, densest label at injection site. Large dots within hatched area, zona in which somata concentrated the labeled proline. BC, brachium conjunctivum; LC, locus ceruleus; Mes V, mesencephalic trigeminal nucleus; MLF, medial longitudinal fasciculus; MRV, motor root of trigeminal nerve; M V, trigeminal motor nucleus; P V, principal trigeminal sensory nucleus; ST V, spinal trigeminal tract. (c‐i): Short lines, orientation of labeled fascicles of fibers. Dots, relative density of label distributions not obviously oriented along axons. Number beside each section, distance in millimeters rostral to injection site. AC, anterior commissure; AHA, anterior hypothalamic area; Amyg, amygdala; BLA, basolateral complex of amygdala; CG, central gray; C‐P, caudate putamen; F, fornix; Fb, fimbria; Hb, habenula; HIT, habenulo‐interpeduncular tract; HPC, hippocampus; IC, internal capsule; IP, inter‐peduncular nucleus; ITR, inferior thalamic radiations; LOT, lateral olfactory tract; MFB, medial forebrain bundle; MG, medial geniculate; ML, medial lemniscus; MIT, mammillothalamic tract; OT, optic tract; PLAC, posterior limb of the anterior commissure; RN, red nucleus; S, septum; SC, superior colliculus; SM, stria medullaris; SN, substantia nigra; SOC, supraoptic commissure; ST, stria terminalis, STN, subthalamic nucleus; V, ventricle; Vb, ventrobasal complex; VMH, ventromedial nucleus of the hypothalamus; ZI, zona incerta.

From Norgren
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Ralph Norgren. Central Neural Mechanisms of Taste. Compr Physiol 2011, Supplement 3: Handbook of Physiology, The Nervous System, Sensory Processes: 1087-1128. First published in print 1984. doi: 10.1002/cphy.cp010324