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Direct connections of primary trigeminal afferent axons with trigeminocerebellar projection neurons in the border zone of rat trigeminal nucleus oralis
.'~'eurosciem'c Letters. 83 11987) 247 252 Elsevier Scientilic Publishers Ireland Ltd.
247
NSI. 1151)20
Direct connections of primary trigeminal afferent axons with trigeminocerebellar projection neurons in the border zone of rat trigeminal nucleus ora|is William M. Falls l),'partment ol Anatomy, Michigan State University, East Lansing, M148b;24-1316 ~/.'.S.A. j (Reccivcd 29 June 1987" Revised ,,ersion received 7 August 1987; Accepted 8 September 1987) K~'.~"words." Rat: Trigeminal nucleus oralis; Primary afferent axon: Trigeminocerebellar neuron: t lorscradish peroxidase: Degeneration This study presents electron microscopical e,,idence for direct synaptic contact between primar) trigem~nal axonal (PR) endings and trigeminocerebellar projection neurons (TCPNs), in the border zone (BZ) of rat trigeminal nucleus oralis. The combined techniques of anterograde degeneration following trigemihal sensory root rhizotomy and retrograde transport of horseradish peroxidase subsequent to injections into the orofacial tactile portions ofcrura I and 11 of the cerebellar hemispheres were utilized. Degenerating PR endings lie centrally in glomeruli where they Ibrm axe, dendritic synapses on higher order dendrites of identilied BZ T('PNs. It is at these synapses that the PR endings are probably transferring tactile input to t'~Z T('PNs for direct relay to the ccrebellar cortex. Transmitter release at the axodendntic synapses may be modilicd within the glomeruli by axoaxonic synapses between terminals containing flattened s',naptic ,,csicles and the PR ending.
Rat trigeminal nucleus oralis (Vo) contains neurons which project directly to orofacial portions of 4 major tactile areas o f the ipsilateral cerebellar cortex (crura l and II and the paramedian Iobule of the hemispheres and the uvula of the vermis) 16.7, 12]. These trigemino-cerebellar projection neurons (TCPNs) are situated in the dorsomedial (DM) and the dorsal one-half of the border zone (BZ) subdivisions [6, 7]. These subdivisions also receive the terminal arborizations of large and small diameter myelinated primary trigeminal axons, i.e. fibers which conduct in the All and Act ranges, respectively, and are thought to be involved in conveying innocuous inputs to Vo neurons II, 5, 10]. It has been suggested that terminals from some of these primary axons synapse on the dendrites of Vo T C P N s thus providing a direct pathway for relaying orofacial tactile inputs to the cerebellar cortex [1, 5, 10]. To date, no anatomical evidence has been presented for direct synaptic contact between a
('orre.~pomh'nce W.M. Falls, Department of Anatomy, B-514 West Fee Hall, Michigan State University. East l.ansmg. M 1 48824-1316, U.S.A. ()3()4-3941) 87 $ I)3.5/)@ 1987 Elsevier Scientitic Publishers Ireland l.td.
24X
primary trigeminal (PR) axonal ending and a Vo TCPN. The present study x~il~ t, ndcrtaken to confirm that TCPNs in thc BZ receive synapses from terminals oF primary trigcminal axons FCPNs were retrogradely labeled by making multiple injections of 0.05 0 . 2 l:1 c)l fresh 30?; HRP (Sigma Type VI) in 2?; dimethylsulfoxide into the right side of orof~-
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Fig. I. a: projection drawings of the dorsal surface of the rat brain and a coronal section through the cerebellum at the level of the Vo showing the position and extent of unilateral H R P injection sites, in one animal, in the orofacial portions of crura I and II. b: diagram from a transverse section through the ipsilateral Vo showing the location (arrow) in the dorsal one-half of the BZ of the retrogradely labeled TCPN illustrated in c. ICP, inferior cerebellar peduncle; SVT. spinal V tract; M D M d and M D M v , dorsal and ventral zones of the middle portion of the dorsomedial subdivision of the Vo: VL. ventrolateral subdivision of the Vo; Vll, facial motor nucleus, c: drawing of a typical BZ TCPN stained in Golgi-like fashion ~ith HRP after crura I and II injections. The dendritic arbor occupies a flattened disk and is oriented parallel to the SVT. The numbered arrow on second-order dendnte D indicates the approximate location at which the electron micrographs in Fig. 2a,b v, ere taken.
249
cial tactile portions ofcrura I and II in two rats (Fig. la). These injection sites were chosen based on the results of previous retrograde HRP investigations [6, 7. 12] indicating that the vast majority o f ' l C P N s in the Vo innervate crura I and II. Details concerning anesthesia and surgical procedure have been described previously [7]. Immediately following the HRP injections in the same animals, a trigeminal sensory root rhizotomv was performed. Two days postoperatively, each animal was deeply
Fig. 2. a.b: tv,'~ adjacent clectrcm micrographs selected from a set of serial sections through dendritc D m i'ig Ic and :l prim:tr,,, a×onal (PR) ending, undergoing dark degeneration lhat silt)'& tile conlponclltx and synaptJc connections which are commonl.,, found m and around this primar.v axon glomerulus in the BZ. In a the centrally located PR ending receives and axoaxonic synapse (arrow) from an ending It:) or,ntaming flattened synaptic ,,csicles. In b the PR ending lk~rms a presumed asymmetrical axodendrltJc s'.naps¢ (crossed arrow) on labeled dendrite 1). Outside tile glomerulus [) receives a symmetrical to rulermediate axodendritic synapse (arrov,) ['rom a P ending containing pleomorphic synaptic ,.¢sicles. Fhe I~..rminal labeled R. ',,,ilia rot, rid synaptic vesicles, is also located outside the glomerulus and it) serial sections it was nc',er observed in symiptic contact with either the PR ending or l~tbc'lccl dendrite [ ) A. axtroc~,tic processc,, x 35.495.
250
anesthetized and perfused transcardially with 250 ml of a solution containing 2% glutaraldehyde-2% paraformaldehyde in 0.15 M phosphate buffer (pH 7.3). Serial transverse sections through the cerebellum and brainstem were cut at 100 ltm on an Oxford vibratome, processed for HRP reaction product using the cobalt-glucose oxidase method [14] and prepared tbr light microscopical analysis [6, 7]. Many TCPNs in the ipsilateral Vo were diffusely filled with HRP reaction product to the extent that much of their dendritic arbors could be characterized. Detailed drawings illustrating the morphological features of several of these cells (Fig. It) were made using a x 100 oil immersion objective at a magnification of x 1250. From its location within the dorsal one-half of the BZ (Fig. I b), as well as by the branching pattern of the flattened disk-shaped dendritic arbor oriented parallel to the spinal V tract. the labeled neuron shown in Fig, Ic conforms to the type of TCPN found in the BZ
[61. The section containing the labeled neuron in Fig. lc was subsequently processed for electron microscopy [3]. The dendritic arbor as well as the cell body were serially thin-sectioned throughout their visible length. Anatomical demonstration that this BZ T C P N receives direct primary trigeminal afferent input was established by the identification of 20 axonai endings, each displaying dark degeneration, which made asymmetrical axodendritic synapses on labeled higher order dendrites (Fig. 2b). No degenerating terminals were found to synapse on either the primary dendrites or the cell body. The exact number of degenerating endings synapsing on this BZ TCPN could not be determined absolutely since distal portions of some higher order dendritic branches may not have been labeled in their entirety. The distribution of observed degenerating terminals across dendrites was on the order of two for each higher-order dendritic branch. Based on studies of synaptic circuitry involving primary axonal endings in trigeminal sensory nuclei [3, 4, 8, 9, 13], the group of primary trigeminal axons whose endings are degenerating in this study are most likely delivering input directly to the dendritic arbors of BZ TCPNs resulting in the activation of these cells. The above findings provide anatomical confirmation that information from primary trigeminal axons is conveyed monosynaptically to orofacial tactile portions of crura I and I I of the cerebellar hemispheres via TCPNs in the dorsal one-half of the BZ. Like primary endings in other trigeminal sensory nuclei [3, 5, 8, 9, 13], most of the degenerating PR endings were found in glomeruli. PR endings were found to be centrally located and surrounded by: a labeled T C P N distal dendrite, an unlabeled dendritic shaft, unmyelinated axons, a small unlabeled axonai ending and astrocytic processes (Fig. 2a,b). In addition to synapsing on the labeled TCPN dendrite, the PR ending made at least one asymmetrical axodendritic synapse with the unlabeled dendrite. To date, potential sources for these unlabeled dendrites are cells situated in other subdivisions of the Vo which send a few of their dendrites into the dorsal one-half of the BZ [5, 7]. These include small- and medium-sized TCPNs located in the extreme lateral portion of the DM (e.g. paramedian Iobule and uvula projection neurons) as well as cells situated in the dorsolateral portion of the ventrolateral subdivision (e.g. type III medullary dorsal horn projection neurons). The small (0.5 1.5 /tm). unlabeled axonal ending was generally dome-shaped and contained flattened
251
agranular synaptic vesicles. This non-primary F ending was always presynaptic in an axoaxonic synapse with the PR ending (Fig. 2a). Since the F ending is the only non-primary axonal ending in each glomerulus, it probably plays a major role in influencing primary afferent transmission. In view of these flattened synaptic vesicles the F ending probably belongs to axons derived from at least one source that can presynaptically inhibit or diminish the firing rate of BZ TCPNs in response to primary, input. To date possible cells of origin include those situated in face areas of the somatosensory cortex [2] as well as neurons in the periaqueductal gray and nucleus raphe magnus [I I]. Evidence for F endings in all of the glomeruli studied here as well as in all other primary afferent glomeruli in the BZ analyzed to date [4, 5] could be coupled with the recent demonstration of abundant leucine- and methionine-enkephalin immunorcactivity in fibers and terminals restricted to the rat BZ [I 5] to suggest that some of these endings may be enkephalinergic. Absolute identification of F endings as enkephalinergic awaits ultrastructural immunoperoxidase studies in the rat Vo. The synaptic organization of the glomeruli analyzed in this study is identical to that described for primary trigeminal afferent glomeruli found throughout the BZ with endings of small diameter myelinated axons at their core [5]. These findings strongly suggest that the degenerating PR endings belong to this group of primary trigeminal axons. If this is the case, these data help substantiate previous anatomical [6] and electrophysiological findings [I, 16] that suggest that small myelinated primary trigeminal axons, carrying orofacial tactile inputs, monosynaptically activate BZ T('PNs. This stud,,' was supported by USPHS Grant DE 06724.. The author would like to thank Ms. Brenda Moore for her excellent technical assistance, Ms. Lizabeth Smith for her constructive comments on the manuscript and Ms. Brenda Schmidt for typing the manuscript. I Azcrad, J., Woda, A. and Albe-Fessard, D., Physiological properties of neurons in difl'ercnl parts of the cal trigeminal sensory complex. Brain Res., 246 (I~)g2) 7 2 I. 2 Dunn. R.('., Jr. and Tolbert, D.L., The corticotrigeminal projeclion in the cat. A sludy of the organi,,ation of cortical projections to the spinal trigeminal nucleus, Brain Res., 240 (1982) 13 25. 3 l:alls. \¥.M.. Morphology and synaptic connections of myelinated primary axons in 1he vcnlrolaleral region of rat lrigemmaI nucleus oralis, J. ( ' o m p Neurol., 244 (1986) 9('~ 110. 4 Fall,,, W.M., Morphology and synaptic connections of unmyelinated primary axons in the border zone ~I" ral trigeminal nucleus oralis, Neurosci. [,ell., 70 (19g(~) 342 347. 5 [:alls, "W.M. and Alban, M.N., Morphology and synaplic conneclions of small myelimited primary trigemmal axons arhorizing a m o n g neurons in the border zone of rat trigeminal nucleus oralis. SomaIosens. Res.. 4 (19gt'l) 97 110. 6 Falls, W.M. and Alban. M.M., Morphological features of identilied trigeminocerebellar projection neurons in the border :,one of rat trigeminal nucleus oralis, Somatosens. Res., 4 (1986) I 12. 7 Falls, W.M., Rice. R.E. and Van Wagner, J.P., Fhe dorsomedial portion of trigeminal nucleus oralis (\:o) in lh¢ r,:|l: cytology and projections to the cerebellum, Somatosens. Res., 3 ([995) g9 118. 8 ( ;t>bel, ,%.. 5'),naplic organization of the substantia gelatinosa glomeruli in the spinal trigemmal nucleus of t h e a d u h c a l , J. Neurocytol.,3(1974) 2[9 243. ~ ( ;ohel. 5,.. Dendroaxonic synapses in the substantia gelatinosa glomeruli of the spinal trigeminal nuclc-
252
us of the cat, J. Comp. Ncurol., 16 (1976) 165-17t~. I0 l layashi. It., Differential terminal distribution of single large cutaneous afferent libres i11 tile ~pmai trigemimd nucleus and in the cervical spinal dorsal horn, Brain Res.. 244 (1982) 17~ 17v I I l lu..J, and Sessle. B.J., "[rigeminal nociceptive and non-nociccptive neurons: brain:~tem mtranuclear projections and modulation by orofuciaI periaqucductal gray and nucleus raphe magnus stimuli, Brain Res., 170(1979) 547 552. 12 ttm:rta, M.F., Frankfurter, A and tlarding, J.K., Studies of the principal sensoD and sensoD and ~pinal trigemina] nuclei of the rat: projections to the superior colliculus, inferior oli',e and cerebellum. J Comp. Neurol., 220 (1983) 147-167. 13 Ide. I..S. and Killackey, l t P . , Fine structural survey of the rat's brainstem sensory trigeminal complex. J. Comp. Neurol., 235 (1985) 145-168. 14 Itoh, K., Konishi, A., Nomura, S,, Mizuno, N., Nakamura, Y. and Sugimoto. T., Application ofcou. pied oxidation reaction to electron microscopic demonstratk)n of horseradish peroxidase: coball glucose oxidase method, Brain Res.. 175 (1979) 341 346. 15 Sar, M., Stumpf, W.l-i., Miller. R.J.. Chang, K.-J. and Cuatrecasas, P., lmmunohistochemical localization of enkephalin in rail brain spinal cord. J. ('omp. Neurol., 182 (1978) 17 38. 16 Sessle. BJ. and Greenwood, L.t-'.. Inputs to trigeminal brainstem neurones from l:acial, oral, toolh and pulp and pharyngolaryngeal tissues. I. Responses to innocuous and noxious stimuli, Brain Res.. 117(1976) 211 226.
247
NSI. 1151)20
Direct connections of primary trigeminal afferent axons with trigeminocerebellar projection neurons in the border zone of rat trigeminal nucleus ora|is William M. Falls l),'partment ol Anatomy, Michigan State University, East Lansing, M148b;24-1316 ~/.'.S.A. j (Reccivcd 29 June 1987" Revised ,,ersion received 7 August 1987; Accepted 8 September 1987) K~'.~"words." Rat: Trigeminal nucleus oralis; Primary afferent axon: Trigeminocerebellar neuron: t lorscradish peroxidase: Degeneration This study presents electron microscopical e,,idence for direct synaptic contact between primar) trigem~nal axonal (PR) endings and trigeminocerebellar projection neurons (TCPNs), in the border zone (BZ) of rat trigeminal nucleus oralis. The combined techniques of anterograde degeneration following trigemihal sensory root rhizotomy and retrograde transport of horseradish peroxidase subsequent to injections into the orofacial tactile portions ofcrura I and 11 of the cerebellar hemispheres were utilized. Degenerating PR endings lie centrally in glomeruli where they Ibrm axe, dendritic synapses on higher order dendrites of identilied BZ T('PNs. It is at these synapses that the PR endings are probably transferring tactile input to t'~Z T('PNs for direct relay to the ccrebellar cortex. Transmitter release at the axodendntic synapses may be modilicd within the glomeruli by axoaxonic synapses between terminals containing flattened s',naptic ,,csicles and the PR ending.
Rat trigeminal nucleus oralis (Vo) contains neurons which project directly to orofacial portions of 4 major tactile areas o f the ipsilateral cerebellar cortex (crura l and II and the paramedian Iobule of the hemispheres and the uvula of the vermis) 16.7, 12]. These trigemino-cerebellar projection neurons (TCPNs) are situated in the dorsomedial (DM) and the dorsal one-half of the border zone (BZ) subdivisions [6, 7]. These subdivisions also receive the terminal arborizations of large and small diameter myelinated primary trigeminal axons, i.e. fibers which conduct in the All and Act ranges, respectively, and are thought to be involved in conveying innocuous inputs to Vo neurons II, 5, 10]. It has been suggested that terminals from some of these primary axons synapse on the dendrites of Vo T C P N s thus providing a direct pathway for relaying orofacial tactile inputs to the cerebellar cortex [1, 5, 10]. To date, no anatomical evidence has been presented for direct synaptic contact between a
('orre.~pomh'nce W.M. Falls, Department of Anatomy, B-514 West Fee Hall, Michigan State University. East l.ansmg. M 1 48824-1316, U.S.A. ()3()4-3941) 87 $ I)3.5/)@ 1987 Elsevier Scientitic Publishers Ireland l.td.
24X
primary trigeminal (PR) axonal ending and a Vo TCPN. The present study x~il~ t, ndcrtaken to confirm that TCPNs in thc BZ receive synapses from terminals oF primary trigcminal axons FCPNs were retrogradely labeled by making multiple injections of 0.05 0 . 2 l:1 c)l fresh 30?; HRP (Sigma Type VI) in 2?; dimethylsulfoxide into the right side of orof~-
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Fig. I. a: projection drawings of the dorsal surface of the rat brain and a coronal section through the cerebellum at the level of the Vo showing the position and extent of unilateral H R P injection sites, in one animal, in the orofacial portions of crura I and II. b: diagram from a transverse section through the ipsilateral Vo showing the location (arrow) in the dorsal one-half of the BZ of the retrogradely labeled TCPN illustrated in c. ICP, inferior cerebellar peduncle; SVT. spinal V tract; M D M d and M D M v , dorsal and ventral zones of the middle portion of the dorsomedial subdivision of the Vo: VL. ventrolateral subdivision of the Vo; Vll, facial motor nucleus, c: drawing of a typical BZ TCPN stained in Golgi-like fashion ~ith HRP after crura I and II injections. The dendritic arbor occupies a flattened disk and is oriented parallel to the SVT. The numbered arrow on second-order dendnte D indicates the approximate location at which the electron micrographs in Fig. 2a,b v, ere taken.
249
cial tactile portions ofcrura I and II in two rats (Fig. la). These injection sites were chosen based on the results of previous retrograde HRP investigations [6, 7. 12] indicating that the vast majority o f ' l C P N s in the Vo innervate crura I and II. Details concerning anesthesia and surgical procedure have been described previously [7]. Immediately following the HRP injections in the same animals, a trigeminal sensory root rhizotomv was performed. Two days postoperatively, each animal was deeply
Fig. 2. a.b: tv,'~ adjacent clectrcm micrographs selected from a set of serial sections through dendritc D m i'ig Ic and :l prim:tr,,, a×onal (PR) ending, undergoing dark degeneration lhat silt)'& tile conlponclltx and synaptJc connections which are commonl.,, found m and around this primar.v axon glomerulus in the BZ. In a the centrally located PR ending receives and axoaxonic synapse (arrow) from an ending It:) or,ntaming flattened synaptic ,,csicles. In b the PR ending lk~rms a presumed asymmetrical axodendrltJc s'.naps¢ (crossed arrow) on labeled dendrite 1). Outside tile glomerulus [) receives a symmetrical to rulermediate axodendritic synapse (arrov,) ['rom a P ending containing pleomorphic synaptic ,.¢sicles. Fhe I~..rminal labeled R. ',,,ilia rot, rid synaptic vesicles, is also located outside the glomerulus and it) serial sections it was nc',er observed in symiptic contact with either the PR ending or l~tbc'lccl dendrite [ ) A. axtroc~,tic processc,, x 35.495.
250
anesthetized and perfused transcardially with 250 ml of a solution containing 2% glutaraldehyde-2% paraformaldehyde in 0.15 M phosphate buffer (pH 7.3). Serial transverse sections through the cerebellum and brainstem were cut at 100 ltm on an Oxford vibratome, processed for HRP reaction product using the cobalt-glucose oxidase method [14] and prepared tbr light microscopical analysis [6, 7]. Many TCPNs in the ipsilateral Vo were diffusely filled with HRP reaction product to the extent that much of their dendritic arbors could be characterized. Detailed drawings illustrating the morphological features of several of these cells (Fig. It) were made using a x 100 oil immersion objective at a magnification of x 1250. From its location within the dorsal one-half of the BZ (Fig. I b), as well as by the branching pattern of the flattened disk-shaped dendritic arbor oriented parallel to the spinal V tract. the labeled neuron shown in Fig, Ic conforms to the type of TCPN found in the BZ
[61. The section containing the labeled neuron in Fig. lc was subsequently processed for electron microscopy [3]. The dendritic arbor as well as the cell body were serially thin-sectioned throughout their visible length. Anatomical demonstration that this BZ T C P N receives direct primary trigeminal afferent input was established by the identification of 20 axonai endings, each displaying dark degeneration, which made asymmetrical axodendritic synapses on labeled higher order dendrites (Fig. 2b). No degenerating terminals were found to synapse on either the primary dendrites or the cell body. The exact number of degenerating endings synapsing on this BZ TCPN could not be determined absolutely since distal portions of some higher order dendritic branches may not have been labeled in their entirety. The distribution of observed degenerating terminals across dendrites was on the order of two for each higher-order dendritic branch. Based on studies of synaptic circuitry involving primary axonal endings in trigeminal sensory nuclei [3, 4, 8, 9, 13], the group of primary trigeminal axons whose endings are degenerating in this study are most likely delivering input directly to the dendritic arbors of BZ TCPNs resulting in the activation of these cells. The above findings provide anatomical confirmation that information from primary trigeminal axons is conveyed monosynaptically to orofacial tactile portions of crura I and I I of the cerebellar hemispheres via TCPNs in the dorsal one-half of the BZ. Like primary endings in other trigeminal sensory nuclei [3, 5, 8, 9, 13], most of the degenerating PR endings were found in glomeruli. PR endings were found to be centrally located and surrounded by: a labeled T C P N distal dendrite, an unlabeled dendritic shaft, unmyelinated axons, a small unlabeled axonai ending and astrocytic processes (Fig. 2a,b). In addition to synapsing on the labeled TCPN dendrite, the PR ending made at least one asymmetrical axodendritic synapse with the unlabeled dendrite. To date, potential sources for these unlabeled dendrites are cells situated in other subdivisions of the Vo which send a few of their dendrites into the dorsal one-half of the BZ [5, 7]. These include small- and medium-sized TCPNs located in the extreme lateral portion of the DM (e.g. paramedian Iobule and uvula projection neurons) as well as cells situated in the dorsolateral portion of the ventrolateral subdivision (e.g. type III medullary dorsal horn projection neurons). The small (0.5 1.5 /tm). unlabeled axonal ending was generally dome-shaped and contained flattened
251
agranular synaptic vesicles. This non-primary F ending was always presynaptic in an axoaxonic synapse with the PR ending (Fig. 2a). Since the F ending is the only non-primary axonal ending in each glomerulus, it probably plays a major role in influencing primary afferent transmission. In view of these flattened synaptic vesicles the F ending probably belongs to axons derived from at least one source that can presynaptically inhibit or diminish the firing rate of BZ TCPNs in response to primary, input. To date possible cells of origin include those situated in face areas of the somatosensory cortex [2] as well as neurons in the periaqueductal gray and nucleus raphe magnus [I I]. Evidence for F endings in all of the glomeruli studied here as well as in all other primary afferent glomeruli in the BZ analyzed to date [4, 5] could be coupled with the recent demonstration of abundant leucine- and methionine-enkephalin immunorcactivity in fibers and terminals restricted to the rat BZ [I 5] to suggest that some of these endings may be enkephalinergic. Absolute identification of F endings as enkephalinergic awaits ultrastructural immunoperoxidase studies in the rat Vo. The synaptic organization of the glomeruli analyzed in this study is identical to that described for primary trigeminal afferent glomeruli found throughout the BZ with endings of small diameter myelinated axons at their core [5]. These findings strongly suggest that the degenerating PR endings belong to this group of primary trigeminal axons. If this is the case, these data help substantiate previous anatomical [6] and electrophysiological findings [I, 16] that suggest that small myelinated primary trigeminal axons, carrying orofacial tactile inputs, monosynaptically activate BZ T('PNs. This stud,,' was supported by USPHS Grant DE 06724.. The author would like to thank Ms. Brenda Moore for her excellent technical assistance, Ms. Lizabeth Smith for her constructive comments on the manuscript and Ms. Brenda Schmidt for typing the manuscript. I Azcrad, J., Woda, A. and Albe-Fessard, D., Physiological properties of neurons in difl'ercnl parts of the cal trigeminal sensory complex. Brain Res., 246 (I~)g2) 7 2 I. 2 Dunn. R.('., Jr. and Tolbert, D.L., The corticotrigeminal projeclion in the cat. A sludy of the organi,,ation of cortical projections to the spinal trigeminal nucleus, Brain Res., 240 (1982) 13 25. 3 l:alls. \¥.M.. Morphology and synaptic connections of myelinated primary axons in 1he vcnlrolaleral region of rat lrigemmaI nucleus oralis, J. ( ' o m p Neurol., 244 (1986) 9('~ 110. 4 Fall,,, W.M., Morphology and synaptic connections of unmyelinated primary axons in the border zone ~I" ral trigeminal nucleus oralis, Neurosci. [,ell., 70 (19g(~) 342 347. 5 [:alls, "W.M. and Alban, M.N., Morphology and synaplic conneclions of small myelimited primary trigemmal axons arhorizing a m o n g neurons in the border zone of rat trigeminal nucleus oralis. SomaIosens. Res.. 4 (19gt'l) 97 110. 6 Falls, W.M. and Alban. M.M., Morphological features of identilied trigeminocerebellar projection neurons in the border :,one of rat trigeminal nucleus oralis, Somatosens. Res., 4 (1986) I 12. 7 Falls, W.M., Rice. R.E. and Van Wagner, J.P., Fhe dorsomedial portion of trigeminal nucleus oralis (\:o) in lh¢ r,:|l: cytology and projections to the cerebellum, Somatosens. Res., 3 ([995) g9 118. 8 ( ;t>bel, ,%.. 5'),naplic organization of the substantia gelatinosa glomeruli in the spinal trigemmal nucleus of t h e a d u h c a l , J. Neurocytol.,3(1974) 2[9 243. ~ ( ;ohel. 5,.. Dendroaxonic synapses in the substantia gelatinosa glomeruli of the spinal trigeminal nuclc-
252
us of the cat, J. Comp. Ncurol., 16 (1976) 165-17t~. I0 l layashi. It., Differential terminal distribution of single large cutaneous afferent libres i11 tile ~pmai trigemimd nucleus and in the cervical spinal dorsal horn, Brain Res.. 244 (1982) 17~ 17v I I l lu..J, and Sessle. B.J., "[rigeminal nociceptive and non-nociccptive neurons: brain:~tem mtranuclear projections and modulation by orofuciaI periaqucductal gray and nucleus raphe magnus stimuli, Brain Res., 170(1979) 547 552. 12 ttm:rta, M.F., Frankfurter, A and tlarding, J.K., Studies of the principal sensoD and sensoD and ~pinal trigemina] nuclei of the rat: projections to the superior colliculus, inferior oli',e and cerebellum. J Comp. Neurol., 220 (1983) 147-167. 13 Ide. I..S. and Killackey, l t P . , Fine structural survey of the rat's brainstem sensory trigeminal complex. J. Comp. Neurol., 235 (1985) 145-168. 14 Itoh, K., Konishi, A., Nomura, S,, Mizuno, N., Nakamura, Y. and Sugimoto. T., Application ofcou. pied oxidation reaction to electron microscopic demonstratk)n of horseradish peroxidase: coball glucose oxidase method, Brain Res.. 175 (1979) 341 346. 15 Sar, M., Stumpf, W.l-i., Miller. R.J.. Chang, K.-J. and Cuatrecasas, P., lmmunohistochemical localization of enkephalin in rail brain spinal cord. J. ('omp. Neurol., 182 (1978) 17 38. 16 Sessle. BJ. and Greenwood, L.t-'.. Inputs to trigeminal brainstem neurones from l:acial, oral, toolh and pulp and pharyngolaryngeal tissues. I. Responses to innocuous and noxious stimuli, Brain Res.. 117(1976) 211 226.