Pain. Part 2a: Trigeminal Anatomy Related to Pain.

In order to understand the underlying principles of orofacial pain it is important to understand the corresponding anatomy and mechanisms. Paper 1 of this series explains the central nervous and peripheral nervous systems relating to pain. The trigeminal nerve is the 'great protector' of the most important region of our body. It is the largest sensory nerve of the body and over half of the sensory cortex is responsive to any stimulation within this system. This nerve is the main sensory system of the branchial arches and underpins the protection of the brain, sight, smell, airway, hearing and taste, underpinning our very existence. The brain reaction to pain within the trigeminal system has a significant and larger reaction to the threat of, and actual, pain compared with other sensory nerves. We are physiologically wired to run when threatened with pain in the trigeminal region and it is a 'miracle' that patients volunteer to sit in a dental chair and undergo dental treatment. Clinical Relevance: This paper aims to provide the dental and medical teams with a review of the trigeminal anatomy of pain and the principles of pain assessment.

Cervicogenic headache: evidence that the neck is a pain generator.

This review was developed as part of a debate, and takes the "pro" stance that abnormalities of structures in the neck can be a significant source of headache. The argument for this is developed from a review of the medical literature, and is made in 5 steps. It is clear that the cervical region contains many pain-sensitive structures, and that these are prone to injury. The anatomical and physiological mechanisms are in place to allow referral of pain to the head including frontal head regions and even the orbit in patients with pain originating from many of these neck structures. Clinical studies have shown that pain from cervical spine structures can in fact be referred to the head. Finally, clinical treatment trials involving patients with proven painful disorders of upper cervical zygapophysial joints have shown significant headache relief with treatment directed at cervical pain generators. In conclusion, painful disorders of the neck can give rise to headache, and the challenge is to identify these patients and treat them successfully.

Trigeminal nucleus caudalis anatomy: guidance for radiofrequency dorsal root entry zone lesioning.

OBJECT: This study seeks to improve the accuracy of trigeminal nucleus caudalis dorsal root entry zone (DREZ) radiofrequency lesioning by quantifying the size and orientation of the nucleus caudalis. METHODS: Using serial axial photographs of 6 formalin-fixed cadaver brainstems, digital nucleus caudalis measurements were taken at 1-mm intervals from the level of the obex to the C(2) dorsal nerve roots. RESULTS: From the obex to the C(2) dorsal nerve roots, the nucleus caudalis decreases in width (from 2.6 ± 0.2 to 1.0 ± 0.3 mm) and, excluding superficial tract thickness, decreases in axial nucleus depth (from 2.4 ± 0.3 to 1.7 ± 0.2 mm). At levels between the obex and 10 mm caudal to the obex, the accessory nerve rootlets exit the brainstem at the junction of the spinal trigeminal tract and the dorsal spinocerebellar tract. CONCLUSION: This study details the anatomic dimensions and orientation of the nucleus caudalis for surgeons who perform DREZ lesioning.

Differential responses of rostral subnucleus caudalis and upper cervical dorsal horn neurons to mechanical and chemical stimulation of the parotid gland in rats.

Blockage of the salivary duct can produce pain and inflammation from the build up of saliva in the parotid gland. The processing of parotid inflammation-induced pain, however, is poorly understood. The purpose of this study was to clarify the functional involvement of the trigeminal subnucleus interpolaris/caudalis transition region (Vi/Vc) and upper cervical spinal cord (C1/C2) in processing nociceptive input relevant to parotitis. The effect of capsaicin-induced parotitis was examined on a total of 37 nociceptive neurons isolated from the Vi/Vc (n = 23) and C1/C2 (n = 14) regions. Eight of 23 Vi/Vc neurons responded to mechanical distention of the parotid gland, whereas no C1/C2 neurons responded to the parotid distention. Receptive field characteristics in all neurons were examined following capsaicin injections into the parotid gland. Mechanical and cold responses increased significantly in C1/C2 but not Vi/Vc neurons following capsaicin. Receptive field sizes also increased in C1/C2 but not Vi/Vc neurons. At the Vi/Vc transition region, pinch-evoked activity increased in neurons receiving convergent inputs from the parotid gland and facial skin when compared to non-convergent neurons. The present data indicate that the hyperalgesia and referred pain associated with parotitis may result from sensitization of C1/C2, but not Vi/Vc nociceptive neurons.

Morphological interrelationship between astrocytes and nerve endings in the rat spinal trigeminal nucleus caudalis.

It has been reported that the spinal trigeminal nucleus caudalis (Sp5C), which receives nociceptive information from the oro-facial regions, has four laminae. To clarify the role of glial cells in the transmission of the nociceptive information, the present study was conducted to examine the detailed distribution of astrocytes in each lamina and also to investigate a morphological interrelationship between the astrocytes and nerve endings in the rat Sp5C. After the preparation of the serial cryostat sections, immunohistochemistry for glial fibrillary acidic protein (GFAP) was employed to identify the astrocytes, and immunohistochemistry for substance P (SP), calcitonin gene-related peptide (CGRP), was used for the nerve endings. We also employed double-labeling immunofluorescence and electron microscopic immunohistochemistry for the GFAP/SP or GFAP/CGRP. GFAP-positive reactions were observed in all laminae of the Sp5C, and SP- or CGRP-positive nerve endings were observed in the lamina I and II. Additionally, we clarified the presence of GFAP/SP- or GFAP/CGRP-positive reactions by the double-labeling immunofluorescence and demonstrated the morphological interrelationship between the astrocytes and nerve endings by the double-labeling electron microscopic immunohistochemistry. These findings suggest that astrocytes might play some roles in the transmission of nociceptive information from the oro-facial region.

Postnatal development of excitation propagation in the trigeminal subnucleus caudalis evoked by afferent stimulation in mice.

The postnatal development of nociceptive afferent activity expansion and its modulation features were examined in mice using an optical imaging technique. Developing mice (1-2 weeks old (N1-2 w), 3-4 weeks old (N3-4 w), 5-6 weeks old (N5-6 w) and 7-8 weeks old (N7-8 w)) and neonatally capsaicin-treated mice were used. The propagation of neuronal excitation was measured by changes in fluorescent intensity in horizontal brain stem slices evoked by electrical stimulation to the trigeminal spinal tract. A single-pulse stimulation evoked excitation propagation in the trigeminal caudalis (Vc). The propagation area was larger in N1-2 w than in N7-8 w, and no differences were observed between capsaicin-treated and naive mice in the same age groups. Repetitive stimulation (100 Hz, 30 pulses) elicited long-lasting and widespread excitation propagation. The excitation propagation area was significantly larger in N7-8 w than in N1-2 w, N3-4 w and N5-6 w. This propagation was suppressed by 5 microM L-703.606, an NK1-receptor antagonist, suggesting that the repetitive stimulation-elicited excitation may require substance-P releases. Morphological observations demonstrated that the neural network in the Vc had grown by postnatal week 5. These results suggest that nociceptive afferent activity co-operatively matures with development of the network structure in the Vc, and that a mechanism for prolonged increase in central excitability is established during a later postnatal period.

Ascending projections from the lateral descending and common sensory trigeminal nuclei in python.

The primary sensory trigeminal system of Python is characterized by the presence of an additional nucleus which is involved in processing data obtained by infrared sensors. This so-called lateral descending nucleus (LTTD) is strictly separated from the nuclei of the common sensory trigeminal system. The present study was undertaken in order to establish the relation between the two sensory trigeminal systems and higher brainstem structures. Further we studied whether the projections of these two systems remain separated at higher brainstem levels. It is shown that the organization of particularly the thalamus is characterized by the presence of specific projection areas of each of the two trigeminal systems: a) the ability of infrared preception is reflected particularly in the presence of an unique thalamic nucleus: the nucleus pararotundus and probably also in the enlargement of nucleus rotundus; and b) distinct subnuclei in the thalamic ventral nuclear complex are related to the various nuclei of the common sensory trigeminal system. The main ascending projection of LTTD runs via a distinct tract to the central gray layer (SGC) of the contralateral tectum mesencephali and the nucleus pararotundus (PR). Rostrally, numerous fibres decussate again via the tectal commissure and terminate ipsilaterally in the rostral part of SGC and in PR. The ascending projections of the common sensory trigeminal nuclei resemble those of mammals by gaining thalamic nuclei (ventral nuclear complex). No projections of the tectum nor to the striatum (like in birds) were observed. The two sensory trigeminal systems remain separately organised, in their projections as well as in their structure. No major connection between the two trigeminal system is present.

Corneal and periocular representation within the trigeminal sensory complex in the cat studied with transganglionic transport of horseradish peroxidase.

The central projections of afferent fibers from the cornea, and the infraorbital, infratrochlear, frontal, lacrimal and auriculotemporal nerves were investigated by means of the transganglionic transport of horseradish peroxidase. Afferent projections to the dorsal horn of the medulla are organized along both the rostrocaudal axis and the ventrolateral to dorsomedial margin of the medullary dorsal horn. An inverted but discontinuous facial representation exists through the restrocaudal axis of the dorsal horn of the medulla with perioral and nasal receptive fields innervated by the infratrochlear nerves represented rostral to the progressively more posterior receptive fields innervated by the frontal, lacrimal and auriculotemporal nerves, respectively. The organization of the primary afferents is not uniform over the laminae of the dorsal horn of the medulla; the projections from the different nerves show the least overlap in lamina II, while overlap is most extensive in laminae I and V. The sensory projection from the cornea to the medullary dorsal horn is most dense in laminae I and II. All nerves, including those innervating the cornea, project to the interpolar, oral and principal trigeminal nuclei and are somatotopically organized. Projections to the reticular formation and the contralateral trigeminal sensory complex were not found in this study. These results support the organization of the dorsal horn of the medulla proposed by Déjerine ('14) and show that this organization is most evident for the primary afferent projections to lamina II.

Bulbospinal projections in the primate: a light and electron microscopic study of a pain modulating system.

The projections of the nucleus raphe magnus (NRM) and the immediately adjacent reticular formation were studied in the macaque monkey following injections of the rostroventral medulla with 3H-leucine and examination of the resultant labeled axons and terminals by light and electron microscopic autoradiography. Five monkeys had accurately placed injections, which resulted in fiber pathway labeling that coursed caudally, laterally, and dorsally to project to laminae I, II, and V of subnucleus caudalis of the trigeminal and then traveled in the dorsolateral funiculus of the cord and terminated in similar laminae of the spinal dorsal horn at cervical levels. The pathway was only lightly labeled caudal to the cervical enlargement and could not be readily discerned above background in the thoracic or lumbar cord. Electron microscopy revealed that axons and terminals serving this system constitute a heterogeneous population. Large-diameter myelinated axons (3-6-micron diameter), small myelinated axons (0.75-3-micron diameter), and clusters of nonmyelinated axons were labeled. Terminals in laminae I, II, and V contained mixtures of clear round and granular vesicles or clear pleomorphic and granular vesicles or formed the central element in synaptic glomeruli. The labeled profiles formed asymmetrical or symmetrical synapses on medium and small dendrites; labeled axosomatic synapses were not observed. In rare instances there were contacts between labeled profiles and vesicle-containing structures, which were probably dendritic, but whether the NRM axon was pre- or postsynaptic to such structures could not be determined. It was concluded that the NRM in the monkey is organized in a manner quite similar to that previously described in the cat. The wide variety of fiber types and synaptic terminals serving this system suggests that different classes of neurons participate in it, probably using several transmitter substances that result in varying postsynaptic effects on neurons located in the trigeminal complex and dorsal horn.

The trigemino-olivary projection in the cat: contributions of individual subnuclei.

Anterograde autoradiographic methods were used to determine the projection of the principal sensory trigeminal nucleus and of each of the three spinal trigeminal subnuclei to the inferior olivary complex in the cat. Our data reveal that the principal sensory trigeminal nucleus does not contribute to the trigemino-olivary pathway. Each spinal trigeminal subnucleus has a unique contribution to this pathway: pars oralis projects sparsely to the border between the dorsal accessory and principal olives (DAO-PO), pars interpolaris projects mostly to the rostral medial DAO, and pars caudalis projects mostly to the rostral medial part of the ventral leaf of PO and slightly to the caudal medial accessory olive. In the light of recent physiological and anatomical findings, our data indicate that information from each spinal trigeminal subnucleus reaches a different segment of the contralateral inferior olivary complex, which in turn distributes differentially to the cerebellar cortex.

Spinal and trigeminal dorsal horn projections to the parabrachial nucleus in the rat.

We studied afferents to the parabrachial nucleus (PB) from the spinal cord and the spinal trigeminal nucleus pars caudalis (SNVc) in the rat by using the anterograde and retrograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Injections of WGA-HRP into medial PB retrogradely labeled neurons in the promontorium and in lamina I of the dorsal rostral SNVc, while injections into lateral PB and the Kölliker-Fuse nucleus retrogradely labeled neurons in these areas as well as in lamina I throughout the caudal SNVc and spinal dorsal horn. Injections of WGA-HRP into the caudal SNVc and dorsal horn of the spinal cord resulted in terminal labeling in the dorsal, central, and external lateral subnuclei of PB and the Kölliker-Fuse nucleus, all of which are known to receive cardiovascular and respiratory afferent information. Injections of WGA-HRP into the promontorium and dorsal rostral SNVc resulted in terminal labeling in the same PB subnuclei, as well as in the medial and the ventral lateral PB subnuclei, which are sites of relay for gustatory information ascending from the medulla to the forebrain. The spinal and trigeminal projection to PB may mediate the convergence of pain, chemosensory, and temperature sensibilities with gustatory and cardiorespiratory systems in PB.

Projections of nucleus caudalis and spinal cord to brainstem and diencephalon in the hedgehog (Erinaceus europaeus and Paraechinus aethiopicus): a degeneration study.

In the light of hypotheses related to the evolution of pain-carrying systems in mammals, terminal projection fields in brainstem and diencephalon of efferents of nucleus caudalis (NC) of the spinal trigeminal complex and spinal cord were determined in hedgehog by using Nauta-Gygax and Fink-Heimer silver techniques for degeneration. Unilateral NC lesions resulted in medullary degeneration in the ventral portion of NC contralaterally and bilaterally in cuneate nucleus (CU) and reticular formation. Pontine degeneration was noted ipsilaterally in medial (PBM) and lateral (PBL) parabrachial, facial motor (VII), and interpolar, oral, and main sensory trigeminal nuclei; degeneration in reticular formation was bilateral. Midbrain degeneration was seen bilaterally in caudal superior colliculus (SC), inferior colliculus (IC), periaqueductal gray, and tegmentum. In thalamus, projections to ventroposterior nucleus (VP) were contralateral and concentrated in a crescent extending along the lateral one-third-to-one-half and ventral border of the nucleus. Bilateral degeneration fields were noted in a dorsomedial sector of the "ventral nuclear field," posterior complex (PO), and mediodorsal nucleus (MD), the degeneration always heavier contralaterally in these nuclei. Sparse degeneration was noted in the medial most portions of the medial geniculate nuclei bordering PO and VP. In rostral diencephalon, bilateral degeneration was traced from the inferior thalamic peduncle to the lateral hypothalamic area (LH). Unilateral spinal cord lesions made between C7 and T1 vertebrae resulted in medullary degeneration in NC contralaterally, ipsilaterally in CU and lateral cuneate nucleus, and bilaterally in gracile nucleus, inferior olivary complex, and reticular formation. Pontine degeneration was limited to ipsilateral PBL and bilaterally to VII. Midbrain degeneration was found bilaterally in IC, SC, nucleus sagulum, and tegmentum; a minor projection was noted in interpeduncular nucleus. In thalamus, projections were confined to ipsilateral PO and zona incerta. In rostral diencephalon bilateral fields were noted in LH. NC terminations in PO and VP parallel results of research in hedgehogs on thalamic projections of the dorsal column nuclei (Jane and Schroeder, '71), and particularly the location in VP of most cells responsive to stimulation of the face (Erickson et al., '67). This suggests that somatic input from NC, some of which may be pain-specific, reaches thalamic areas, a portion of whose neurons are characterized as polymodal and at least partially convergent for somatotopy. These results are consistent with the thesis that specific sensory thalamic nuclei evolved from a diffuse sensory region. Response properties of neurons in the dorsomedial portion of the ventral nuclear field, an area which are also received NC efferents, are not known. Last, NC projections to MD and LH implicate the role of "limbic" aspects of nociception.

Oral and facial representation within the medullary and upper cervical dorsal horns in the cat.

Transganglionic transport of HRP was used to study the patterns of termination of somatic afferent fibers innervating oral and facial structures within the trigeminal nucleus caudalis and upper cervical dorsal horn of the cat. In separate animals, the superior alveolar, pterygopalatine, buccal, inferior alveolar, lingual, frontal, corneal, zygomatic, infraorbital, mental, mylohyoid, and auriculotemporal branches of the trigeminal nerve were traced in this experiment. The organization of the primary afferents innervating the oral structures is not uniform across laminae and at different rostrocaudal levels of the nucleus caudalis. The superior alveolar and pterygopalatine nerves mainly terminate in laminae I, II, and V at the level of the rostral one-third of the caudalis. By contrast, the lingual, inferior alveolar, and buccal nerve terminate in laminae I-V of, respectively, the rostral third, the entire length, and caudal two-thirds of the caudalis. In addition, the lingual, buccal, and pterygopalatine nerves terminate in the dorsal and middle parts of the interstitial islands or pockets of lamina I neuropil extending to the rostral levels parallel to the nucleus interpolaris. Mediolaterally, in laminae I, II, and V of the rostral third an extensive overlap of projections was found between the branches from each trigeminal division, and some overlap was observed between projections from the mandibular and maxillary divisions. On the other hand, the projections of primary afferents innervating the facial structures are arranged in a somatotopic fashion in rostrocaudal and mediolateral axes over the laminae (I-IV) through the nucleus caudalis and upper cervical dorsal horn. Fibers from the perioral and perinasal regions terminate most rostrally in caudalis, and fibers from progressively more posterior facial regions terminate at successively lower levels. A mediolateral somatotopic arrangement was observed, with fibers from the ventral parts of face ending in the medial regions and fibers from the progressively more dorsal parts of the face ending in successively more lateral regions of the medullary and upper cervical dorsal horns. Corneal afferent terminals are concentrated in the outer parts of lamina II at the levels of the rostral parts of the caudal two-thirds of the caudalis and the interstitial islands of lamina I. The maxillary division terminates first at the most caudal level of the caudalis, followed by the ophthalmic division descending as far as the C2 segment and the mandibular division reaching the most caudal level of the C2 segment.(ABSTRACT TRUNCATED AT 400 WORDS)

Spinal and trigeminal projections to the nucleus of the solitary tract: a possible substrate for somatovisceral and viscerovisceral reflex activation.

This study used the retrograde transport of a protein-gold complex to examine the distribution of spinal cord and trigeminal nucleus caudalis neurons that project to the nucleus of the solitary tract (NST) in the rat. In the spinal grey matter, retrogradely labeled cells were common in the marginal zone (lamina I), in the lateral spinal nucleus of the dorsolateral funiculus, in the reticular part of the neck of the dorsal horn (lamina V), around the central canal (lamina X), and in the region of the thoracic and sacral autonomic cell columns. The pattern of labeling closely resembled that seen for the cells at the origin of the spinomesencephalic tract and shared some features with that of the spinoreticular and spinothalamic tracts. Labeled cells in lamina IV of the dorsal horn were only observed when injections spread dorsally, into the dorsal column nuclei, and are thus not considered to be at the origin of the spinosolitary tract. They are probably neurons of the postsynaptic fibers of the dorsal column. Retrogradely labeled cells were also numerous in the superficial laminae of the trigeminal nucleus caudalis, through its rostrocaudal extent. The pattern of marginal cell labeling appeared to be continuous with that of labeled neurons in the paratrigeminal nucleus, located in the descending tract of trigeminal nerve. Since the NST is an important relay for visceral afferents from both the glossopharyngeal and vagus nerves, we suggest that the spinal and trigeminal neurons that project to the NST may be part of a larger system that integrates somatic and visceral afferent inputs from wide areas of the body. The projections may underlie somatovisceral and/or viscerovisceral reflexes, perhaps with a significant afferent nociceptive component.

Trigeminal afferents to the diencephalon in the rat.

Trigemino-diencephalic connections were studied in the rat using wheat-germ agglutinin conjugated to horseradish peroxidase as an anterogradely transported axonal tracer. Injection of the tracer into the subnucleus principalis produced two foci of dense labelling: one ventromedial: and one dorsal within the medial part of the ventrobasal complex. Other diencephalic structures containing granules of reaction product were the medial part of the medial geniculate body, the ventral area of the zona incerta and the nucleus lateralis posterior, pars lateralis. Injection of the tracer into the subnucleus interpolaris labelled the same structures, but less densely. After an injection into the subnucleus caudalis, labelling was observed in the same thalamic areas, although projections to the zona incerta or the lateralis posterior were not consistent. Additional labelling was observed in the subfascicular area of the mesodiencephalic junction, the nucleus submedius and the intralaminar nuclei centralis medialis and lateralis. In those cases of injection into the subnuclei principalis and interpolaris, all observed thalamic sites of projection were contralateral to the injection site. Following injection into the subnucleus caudalis, projections toward lateral thalamic structures were contralateral, but the nucleus submedius and the intralaminar nuclei exhibited bilateral labelling. Using high magnification (1250 X) with bright-field illumination, an analysis of the morphology of some terminal arborizations was attempted. Despite some technical limitations, the analysis indicated that in the ventrobasal complex, some terminal ramifications of axons originating from the three trigeminal subnuclei under study arborize so as to encompass a rounded area, the diameter of which could be as large as 100 microns, thereby resembling the classically described "bushy arbors". Such arborizations could not be distinguished in the axons projecting to the medial part of the medial geniculate body. In this latter nucleus, the terminals appeared to arise from a stem fiber as short side branches at approximately right angles to the parent stem axon. In the other areas where afferent terminal labelling was observed, the density of the network of the labelled fibers often complicated the analysis of morphological features. However, arborizations such as those observed in the ventrobasal complex or the medial geniculate nucleus could not be distinguished.(ABSTRACT TRUNCATED AT 400 WORDS)

The efferent connections of the nuclei of the descending trigeminal tract in the mallard (Anas platyrhynchos L.).

The efferent and intranuclear connections of the nuclei of the descending trigeminal tract of the mallard have been studied with lesion methods, and by axonal transport techniques following injections of tritiated leucine, and of horseradish peroxidase. The large subnucleus oralis neurons, including those belonging to the nucleus of the ascending glossopharyngeal tract, have proven to be the sole origin of trigeminocerebellar connections. The cerebellar afferents are of the mossy fiber type, and terminate predominantly in lobules V, VI and VII, and possibly, lobule IV. Trigeminocerebellar projections are ipsilateral except for the vermal area. Subnucleus interpolaris is the main source of intratrigeminal fibers that terminate in subnucleus oralis and the ventral part of the main sensory nucleus. These intranuclear connections are bilateral, but the medium-celled caudal part of subnucleus interpolaris in particular contains the majority of bi- and/or contralaterally projecting neurons. Additionally, the small cells in the rostral part of subnucleus interpolaris project ipsilaterally upon the parabrachial region, and upon the lateral reticular formation. Projections upon the parabrachial region furthermore emanate bilaterally from layer I of the rostral subnucleus caudalis. A minor part of layer I neurons sends its axons contralaterally along with those of the dorsal column nuclei toward the thalamic nucleus dorsolateralis posterior. Associated with the medial lemniscus, contralateral termination is also present in the lateral part of the ventral lamella of oliva caudalis, in the marginal zone of nucleus mesencephalicus lateralis, pars dorsalis and immediately surrounding intercollicular grey and, finally, in the nucleus intercalatus thalami. Furthermore, a bilaterally descending projection from subnucleus caudalis upon layers I and II of the rostral cervical cord was observed. Close to their origin subnucleus caudalis neurons project upon the adjoining caudal part of the lateral reticular formation.

Neural pathways mediating the corneal blink reflex and Bell's phenomenon in the cat.

Two central projections from the corneal representation of the sensory trigeminal complex in the cat were demonstrated with horseradish peroxidase, autoradiographic and Golgi methods: (1) to the dorsal subdivision of the ipsilateral facial nucleus that innervates the orbicularis oculi muscle; and (2) to the bordering area between the contralateral central gray matter and the oculomotor nucleus, which receives dendrites of the oculomotor cells innervating the contralateral superior rectus muscle. These two routes probably mediate early responses of the corneal blink reflex and Bell's phenomenon, respectively.

Direct projections from the caudal spinal trigeminal nucleus to the striatum in the cat.

The results of a WGA-HRP and HRP study in the cat indicated that some neurons in the marginal zone (lamina I) of the caudal spinal trigeminal nucleus sent their axons contralaterally to the striatum; mainly to the dorsal part of the putamen, and additionally to the ventrolateral part of the caudate nucleus, at the stereotaxic rostrocaudal levels of A 13.0-A 15.5.

Laminar-related projection of primary trigeminal fibers in the caudal medulla demonstrated by transganglionic transport of horseradish peroxidase.

The mode of termination of primary afferent fibers within the cat trigeminal nucleus caudalis was investigated by means of the transganglionic transport of horseradish peroxidase (HRP). Several types of laminar-related labeling were observed, depending upon the survival time after HRP application. At the earliest survival time (28-34 h) the highest density of labeling was found in laminae I and II. At 2 and 3 days survival laminae III and IV were heavily labeled, in addition to laminae I and II where the amount of labeling was greatly increased in lamina I, but not in lamina II. At 5 days survival time an abrupt drop of labeling occurred in laminae I and II, while this pattern was not predominant in laminae III and IV. In lamina V the pattern of labeling was less intense and not changeable through all survival times observed. These findings indicating a differentiation of the primary afferent terminals have good correspondence with a functional specialization of neuronal locations since the functional properties of neurons vary according to their locations.

Toxic ricin demonstrates a dual dental projection.

Toxic ricin was used to study the central distribution of dental afferents in the cat. Following intrapulpal ricin injections ganglion cell degeneration is seen in the II and III ganglion divisions. Central argyrophilic degeneration occurs in the dorsal portion of all ipsilateral trigeminal nuclei. Ventral degeneration is seen in the pars interpolaris and pars caudalis. No contralateral degeneration was observed. The results are discussed with regard to previous studies of the central location of dental afferents.