Two types of tooth pulp units in the bulbar lateral reticular formation

A novel computerized system for thermal stimulation of tooth in ferrets

A dual thermal and electrical stimulator was developed to examine the central pathways that transmit noxious stimuli for intact dentition. This system allows computer-controlled stimulation of the canines of ferrets with either noxious heat or electrical stimuli. A set of in vitro studies demonstrated that the application of thermal stimuli to an intact tooth can produce pulpal temperatures above 43 °C, which is perceived as a painful stimulus in humans. In a set of in vivo studies, it was demonstrated that heating an intact tooth at temperatures of at least 40 °C, excited trigeminal brainstem neurons. Only 15% of the neurons activated by electrical stimulation responded to noxious heat applied to the canine. Eight of the 23 neurons were classified as nociceptive specific neurons and responded only to noxious stimulation of their cutaneous receptive fields. Fifteen of the 23 neurons were classified as wide dynamic range neurons and responded to both noxious and non-noxious stimulation applied to their cutaneous receptive fields. This new device can accurately deliver both thermal and electrical stimuli to an intact tooth, which allows an evaluation of the central neural circuits that respond to noxious stimulation of the dentition.

Neuronal activity related to spontaneous and capsaicin-induced rhythmical jaw movements in the rat

Intraoral capsaicin induced rhythmical jaw movements (RJM) in anesthetized rats. Neurons in the trigeminal spinal nucleus caudalis or the cortico-peduncular (CP) axons were extracellularly recorded. Capsaicin excited dose-dependently most caudalis neurons, which were activated by stimulation of the oral cavity and/or the tooth pulp and activated during spontaneous or induced RJM. Ten of 55 CP axons were antidromically activated by stimulation of the contralateral trigeminal motor nucleus. All antidromic and 29 other CP axons discharged prior to the spontaneous RJM, but most of them did not during capsaicin-induced RJM. These neuronal activities possibly initiate spontaneous RJM although the activities of caudalis neurons are necessary for capsicin-induced RJM.

Differential distribution of four types of tooth pulp neurons in the caudal medulla oblongata of the cat

The medulla oblongata caudal to the obex was explored for neurons responsive to tooth pulp (TP) stimulation in cats. Four different classes of TP neurons were found. They were TP specific neurons, trigeminal wide dynamic range neurons with TP input, trigeminal subnucleus reticularis ventralis (SRV) neurons with TP input and convergent reticular formation with TP input. They were differentially distributed within the caudal medulla oblongata.

The central projections of the monkey tooth pulp afferent neurons

Transganglionic transport of horseradish peroxidase conjugated to wheatgerm agglutinin (HRP:WGA) entrapped in hypoallergenic polyacrylamide gel was used to study the patterns of termination of primary afferents that innervate the upper and lower tooth pulps within the trigeminal sensory nuclear complex (TSNC) of the monkey. HRP:WGA injections were also made into the lower incisors and molars, in order to examine the topographic arrangement of pulpal afferent projections. HRP-labeled pulpal afferents innervating lower and upper teeth projected ipsilaterally to the rostral subnucleus dorsalis (Vpd) and caudal subnucleus ventralis (Vpv) of the nucleus principalis (Vp); the rostrodorsomedial (Vo.r) and dorsomedial (Vo.dm) subdivisions of the nucleus oralis (Vo); the dorsomedial subdivision of the nucleus interpolaris (Vi); and laminae I-II and/or V of the nucleus caudalis (Vc) at its rostralmost level. The HRP-labeled terminals from upper and lower pulpal afferents formed a rostrocaudal column from the midlevel of Vp to the rostral tip of Vc. The label in Vp and Vo was considerably dense, but the column of terminals was interrupted at the Vpd-Vpv transition. The label in Vi and Vc was much less dense compared to that in the rostral nuclei, and the column of terminals was interrupted frequently. The representation of the upper and lower teeth in TSNC was organized in a somatotopic fashion that varied from one subdivision to the next, though their terminal zones overlapped within Vpd. The upper and lower teeth were represented in Vpv, Vo.r, Vo.dm, Vi, and Vc in a ventrodorsal, dorsoventral, lateromedial, lateromedial, and lateromedial sequence, respectively. Topographic arrangement was also noticed for the projections of pulpal afferents from the lower incisors and molars: The representations of the lower incisors and molars in Vpv, Vo.r, Vo.dm, Vi, and Vc were organized in a lateromedial, dorsoventral, ventrodorsal, ventrodorsal, and lateromedial sequence, respectively. The present results indicating sparse projections from pulpal afferents in the monkey's Vc are in good correspondence with a clinical report that trigeminal tractotomy just rostral to the obex has no significant effect on dental pain perception in patients. Furthermore, the present study indicates that projection patterns of pulpal afferents--which include the termination sites, the density of terminations between nuclei, and topographic arrangement--differ among animal species.

Somatotopic organization of tooth pulp primary afferent neurons in the cat

Transganglionic transport of horseradish peroxidase-wheat germ agglutinin conjugate (HRP-WGA) was used to study the somatotopic organization of pulpal afferent neurons innervating the different types of teeth in the trigeminal ganglion and trigeminal sensory nuclear complex (TSNC). In separate animals, the upper first 3 incisors (UI1-3), canine (UC), second premolar (UP2) and third premolar (UP3), and the lower first three incisors (LI1-3), canine (LC), first premolar (LP1), second premolar (LP2) and molar (LM) were traced in this experiment. Cell bodies innervating posterior teeth were found with greater frequency in dorsal maxillary ganglion regions, while somata supplying more anterior teeth were predominant ventrally. In contrast, cell bodies innervating the lower teeth were not arranged in a somatotopic fashion in the mandibular subdivision. Each pulpal afferent from lower and upper teeth projected to the subnucleus dorsalis (Vpd) of the pars principalis, the rostrodorsomedial (Vo.r) and dorsomedial parts (Vo.dm) of the pars oralis (Vo), the medial regions of the pars interpolaris (Vi), and laminae I, II, and V of the medullary dorsal horn, and terminal fields between the upper and lower teeth were separated in each subdivision. Pulpal projections from both the upper and lower teeth to each subdivision were organized in a somatotopic manner, while an extensive overlap in projections was noted between the adjoining teeth. In the Vpd, the upper and lower teeth were represented dorsoventrally, and projections from the anterior to posterior teeth in the upper jaw were arranged in both rostrocaudal and ventrodorsal sequences whereas those in the lower jaw were organized caudarostrally and lateromedially. In the Vo.r and Vo.dm, the upper and lower teeth were represented in a mediolateral sequence and projections from the anterior to posterior teeth were organized in a ventrolateral to dorsomedial sequence. In the Vi, pulpal projections were organized in a topographic fashion similar to that observed in the Vo.r and Vo.dm. In the medullary dorsal horn, the upper and lower teeth were represented in laminae I, II and V in a lateromedial sequence. Their projections to laminae I and V were topographically organized in a mediolateral and rostrocaudal sequence.(ABSTRACT TRUNCATED AT 400 WORDS)

Uptake and transneuronal transport of horseradish peroxidase-wheat germ agglutinin by tooth pulp primary afferent neurons

Horseradish peroxidase-wheat germ agglutinin (HRP-WGA) applied to proximal stumps of tooth pulp primary afferent neurons in rats was taken up and transported transneuronally to neurons in the ipsilateral trigeminal brainstem nuclear complex. The results of this study suggest that HRP-WGA transport may be a novel means of labeling both primary and higher order neurons that transmit tooth pulp sensory information in the rat and may be used to investigate the fine structure and synaptic contacts of central nervous system neurons that receive tooth pulp afferent input.

Topographic representation of lower and upper teeth within the trigeminal sensory nuclei of adult cat as demonstrated by the transganglionic transport of horseradish peroxidase

Transganglionic transport of horseradish peroxidase-wheat germ agglutinin conjugate (HRP-WGA) entrapped in hypoallergenic polyacrylamide gel was used to study the patterns of termination of primary afferents that innervate the lower and upper tooth pulps within the trigeminal sensory nuclear complex (TSNC). HRP injections were made into the inferior and superior alveolar nerves in order to compare the central projections of the whole nerve with those from tooth pulps. In addition, the relationship between the distribution of the trigeminothalamic tract cells and the projection sites of the tooth pulp afferents was investigated by injecting HRP into the posterior ventral thalamus. HRP-labeled tooth pulp afferent fibers innervating the lower and upper teeth projected to the subnucleus dorsalis (Vpd) of pars principalis, the rostrodorsomedial part (Vo.r) and nucleus dorsomedialis (Vo.dm) of pars oralis, the medial regions of pars interpolaris, and laminae I, II, and V of pars caudalis. Terminal fields of the lower tooth pulp afferents formed a rostrocaudally running, uninterrupted column from the midlevel of Vpd to the caudal tip of caudalis. In contrast, the column of termination of upper tooth pulp afferents was discontinuous at the Vpd/Vo.r transition, and ended at the more rostral level of the caudalis than that of the lower tooth pulp afferents. The representation of the lower and upper teeth in the TSNC was organized in a somatotopic fashion which varied from one subdivision to the next, although terminal zones of the inferior and superior alveolar nerves overlapped within the Vo.r, Vo.dm, and dorsomedial part of rostral pars interpolaris. The lower and upper teeth were represented in the Vpd, Vo.r, Vo.dm, medial region of pars interpolaris, and laminae I, II, and V, in a ventrodorsal or caudorostral, dorsoventral, lateromedial, dorsoventral, and mediolateral or dorsomedial-ventrolateral sequence, respectively. The smaller, more focal terminal areas of the teeth contrasted sharply with more extensive terminal fields of the alveolar nerves. The HRP injections within the thalamus indicated that neurons in Vpd, the caudal pars interpolaris, and laminae I/V of caudalis, which are subdivisions of TSNC that receive pulpal projections, sent their axons to the ipsilateral and contralateral posterior ventral thalamus.(ABSTRACT TRUNCATED AT 400 WORDS)

Functional difference of tooth pulp-driven neurons in oral and facial areas of the somatosensory cortex (SI) of the cat

Single-unit discharges were recorded in the oral and facial areas of the cat somatosensory cortex (SI) while electrical stimuli were individually delivered to eight tooth pulps. The incidence of the tooth pulp-driven (TPD) neurons was 44.7% in the oral area, but only 17.3% in the facial area. Both sets of neurons were also excited by nonnoxious stimulation of the oral structures or of the facial hair, and thus were polymodal. These TPD neurons were confirmed histologically to be in area 3b and were classified into monotooth input type and multitooth input type according to their response to stimulation. Neurons of the monotooth input type appeared three times more frequently in the oral area than in the facial area. The input(s) to the TPD neurons in the former area were slightly stronger from the canine(s) than from the molar(s), but the opposite was the case in the facial area. In the oral area, 83% of the TPD neurons responded with brisk discharges of short latency, whereas 54% of the TPD neurons in the facial area responded with those of a long latency. These findings suggest that the pulpal information to the somatosensory cortex is conveyed by pathways that appear, at least at certain points in the nervous system, to be spatially separated.

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)

An electrophysiological study of canine, premolar and molar tooth pulp afferents and their convergence on medullary trigeminal neurons

The two main aims of this study were (1) to compare the conduction velocities of tooth pulp (TP) afferents innervating the cat canine, premolar and molar teeth and (2) to determine the degree of convergence of afferent input originating in these different teeth on medullary dorsal horn (MDH) neurons. Experiments were conducted on 10 cats anesthetized with chloralose. Single unit extracellular recordings were obtained from the tooth pulp or the MDH. The distribution of conduction velocities of afferents originating in the mandibular and maxillary canines, premolars and mandibular molar were all found to be similar except that the mean conduction velocity of canine afferents was slightly higher than the means for the other teeth. A total of 48 MDH neurons excited by TP stimulation was studied. Most MDH neurons activated by electrical stimulation of one of the TPs could also be activated by stimulation of one or more of the other TPs. In addition to the marked convergence from different teeth, most of the TP-activated neurons also had convergent inputs from facial skin and/or intraoral mucosa.

The central projections of tooth pulp afferent neurons in the rat as determined by the transganglionic transport of horseradish peroxidase

Transganglionic transport of horseradish peroxidase (HRP) or horseradish peroxidase-wheat germ agglutinin conjugate (HRP-WGA) was used to map in detail the central projections of trigeminal primary afferent neurons that innervate the dental pulp organ of the rat. In each of ten animals, 0.5-2.0 microliters of enzyme solution was injected into the pulp chamber of the first maxillary molar tooth. Postmortem examination of the decalcified teeth in all cases showed that the HRP/HRP-WGA remained confined to the pulp chamber and pulp roots, with no spread of enzyme into periapical tissues. HRP-labeled tooth pulp afferent fibers projected to all four rostrocaudal subdivisions of the ipsilateral trigeminal brainstem nuclear complex (TBNC) and to the upper cervical spinal cord. The labeled terminal fields formed a column that stretched relatively uninterrupted from just caudal to the rostromedial tip of the trigeminal principal sensory nucleus to at least the C2 segment of the spinal cord. The density of the afferent projection varied markedly from one rostrocaudal level of the TBNC to the next but was heaviest in an area encompassing the caudal one-half of the principal sensory nucleus and the rostral two-thirds of pars oralis. Fibers projected only lightly to pars caudalis, where they terminated preferentially in laminae I, IIa, and the junctional zone between laminae IV and V. HRP-labeled terminals in C1 and C2 were located almost exclusively in laminae I. In the dorsoventral axis, the terminal fields in the TBNC were located in a surprisingly dorsal part of the complex, well within what has been shown by others to be largely an area of termination for mandibular division fibers. Most fibers ended in medial parts of the TBNC, with the exception of two modestly labeled terminal fields located in the lateral aspects of rostral pars oralis and rostral pars caudalis. No labeled fibers terminated in the contralateral TBNC or contralateral cervical spinal cord.

Responses to corneal stimulation in vestibulospinal units of nucleus Deiters

In precollicular decerebrated cats, the response patterns of antidromically identified vestibulospinal Deiters's neurons to stimulation of corneal receptors were investigated. These patterns were compared with the responses of the somatosensory receptors of the neck and limbs as well as with the vestibular input of the horizontal semicircular canals. Of the 162 antidromically driven Deiters's units, 23 were influenced, mainly bilaterally, from corneal receptors. Response latencies evoked by electrical stimulation of the cornea ranged from 6 to 16 ms (mean 12 ms). In 99 Deiters's cells, the influence from the limbs was examined: 51 revealed primarily an ipsilateral modulation from proximal joint receptors. Convergence with joint receptor input was found in 16 of the 18 corneally driven cells tested. Of the 115 cells tested, 15 neurons responded to neck rotation in the horizontal plane. A contribution from the lateral semicircular canals was found in 7 of the 101 examined Deiters's neurons. The connection of corneal receptors with the spinal motor system, via a vestibulospinal pathway, may mediate a nociceptive reflex protecting the eyes and the face.

Tooth pulp input to the spinal trigeminal nucleus: a comparison of inhibitions following segmental and raphe magnus stimulation

In rats and cats anaesthetized with urethane a comparison was made of the inhibitory effects of raphe magnus (NRM) and segmental (facial skin) stimulation on neurones in nucleus caudalis excited by tooth pulp stimulation. The upper and lower ipsilateral incisor teeth were used in rats (176 neurones) and the corresponding canine teeth in cats (34 neurones). The recording sites were located in all layers of nucleus caudalis and in the underlying reticular formation. Both the evoked responses and the conditioning effects were similar in the two species. Both forms of conditioning inhibited about half the neurones tested but only as small proportion was influenced from both sources. NRM stimulation had almost identical effects on neurones driven from upper teeth or from lower teeth and tended to act on those cells with longer latencies. Segmental stimulation influenced the majority of shorter latency cells and produced greater inhibitions of upper tooth pulp neurones. Diffuse noxious inhibitory controls were also observed for certain neurones.

An HRP study of the central projections of primary trigeminal neurons which innervate tooth pulps in the cat

Trigeminal ganglia and brain stem of adult cats were studied following HRP injections into tooth pulps or after exposure of the cut end of the inferior alveolar nerve to HRP. Ipsilateral ganglion cells within a wide range of sizes were labeled in both experimental situations, whereas no labeled cells were observed in the contralateral ganglion in any animal. Labeled central branches of tooth pulp and inferior alveolar neurons were observed in all subdivisions of the ipsilateral trigeminal sensory complex. Terminal labeling in the tooth pulp experiments was confined to the dorsomedial parts of the main sensory nucleus and subnuclei oralis and interpolaris. Caudal to the obex terminal labeling was restricted to the medial halves of laminae I, IIa and V of the medullary dorsal horn. In the inferior alveolar nerve experiments dense terminal labeling was observed in the dorsal parts of the main sensory nucleus and subnuclei oralis and interpolaris. Caudal to the obex terminal labeling was located throughout laminae I to V in contrast to the tooth pulp experiments. Neither of the two experimental situations offers any evidence for a bilateral or contralateral brain stem projection of primary trigeminal neurons.

Development of the brain stem in the rat. I. Thymidine-radiographic study of the time of origin of neurons of the lower medulla

Groups of pregnant rats were injected with two successive daily doses of 3H-thymidine from gestational days 12 and 13 (E12 + 13) until the day before birth (E21 + 22). In adult progeny of the injected rats the proportion of neurons generated on specific days was determined quantitatively in the major nuclei of the lower medulla. The earliest generated cells form two motor nuclei: the hypoglossal and dorsal vagal nuclei. The bulk of hypoglossal neurons are produced on day E12, with a small proportion earlier; the bulk of dorsal vagal neurons are produced, likewise, on day E12, with a small proportion on day E13. The neurons of the third motor nucleus of the region, the ambiguous, are generated later, with a peak on day E15. Neurons of the sensory relay nuclei, the gracilis, cuneatus, and solitarius are produced over a more extended period, with peaks on day E13; the exception was the external cuneate nucleus in which peak generation time was on day E15. In the caudal nucleus of the trigeminal complex, neurons of the subnucleus magnocellularis arise earliest, with a peak on day E14, and those of the subnucleus marginalis last, with a peak on day E15, and extending into day E16. The neurons of the nuclei raphe pallidus and obscurus, and of the dorsal and ventral portions of the caudal medullary reticular formation, are produced between days E12 and E15, without any obvious peaks. The neurons of the nucleus parasolitarius and the nucleus of Roller are produced relatively late, and the area postrema contains a germinal cell population throughout the embryonic period, presumably supplying cells to the choroid plexus of the fourth ventricle. On the basis of absolute datings, duration of neuron production, intranuclear and internuclear gradients, and other criteria, it is postulated that the neurons of the lower medulla are derived from at least eight different cytogenetic zones.

Cell responses evoked by tooth pulp stimulation above the marginal layer of the cat's trigeminal nucleus caudalis

In cats anesthetized with urethane, all-or-nothing, synaptically evoked recordings were made from 80 separate units in the descending spinal tract of the trigeminal nerve above the left trigeminal nucleus caudalis, at depths not exceeding 50 micrometer from the surface of the medulla. The units were excited by the left upper (21), left lower (25), either (28), or only on simultaneous stimulation of both (six) canine tooth pulps. There was no somototopic distribution. The latency of responses ranged from 4 to 82 msec. For the group of 28 units excited by upper and lower tooth pulps, there was close matching of response latencies from the two teeth. An abrupt decrease in latency upon increasing stimulation strength ("jumping"), and a gradual increase in latency during repetitive stimulation at a frequency between 1 and 20 Hz ("drifting") was characteristic of most, but not all, responses. Units evoked by stimulation of the inferior dental or infraorbital nerves had similar characteristics. Stimulation of a tooth pulp at threshold for a particular unit was used to test the excitability of that unit after suprathreshold stimulation of the same or a different canine tooth pulp. Stimulation of the upper left canine tooth pulp was generally only facilitatory, while stimulation of the lower left canine tooth pulp was initially facilitatory and later inhibitory. Stimulation of the upper or lower right canine tooth pulps did not excite but could inhibit units excited by the left canine tooth pulps. There was a significant correlation between the frequency at which a unit would follow repetitive stimulation and the duration of the inhibition generated by the first of a pair of stimuli. Long inhibition was associated with poor frequency following.

Efferent projections from temperature sensitive recording loci within the marginal zone of the nucleus caudalis of the spinal trigeminal complex in the cat

The efferent projections from nucleus caudalis of the spinal trigeminal complex in cats were studied with retrograde and anterograde axonal transport techniques combined with localization of recording sites in the thalamus and marginal zone of nucleus caudalis to innocuous skin cooling. Results showed brainstem projections from nucleus caudalis to rostral levels of the spinal trigeminal complex, to the ventral division of the principal trigeminal nucleus, the parabrachial nucleus, cranial motor nuclei 7 and 12, solitary complex, contralateral dorsal inferior olivary nucleus, portions of the lateral reticular formation, upper cervical spinal dorsal horn and, lateral cervical nucleus. Projections to the thalamus included; a dorsomedial region of VPM (bilaterally) and to the main part of VPM and PO contralaterally. Neuronal activity was recorded in the dorsomedial region of VPM to cooling the ipsilateral tongue. HRP injections in this thalamic region retrogradely labeled marginal neurons in nucleus caudalis. These results show that marginal neurons of nucleus caudalis provide a trigeminal equivalent of spinothalamic projections to the ventroposterior nucleus in cats.

Branched afferent nerves supplying tooth-pulp in the cat

1. Recordings have been made from nerve terminals in canine and third incisor teeth of cats. 2. Ninety-three tooth-pulp units in ten cats could be excited either by mechanical or electrical stimulation of adjacent mucous membrane or by electrical stimulation of the pulp of a nearby tooth. 3. Section of the trigeminal nerve centrally did not abolish these responses. 4. Results for twelve out of thirteen units tested with collision techniques indicated that the nerves were branched.

Analysis of the circuitry responsible for primary afferent depolarization in the trigeminal spinal nucleus caudalis of cats

Depth analysis was performed on the field potential evoked by stimulation of the infraorbital nerve in the trigeminal spinal nucleus caudalis and the subjacent lateral reticular formation of cats. It was shown by dye marking of the recording positions that each subnucleus of the nucleus caudalis (subnucleus marginalis, gelatinosus and magnocellularis) and the reticular formation could be differentiated from one another by the characteristics of the peripherally evoked field potentials. Responses of neurons were extracellularly recorded in the subnuclei gelatinosus and magnocellularis of the nucleus caudalis and in the reticular formation to stimulation of the trigeminal sensory branches (the frontal, infraorbital and lingual nerves), the nucleus ventralis posteromedialis of the thalamus and the cerebral cortex. The properties of the neurons were studied in relation to their thresholds, latencies, receptive fields (sensory branches effective for spike generation) and frequency-following capacities. These responses were then compared with properties of the PAD induced in the fibers terminating in the nucleus caudalis by similar peripheral and central stimulation. It was found that the neurons in the subnucleus magnocellularis were the most likely candidates for the interneurons mediating the peripherally evoked disynaptic PAD in the trigeminal nerve fibers terminating in the nucleus caudalis.