JOURNAL OF MORPHOLOGY 198:131-147 (1988)

Afferent Projections of the Trigeminal in the Goldfish, Carassius auratus R.L. PUZDROWSKI Department ofBioZogy, The University of Michigan, Ann Arbor. Michigan 48109 ABSTRACT The horseradish peroxidase (HRP) histochemical technique was used to examine the peripheral distribution and afferent projections of the in the goldfish, Carassius auratus. Sensory fibers of the trigeminal nerve distribute over the head via four branches. The ophthalmic branch distributes fibers to the region above the eye and naris. The maxillary and mandibular branches innervate the regions of the upper and lower , respectively. A fourth branch of the trigeminal nerve was demonstrated to be present in the hyomandibular trunk. Upon entering the medulla the trigeminal afferent fibers divide into a rostro- medially directed bundle and a caudally directed bundle. The rostromedially directed bundle terminates in the sensory trigeminal (STN) located within the rostral medulla. The majority of fibers turn caudally, forming the descending trigeminal tract. Fibers of the descending trigeminal tract termi- nate within three medullary nuclei: the nucleus of the descending trigeminal tract (NDTV), the spinal trigeminal nucleus (Spv), and the medial funicular nucleus (MFn). All projections, except for those to the MFn, are ipsilateral. Contralateral projections were observed at the level of the MFn following the labeling of the ophthalmic and maxillomandibular branches. All branches of the trigeminal nerve project to all four of the trigeminal medullary nuclei. Projections to the STN and MFn were found to be topographically organized such that the afferents of the ophthalmic branch project onto the ventral portion of these nuclei, while the afferents of the maxillo- and hyomandibular branches project to the dorsal portion of these nuclei. Cells of the mesencephalic trigeminal nucleus were retrogradely labeled following HRP application to the ophthalmic, maxillary, and mandibular branches of the trigeminal nerve. In addition to demonstrating the ascending mesencephalic trigeminal root fibers, HRP application to the above-mentioned branches also revealed descending mesencephalic trigeminal fibers. The de- scending mesencephalic trigeminal fibers course caudally medial to the bran- chiomeric motor column and terminate in the ventromedial portion of the MFn.

As in most anamniotic gnathostomes, the '67; Harrison, '81). trigeminal in teleosts consists In teleosts the afferent fibers of the trigem- of maxillomandibular branches, which dis- inal nerve enter the as a single tribute fibers to the oral region, and an root that distributes fibers to the trigeminal ophthalmic branch, which distributes fibers medullary nuclear complex (Woodburne, '36; to the region dorsal to the and naris. In Luiten, '75). In addition to these sensory af- addition, in teleosts a third branch of the ferents, fibers from the cells of the mesence- trigeminal nerve, termed the ramus com- phalic trigeminal nucleus exit the brainstem municans, has been described coursing pe- through the trigeminal root and course pe- ripherally along the rostral operculum as a ripherally in the branches of the trigeminal part of the hyomandibular trunk (Herrick, nerve. 1899; Allis, '03; Maheshwari, '65; Saxena, There have been relatively few studies uti-

G 1988 ALAN R. LISS, INC 132 R.L. PUZDROWSKI lizing modern tracing techniques to investi- trigeminal nucleus. A third specimen (ap- gate the primary afferents of the sensory trigeminal nerve in teleosts. Using degener- A hhreviatwns ation methods to investigate the projections of the entire trigeminal root in the carp (Cy AG anterior prinus carpw), Luiten ('75) reported that the ALLN root of the anterior lateral line nerve descending trigeminal tract gives off ALLNd dorsal root of the anterior lateral line nerve ALLN, ventral root of the anterior lateral line to four different cell groups, which he consid- nerve ered subdivisions of the nucleus of the de- BS bulbospinal tract scending trigeminal tract. There has been Cb only a single study of the trigeminal sensory cc cerebellar crest afferents utilizing horseradish peroxidase CL caudal lobe of the cerebellum DTN dorsal tegmental nucleus (HRP) techniques in two species of teleosts, DTV descending trigeminal tract Hypsoblennius and Oxylebius (von Bartheld EG eminentia granularis and Meyer, '85). That study was limited to FL facial lobe Fm facial an examination of the afferent projections of FR fasciculus retroflexus the trigeminal nerve branches innervating Fs facial sensory nerve the cirri (finger-like projections found on the HYO hyomandibular trunk dorsal surface of the head in these genera) I11 third ventricle IL inferior lobe of the hypothalamus and its findings agreed, for the most part, I0 infraorbital trunk with the description given by Luiten ('75). IR inferior The mesencephalic trigeminal nucleus has Iv fourth ventricle been described in a number of teleosts (Wein- LLF lateral longitudinal fasciculus MA Mauthner berg, '28; Woodburne, '36; Luiten, '75, '79). MAN mandibular branch of the infraorbital trunk However, the peripheral distribution of the MAX maxillary branch of the infraorbital trunk afferents of these cells has never been fullv MFn medial funicular nucleus examined, nor has a mesencephalic trigemi- medial longitudinal fasciculus MR medial reticular formation nal root ever been experimentally demon- Mv cells of the mesencephalic trigeminal strated in a teleost. nucleus The purpose of the present study was to nC commissural nucleus of Cajal provide a complete description of the primary Nm nucleus of the descending trigeminal tract NMLF nucleus of the medial longitudinal sensory afferents of the trigeminal nerve in fasciculus a teleost, the goldfish Carassius auratus, uti- oPv ventricle of the optic tectum lizing HRP techniques. An abstract of this OT optic tectum work has been published previously Cpuz- pal palatine ramus of the facial sensory nerve PC posterior commissure drowski, '85). PCT posterior cerebellar tract PG preglomerular complex MATERIALS AND METHODS PN periventricular nucleus of the inferior lobe The results of this study are based on ob- rcl Communicating ramus of the trigeminal nerve servations made using 32 goldfish, C. aura- rosl lateral line ramus of the supraorbital trunk tus, ranging in size from 1-12 cm in total ros, trigeminofacial ramus of the supraorbital length. All surgical and perfusion procedures v+vii trunk otic ramus of the lateral line nerve utilized animals that were anesthetized with Et recurrent ramus of the facial sensory nerve tricaine methane sulfonate (Sigma Chemical ~IIroot of the facial sensory nerve Co., St. Louis, MO). SG secondary gustatory tract Five animals were used for survey pur- SGN secondary supraorbital trunk poses. Two specimens were transcardially so spinal trigeminal nucleus perfused with 0.04 M phosphate buffer and superior reticular formation MA(90 ml80% ethanol, 5 ml stock formal- STN sensory trigeminal nucleus dehyde, 5 ml glacial ascetic acid); the T dorsal torus longitudinalis were removed, embedded in Paraplast &an- :L root of the trigeminal nerve cer, Distributors, St. Louis, MO), and serially Val" valvula of the cerebellum sectioned in the transverse plane at 10 pm. VII root of the facial sensory nerve These sectioned brains were subsequently VIIm vagal lobe stained with cresyl violet and used to exam- ;k ine the normal anatomy of the brainstem Xm vagal motor nucleus trigeminal centers and the mesencephalic XN Fig. 1. Lateral view of the head of Carassius auratus HRP application. The outline of the is indicated illustrating the distribution of the supraorhital, infraor- by dots; the outline of the body is indicated by dashes. hital, and hyomandihular nerve trunks over the head. Bar scale = 1 mm. Arrowheads indicate points of nerve transection and proximately 1 cm in length) was prepared Hawk International, Distributors, Montreal, according to the Bodian reduced silver pro- Canada). The location of HRP application to cedure and used to examine the peripheral each trunk is illustrated in Figure 1. When distribution of the main nerve trunks carry- labeling the that course along the ing trigeminal rami. The peripheral distri- walls of the orbit (SO, 10, MAX, and MAN, bution of the nerve trunks of the head was see Fig. 1) the eyeball was retracted, the also examined in two specimens that had nerve transected and labeled with HRP, and been prepared by a modification of the Sihler the eyeball then replaced and covered with technique (Freihofer, '66). Specimens pro- histoacryl. The hyomandibular trunk was cessed by this technique are rendered trans- transected and labeled with HRP at the point parent, except for the nerves, which are in the rostrodorsal wall of the opercular cav- stained a dark purple. ity where it courses superficially before en- Surgical procedures were performed with tering the hyomandibular bone. The palatine the aid of a Zeiss stereozoom operating mi- ramus was labeled with HRP at a point croscope. Following anesthetization, the fish where it courses along the posterior wall of was wrapped in moistened cheesecloth and the orbit in order to establish that it was a individual nerve trunks were exposed and branch of the facial sensory nerve containing transected. A gelfoam pledget soaked in HRP no trigeminal fibers (Puzdrowski, '87). (Sigma VI, St. Louis, MO) was placed on.the Following a survival time of 7-14 days at proximal stump of the transected nerve, and 22-26"C, the animals were reanesthetized the wound was closed with histoacryl (Tri and transcardially perfused with cold 0.04 M 134 R.L. PUZDROWSKI phosphate buffer followed by 4% glutaralde- tracing the cranial nerve roots of the trigem- hyde. The brains were washed for 8 hr in inal, facial, and anterior lateral line nerves 20% sucrose-phosphate buffer, the to their respective portion of the complex were removed, and the brains were embed- (Fig. 2). The is charac- ded in gelatin. The gelatin blocks were fixed terized by particularly large, pseudounipolar for an additional 4 hr in 4% glutaraldehyde cells, and constitutes the extreme rostral and in 20% sucrose-phosphate buffer. The blocks the rostrodorsal intermediate portion of the were then rinsed in 20% sucrose-phosphate ganglionic plexus. Separate ophthalmic and buffer for 8 hr to remove any excess fixative. maxillomandibular ganglia could not be Forty-micrometer transverse sections of the distinguished. gelatin-embedded brains were cut on a slid- The three that make ing microtome, collected in 0.04 M phosphate up the anterior ganglion distribute rami over buffer, and stored at 4°C. The sections were the head through three major nerve trunks: processed according to the Mesulam ('78) tet- the supraorbital, infraorbital, and hyoman- ramethylbenzidine (TMB) protocol, or by the dibular (Fig. 1).Application of HRP to these Hanker-Yates protocol (Hanker et al., '77) for trunks demonstrated that each contains fi- visualization of HRP. bers that enter the medulla through the tri- A total of 27 experimental cases were used geminal root and project to the trigeminal in this study. Since initial unilateral experi- sensory nuclei of the medulla. mental cases demonstrated that the trigemi- The trigeminal fibers of the supraorbital nal projections were for the most part ipsi- trunk represent the ophthalmic branch of the lateral, subsequent HRP labelings were trigeminal nerve. The ophthalmic branch made bilaterally. Bilateral labelings in which arises from the rostral-most portion of the different nerve branches were labeled on each ganglionic complex together with a bundle of side of the head facilitated comparison of the facial sensory fibers (Fig. 2). This trigemino- relative positions of the terminal fields of facial complex courses rostrally along the op- individual nerves. Of the 27 experimental tic tectum and exits the braincase, along with cases, there were 8 cases in which the su- the , through a foramen lat- praorbital trunk (the ophthalmic branch) was eral to the rostral pole of the tectum. After labeled; 5 cases in which the infraorbital emerging from the cranium, this nerve com- trunk was labeled; 4 cases in which the max- plex is accompanied by the supraorbital illary branch was labeled on one side and the branch of the anterior lateral line nerve, thus mandibular branch was labeled on the other forming the supraorbital trunk. Unlike other side; 5 cases in which the infraorbital trunk teleosts that have been described (Herrick, was labeled on one side and the supraorbital 1899, 1900), the supraorbital branch of the trunk was labeled on the opposite side; and 5 anterior lateral line nerve does not anasto- cases of the hyomandibular trunk, 3 in which mose with the rest of the supraorbital trunk. the infraorbital trunk was labeled on the op- The supraorbital trunk courses rostrally posite side. along the roof of the orbit, distributing sen- sory fibers to the of the rostrodorsal RESULTS surface of the head (Fig. 1). Upon reaching Normal anatomy the front of the orbit, the supraorbital trunk passes through a foramen in the frontal bone Peripheral distribution to distribute fibers in the region of the nasal The trigeminal nerve in C. auratus enters aperture. Although branches of this trunk the brainstem as a single root comprising were observed coursing closely beneath the both sensory and motor fibers. The motor , no fibers were observed fibers compose the ventral portion and the entering the epithelium. Furthermore, no fi- sensory component the dorsal portion of the bers from the supraorbital trunk were traced trigeminal root. As in many teleosts, during to the oral cavity or . embryogenesis the ganglia of the trigeminal The infraorbital trunk is composed of fibers nerve fuse with the ganglia of the anterior from the trigeminal ganglion, the dorsal gan- lateral line and facial nerves to form a gan- glion of the anterior lateral line nerve, and glionic complex termed the anterior gan- the facial ganglion. The trigeminal fibers of glion. Using the nonexperimental, serially the infraorbital trunk comprise the maxillo- sectioned head, it was possible to distinguish mandibular branches of the trigeminal nerve the components of the ganglionic complex by (Fig. 2). The infraorbital trunk courses pe- TRIGEMINAL AFFERENT PROJECTIONS IN GOLDFISH 135

......

Fig. 2. Schematic drawing of the lateral view of the oblique lines; large dots indicate the position of the tri- anterior ganglion of Carassius auratus. Rostra1 is to the geminal ganglion. The facial component is represented left. To simplify the diagram only sensory components by horizontal lines; squares illustrate the extent of the are illustrated. The relations of the trigeminal, facial, facial ganglion. The lateral line component is shown and anterior lateral line components are indicated dia- without highlight; triangles indicate the position of the grammatically. The outline of the brain is shown in dorsal and ventral ganglia. Bar scale = 0.3 mm. small dots. The trigeminal component is represented by ripherally along the caudal wall and floor of lar trunk (rcl, Fig. 2). Furthermore, a small the orbit and divides distally into two main bundle of fibers separates off from the in- branches, the maxillary and mandibular (Fig. fraorbital trunk soon after this trunk exits I). Application of HRP to these branches the braincase. These fibers turn caudolater- demonstrated that each contains fibers that ally and join the hyomandibular trunk just enter the medulla through the trigeminal before it enters the hyomandibular bone. A root and project to trigeminal medullary cen- similar bundle of fibers, termed the ramus ters. The maxillary branch distributes sen- communicans, has been described in several sory fibers to the anterior roof of the mouth, other species of teleosts (Herrick, 1899; Allis, the dorsal lip, and the region rostral to the '03; Maheshwari, '65; Saxena, '67; Harrison, eye. The mandibular branch distributes tri- '81). Herrick claimed that the ramus com- geminal motor fibers to the various compo- municans of Menidia is composed of trigemi- nents of the m. adductor mandibulae, as well nal fibers. Whether this ramus in Carassius as sensory fibers to the floor of the mouth, contains trigeminal fibers could not be deter- lower lip, and rostral cheek region. mined conclusively in the present study. The The trigeminal ganglion extends sensory peripheral distribution of the trigeminal fi- fibers to the hyomandibular trunk by two bers of the hyomandibular trunk was not routes. A small bundle of fibers from the determined in this study. However, the hyo- trigeminal portion of the ganglionic plexus mandibular trunk distributes fibers to the passes ventrally along the lateral edge of the opercular, gular, and cheek regions, as shown trigeminal ganglion to join the hyomandibu- in Figure 1. 136 R.L. PUZDROWSKI

Figure 3 TRIGEMINAL AFFERENT PROJECTIONS IN GOLDFISH 137

The trigeminal nuclei sible, therefore, to trace all fibers to their respective roots and thus identify the root of The root of the trigeminal nerve enters the origin of any particular projection. Labeled brainstem in a caudomedial direction and fibers in the roots of the trigeminal, facial, forms a cap over the ascending secondary gustatory tract (Fig. 3A). The trigeminal mo- and anterior lateral line nerves were ob- served with peripheral labeling of any of the tor fibers continue medially and ventrally three nerve trunks. The densest trigeminal around the gustatory tract to their cells of projections (i.e., the greatest number of la- origin in the trigeminal motor nucleus (Fig. 3A). The sensory portion of the trigeminal beled fibers) were observed following HRP application to the infraorbital trunk; the nerve comprises two components: the fibers sparsest projections were observed following to the sensory trigeminal nuclei of the me- HRP application to the hyomandibular dulla, and the fibers of the mesencephalic trigeminal cells. The nomenclature used for trunk. Each of the branches of the trigeminal the trigeminal sensory nuclei is based on that nerve project ipsilaterally to all medullar nu- clei (Fig. 4). Since the observed medullar pro- of Herrick ('06) and Woodburne ('36). The jections of the maxillary and mandibular sensory trigeminal nuclei of the medulla con- sist of the sensory trigeminal nucleus (STN), branches were similar to those following la- beling of the entire infraorbital trunk, a de- the nucleus of the descending trigeminal tailed description of the infraorbital trunk tract (NDTV), the spinal trigeminal nucleus projections is given and exceptions to this (Spv), and the medial funicular nucleus (MFn) pattern in the maxillary and mandibular (Fig. 3). A more complete description of these projections are noted. Ipsilateral retrograde nuclei is given with the description of the labeling of the mesencephalic trigeminal experimental results. The mesencephalic tri- cells was observed with HRP application to geminal nucleus in the goldfish is a diffuse the supra- or infraorbital trunk. The efferent nucleus consisting of oval-shaped cells (20- cells of the anterior lateral line nerves and 40 pm, long axis) which lie scattered along the cells of the trigeminal and facial motor the floor of the ventricle of the optic tectum. nuclei also were labeled in this study. The The cells are found either singly or in groups organization of these centers will be de- of four or five in the region between the pos- scribed in later papers. terior commissure and the nucleus of the me- dial longitudinal fasciculus. The number of Medullar projections of the trigeminal cells on each side of the brain varies from 8 sensory root to 12. The mesencephalic trigeminal root was The sensory trigeminal nucleus. Upon en- not apparent in the normal material exam- tering the brainstem a portion of the trigem- ined in this study. inal sensory fibers passes dorsomedially and Experimental results rostrally into a nucleus of densely packed, medium-sized (5-10 pm), round cells (Fig. 3A), Since fibers from the facial and the ante- which extends from the level of the entrance rior lateral nerves intermingle with the tri- of the trigeminal nerve rostrally beneath the geminal rami to form complex nerve trunks, medial portion of the secondary gustatory peripheral HRP labeling of the nerve trunks nucleus to the level of the commissure of the results in labeling not only the trigeminal secondary gustatory nucleus. This nucleus fibers but all fibers associated with the com- has been termed the sensory trigeminal nu- plex. Intracranially, the fibers of each cranial cleus (STN) (Woodburne, '36). The branches nerve segregate and enter the brainstem of the trigeminal sensory nerve project in a through distinctly separate roots. It was pos- rough topography onto the STN. The trigem- inal sensory fibers of the ophthalmic branch project to the more ventral portion of the Fig. 3. Photomicrographs of cresyl violet-stained STN, whereas the trigeminal fibers of the transverse sections through the brainstem of Curussius infraorbital and hyomandibular trunks pro- aurntus, illustrating the nuclei (outlined in dashed lines) ject to the more dorsal and rostral portion of of the trigeminal medullary nuclear complex at succes- sive levels from A (most rostral) to D (most caudal). the STN (Fig. 4A). The terminal fields of the Medial is to the left. Asterisks indicate the position of infraorbital and hyomandibular branches ex- the DTV. A: Level of the entrance of the trigeminal hibit considerable overlap within this nerve showing the STN. B: Level of the facial motor nucleus. nucleus illustrating the NDTV. C: Level of the caudal vagal root showing the Spv. D: Level of the rostral spinal The nucleus of the descending trigeminal cord illustrating the MFn. Bar scale in D = 0.15 mm. tract. The majority of the trigeminal sensory 138 R.L. PUZDROWSKI

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Fig. 4. Drawings of transverse sections through the wavy lines represent descending mesencephalic trigem- brainstem of Carassius auratus illustrating the afferent inal fibers. Lateral view of the brain in the upper right projections of the ophthalmic (open circles), infraorbital hand indicates the level of each transverse section. Bar (dots), and hyomandihular (triangles) branches of the scale = 0.5 mm. trigeminal nerve. The DTV is shown in solid black. Short fibers turn caudally forming the descending width at midsoma) fusiform-shaped cells (Fig. trigeminal tract (DTV), which descends 3B). Small (2-5 pm) round cells also were through the medulla along the dorsolateral observed among the fibers of the descending surface of the ascending secondary gustatory tract. These cells have been considered to be tract (Fig. 4B-D). In its course caudally the part of the NDTV by previous investigators DTV is divided into dorsal and ventral fasci- (Woodburne, '36; Nieuwenhuys and Pouwels, cles by the entering fibers of the facial motor '83). root. These fascicles recombine to form a sin- Application of HRP to any of the trigemi- gle tract immediately caudal to the facial nal nerve branches resulted in labeled fibers motor root. throughout the mediolateral extent of the The DTV is accompanied by a medially DTV. As it courses caudally, the DTV sends situated, longitudinal column of cells termed fibers medially into the NDTV (Figs. 4,5A,B). the nucleus of the descending trigeminal Whether these fibers represent collaterals of tract (NDTV). The NDTV is a diffuse nucleus fibers within the DTV could not be deter- consisting of medium-sized (5-10 pm) and mined. The projections are most extensive at large (15-20 pm) round cells and large (10- four levels, which correspond to the levels of 15 pm width at midsoma) and small (5 pm the nuclei of the branchiomeric motor col- TRIGEMINAL AFFERENT PROJECTIONS IN GOLDFISH 139 umn. Each level receives projections from fascicle. The ventral fascicle projects medi- all trigeminal sensory branches examined. ally along the ventral border of the nucleus. There was considerable overlap in the projec- The fibers of the dorsal fascicle project medi- tions of the trigeminal nerve branches onto ally along the dorsal border of the nucleus the NDTV. However, in general the projec- and terminate densely in the dorsomedial tions of the ophthalmic branch are restricted portion of the MFn (Figs. 4F, 6B). Consider- to the dorsal portion of the NDTV, whereas able overlap occurs between the ventral por- those of the infraorbital and hyomandibular tion of the ophthalmic branch and the ventral trunks project to both the dorsal and ventral fascicle of the infraorbital trunk. The trigem- portions of the NDTV (Figs. 4B,C, 5A,B). inal sensory fibers of the hyomandibular Additionally, following labeling of the en- trunk project to the dorsolateral portion of tire infraorbital trunk, fibers were observed the MFn, overlapping the dorsolateral por- coursing through the ventral portion of the tion of the projection area of the infraorbital NDTV to terminate among the cells of the trunk. trigeminal and facial motor nuclei (Fig. 5C). Furthermore, following HRP application to This projection was not seen following indi- the infraorbital or ophthalmic branches of vidual labeling of the maxillary or mandibu- the trigeminal nerve, a small number of fi- lar branches. A projection to the NDTV from bers was observed crossing the midline and the ventral fascicles of the facial sensory root coursing rostrally a short distance before ter- was observed following HRP labeling of any minating in the medial portion of the contra- of the three nerve trunks. This projection lateral MFn. was heaviest following labeling of the in- fraorbital trunk. A projection from the DTV The mesencephalic trigeminal root and to the ventral aspect of the facial lobe also nucleus was seen at this level with HRP application Labeled mesencephalic trigeminal root fi- to the infraorbital trunk Figs. 4C, 5D). These bers and cells were observed following the projections are apparent with the labeling of application of HRP to the ophthalmic branch the maxillary and mandibular branches, al- or the maxillary and mandibular branches of though in the former case they are extremely the infraorbital trunk (Fig. 7). The mesence- light. phalic trigeminal root consists of large-di- The spinal trigeminal nucleus and medial ameter fibers that enter the brainstem with funicular nucleus. The majority of fibers of the trigeminal motor root coursing medially the DTV descend into the caudal medulla to and ventrally toward the trigeminal motor terminate in the spinal trigeminal nucleus nucleus. Although the mesencephalic tri- (Spv) and the medial funicular nucleus (MFn). geminal root fibers are few in number, their Both of these nuclei consist mostly of round ascending course to the mesencephalon could cells (5-10 pm) with a few small, fusiform- be easily followed due to their large diameter shaped cells (Fig. 3C,D). The Spv and MFn (Figs. 7, 8). The mesencephalic trigeminal are distinguished from the NDTV based on root does not compose a defined fiber bundle, the lack of large, round, and fusiform-shaped but rather consists of scattered fibers that cells, which are present in the NDTV. Most are spread in the mediolateral plane between of the cells of the MFn are concentrated in the medial and lateral longitudinal fasciculi the medial portion of the nucleus and are (Fig. 8). As they ascend, the mesencephalic more densely packed than those of the Spv. trigeminal fibers gradually move dorsally to The MFn extends into the rostra1 course beneath the dorsal tegmental nucleus to the level of the first spinal nerve. toward the floor of the ventricle of the optic As in the NDTV, there is a great deal of tectum. Furthermore, following application overlap between the projections of the tri- of HRP to the ophthalmic or infraorbital geminal branches onto the Spv (Fig. 4E). In trunk branches, large-diameter fibers were contrast, the individual branches of the tri- observed coursing caudally, dorsolateral to geminal nerve project onto the MFn in a the reticular formation and ventromedial to distinctly topographic manner (Figs. 4F, 6). the NDTV (Figs. 4B-F, 5). In their course The trigeminal sensory fibers of the oph- caudally, these fibers pass along the dorso- thalmic branch project to the region between medial edge of the facial motor nucleus (Fig. the two fascicles of the infraorbital trunk 5A,C,D). Whether or not these fibers make (Figs. 4F, 6A). The projections of the infraor- synaptic contacts with the cells of this motor bital trunk onto the MFn are divided into a nucleus could not be determined. The de- large dorsal fascicle and a smaller ventral scending mesencephalic trigeminal fibers Figure 5 TRIGEMINAL AFFERENT PROJECTIONS IN GOLDFISH 141 were traced into the ventromedial portion of the caudomedial portion, and the anterior the MFn. lateral line ganglia lie on the dorsal and lat- There was one labeled cell in the mesen- eral edges of the anterior ganglionic com- cephalon for each labeled ascending mesen- plex. The organization of the anterior gan- cephalic trigeminal root fiber. The number of glion in the goldfkh is in accordance with labeled cells following HRP application to descriptions of the anterior ganglion of other the entire infraorbital trunk ranged from 9 teleosts (Herrick, 1899,1900, '01; Luiten, '79). to 12 (n = 4). In contrast, following applica- Further experimental investigations involv- tion of HRP to the ophthalmic branch, only 2 ing labeling of the various roots of the cra- to 4 cells (n = 4) were labeled. Furthermore, nial nerves whose ganglia compose the individual HRP labelings of the maxillary anterior ganglionic complex are necessary to and mandibular branches of the infraorbital confirm these descriptions. trunk resulted in fewer labeled mesence- As in other anamniotic gnathostomes, the phalic trigeminal cells (6 to 9, n = 4) than trigeminal sensory nerve consists peripher- when the entire trunk was filled more proxi- ally of an ophthalmic branch and a maxillo- mally. Application of HRP to the mandibular mandibular branch. An additional branch of branch resulted in labeling 4 to 6 mesence- the trigeminal nerve, the ramus communi- phalic trigeminal cells (n = 3), whereas HRP cans, has been reported in a variety of tele- application to the maxillary branch labeled osts (Herrick, 1899, 1900, '01; Allis, 03; only 2 to 3 cells (n = 3). Maheshwari, '65; Saxena, '67; Harrison, '81). This branch has been described as coursing DISCUSSION peripherally as part of the hyomandibular In the present investigation, the organiza- trunk. All of these reports involved nonex- tion of the primary trigeminal sensory sys- perimental observations of the distribution tem of the goldfish, C. auratus, was exam- of the . The results of the pres- ined. The results of this study demonstrate ent study confirm the presence of trigeminal that the overall organization of the primary sensory fibers within the hyomandibular trigeminal sensory system in Carassius is trunk. The area of innervation of these tri- similar to that in other (Ariens geminal sensory fibers remains uncertain. Kappers et al., '36). Furthermore, the pres- A sensory trigeminal complex, which re- ent study serves to clarify and extend the ceives primary trigeminal afferent projec- results obtained in previous investigations of tions, was found in Carassius extending as the trigeminal sensory systems of other spe- a continuous cell column from the isthmic cies of teleosts (Herrick, '06; Woodburne, '36; region to the rostral spinal cord. This orga- Luiten; '75; Luiten and van der Pers, '77; von nization is in general agreement with de- Bartheld and Meyer, '85). scriptions of the trigeminal system in other In Carassius the trigeminal ganglion teleosts (Luiten, '75; von Bartheld and Meyer, makes up the rostral and rostrodorsal inter- '85) and is similar to descriptions of the pri- mediate portions of the anterior ganglionic mary trigeminal systems in other verte- complex. The large facial ganglion composes brates. In Carassius the cell column can be divided into four distinct nuclei (STN, NDTV, Spv, MFn) based on cytoarchitectural crite- ria. There has been some uncertainty as to Fig. 5. Photomicrographs of transverse sections through the brainstem of Carassius auratus at the level whether the STN should be considered a sep- of the facial motor nucleus. A$: Sections illustrating arate nucleus or simply a rostral continua- the projections to the NDTV 6 days after bilateral appli- tion of the NDTV. In his initial study on the cation of HRP to the ophthalmic branch (A) and infraor- trigeminal sensory system in the carp, Cypri- bital trunk (B). Small asterisks in both A and B indicate the shrinkage artifact. C: Higher-power view of the fa- nus carpio, Luiten ('75) considered the STN cial motor nucleus region of the section shown in B, to be a subdivision of the NDTV. In a subse- illustrating labeled fibers (large arrows) of the DTV quent paper, Luiten and van der Pers ('77) coursing through the NDTV and among the cells of the reported that the cells of the rostral portion facial motor nucleus 6 days after HRP application to the infraorbital trunk. D: Section slightly rostral to that of the STN, cells in the NDTV, SPv, and MFn shown In A and B, illustrating a bundle of fibers (arrows) project to the trigeminal and facial motor projecting dorsomedially from the DTV toward the ven- nuclei. Based on the fact that labeled cells trolateral portion of the facial lobe. Small arrowheads in were found only in the rostral portion of the A, C, and D indicate labeled descending mesencephalic trigeminal fibers. Medial is to the left in B-D and to the STN it was concluded that the STN is a dis- right in A. Bar scale for A and B = 0.15 mm; for C = tinct nucleus separate from the rest of NDTV 0.04 mm; and for D = 0.10 mm. (Luiten and van der Pers, '77). In Carassius 142 R.L. PUZDROWSKI

Fig. 6. Photomicrographs of the right and left half of on the left side (A) and the infraorbital trunk on the a single transverse section through the cervical spinal right side (K)of the animal. Note the division of the cord of a goldfish illustrating the projections to the MFn projections of the infraorbital trunk into dorsal and ven- 6 days after HRP application to the ophthalmic branch tral fascicles (arrowheads). Bar scale in A = 0.15 mm. the STN can be distinguished further on the (Northcutt, '79; Ronan, '85), the STN may basis of its cytoarchitectural organization. be a shared derived character of jawed This nucleus is composed of densely packed, vertebrates. round cells, whereas in the remaining por- The results of the present study extend in tion of the trigeminal nuclear complex the several important ways the results obtained nuclei consist of relatively diffusely orga- in previous nonexperimental and experimen- nized round and fusiform-shaped cells. Fur- tal studies of the trigeminal nerve afferent thermore, in Carassius the STN has been projections in other species of teleosts (Wood- shown to project heavily to the valvula of the burne, '36; Luiten, '75; von Bartheld and cerebellum, whereas the NDTV does not Meyer, '85). Woodburne ('361, using nonex- (Wullimann and Northcutt, '86). These obser- perimental methods to describe the trigemi- vations support the separation of the STN as nal projections in trout and carp, reported a a nucleus distinct from NDTV. A topologi- segregation of the fibers within the descend- cally similar primary trigeminal nucleus has ing trigeminal tract such that the fibers of been described in elasmobranchs (Smeets and the ophthalmic branch course in the medial Nieuwenhuys, '76; Northcutt, '781, sturgeon portion of the tract and the fibers of the max- (New and Northcutt, '841, bullfrogs (Fuller illomandibular branch course in the lateral and Ebbesson, '73), reptiles (Molenaar, '78; portion of the tract. In the present study no Barbas-Henry and Lohman, '861, birds (Dub- such segregation of the fibers of the DTV was beldam and Karten, '78; Dubbeldam, 'go), observed. Labeled fibers were present and mammals (Olszewski, '50; Kerr, '63; throughout the mediolateral extent of the Marfurt, '81). Since hagfkhs and lampreys DTV following HRP application to any of the appear to lack a topologically similar nucleus branches of the trigeminal nerve. Further- TRIGEMINAL AFFERENT PROJECTIONS IN GOLDFISH 14 3 more, Woodburne ('36) reported a projection Oxylebius. The results of the present study from the DTV to the vagal lobe. In Carassius in Carassius generally concur with these pre- no projection from the DTV to the vagal lobe vious descriptions of the course and termi- was observed, nor have such projections been nation of the trigeminal afferents. Further- reported in other recent experimental stud- more, Luiten reported a projection from the ies of the trigeminal system in other species DTV to the ventrolateral aspect of the facial of teleosts (Luiten, '75; von Bartheld and lobe. This projection also was found in Car- Meyer, '85). assius with HRP labeling of the infraorbital Luiten ('75), utilizing degeneration tech- trunk. In Carassius the ventrolateral portion niques, investigated the afferent projections of the facial lobe also receives projections of the entire trigeminal nerve in the carp, from the mandibular branch of the facial sen- Cyprinus carpio. In this study Luiten re- sory nerve (Puzdrowski, '87). The conver- ported that the DTV gives off fibers medially gence of facial sensory and trigeminal to five terminal fields, which he considered sensory fibers on the ventrolateral portion of to be subdivisions of the NDTV. Similar re- the facial lobe indicates that this portion of sults were reported by von Bartheld and the lobe may represent an integration center Meyer ('85) in their HRP study of the central for and touch information. Luiten and projections of the cirrus nerve (a branch of van der Pers ('77) have shown that cells in the supraorbital trunk) in Hypoblennius and this portion of the facial lobe project to the

Fig. 7. Photomicrographs of transverse sections branch. B: Section at the level of posterior commissure through the mesencephalon of Curussius uumtus. Me- showing the labeled soma of two mesencephalic trigemi- dial is to the left. A Section at the level of the dorsal nal neurons 10 days after HRP application to the tegmental nucleus (DTN) illustrating labeled fibers (ar- ophthalmic branch. Arrows indicate two mesencephalic rows) of the ascending mesencephalic trigeminal root trigerninal fibers whose cell bodies lie more rostrolater- coursing rostrally between the nucleus of the MLF and ally. Bar scale = 0.15 mm. LLF 10 days after HRP application to the ophthalmic 144 R.L. PUZDROWSKI ABC D I I '1f

B

Fig. 8. Drawings of transverse sections through the bodies (large dots) located along the floor of the ventricle cerebellum and mesencephalon of Curassius uuratus, il- of the optic tectum. Lateral view of the brain in the lustrating the course of the ascending mesencephalic upper right hand indicates the level of each transverse trigeminal root fibers (short, wavy lines) to their cell section. Bar scale = 1 mm. trigeminal and facial motor nuclei. The con- muscles (Luiten, '75, '79; Luiten and van der vergence of tactile and gustatory information Pers, '77). The projection of the afferents of upon these cells would provide a quick eval- the ophthalmic branch to the NDTV has im- uation of the palatability of a food item and portant implications in this regard. Unlike allow rapid ingestion of particularly palata- the infraorbital and hyomandibular trunks, ble morsels. the supraorbital trunk (of which the ophthal- A topographic organization of the primary mic branch is a part) innervates no respira- trigeminal afferents has been reported in a tory musculature. The projection of the affer- variety of amniotes (Torvik, '56; Kerr, '63; ent fibers of the ophthalmic branch to the Grant and Arvidsson, '75; Molenaar, '78; NDTV suggests that at least a part of this Dubbeldam and Karten, '78; Dubbeldam, '80; nucleus serves some sensory function (i.e., Barbas-Henry and Lohman, '86). In general touch, temperature, ) other than muscle the maxillomandibular afferents are de- . scribed as projecting to the dorsal portion of The projections from the DTV to the tri- the trigeminal medullar nuclear complex and geminal and facial motor nuclei following the ophthalmic afferents to the ventral por- labeling of the infraorbital trunk may repre- tion. In Carassius the organization of the sent a monosynaptic proprioceptive circuit. trigeminal afferent projections to the STN Such a circuit was proposed by Luiten ('79) and MFn is in accordance with this pattern; based on electrophysiological recordings from the ophthalmic fibers project to the ventral the trigeminal and facial motor nuclei in the portion of these nuclei and the maxilloman- carp. dibular afferents project to the dorsal portion. Contralateral trigeminal projection. at cer- The cells of the NDTV have been consid- vical levels have been described in a variety ered to function as in the pro- of tetrapods (Kerr, '63; Fuller and Ebbesson, prioceptive reflex circuits of the respiratory '73; Molenaar, '78; Dubbeldam and Karten, TRIGEMINAI, AFFERENT PROJECTIONS IN GOLDFISH 145 '78; Marfurt, '81; Barbas-Henry and Loh- branches of the trigeminal nerve. This distri- man, '86). In Curassius projections to the con- bution of mesencephalic trigeminal fibers tralateral MFn were observed with HRP agrees with more recent reports on the distri- application to the ophthalmic and infraorbi- bution of the mesencephalic trigeminal fi- tal branches of the trigeminal nerve. Contra- bers in other vertebrates (monitor lizard, lateral projections were not observed in Barbas-Henry and Lohman, '86; cat, Corbin, previous experimental investigations of the '40; rat, Matesz, '81; dog, Ryu et al., '83). trigeminal afferents in other species of tele- While there is ample evidence that the osts (Luiten, '75; von Bartheld and Meyer, mesencephalic nucleus in mammals is in- '85). However, contralateral trigeminal pro volved in proprioception of the masticatory jections at cervical levels are present in Zctal- musculature, teeth, , and possibly also urus punctatus and Lepomis cyanellus (per- of the extrinsic eye muscles (Corbin and Har- sonal observations). Therefore, contralateral rison, '40; Dale Smith and Marcarian, '68; projections at cervical levels appear to be Dale Smith, '69; Alvarado-Mallart et al., '79, more widespread among teleosts than pre- the function of this nucleus in nonmammals vious accounts would indicate. is less well established. Roberts and Witkov- The failure of previous investigators to re- sky ('75) demonstrated that the mesence- port contralateral cervical trigeminal projec- phalic trigeminal neurons in Sycliorhinus tions in teleosts is probably due to the and Mustelus do not respond to movements sparseness of these projections and to differ- of the jaw, but rather are responsive to me- ences in techniques rather than to real spe- chanical stimulation of the teeth and peri- cies differences. The trigeminal afferents in oral skin. Luiten ('79) labeled sensory and sturgeons, on the other hand, are entirely motor cells in the brainstem of the carp, Cy- ipsilateral (New and Northcutt, '84). Thus, prinus carpio, by intramuscular injection of there are two patterns for cervical trigeminal HRP into the masticatory and extraocular projections among osteichthyans. It is uncer- musculature. None of these injections re- tain which of these conditions represents the sulted in the labeling of mesencephalic tri- primitive condition for this group. Based on geminal cells. Following experiments the lack of contralateral cervical projections involving intranerve HRP injection into the in hagfkhs (Ronan, '85) and lampreys maxillary and mandibular branches of the (Northcutt, '791, it is most parsimonious to infraorbital trunk, which resulted in the la- conclude that contralateral cervical trigemi- beling of the mesencephalic trigeminal neu- nal projections were not present in the com- rons, Luiten ('79) concluded that the mon ancestor of lampreys and gnathostomes. mesencephalic trigeminal neurons serve a It is possible that the condition seen in tet- perioral mechanoreceptive function. The rapods and teleosts represents an indepen- presence of mesencephalic trigeminal neu- dently derived condition, the lack of contra- rons in the supraorbital trunk, which in Car- lateral trigeminal projections at cervical assius distributes fibers to the rostrodorsal levels being the primitive condition for oste- head region, shows that the mesencephalic ichthyans. On the other hand, it is also pos- trigeminal nucleus has a broader peripheral sible that the pattern seen in sturgeons field of innervation than just the perioral represents the loss of contralateral projec- region. tions at cervical levels, the condition found The course of the ascending mesencephalic in teleosts and tetrapods representing the trigeminal fibers in Curassius is similar to primitive condition for osteichthyans. Decid- what has been described in Salmo and Sal- ing between these hypotheses will require an velinus (Woodburne, '36). Projections from the investigation of the primary trigeminal af- mesencephalic trigeminal neurons to the ros- ferents in polypteriforms and in elas- tral spinal cord have been described in lepi- mobranchs. dosirenid lungfishes (Ronan and Northcutt, Classically, the mesencephalic trigeminal '85), toads (Corvaja and d'Ascanio, '811, tur- fibers were thought to convey proprioceptive tles (ten Donkelaar et al., '80), lizards (Gold- impulses from the and stein and Jacobs, '69; Ebbesson, 'Bl), snakes to course peripherally exclusively in the (Dacey, '821, and rats (Matesz, '81). In Curas- mandibular branch of the trigeminal nerve sius, Harold Sloan (personal communication) (Ariens Kappers et al., '36). In Carussius the has labeled the mesencephalic trigeminal mesencephalic trigeminal sensory fibers are neurons with HRP application to the rostra1 also present in the ophthalmic and maxillary spinal cord. Large-diameter fibers, similar to 146 R.L. PUZDROWSKI the ascending fibers of the mesencephalic tri- Comp. Neurol. 204t268-279. geminal root, were found in Carassius pass- Dale Smith, R. (1969) Location of the neurons innervat- ing tendon spindles of masticator muscles. Exp. Neu- ing caudally from the level of the entrance of rol. 25646-654. the trigeminal nerve to terminate in the ven- Dale Smith, R., and H.Q. Marcarian (1968) Centripetal tromedial MFn. Herrick (’06) considered the localization of tooth and tongue tension receptors. J. MFn to be a coordination center for facial Dent. Res. 47t616-621. Dubbeldam, J.L. (1980) Studies on the somatotopy of the gustatory sensation, mediated via the de- trigeminal system in the mallard, AnaspEatyrhynchos. scending secondary gustatory tract, and for 11. Morphology of the principal sensory nucleus. J. touch sensation, mediated via the descending Comp. Meurol. 191t557-571. trigeminal tract. If mesencephalic trigemi- Dubbeldam, J.L., and H.J. Karten (1978)The trigeminal system in the pigeon (CoEumba Eiuia). I. Projections of nal neurons are involved in mechanorecep- the gasserian ganglion. J. Comp. Neurol. 18Ot661-678. tive sensation from the rostra1 head and lips, Ebbesson, S.O.E. (1981) Projections of the optic tectum then it is reasonable that they should project and the mesencephalic nucleus of the trigeminal nerve to the coordination center of taste and touch in the tegu lizard (Tupinambis nigropurzctatus). Cell Tissue Res. 216t151-165. for the head region. Freihofer, W.C. (1966) The Sihler technique of staining nerves for systematic study especially of fishes. Copeia ACKNOWLEDGMENTS 4t470-475. I thank Dr. R. Glenn Northcutt for his Fuller, P.M., and S.O.E. Ebbesson (1973) Central projec- tions of the trigeminal nerve in the hull frog (Rana guidance in every aspect of the research for catesbeiana),J. Comp. Neurol. 152: 193-200. and preparation of this paper. I also thank Goldstein, M.H., and V.L. Jacobs (1969) The central pro- Dr. M.F. Wullimann, Dr. N.E. Kemp, Dr. B. jections of the mesencephalic root of the trigeminus in Oakley, Thomas Schilling, Georg Striedter, a lizard (Lacerta uiridis). Brain Res. 14:307-320. Grant, G., and J. Arvidsson (1975) Transganglionic de- and Jiakun Song for critically reading the generation in trigeminal primary sensory neurons. manuscript, and Martha Vogel for technical Brain Res. 95:285-279. assistance in the preparation and sectioning Hanker, J.S., P.E. Yates, C.B. Metz, and A. Rustioni of Paraplast-embedded brains used in this (1977) A new specific, sensitive and non-carcinogenic reagent for the demonstration of horseradish peroxi- study. This report represents a portion of the dase. J. Histochem. 9t789-792. author’s doctoral dissertation in the Horace Harrison, G. (1981) The cranial nerves of the teleost H. Rackham School of Graduate Studies at Trichiurus lepturus. J. Morphol. 167t119-134. The University of Michigan. This research Herrick, C.J. (1899) The cranial and first spinal nerves of Menidia; a contribution upon the nerve components was supported in part by NIH grants of the bony fishes. J. Comp. Neurol. 9:153-455. NSllOON and EY02485 to Dr. R.G. Herrick, C.J. (1900) A contribution upon the cranial Northcutt. nerves of the codfish. J. Comp. Neurol. l0:265-318. Herrick, C.J. (1901) The cranial nerves and cutaneous LITERATURE CITED organs of the North American siluroid fishes. J. Comp. Neurol. llt177-249. Allis, E.P. (1903) The skull and the cranial and first Herrick, C.J. (1906) On the centers for taste and touch spinal muscles and nerves in Scomber scomber. J. Mor- in the of fishes. J. Comp. Neurol. phol. 18:45-328. 16t403-439. Alvarado-Mallart, M.R., C. Batini, C. Buisseret-Delmas, Kerr, F. (1983) The divisional organization of the affer- and J. Corvisier (1975) Trigeminal representations of ent fibers of the trigeminal nerve. Brain 86t721-732. the masticatory and extraocular proprioceptors as re- Luiten, P.G.M. (1975) The central projections of the tri- vealed by horseradish peroxidase retrograde transport. geminal, facial and anterior lateral line nerves in the Exp. Brain Res. 23t167-179. carp (Cyprinus carpio L.). J. Comp. Neurol. 160t399- Ariens Kappers, C.U., G.C. Huber, and E.C. Crosby 418. (1936) The Comparative Anatomy of the Nervous Sys- Luiten, P.G.M. (1979) Proprioceptive reflex connections tem of Vertebrates, Including Man. Vol. I. New York: of head musculature and the mesencephalic trigemi- Macmillan. Reprinted 1960: New York: Hafner. nal nucleus in the carp. J. Comp. Neurol. 183:903-912. Barbas-Henry, H.A., and A.H.M. Lohman (1988) The Luiten, P.G.M., and J.N.C. van der Pers (1977) The con- motor complex and primary projections of the trigemi- nections of the trigeminal and facial motor nuclei in nal nerve in the monitor lizard, Varanus exanthemati- the brain of the carp (Cyprinus carpio L.) as revealed cus. J. Comp. Neurol. 254t314-329. by anterograde and retrograde transport of horserad- Corbin, K.B. (1940) Observations on the peripheral dis- ish peroxidase. J. Comp. Neurol. 174375-590. tribution of fibers arising in the mesencephalic nu- Maheshwari, S.C. (1965) The cranial nerves of Mastu- cleus of the fifth cranial nerve. J. Comp. Neurol. cembelus armatus. Jpn. J. Ichthyol. 12t89-98. 73; 153- 177. Marfurt, C.F. (1981)The central projections of trigeminal Corbin, K.B., and F. Harrison (1940) Function of mesen- primary afferent neurons in the cat as determined by cephalic root of fifth cranial nerve. J. Neurophysiol. the transganglionic transport of horseradish peroxi- 3t423-435. dase. J. Comp. Neurol. 203t785-798. Corvaja, N., and P. d’Ascanio (1981) Spinal projections Matesz, C. (1981) Peripheral and central distribution of from the mesencephalon in the toad. Brain Behav. fibers of the mesencephalic trigeminal root in the rat. Ed.19:205-213. Neurosci. Lett. 27t13-17. Dacey, D.M. (1982) Axon morphology of mesencephalic Mesulam, M.M. (1978) Tetramethyl benzidine for horse- trigeminal neurons in a snake, Thamnophis sirtalis. J. radish peroxidase neurohistochemistry. A non-carci- TRIGEMINAL AFFERENT PROJECTIONS IN GOLDFISH 147

nogenic blue reaction-product with superior sensitivity Pacific . Sac. Neurosci. Ahstr. 11t1311. for visualizing neural afferents and efferents. J. Histo- Ronan, M., and R.G. Northcutt (1985) The origins of chem. Cytochem. 28t108-117. descending spinal projections in lepidosirenid lung- Molenaar, G.J. (1978) The sensory trigeminal system of fishes. J. Comp. Neural. 241t435-444. a snake in the oossession of infrared receotors. 11. The Ryu, K., K. Watanahe, and E. Kawana (1983) The mes- central projections of the trigeminal nerve. J. Comp. encephalic root fibers of the trigeminal nerve in thc Neurol. 179:137-152. dog. Acta Anat. 116:28-36. New, J.G., and R.G. Northcutt (1984) Primary projec- Saxena, P.K. (1967) The cranial nerves of the mud eel tions of the trigeminal nerve in two species of stur- Amphipnous cuchia. Acta Anat. 87t306-320. geon: Acipenser oxyrhynchus and Scaphirhynchus Smeets, W.J.A.J., and R. Nieuwenhuys (1978) Topologi- platyrhynchua J. Morphol. 182t125-136. cal analysis of the hrain stem of the sharks Squalus Nieuwenhuys, R., and E. Pouwels (1983) The brain stem acanthias and Scyliorhinus canicula. J. Comp. Neurol. of actinopterygian fishes. In R.G. Northcutt and R.E. 106:51-142. Davis (eds): Fish Neurobiology. Vol. 1. Ann Arbor, ten Donkelaar, H.J., A. Kusuma, and R. de Boer-van Michigan: University of Michigan Press, pp. 25-87. Huizen (1980) Cells of origin of pathways descending Northcutt, R.G. (1978) Elasmobranch brain organiza- to the spinal cord in some quadrupedal reptiles. J. tion. In S. Hodgson and R.F. Mathewson ieds): Sensory Comp. Neural. 192327-851. Biology of Sharks, Skates and Rays. Arlington, Vir- Torvik, A. (1956) Afferent connections to the sensory ginia: U.S. Government Printing Office, pp. 117-193. trigeminal nuclei, the nucleus of the solitary tract and Northcutt, R.G. (1979) Experimental determination of adjacent structures. An experimental study in the rat. the primary trigeminal projections in lampreys. Brain J. Comp. Neurol. 106:51-142. Res. 163r323-327. von Bartheld, C.S., and D.L. Meyer (1985) Trigeminal Olszewski, J. (1950) On the anatomical and functional and facial innervation of cirri in three teleost species. organization of the spinal trigeminal nucleus. J. Comp. Cell Tissue Res. 241t615-622. Neurol. 92:401-413. Weinherg, E. (1968) The mesencephalic root of the fifth Puzdrowski, R.L. (1985) The primary projections of the nerve. A comparative anatomical study. J. Comp. Neu- trigeminal nerve in goldfish, Carassius auratus. SOC. rol. 48249-405. Neurosci. Abstr. 11t1311. Woodhurne, R.T. (1936) Phylogenetic considerations of Puzdrowski, R.L. (1987) The peripheral distribution and the primary and secondary centers and connections of central projections of the sensory rami of the facial the trigeminal complex in a series of vertebrates. J. nerve in goldfish, Carassius auratus. J. Comp. Neurol. Comp. Neurol. 65t403-493. 259t382-392. Wullimann, M.F., and R.G. Northcutt (1986) Afferent Roberts, R.L.. and P. Witkovskv (1975) A functional connections of the valvula cerehelli in the goldfish, analysis of the mesencephalic n;cleus of the fifth nerve Carassius auratus. In Splechtna and Hilgers (eds): 2nd in the selachian hrain. Proc. R. Sac. Land. B, 190t473- Int. Symp. Vert. Morphol. Vienna, Fortschr. Zool. No. 495. 35. Stuttgart; Fischer (in press). Ronan, M. (1985) Primary trigeminal projections in the