Ascending Spinal Systems in the Fish,Prionotus carolinus

THOMAS E. FINGER*

Department of Cellular and Structural Biology,University of Colorado Health Sciences Center, Denver, Colorado 80262

ABSTRACTThe fin rays of the pectoral fin of the sea robins (teleostei) are specialized chemosensory

organs heavily invested with solitary chemoreceptor cells innervated only by spinal nerves. Therostral spinal cord of these animals is marked by accessory spinal lobes which are uniqueenlargements of the dorsal horn of the rostral spinal segments receiving input from the fin raynerves. Horseradish peroxidase (HRP) and 1,19-dioctadecyl-3,3,39,39-tetramethylindocarbo-cyanine perchlorate (diI) were used as anterograde and retrograde tracers to examine theconnectivity of these accessory lobes and the associated ascending spinal systems in the sea robin,Prionotus carolinus. The majority of dorsal root fibers terminate within the accessory lobes at ornearby their level of entrance into the spinal cord. A few dorsal root axons turn rostrally in thedorsolateral fasciculus to terminate in the lateral funicular complex situated at the spinomed-ullary junction. The lateral funicular complex also receives a heavy projection from the ipsilateralaccessory lobes. In addition, it contains a few large neurons that project back onto the accessorylobes. Injections of either diI or HRP into the lateral funicular complex label fibers of the mediallemniscus which crosses the midline in the caudal medulla to ascend along the ventral margin ofthe contralateral rhombencephalon. Within the medulla, fibers leave the medial lemniscus toterminate in the inferior olive and in the ventrolateral medullary reticular formation. Uponreaching the midbrain, the medial lemniscus turns dorsally to terminate heavily in a lateraldivision of the torus semicircularis, in the ventral optic tectum, and in the lateral subnucleus ofthe nuc. preglomerulosus of the thalamus. Lesser projections also reach the posterior periven-tricular portion of the posterior tubercle with a few fibers terminating along the ventral, posteriormargin of the ventromedial (VM) nucleus of the thalamus. The restricted projection to the ventraltectum is noteworthy in that this part of the tectum maintains the representation of the ventralvisual field, that is, the area in which the fin rays lie. A prominent spinocerebellar system is alsoevident. Both direct and indirect spinocerebellar fibers can be followed through the dorsolateralfasciculus, with or without relay in the lateral funicular nucleus and terminating in a restrictedportion of the granule cell layer of the ipsilateral corpus cerebelli. The similarities in connectivityof the spinal cord between the sea robins and other vertebrates are striking. It is especially notablebecause sea robins utilize the chemosensory input from the fin rays to localize food in the environ-ment. Thus, although these fish use their spinal chemosense as other fishes use their external tastesystems, the spinal chemosense apparently relies on the medial lemniscal system to guide thischemically driven feeding behavior. J. Comp. Neurol. 422:106–122, 2000. © 2000 Wiley-Liss, Inc.

Indexing terms: spinal cord; medial lemniscus; taste; solitary chemoreceptor cell; cerebellum;

funicular nucleus; optic tectum

Various fishes utilize external chemoreceptors on ex-ploratory appendages (e.g., barbels or fin rays) to localizefood in their surroundings. Most of these systems, how-ever, involve elaboration of the external gustatory systemand thus are innervated by branches of the facial nerve,e.g., barbels in catfishes, goatfish, and cods; pelvic fins ofgadids; or the modified pectoral fins of gouramis (Scharreret al., 1947). The sea robins and gurnards, which areadvanced marine teleosts (Scorpaeniformes), have aunique specialization involving the rostralmost fin rays ofthe pectoral fin (Figs. 1A,B). Although the free fin rays are

chemosensory and are utilized like a gustatory system forlocalizing food in the environment (Scharrer et al., 1947;

Grant sponsor: National Science Foundation; Grant number: NSF BNS-79-04310.

*Correspondence to: Thomas Finger, Department of Cellular and Struc-tural Biology, University of Colorado Health Sciences Center B-111, 4200E. Ninth Ave., Denver, CO 80262. E-mail: tom.finger@uchsc.edu

Received 5 August 1999; Revised 11 February 2000; Accepted 11 Febru-ary 2000

THE JOURNAL OF COMPARATIVE NEUROLOGY 422:106–122 (2000)

© 2000 WILEY-LISS, INC.

Bardach and Case, 1965; Silver and Finger, 1984), theseorgans are innervated only by spinal nerves and possesssolitary chemoreceptor cells (SCCs) rather than taste buds(Morrill, 1895; Whitear, 1971). The spinal nerves inner-vating the fin rays terminate in dorsal enlargements of thedorsal horn, called accessory spinal lobes (Morrill, 1895;Herrick, 1907), situated in the rostral spinal cord. Thisspinally innervated chemosense is part of neither the gus-tatory nor olfactory senses. Rather it is an elaboration ofthe SCC system present in all aquatic anamniote verte-brates (Whitear, 1992; Finger, 1997).

In most fishes, the SCCs are distributed widely acrossepithelial surfaces of the trunk, head, fins, and gill arches(Whitear, 1992). The SCCs may be innervated by any ofthe cutaneous nerves, including trigeminal, facial, or spi-nal (Whitear, 1992). This diversity of innervation is re-flected in the diverse primary sensory nuclei that servethis modality. For example, in the gadid Ciliata mustela,the SCCs of the specialized dorsal fin are innervated bythe facial nerve and are represented in a particular sub-nucleus (dorsal lobule of the facial lobe) of the medullaryvisceral sensory column (Kotrschal and Whitear, 1988). Incontrast, the SCCs on the pectoral fin rays of sea robinsare innervated by spinal nerves and are represented insomatotopically organized accessory spinal lobes (Finger,1982).

The older descriptions of the spinal accessory lobes(Tiedemann, 1816; Morrill, 1895; Herrick, 1907) delineatesix pairs of accessory lobes within the rostral spinal cord.Indeed, cursory examination of the rostral cord and closedmedulla reveals six dorsal enlargements. These early de-

scriptions are, however, problematic in including the me-dial funicular nucleus—actually a part of the spinal tri-geminal complex—as a spinal lobe, and in dividing one ofthe more caudal lobes into two parts because of the pres-ence of a furrow containing a prominent blood vessel.Closer examination (Finger, 1982) reveals three majorpaired accessory lobes (ALM) caudally and one minorpaired lobe (ALm) situated between the major lobes andthe medial funicular nucleus (Fig. 2D). The fin rays arerepresented in the three major lobes, whereas the pectoralfin proper is represented in the minor accessory lobe (Fig.1C).

How the fin ray chemosense of sea robins is representedwithin the central nervous system above the level of thespinal cord has not been resolved. Behaviorally, this senseis used like an external gustatory system; in terms ofperipheral innervation and central projections of the pri-mary sensory fibers, the sense is organized like the so-matosensory system. In a fish (gadidae: Ciliata) with aspecialized SCC system (Kotrschal and Whitear, 1988),the SCCs are innervated by a recurrent branch of thefacial nerve. This nerve projects centrally into a distinctportion of the facial lobe which forms the rostral end of thevisceral sensory column. Rostral relays of this faciallyinnervated SCC system are largely indistinguishable fromthose of adjacent gustatory compartments of the faciallobe (Kotrschal and Finger, 1996). In the rocklings, how-ever, the SCC system is not used for detection and local-ization of food, but rather may serve as a “predator detec-tor” (Peters et al., 1991). Whether the spinally innervatedSCCs on the fin rays of sea robins maintain a unique

Abbreviations

2Gt secondary gustatory tract4th Vent fourth ventricleALM major spinal accessory lobeALm minor spinal accessory lobeAT anterior thalamic nucleusBC brachium conjunctivumCb cerebellumCbg granule cell layer of the cerebellumCbm molecular layer of the cerebellumCC crista cerebellarisCG corpus (nucleus) glomerulosusDLF dorsolateral fasciculusDMNX dorsal motor nucleus of the vagusEG eminentia granularisFL facial lobeHC horizontal commissureHy hypothalamusiaf internal arcuate fibersIL inferior lobeiO inferior oliveLFB lateral forebrain bundleLFn lateral funicular nucleusLFn-d dorsal posterior nucleus of the lateral funicular complexLFn-mag magnocellular nucleus of the lateral funicular complexlg large neurons of the accessory lobesLL lateral lemniscusmed medium-sized neurons of the accessory lobesMFn medial funicular nucleusML medial lemniscusMLF medial longitudinal fasciculusnA nucleus ambiguusNALL anterior lateral line nerveNCC commissural nucleus of CajalnD nucleus diffususnM nucleus medialis of the octavolateral areannc nonneural cell

nPGl lateral preglomerular nucleusnPGlex exterolateral portion of the lateral preglomerular nucleusnPGm medial preglomerular nucleusnPT posterior thalamic nucleusNV trigeminal nerveNVII facial nerveNVm trigeminal motor nucleusNX vagus nervePC posterior commissurePO preoptic nucleusPPv periventricular pretectal nucleus, ventral partRF reticular formationSAC stratum album centrale of the optic tectumSFGS stratum fibrosum et griesium superficiale of the optic tec-

tumsm small neurons of the accessory lobesSMn sonic motor nucleusSpCt spinocerebellar tractSpN 1 first spinal rootSpN 2 second spinal rootSpN 3 third spinal rootSpVt spinal trigeminal tractTBt tectobular tractTel telencephalonTeO optic tectumTL torus longitudinalisTPp periventricular nucleus of the posterior tubercleTrO optic tractTS torus semicircularisTSe external nucleus of the torus semicircularisVent ventricleVH ventral hornVL vagal lobeVLF ventral longitudinal fasciculusVM ventromedial thalamic nucleus

107SPINAL CONNECTIONS IN SEA ROBIN

lemniscal system, or utilize either gustatory or spinallemniscal pathways is unknown.

Ascending pathways from the spinal cord have beenstudied in only a limited number of teleost fish (Hayle,1973a; Murakami and Ito, 1985; Oka et al., 1986; Beckeret al., 1997; Prasada Rao and Sharma, 1999). These stud-ies generally reveal ascending systems terminating in themedullary ventrolateral reticular formation, vestibularnuclei, and cerebellum. In addition, recent studies on os-tariophysean fishes (goldfish and zebrafish) report directspinal projections to the diencephalon and telencephalon,but these have not been confirmed by retrograde methods(Becker et al., 1997; Hanna et al., 1998; Prasada Rao andSharma, 1999). A multisynaptic thalamopetal spinal lem-niscus arising from rostral cervical levels has been re-ported in another scorpaeniform teleost (Ito et al., 1986).Despite this, it is unclear whether there exists a distinctmultisynaptic spinal lemniscal system equivalent to thedorsal column-medial lemniscus in tetrapod vertebrates(Munoz et al., 1997). To examine this issue, anterogradeand retrograde tracers were placed into the dorsal roots,accessory spinal lobes, and other nuclei along the ascend-ing system arising from the spinal accessory lobes in a sea

robin. A portion of these findings have been presented inabstract form (Finger, 1981, 1983a,b) and have been sum-marized in Finger (1997).

MATERIALS AND METHODS

Adult sea robins, Prionotus carolinus, gathered by thecollecting service of the Marine Biological Laboratory(Woods Hole, MA), were utilized in these studies. The fishranged in size from about 15 to 35 cm in total length. Theanimals, gathered by trawl from local waters, were keptfor up to 6 weeks in large holding tanks supplied withrunning sea water. All procedures were carried out inaccordance with guidelines of the University of ColoradoInstitutional Animal Care and Use Committee.

As detailed in Table 1, the majority (63 total) of tract-tracing studies were accomplished by means of in vivotracing with horseradish peroxidase (HRP). Four addi-tional cases relied on postmortem tracing with 1,19-dioctadecyl-3,3,39,39-tetramethylindocarbocyanine per-chlorate (diI). In addition, the brains of five animals wereavailable for conventional histological analysis includingcresyl-violet Nissl stain and reduced silver techniques.

Fig. 1. A,B: The sea robin, P. carolinus. The free fin rays of thepectoral fin especially noticeable in panel B are used as exploratoryorgans. The specimen in this photograph was approximately 15 cm intotal length. Adults of this species can attain a length of 40 cm ormore. Photo courtesy of MBL Photographic Services. C: Drawing of amacerated specimen of P. palmipes Storer modified to reflect modern

terminology and reprinted from Morrill (J Morphol 1895) with per-mission from Wiley-Liss, Inc., a subsidiary of John Wiley & Sons, Inc.The terminology for the accessory spinal lobes has been updated. Forabbreviations, see list. Panel B reprinted from Finger (1997) withpermission of the publisher, S. Karger AG, Basel.

108 T.E. FINGER

For all surgical procedures, the animals were anesthe-tized in a 1:10,000 dilution of MS222 in seawater. At thisconcentration, spontaneous respiration largely or com-

pletely ceases. The fish were then transferred to a surgicalchamber which permitted artificial respiration of the an-imal with a recirculating solution (1:20,000) of the same

Fig. 2. A–C: Transverse sections through the lateral funicularcomplex of P. carolinus from rostral (A) to caudal (C). D: Dorsal viewof the brain and upper spinal cord of a sea robin showing the funicularnuclei and accessory spinal lobes. The levels of panels A–C are indi-cated to the right of the brainstem. The levels of the chartings inFigures 6 and 8 are indicated by the dotted lines at the left side of the

brain. E: Composite charting of three horizontal sections takenthrough the brainstem of a sea robin following application of HRP tothe trigeminal nerve illustrating the trigeminal projection to theMFn. The lines between panels E and D indicate the approximateanteroposterior extent of the charting shown in panel E. For abbre-viations, see list. Scale bar 5 250 mm.

109SPINAL CONNECTIONS IN SEA ROBIN

anesthetic. Following surgery, the animals were revivedby running fresh seawater through the mouths and overthe gills. Animals generally revived within a few minutesof this treatment. The animals were maintained postop-eratively in individual holding tanks (approximately 100–200 liters each). During this period, the animals were fedpieces of cut squid every day or two. Generally, the ani-mals readily accepted food within 12–24 hours of surgery.

For in vivo tracing, HRP (Type VI, Sigma, St. Louis,MO) paste with or without 1% lysolecithin (Bell et al.,1981) on the end of an insect pin was applied to superficialstructures. In the cases of deep diencephalic application, itwas injected stereotaxically as approximately a 10% solu-tion (Table 1). Following HRP injection, the wound wasclosed with gelfoam and Vaseline; the skin was sutured orthe skull sealed with cyanoacrylate glue.

Survival times were determined empirically but gener-ally ranged from 3 to 7 days (Finger, 1981). Somewhatlonger times were used in larger fish (Table 1). After thistime, the fish were reanesthetized in MS222 and perfusedthrough the conus arteriosus with 50 ml of marine teleostRinger’s solution (Forster and Taggart, 1959) followed by100 ml of 4% glutaraldehyde in 0.1 M phosphate buffer(pH 7.2–7.4). The brain and upper spinal cord were re-moved and embedded in gelatin after a 1–2-hour postfix.After an additional 2–4 hours of fixation, the blocks wereplaced overnight into 20% sucrose in phosphate buffer.

Frozen sections (35 or 40 mm) were taken in the trans-verse, horizontal, or sagittal planes. Every third sectionwas processed for HRP histochemistry according to theHanker-Yates method (Bell et al., 1981). Some sectionswere also processed by using tetramethylbenzidine (Me-sulam, 1978).

To extend or confirm observations made primarily withthe HRP material, diI was employed as a postmortemtracer. For these experiments, fish were anesthetized andperfused as above, except that 4% buffered paraformalde-hyde (pH 7.2–7.4) was used as the primary fixative. Thebrains and spinal cords were stored for up to 4 years at4°C in this fixative before the dye was placed. DiI crystalswere picked up on the end of a glass needle and insertedinto the target region. Agar was flowed across the brainsurface to hold the dye in place and to prevent migrationof the crystals from the application site. After dye place-ment, the brains were returned to 4% buffered parafor-maldehyde and were permitted to stand at room temper-

ature for 1 month to 2 years. After that period, the brainswere sectioned at 50 mm on a vibratome and examinedwith an epifluorescence microscope. The diI cases includedtwo in which the dye was placed in the lateral funicularnucleus (LFn), one on the dorsal root of the second spinalnerve and one in which the dye was placed in the cere-bellum.

A potential confound with the dense labeling obtainedwith both the HRP and DiI tracers is the filling of collat-erals of retrogradely labeled neurons. Thus, an effort wasmade to confirm connections reported herein by reciprocalinjections, i.e., in both the efferent nucleus and the sup-posed target.

Photomicrographs for Figures 1–8 of this manuscriptwere captured initially on TMax film. Selected film imageswere then scanned at 1,500–2,400 dpi on a Minolta Mul-tiscan attached to a Power MacIntosh 9500 computer. Ahigh pass filter (150–200 pixels) was applied to low powerimages to eliminate unevenness in background illumina-tion. Otherwise, only contrast and brightness were ad-justed on the scanned images. Micrographs for Figure 9were captured directly with a Spot CCD camera utilizingflatfield correction to achieve even background illumina-tion.

RESULTS

Lateral funicular complex

Although the brainstem of sea robins has been de-scribed in some detail (Herrick, 1907), there has not beenan adequate description of the lateral funicular complex,which is crucial for this study. The lateral funicular nu-clear complex extends caudally from the level of the obexinto the rostral spinal cord. Caudally, the lateral funicularnuclear complex is continuous with the spinal dorsolateralfasciculus (DLF) which is prominent in this genera. In-deed, as described below, the fibers of the DLF largelyterminate in the lateral funicular complex.

The main part of the lateral funicular nuclear complexis roughly spherical, approximately 500 mm in diameter.The rostral end of the lateral funicular complex is lateralto the commissural nucleus of Cajal near the obex.Throughout its caudal two thirds, the lateral funicularcomplex is bordered by the medial funicular nucleus (Fig.2), which, in this species, comprises areas of neuropilinterspersed with aggregates of granule-like neurons(Figs. 2B,C). The medial funicular nucleus had previouslybeen named “accessory lobe 1” by Herrick (1907) but thisstructure is clearly dissimilar from the spinal accessorylobes, both in terms of structure and connectivity. Accord-ingly, I have adopted the nomenclature used for otherteleost species (Herrick, 1906; see Finger, 1982 for a morecomplete discussion of the nomenclature for accessorylobes).

The lateral funicular complex is not homogeneous. Al-though it is difficult to determine the exact disposition ofall the component nuclei without further study, at leastthree regions are noticeable (Figs. 2A–C) in terms of struc-ture and connectivity: (1) posterior magnocellular nucleus(LFn-mag), (2) dorsal posterior nucleus (LFn-d), and (3)principal nucleus (LFn). The posterior magnocellular nu-cleus is characterized by large (10340 – 15350 mm) cellsscattered along the ventromedial margin of the complex atand below the level of the sonic motor nucleus (Fig. 2C).

TABLE 1. Location and Number of HRP Injections in the ExperimentalCases Used for This Study1

Structure No. casesSurvival time range

(in days)

Spinal ganglia or nerves 16 1–7Spinal cord (not accessory lobe) 5 3–10Spinal cord accessory lobe 23 1–12Lateral funicular nucleus 5 2–6Medial funicular nucleus 2 5 and 9Trigeminal nerve 2 2 and 3Cerebellum 2 3 and 7Preglomerular nucleus 5 4–5Diencephalon (control) 1 6

1Survival time shows the range of times (in days) used for the various cases. The“Preglomerular nucleus” injections were made stereotaxically; the “Diencephalon con-trol” case was an attempted preglomerular injection but missed the preglomerulartarget. The injection coordinates, which used the midline of the anterior margin of thetectal commissure as a zero point, were: 0.4–0.6 mm anterior of the commissure, 2.2mm vertically down from the commissure, and 1.1–1.2 mm lateral of the midline.Injection volumes were 30–100 nl of approximately a 10% solution of HRP in water.

110 T.E. FINGER

The dorsal posterior nucleus consists of large neurons(10325 – 15330 mm) arrayed along the dorsal margin ofthe posterior half of the complex (Fig. 2B). The dendritesof these cells tend to be oriented concentric to the curvedlateral surface of the funicular complex. Finally, the prin-cipal nucleus consists of a mixed population of neurons,including many large neurons (10 3 30 mm) intermingledwith the fibers of the DLF just caudal to the level of theobex and extending into the closed medulla (Fig. 2A).

Central projections of spinal roots

As described by Morrill (1895), the fin rays are inner-vated by the nerves issuing from the third spinal ganglionwith the remainder of the pectoral fin being innervated bythe second spinal root (SpN 2). After HRP had been ap-plied to either the second or third spinal root (SpN 3), themajority of labeled fibers terminated in the accessory spi-nal lobes as reported previously (Finger, 1982). Theselobes are elaborations of portions of the dorsal horn (Her-rick, 1906; Finger, 1982; Finger and Kalil, 1985). In somecases, including the diI case, a few, scarce labeled fiberswere observed within the DLF of the rostral spinal cordextending to the level of the caudal end of the lateralfunicular complex. A few labeled axons could be followedto a terminus in the medial face of the lateral funicularcomplex, in the vicinity of the posterior magnocellularnucleus. It was unclear, however, whether these primaryafferent fibers were terminating in the magnocellular nu-cleus or in the medially adjacent medial funicular com-plex. Conversely, when diI was applied to the lateral fu-nicular nucleus, a few labeled ganglion cells (Fig. 3) occurwithin the ipsilateral third spinal ganglion, i.e., the oneproviding innervation to the fin rays. Although the major-ity of primary spinal afferent fibers terminate near theirlevel of entrance into the spinal cord, a few primary affer-ents ascend within the DLF to reach the vicinity of thelateral funicular complex at the spinomedullary junction.

Trigeminal nerve. Following application of HRP tothe anterior cranial nerve complex including the trigemi-nal nerve, anterior lateral line nerve, and facial nerve,labeled fibers could be traced entering the trigeminal rootto reach the spinal trigeminal tract (SpVt). The descend-ing fibers of this tract terminate within the medial funic-ular nucleus (Fig. 2E).

Accessory lobes

The three principal accessory lobes maintain mostlyidentical ascending connections; the rostral, minor acces-sory lobe has similar parallel connections with exceptionsas noted below. The following description is drawn pri-marily from a case in which the second accessory lobe hadreceived an injection of HRP but applies also to othermajor accessory lobe cases.

Intraspinal connections. Following injection of HRPinto any of the principal accessory lobes, labeled fibersemerge from the ventral face of the lobe to terminate inthree intraspinal targets: (1) ipsilateral dorsal horn andaccessory lobes, (2) ipsilateral ventral horn (Finger andKalil, 1985), and (3) contralaterally in the dorsal horn andaccessory lobe after crossing the midline in an intrinsicspinal commissure just rostral to the injected lobe. Themajority of labeled fibers join the ipsilateral DLF (seeFigs. 6B,C). Retrogradely labeled elongate interneurons(10 3 15 mm) occur sparsely within the ipsilateral inter-mediate gray, just dorsal to the ventral horn motor pools.Much larger (15 3 20 mm), multipolar neurons are retro-gradely labeled within the ipsilateral major accessorylobes; fewer occur in the contralateral accessory lobe. Suchlarge multipolar neurons are rarely labeled followingtracer injections into the minor accessory lobe. Through-out the length of the rostral spinal cord, labeled fibersassociated with the DLF turn medially to terminatewithin the neuropil of the adjacent ventral horn.

Dorsolateral fasiculis. The labeled fibers of the DLFcontinue rostrally beyond the major and minor accessorylobes to terminate throughout the lateral funicular com-plex. The projection to the principal nucleus of the lateralfunicular complex is much larger with major accessorylobe injections than after injections into the minor acces-sory lobe. Following HRP injections into the lateral funic-ular complex, numerous large and small retrogradely la-beled neurons (Fig. 4D) occur in the major lobe whereaslabeled neurons are scarce in the minor accessory lobe.Following injection of either the major or minor accessorylobes, the large neurons of the posterior magnocellularnucleus of the lateral funicular complex are retrogradelylabeled bilaterally, but are more numerous and moreheavily labeled ipsilaterally. Their long dendrites can beseen extending for 200–300 mm throughout much of thelength and width of the principal nucleus of the complex(Figs. 4A–C).

A small contingent of labeled small and medium-sizedfibers (0.5–1.5 mm) continue rostrally past the funicularnuclear complex through the length of the medulla, finallyterminating in the granule cell layer of the ipsilateralcerebellum. Within the medulla, the rostral continuationof the dorsolateral fascicular fibers is situated between thespinal trigeminal tract and the secondary gustatory tract(2Gt). The ascending labeled fibers continue as part of thespinocerebellar tract (for location, see SpCt, Figs. 6E–I)which includes cerebellopetal fibers from the funicularcomplex as well as from the spinal cord proper. At mid-pontine levels, the spinocerebellar tract curves dorsally toenter the cerebellum immediately caudal to the superiorsecondary gustatory nucleus. Upon entering the cerebel-lum, the labeled fibers arising from the spinal cord turndorsally and recurve caudally to terminate as mossy fiberswithin the corpus cerebelli at the level of the pontomed-ullary junction (Figs. 5A,B). These direct spinocerebellar

Fig. 3. Fluorescence micrograph of a horizontal section throughthe third spinal ganglion (SpN 3 of Fig. 1) following application of DiIto the LFn. A few labeled ganglion cells, similar to that shown, can befound dispersed throughout the ganglion.

111SPINAL CONNECTIONS IN SEA ROBIN

Fig. 4. A: Transverse section through the magnocellular nucleusof the lateral funicular complex (LFn-mag) following HRP applicationto a major accessory lobe of the spinal cord. Approximately the level ofFigure 2C. B: Higher magnification of the retrogradely labeled cells inpanel A. The long, lateral dendrites fan out dorsally and ventrallyamong the labeled axons of the DLF. Same magnification as panel C.C: Horizontal section through the LFn of another fish in which HRP

had been applied to a major accessory lobe. The lateral dendrites ofthe magnocellular nucleus can be seen to extend anteriorly as well aslaterally within the DLF. D: Transverse section through a majoraccessory lobe following application of HRP to the LFn. Diverse sizesof neurons (sm, small; med, medium; lg, large) are retrogradely la-beled. Compare this image to Figure 5C showing that only the largeneurons project past the lateral funicular complex to the cerebellum.

Fig. 5. A: Transverse section through the mid-cerebellum of a searobin following injection of HRP into the LFn. The labeled fibers of thedirect and indirect spinocerebellar systems end in the granule celllayer within a restricted portion of the cerebellar corpus. B: Highermagnification of mossy fiber terminals between the granule cells.C: Transverse section of a major accessory lobe following application

of HRP to the cerebellar corpus. Retrogradely labeled large neurons(lg) occur in the deep portions of the lobe. A few nonneuronal cells(nnc) also exhibit reaction product in this preparation. D: Horizontalsection showing retrogradely labeled large neurons (arrows) of thedorsal subnucleus of the lateral funicular complex following HRPinjections into the cerebellum. For abbreviations, see list.

Fig. 6. Series of chartings of transverse sections through the brainof a sea robin following injection of HRP into the lateral funicularnucleus (B). The levels of section for these drawings are shown inFigure 2D. The medial lemniscus can be seen to decussate (B–D) toascend along the ventral margin of the medulla and pons (E–H) to

reach terminal sites in the midbrain (J,K) and thalamus. Note thatthe projection to the optic tectum is restricted to the ventral midan-terior portion. A more detailed charting of the thalamus, rostral toPanel L of this figure, is shown in Figure 8. For abbreviations, see list.

projections reach a restricted zone within the granulelayer roughly midway between the midline and the lateralsurface of the cerebellum. The region of cerebellum receiv-ing direct spinocerebellar input is contained within thearea receiving indirect spinocerebellar input via the lat-eral funicular complex (see below and Fig. 5A). The spino-cerebellar projections are more substantial from the majoraccessory lobes than from the minor lobe.

Although direct spinocerebellar fibers could be identi-fied, the majority of ascending spinal fibers arising fromthe accessory lobes terminate in the lateral funicular nu-cleus. Essentially, no direct projection from the accessorylobes to the ventrolateral medullary reticular formationwas observed, although substantial projections to the ven-trolateral reticular formation occur following injections ofHRP that involve the more ventral areas of the spinal cord(data not shown).

Following injections of HRP into the cerebellum, numer-ous retrogradely labeled neurons occur in the accessorylobes as well as in the dorsal horn of more caudal portionsof the spinal cord. The cerebellopetal neurons of the ac-cessory lobes generally are medium-sized neurons (up to8 3 20 mm) situated around the white matter deep in thelobe (Fig. 5C).

Lateral funicular complex

Connections of the lateral funicular complex were deter-mined using two separate methods: in vivo injection of HRPand postmortem application of diI. In both situations, adorsolateral approach to the lateral funicular complex wasemployed. In two cases, the injection was confined almostentirely to the principal and dorsal posterior nuclei of thecomplex and these connections are described below.

Following either HRP injections or diI applications tothe lateral funicular complex, labeled axons are evident inthe DLF of the spinal cord. Although some of these fibers

continue caudally beyond the accessory spinal lobes, themajority turn dorsolaterally to enter the accessory spinallobes. Numerous retrogradely labeled small and medium-sized somata (4310 – 7320 mm) are scattered throughoutthe substance of the lobes, including some in the mostsuperficial layer of small unipolar neurons (Fig. 4D, sm).Still larger retrogradely labeled neurons are situateddeeper in the lobe. Presumably, the majority of labeledaxons of the DLF represent retrogradely labeled axons,but reciprocal funiculospinal projections exist (see above),so some anterogradely labeled axons probably contributeto this labeled bundle. In addition, some labeled axons ofthe DLF turn laterally to exit the cord with the spinaldorsal roots. These labeled axons can be followed into thelarge spinal ganglia of the first spinal roots wherein la-beled ganglion cell somata can be identified (see above andFig. 3). This confirms the presence of a few primary spinalafferent fibers that project directly to the area of thelateral funicular complex.

At the level of the injection of tracer, labeled fibers can beseen emerging from the medial face of the principal nucleusof the lateral funicular complex (Fig. 6). Two major ascend-ing fiber systems originate in the lateral funicular nucleus.One, the funiculocerebellar tract, joins the spinocerebellartract to ascend ipsilaterally along the lateral margin of themedulla; the other, apparently the medial lemniscus (ML),emerges from the ventrolateral edge of the lateral funicularnucleus and heads ventromedially to cross the midline be-tween the medial longitudinal fasciculus (MLF) and the ven-tral longitudinal fasciculus (VLF) in the brainstem (Figs.6B–H). At the level of the caudal vagal lobe (VL), this fiberbundle assumes a position along the ventral surface of thebrainstem, lateral to the VLF and medial to the brainstembranchiomotor nuclei. A few labeled fibers occur in a similarposition ipsilaterally.

Fig. 7. Transverse section through the midbrain following injec-tion of HRP into the lateral funicular nucleus. A: The torus semicir-cularis at the level of Figure 6K. Sparsely distributed labeled fibersoccur in the ventral layers of the main toral nucleus whereas a dense

terminal plexus occurs in the external nucleus of the torus semicir-cularis (TSe). B: The ventral part of the optic tectum showing labelingof fibers in the stratum album centrale (SAC). For abbreviations, seelist.

115SPINAL CONNECTIONS IN SEA ROBIN

Cerebellar connections. The funiculocerebellar fiberscourse rostrally through the medulla between the spinaltrigeminal tract and the ascending secondary gustatorytract, i.e., along with the direct spinocerebellar fibers(Figs. 6D–H). In contrast to the direct spinocerebellarfibers, the funiculocerebellar system includes a handful oflarge axons (3–4 mm in diameter) intermingled with aplethora of small and intermediate-sized (1–2 mm) fibers.At the junction of the pons and medulla, both the second-ary gustatory tract and the funiculocerebellar tract lieventrolateral of the widest lateral extension of the fourthventricle. Both fiber systems continue rostrally in thisposition until reaching the level of the secondary gusta-tory nucleus, ventral to the cerebellar corpus at the levelof the caudal pole of the optic tectum (Fig. 6I). Whereasthe secondary gustatory tract terminates within its nu-cleus, the funiculocerebellar fibers, like the direct spino-cerebellar tract, recurve caudally within the inferior cer-ebellar peduncles to terminate as mossy fibers within thegranule layer of the cerebellar corpus just caudal to thelevel of the secondary gustatory nucleus (Figs. 6G,H). Thearea of funiculocerebellar fiber termination is restricted toa 200–300-mm wide zone about one-fourth of the distancefrom the midline and the lateral edge of the granule layer(Figs. 5A, 6G). This area includes, but is larger than, thearea in which direct spinocerebellar fibers terminate.

Injections of tracer into the cerebellum retrogradely labela population of large (7 3 40 mm) cells situated along themedial edge of the DLF at the caudal end of the lateralfunicular complex (Fig. 5D). The dendrites of these cellsextend laterally into the DLF and funicular nuclear complex.

Medial Lemniscus. The medial lemniscus ascendsalong the ventrolateral margin of the brainstem untilreaching the midbrain (Fig. 6J). At that point, the tract issituated just ventromedial to the tectospinal and tectob-ulbar tracts. Throughout the pons, a few labeled axons canbe found outside of the compact tract and within thenearby ventrolateral reticular formation. In addition,throughout the length of the medulla, a small contingentof fibers parallels the medial lemniscus but in a moredorsomedial position (Figs. 6F–H). These fibers coursewithin the longitudinal bundles of the medullary reticularformation but join the lateral lemniscus (LL) upon reach-ing mesencephalic levels. This contingent of labeled fibersappears to rejoin the medial lemniscus as it terminates inthe midbrain and diencephalon. As the medial lemniscuscourses along the ventral margin of the medulla, below thevagal lobe, it gives rise to a small terminal field within theinferior olivary complex (Fig. 6E).

At the level of the rostral end of the valvula cerebelli, themedial lemniscus turns dorsalward and emits a small groupof fibers that terminate sparsely in the ventral layers, andmore heavily in the external nucleus, of the torus semicircu-laris (TSe, Figs. 6K, 7A). The majority of labeled fibers con-tinue rostralward and divide into two groups: one turnslaterally to enter the optic tectum (Fig. 7B), the other turnsmedially to enter the diencephalon especially targeting thepreglomerular complex (Figs. 6L, 8).

The tectopetal fibers enter the stratum album centraleof the optic tectum and turn caudally within the tectum toreach more caudal portions of that structure (Fig. 7B). Nolabeled fibers could be traced into dorsal portions of theoptic tectum. The zone of termination is restricted to theventral third of the midtectum. The area of the optictectum in which the funiculotectal fibers terminate corre-

sponds to the anteroventral portion of the visual field, i.e.,the area expected to contain the portion of the visual fieldin which the fin rays lie (Fig. 1B).

The majority of the diencephalic-directed fibers ends asa dense terminal network within the rostrolateral part ofthe preglomerular nuclear complex (Fig. 8A,C). This is theprincipal diencephalic target of the ascending spinal lem-niscal system. The preglomerular target of medial lemnis-cus in sea robins is a component of the lateral preglomeru-lar nucleus (nPGl) identified in other teleosts (Peter andGill, 1975; Peter et al., 1975; Bradford and Northcutt,1983) but does not form the entirety of the nPGl.

Figure 9 shows a series of sections through the posteriordiencephalon of a sea robin. The caudal end of the preglo-merular complex (Fig. 9A) lies medially to the corpus(nucleus) glomerulosus (CG) and dorsal to the lateral fore-brain bundle (LFB). At this level, the preglomerular com-plex consists only of the medial subnucleus (nPGm), char-acterized by small, densely packed somata. Just caudal tothe posterior face of the corpus glomerulosus, the posteriorthalamic nucleus (nPT; Yoshimoto et al., 1998) appearsdorsolaterally but remains distinct from the preglomeru-lar complex. Immediately rostral to the corpus glomerulo-sus (Fig. 9B), the preglomerular nucleus becomes morecomplex. The compact, parvocellular medial subnucleuscontinues rostrally, dorsal to the LFB. The lateral nucleusof the preglomerular complex (nPGl) appears, laterallyabutting the nPGm. The lateral nucleus contains small tomedium-sized cells, not as densely packed as the medialnucleus. This is the spinorecipient subnucleus. The lateralnucleus in sea robins extends far rostrolaterally, comingto lie just ventromedial to the optic tract (Tr0; Figs. 9C,D).An additional, comma-shaped subnucleus of the nPGl ap-pears along the lateral edge of the rostral half of thespinorecipient component. Provisionally, this is called theexterolateral subnucleus of the lateral preglomerular nu-cleus (nPGlex). This subnucleus does not receive ascendinginput from the medial lemniscus.

At the level of the posterior commissure, a small fascicleof fibers from the medial lemniscus sweeps medially dor-sal to the preglomerular complex to end in the periven-tricular nucleus of the posterior tubercule (Fig. 8A, TPp).In some cases, a few labeled fibers can be followed throughthe dorsal margin of this nucleus to terminate along theventralmost margin of the VM nucleus of the thalamus(Fig. 8B). Some labeled fibers can be followed through thehorizontal commissure apparently reaching contralateraldiencephalic targets. Injections of HRP into the preglo-merular complex retrogradely label numerous medium-sized cells (10 3 30 mm) within the principal nucleus ofthe lateral funicular complex (Fig. 8D).

In summary, the main medullary target of ascendingspinal information is the lateral funicular nuclear com-plex. This complex projects in turn to the ipsilateral cer-ebellum and to the contralateral optic tectum, torus semi-circularis, and a nucleus of the preglomerular complex.This crossed fiber system and its targets resembles, inmost respects, the medial lemniscus system described intetrapod vertebrates.

DISCUSSION

The present study details the ascending spinal path-ways emanating from the accessory spinal lobes of therostral spinal cord of a sea robin. These lobes serve as the

116 T.E. FINGER

Fig. 8. A: Charting of a transverse section through the mid-thalamus, continuing the series of chartings shown in Figure 6. Thedense terminal plexus in the preglomerular complex (nPG) is shownas is the less extensive projection to the TPp. B: Photomicrograph ofthe area indicated by the guidelines around rectangle B in panel A,showing the thalamus. Labeled fibers are indicated by the arrowsalong the dorsal border of TPp where it abuts the VM nucleus of the

thalamus. C: Low power micrograph showing much of the sectioncharted in panel A. The dense terminal network in the preglomerularnucleus (nPG) is easily visible. D: Photomicrograph of a transversesection through the lateral funicular nucleus showing medium-sizedretrogradely labeled neurons following injection of HRP into the thal-amus including the preglomerular nuclear complex, TPp, and pretec-tum. For abbreviations, see list.

primary target for sensory information arriving on thenerves innervating the fin rays and pectoral fin proper.Although these nerves convey chemosensory information(Bardach and Case, 1965; Silver and Finger, 1984), theyalso carry substantial tactile and proprioceptive afferents(Silver and Finger, 1984 and unpublished observations)from the fin and fin rays. Thus, although no direct elec-trophysiological evidence is available, the accessory spinallobes probably deal with all three types of information.The fin ray chemosense is most likely represented in theaccessory spinal lobes because these lobes are present onlyin fishes with the pectoral fin specialization and only atthe level of termination of the nerves innervating this fin.The dorsal spinal gray matter ventral to the accessorylobes appears similar to that of the dorsal horns along theremainder of the spinal cord. It seems likely that thisnonspecialized spinal dorsal gray serves a nonspecializedfunction unrelated to the specialized chemosense of thepectoral fins.

The unique characteristics of the upper spinal cord ofsea robins were noted perhaps as early as 1685 by Samuel

Collins (Collins, 1685).1 Tiedemann (1816) provided thefirst clear description of the existence of the spinal acces-sory lobes and their relationship to the free pectoral ap-

1 The Collins work is cited by Tiedemann (1816) and, later, by Scharreret al. (1947) as first depicting the spinal enlargements of the rostral spinalcord in sea robins (gurnards). Examination of the Collins work, however,shows an illustration of a brain—perhaps not even a sea robin brain—lacking such spinal enlargements. The text describes “five processes”(which in context means lobes or masses) of the brain, which might betaken to include the spinal enlargements. However, the text goes on tostate that “. . . the anterior (processes) are a pair much larger then (sic) therest . . . out of (which) . . . arise the optic nerves.” The first of these fiveswellings thus referred to the optic tecta. Reading in context then, thesecond pair were the lateral expansions of the cerebellum, the third thefacial/vagal lobes, or perhaps even the expansions of the medial octavolat-eral nuclei (lateral line lobes). In any event, this leaves insufficient “pro-cesses” to account for the spinal accessory lobes. Thus, although the text ofthe Collins work is promising in referring to five expansions on the brain,both the accompanying illustration (Table 70, Fig. 3) and the subsequenttext fail to indicate that Collins had any knowledge of the spinal accessorylobes.

Fig. 9. A series of transverse sections through the preglomerularcomplex of the sea robin from (A) caudal to (D) rostral; approximately200 mm spacing between sections. Lateral is to the right as in Figures6 and 8 to which this figure should be compared. Figure 8C lies about100 mm rostral to the level of Figure 9D. The exterolateral subnucleus(nPGlex) of the lateral preglomerular nucleus (nPGl) contains a some-

what higher density of cells than the main part of the nucleus. As canbe seen in Figure 8C, the nPGlex does not receive input from themedial lemnisus and may be a tertiary gustatory nucleus (see Discus-sion). Arrowheads mark boundaries between subdivisions of the pre-glomerular complex. For abbreviations, see list.

118 T.E. FINGER

pendages. The one-to-one relationship between the majoraccessory lobes and the fin rays was implied first by Ari-ens Kappers et al. (1967) who noted that in a specimenlacking one free fin ray, there was a corresponding ab-sence of one spinal accessory lobe (Fig. 82, p 176 of AriensKappers et al., 1967). A more recent study (Finger, 1982)demonstrated the reversed anterior-posterior somatotopyin this system. The three fin rays are represented in thecaudal, major accessory lobes, while the bulk of the fin isrepresented in the more rostral, minor accessory lobe.

Primary dorsal root fibers

The majority of primary spinal afferent fibers in searobins terminates at or near their level of entrance intothe spinal cord (Finger, 1982). Sea robins do, however,have a population of dorsal root ganglion cells that projectdirectly to the lateral funicular nucleus of the caudalmedulla. Such a direct ascending system from dorsal rootganglia has not been reported previously in adult teleosts.There is, however, a description of direct ascending pri-mary afferent axons in zebrafish reaching up to 10 seg-ments above their level of entrance, presumably atmidlevels of the cord (Bernhardt et al., 1990). Possibly, thedorsal root fibers reaching the lateral funicular complex insea robins are merely a reflection of the normal rostral-ward extension of dorsal root ganglion cells as occurs inother teleosts. The presence of long ascending fibers fromdorsal root ganglion cells appears to be a common featurein other vertebrates (summarized in Munoz et al., 1997)including hagfish (Ronan and Northcutt, 1990) and am-phibians (Joseph and Whitlock, 1968; Munoz et al., 1997).

In tetrapods, the ascending dorsal root axons runlargely within the dorsal columns (Munoz et al., 1997). Inadult agnathans and in larval amphibians and fishes, thisis also the position of the ascending axons of the spinaldorsal cells (Bernhardt et al., 1990; Ronan and Northcutt,1990), which serve as primary sensory neurons analogousto dorsal root ganglion cells. The sea robins, like mostother teleost fishes, appear to have little or no dorsalcolumn system in the spinal cord (Hayle, 1973a; Oka etal., 1986; Ronan and Northcutt, 1990). In sea robins andhagfish, the ascending dorsal root fibers course eitherentirely (sea robin) or largely (hagfish) within the DLF.Whether a difference in position of a fiber system withinthe spinal white matter is significant is unclear. In vari-ous mammals, the position of the corticospinal tract variesfrom dorsal to lateral funiculi without apparent functionalconsequence (Armand, 1982).

Ascending spinal connections

The principal targets for primary sensory informationfrom the fin rays are the major accessory spinal lobes.These lobes are homologous to portions of the dorsal hornbut do not include all the neuronal populations of thespinal dorsal horn (Finger, 1982). The ascending connec-tions reported in this study are limited to those arisingfrom the accessory lobes and therefore may not be repre-sentative of all modalities of somatosensory inputs to thecord.

The ascending output from the spinal accessory lobesends in two main targets: the cerebellum and lateral fu-nicular nucleus. Fibers reaching these targets travel inthe DLF. In other vertebrates, including both agnathansand tetrapods, three ascending fiber systems can be fol-lowed from the spinal cord to a variety of extracerebellar

supraspinal targets (Ronan and Northcutt, 1990; Munozet al., 1997): (1) dorsal column-medial lemniscus systemultimately relaying to reach the diencephalon, (2) DLF tothe lateral cervical nucleus and cerebellum, and (3) ven-tral and ventrolateral funicular pathways reaching theventrolateral reticular formation, midbrain, and dien-cephalon.

It is striking that the output from the accessory lobes insea robins runs only in the dorsolateral system. Injectionscaudal to the accessory lobes, including the ventral por-tion of the cord, demonstrate a substantial spinoreticularprojection as in other vertebrates. This projection system,however, does not arise from the accessory lobes. Like-wise, no ascending dorsal column system arises from theaccessory lobes. However, in other teleosts, the dorsalcolumn system is negligible (Hayle, 1973a; Munoz et al.,1997) except in mormyrids (Szabo et al., 1991) where thedorsal columns participate in a precerebellar relay sys-tem. Thus, the absence of a prominent dorsal columnsystem in the sea robin is not surprising. Because ag-nathan fish have substantial dorsal columns (Ronan andNorthcutt, 1990), the absence of this system in teleostsshould be viewed as an apomorphic rather than as aprimitive vertebrate trait.

Spinocerebellar systems. In sea robins, informationfrom the accessory spinal lobes reaches the ipsilateral cere-bellum both directly and indirectly. A small contingent offibers can be traced directly from the accessory lobes to thegranule cell layer of the ipsilateral corpus cerebelli. Thissame area of the cerebellum receives a more substantialprojection from a group of large neurons situated along theedge of the lateral funicular nucleus, which apparently re-ceive ascending spinal information traveling in the DLF. Asimilar group of cerebellopetal neurons of the caudolateralmedulla has been identified in several teleosts (Finger, 1978;Wullimann and Northcutt, 1988; Szabo et al., 1991). Like-wise, direct spinocerebellar neurons are described in severalteleost species (Finger, 1978; Wullimann and Northcutt,1988).

The spinocerebellar input is limited to a relatively narrowlongitudinal strip of the corpus cerebelli situated roughlytwo-thirds of the way from the midline to the lateral marginof the structure. This spinal recipient region appears to beadjacent to the portion of the cerebellum that receives oc-tavolateral inputs (unpublished observations). Whetherthese two areas are contiguous or not in sea robins remainsto be determined. In a salmonid (Hime salmon: Oka et al.,1986) and in a cypriniform (zebrafish: Becker et al., 1997),the spinocerebellar inputs terminate in the vestibulolateralcerebellum (eminentia granularis) as well as in the cerebel-lar corpus. This is different from the situation in a rockfish(Sebasticus marmoratus; Murakami and Ito, 1985) and searobin, both scorpaeniformes, where spinocerebellar projec-tions reach only the corpus cerebelli.

In addition to the direct spinocerebellar system, searobins also possess an indirect system with a relay in thegigantocellular component of the lateral funicular com-plex. An apparently similar funiculocerebellar relay hasbeen reported in several teleost species including catfish(Finger, 1978), sunfish (Wullimann and Northcutt, 1988),and mormyrids (Szabo et al., 1991).

The types of sensory information conveyed to the cere-bellum in sea robins is unknown. It seems likely that thespinocerebellar system in this species would carry propri-oceptive and tactile information as in other vertebrates.

119SPINAL CONNECTIONS IN SEA ROBIN

Whether the cerebellum of sea robins also receives chemo-sensory information is of interest. The chemosensory mo-dalities of taste and smell are not well represented in thecerebella of other species, although responses to tastestimuli have been reported in the cerebellum of a frog(Hanamori and Ishiko, 1987).

Medial lemniscus. The lateral funicular nucleus inPrionotus gives rise to a prominent ascending fiber systemreminiscent of the medial lemniscal system in other ver-tebrates. The systems are similar in terms of region oforigin, decussation and course along the ventral tegmen-tum, and diencephalic and mesencephalic targets. In searobins, frogs (Munoz et al., 1995), and pigeons (Wild,1989), the medial lemniscus originates from funicular nu-clei hodologically equivalent to the dorsal column nuclei,lateral cervical nuclei, and external cuneate nucleus.Prominent medial lemniscal targets include the inferiorolive, subnuclei of the inferior colliculus/torus semicircu-laris, and the optic tectum, as well as several thalamicnuclei.

As in other vertebrates (Wild, 1989; Munoz et al., 1997),ascending spinal pathways in Prionotus terminate in sev-eral areas of the midbrain including the torus semicircu-laris and optic tectum. The projection to the lateralmostportion of the torus semicircularis in Prionotus ends in anarea which, in a salmon, receives direct input from therostral spinal cord (Oka et al., 1986). Likewise, in severalamphibia, the spinal cord projects directly to the torussemicircularis (Munoz et al., 1997). These direct and indi-rect spinal connections to the lateral torus semicircularishave been likened to the spinal projection to the intercol-licular nucleus in amniotes (Munoz et al., 1997).

The limited rostral extent of direct ascending spinalprojections in sea robins is striking in comparison to otherfishes. In salmon, sharks, and amphibians, the spinal cordprovides a substantial direct input to the midbrain tar-gets. In Prionotus, virtually all of the spinal input to themidbrain is relayed through the lateral funicular nuclearcomplex. Whereas this may represent a specialization inPrionotus, several methodological issues must be takeninto consideration. First, the present study is limited toinvestigation of ascending systems arising from the acces-sory spinal lobes. Whereas these are elaborations of thespinal dorsal horn, they do not include all parts of thedorsal horn. For example, enkephalin-like immunoreac-tivity is limited to a relatively small wedge of cells situ-ated ventral to the accessory lobes (Finger, unpublishedobservations). Thus, processing of pain and possibly othertypes of somatosensory information may be restricted togray matter of the spinal cord ventral to the accessorylobes. Because the tracer injections involved only the ac-cessory lobes, it is possible that other, longer spinal pro-jection systems may arise from the more ventral spinalgray not included in the injection sites. Indeed, injectionsof HRP involving the entire dorsoventral extent of the cordcaudal to the accessory lobes show far more extensiveanterograde labeling than those involving just the acces-sory lobes. These cases are, however, confounded by theextensive retrograde labeling of collaterals arising fromspinopetal neurons throughout the brainstem.

Recently, direct connections from the spinal cord to thetelencephalon have been described on the basis of antero-grade tracing experiments in two cypriniform teleosts:zebrafish (Becker et al., 1997) and goldfish (Hanna et al.,1998). However, in neither case were these extensive ros-

tral projections confirmed by retrograde methods. A directspinotelencephalic system was not observed originatingfrom the major accessory lobes in sea robins. However,telencephalopetal fibers crossing in the horizontal commis-sure do arise from the lateral funicular complex. It is possiblethat use of a more sensitive tracer, e.g., biotinylated dextranamine, in sea robins might reveal projections to the basalforebrain in the vicinity of the horizontal commissure. Alter-natively, the direct spinotelecephalic projection may be apo-morphic to cypriniform teleosts and may not be present inpercimorphs.

Thalamic projections. The medial lemniscus systemin Prionotus ultimately terminates in the preglomerularcomplex and the TPp of the thalamus. Both of these areportions of the posterior tubercle—a portion of the thala-mus well developed in fishes, but relatively smaller inamniotes (Braford and Northcutt, 1983). The diencepha-lon of fishes is a complex area with large interspeciesdifferences and unclear boundaries between some nucleargroups. Accordingly, it is difficult to draw equivalencesbetween nuclear groups in fishes no less between fish andamniotes.

The medial lemniscus of sea robins terminates mas-sively in a part of the lateral preglomerular nucleus. Thepreglomerular nuclear complex in teleosts is quite vari-able between species (Braford and Northcutt, 1983; Mu-rakami et al., 1986). Two components are generally recog-nized, medial and lateral nuclei, although others havebeen described under a variety of names. Figure 9 pre-sents a series of sections through the preglomerular com-plex of a sea robin. Based on Nissl-staining characteristicsas well as on hodological data, the complex can be dividedinto at least three territories. Following the previouslyestablished nomenclature, I describe the primary divisionof the complex into a medial nucleus, containing denselypacked small cells, and a lateral nucleus, containing moreintermediate-sized somata and a more heterogeneous neu-ropil. A more ventrolateral subdivision of the lateral nu-cleus can be discerned in Nissl-stained material (nPGlex),but this division is more obvious in the labeled material.The medial lemniscus terminates in the principal part ofthe lateral nucleus, but not in the exterolateral subnu-cleus.

How these different parts of the lateral preglomerularnucleus compare to previous hodological descriptions of thisregion in other species is not entirely clear because this areais quite variable across species. In sea robins, the lateralpreglomerular nucleus appears much larger than in otherpercomorphs (c.f. rockfish, Murakami et al., 1986), perhapsdue to the relatively large spinal lemniscal system. In an-other percomorph teleost (Oreochromis), ascending second-ary gustatory inputs terminate in the “preglomerular ter-tiary gustatory nucleus” (following the nomenclature ofWullimann, 1988), apparently part of the lateral preglo-merular complex (Yoshimoto et al., 1998). The area of ter-mination of these gustatory inputs is, however, far lateral inthe lateral preglomerular nucleus and may correspond to theregion of the exterolateral subnucleus described here for searobins. Preliminary data from sea robins reveal ascendinggustatory inputs terminating in the posterior thalamic nu-cleus and the area of loose neuropil ventrolateral to thenPGlex (Finger, 1983a,b).

In another scorpaeniform fish (Sebasticus), the ventrome-dial (VM) nucleus of the thalamus is described as a target forascending somatosensory information (Murakami and Ito,

120 T.E. FINGER

1985; Ito et al., 1986). In sea robin, I find only a meagerprojection to VM and that to a far posterior ventral portionwhich borders the TPp. The bulk of the medial lemniscalinput to this vicinity terminates in the immediately subja-cent, TPp. Unfortunately, the figures provided by Ito andcoworkers (Murakami and Ito, 1985; Ito et al., 1986) do notpermit assessment of whether the ascending spinal inputthey describe also terminates predominantly in the TPp.These authors do state that the somatosensory input to thisregion terminates in the most posterior and ventral portionsof VM, i.e., the area adjacent to the TPp. The tracer injec-tions into the VM in the later work (Ito et al., 1986) includethe TPp and so offer no help in distinguishing between thesetwo potential target sites for the lateral funicular nucleus.

In amniote vertebrates, the dorsal thalamus serves asthe principal lemniscal relay to the telencephalon. Infishes, the posterior tubercle may play an equivalent role.Although these both are constituents of the thalamus,they are not considered to be homologous (Braford andNorthcutt, 1983; Northcutt, 1995). In both sea robins andamniotes, the medial lemniscus targets numerous tha-lamic target nuclei (Wild, 1989), thus it is simplistic toequate the principal thalamic target nucleus of one spe-cies, e.g., nuc. preglomerulosus in the sea robin, with theprincipal thalamic target of another, e.g., dorsal interme-diate ventral anterior nucleus in pigeon or the ventropos-terolateral nucleus (VPL) in mammals.

Multimodal sensory representation in the optic tec-

tum. A prominent indirect spinotectal system was ob-served in Prionotus. Direct and indirect spinotectal connec-tions are present in many other chordates, including hagfish(Ronan and Northcutt, 1990), sharks (Hayle, 1973b), lung-fish (Ronan and Northcutt, 1990), and most tetrapods stud-ied to date (summarized in Ronan and Northcutt, 1990;Munoz et al., 1997). Of particular interest is the fact that thespinotectal system arising from the accessory lobes in Pri-onotus terminates predominantly in the ventral, rostral, andmidtectum. This region contains the representation of theventral visual field, i.e., the portion in which the fin rayswould be expected to lie (Fig. 1B). Thus, this region of tectumis likely to contain a multimodal representation of the ven-tral region of visual space. This seems analogous to thesituation in pit vipers where the optic tectum receives con-vergent information from the visual system and the infrareddetection system innervated by the trigeminal nerve (Hart-line et al., 1978; Newman and Hartline, 1981). Likewise, thesuperior colliculus in cats and primates (Stein, 1998) con-tains multimodal neurons that respond optimally to somato-sensory and visual stimuli arising simultaneously from onelocus. Whether deep cells in the ventral tectum of sea robinsrespond to the chemical nature of a stimulus delivered to thefin rays or are merely responsive to the tactile and proprio-ceptive cues is of interest. Chemical responsiveness has notbeen reported in the optic tectum of other vertebrates. How-ever, few other vertebrates are likely to be able to perceivethe chemical nature of a visual target.

In summary, the ascending fin ray chemosensory informa-tion is transmitted over the medial lemniscus, i.e., a somato-sensory pathway in every other vertebrate studied to date.There is no overlap in central targets between the ascendingfin ray chemosensory nuclei and those devoted to the gusta-tory sense (Finger, 1983b), at least through diencephaliclevels. So despite behavioral similarities in the way Priono-tus uses the fin ray chemosense and how catfish or cods usetheir external gustatory sense, there is no similarity of cen-

tral organization of these two modalities. Thus, in Prionotus,as well as in rocklings (Kotrschal and Finger, 1996), themanner in which a sensory system is used behaviorally doesnot dictate or limit the overall central organization of thesystem. Rather, the anatomy of the peripheral innervationand endorgans appears to be the determining factor for cen-tral organization. So, for example, the SCCs on the dorsal finof rocklings, although not used for acquisition of food, areinnervated by the facial nerve and project centrally like agustatory system (Kotrschal and Finger, 1996). In contrast,the SCCs of sea robins, although utilized behaviorally likethe gustatory sense, are innervated by spinal nerves andtherefore connect centrally like a somatosensory modality.

ACKNOWLEDGMENTS

Thanks are due to R.G. Northcutt and M. Wulliman forassistance regarding identification of diencephalic cellgroups. The bulk of the histological work for this study wasperformed by Peggy Leong, Cheryl Adams, and Barbel Bottger.

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