Distribution and innervation of lateral line organs in the channel catfish

Anterodorsal lateral line nerve.

The ganglion of the anterodorsal lateral line nerve constitutes the most dorsolateral group of sensory neurons within the anterior ganglionic complex (Figs. 10, 12B, 13A). The ganglionic cells of this lateral line nerve (Fig. 13A) can be distinguished easily from the other neurons in the complex by their large size. Measurements of a random sample of anterodorsal lateral line ganglionic cells in a single 39-cm-long individual reveal a mean diameter of 18.7 μm ± SE 0.32 (N50). Not only are these sensory neurons among the largest ganglionic cells in the complex, they are scattered among fibers of the nerve trunk, which forms a separate, distinct bundle as it passes along the lateral surface of the ganglionic complex until its root joins that of the anteroventral lateral line nerve (Fig. 10). At this point, the combined roots enter the medulla and turn dorsally to terminate within the electroreceptive lateral line lobe, the medial octavolateral nucleus, and the eminentia granularis (Fig. 12A,C).

Superficial ophthalmic and buccal rami (Figs. 5, 6) arise from the rostral pole of the ganglion of the anterodorsal lateral line nerve (Fig. 10). As the superficial ophthalmic ramus passes rostrally, it exits the neurocranium by means of a separate foramen and continues rostrally above the orbit (Figs. 5, 6). Almost immediately, the superficial ophthalmic ramus gives rise to a ramule that innervates the first neuromast in the supraorbital canal (Figs. 1A, 6), and it also divides into several smaller fascicles that ramify in the skin. The profundal nerve also contributes fibers to the first ramule of the superficial ophthalmic ramus as this ramule emerges from the ramus (Fig. 6). Because the profundal nerve is joined by a sizable component of facial sensory fibers before its exiting the neurocranium, it is likely that this nerve consists of somatic and visceral sensory fibers. Both of these fiber classes probably also join the first ramule of the superficial ophthalmic ramus. If so, fibers of the first ramule innervate the skin and taste buds, as well as the first supraorbital neuromast and the ampullary organs closely associated with this segment of the supraorbital canal. As the superficial ophthalmic ramus passes dorsal to the eye, it issues a second ramule, which innervates the second supraorbital neuromast (Fig. 1A). This ramule also divides into several smaller fascicles that ramify in the skin. As the superficial ophthalmic ramus continues rostrally beyond the orbit to innervate the remaining two supraorbital neuromasts and the single ethmoid neuromast (Fig. 1A), it will anastomose with profundal rami at least two more times. Thus, it is likely that each superficial ophthalmic ramule also innervates skin and taste buds. Not surprisingly, application of HRP to the stump of the superficial ophthalmic ramus after transection at a midorbital level retrogradely labels cells in the anterodorsal lateral line ganglion, as well as cells in the facial and trigeminal ganglia whose labeled fibers can be traced into the facial lobe and descending trigeminal tract, respectively.

One additional ramule that arises from the dorsocaudal surface of the anterodorsal lateral line ganglion (Figs. 6, 10) is interpreted as an anterior ramule of the superficial ophthalmic ramus. In most bony fishes, this ramule arises from the proximal end of the superficial ophthalmic ramus rather than the position noted in catfish. However, in the latter, it is interpreted as the anterior ramule because it can be seen to innervate a single neuromast within a canal that arises from the caudal end of the supraorbital canal, as well as two superficial neuromasts (Figs. 1A, 6). In other fishes, this line of neuromasts is termed the anterior pit line and is always innervated by an anterior ramule of the superficial ophthalmic ramus. The anterior ramule is also joined by a small bundle of fibers that arises from the dorsal surface of the facial ganglion and, a little more distally, by an additional bundle of fibers that arises from the dorsal surface of the trigeminal ganglion (Fig. 10). Thus, the anterior ramule, like the other ramules of the superficial ophthalmic ramus, carries somatic and visceral sensory fibers that presumably innervate the skin and taste buds in the vicinity of the anterior pit line.

The buccal ramus of the anterodorsal lateral line nerve (Figs. 6, 10) also arises from the rostral pole of the anterodorsal lateral line ganglion. Unlike the superficial ophthalmic ramus, which courses dorsally to exit the neurocranium high on the wall of the orbit, the buccal ramus turns ventrally and exits the neurocranium as the most dorsolateral component of the superolateral strand (Fig. 10). In catfishes, most sensory and motor fibers initially emerge from the anterior ganglionic complex in one of three major bundles: the inferomedial strand, the superolateral strand, or the hyomandibular trunk (Fig. 10). Both the inferomedial and superolateral strands are composed of facial and trigeminal fibers. As the strands exit the neurocranium, some fibers are exchanged, but the inferomedial strand seems to be primarily composed of fibers that form the maxillary ramus of the trigeminal nerve and the palatine ramus of the facial nerve, whereas the superolateral strand is primarily composed of fibers that form the buccal ramus of the anterodorsal lateral line nerve and the mandibular ramus of the trigeminal nerve. In each case, the buccal, maxillary, and mandibular rami are composed, in part, of facial fibers that will innervate external taste buds. The hyomandibular trunk, the third major bundle of fibers to leave the anterior ganglionic complex, is formed by fibers of the anteroventral lateral line nerve, trigeminal somatic sensory fibers, and facial gustatory fibers.

The facial fibers that constitute part of the buccal ramus of the anterodorsal lateral line nerve arise from the dorsolateral surface of the superolateral strand and initially cap the buccal ramus (possibly a part of the accessory maxillary nerve of Herrick, 1901). The buccal ramus also seems to be joined by trigeminal fibers that arise from the ventrolateral surface of the superolateral strand and merge with the ventral surface of the buccal ramus. Unfortunately, all of the experimental cases that might have corroborated these observations included both the buccal and maxillary rami. In each of these cases, retrogradely labeled cells were observed in the facial and trigeminal ganglia, as well as in the anterodorsal lateral line ganglion, but labeled cells would have been expected in the facial and trigeminal ganglia if only the maxillary ramus were labeled.

The buccal ramus exits the neurocranium adjacent to the dorsolateral surface of the superolateral strand (Fig. 11). At this point, fibers are still being exchanged between the superolateral and inferomedial strands, but this exchange stops abruptly, and the maxillary and mandibular rami of the trigeminal nerve emerge as distinctly separate fiber bundles. As the maxillary ramus continues rostrally and ventrally, the buccal ramus remains adjacent to the dorsolateral surface of the maxillary ramus, then continues rostrally and divides into outer (ob, Fig. 6) and inner (ib, Fig. 6) branches, historically termed the outer and inner buccal rami (Herrick, 1901). The outer ramus turns laterally to pass under the m. levator arcus palatini and issues its first ramule, which innervates the first neuromast within the infraorbital canal (Figs. 1A, 6). Several additional branches issue from the first ramule (Fig. 6) and ramify within the skin innervating ampullary organs and taste buds in the vicinity of the caudal infraorbital canal. The remaining fibers of the outer buccal ramus continue rostrally to innervate the second neuromast of the infraorbital canal (Figs. 1A, 6). As these fibers course rostrally, numerous small twigs enter the skin so that most of the skin and associated sensory receptors located between the posterior half of the infraorbital and preoperculomandibular canals are innervated by the outer buccal ramus. The most rostrally coursing fibers of the outer buccal ramus anastomose with one or two branches of the inner buccal ramus to innervate the third neuromast of the infraorbital canal (Figs. 1A, 6).

Whereas the outer buccal ramus initially courses laterally and begins to innervate neuromasts of the caudal infraorbital canal and ampullary organs adjacent to the canal (Fig. 1B), the inner buccal ramus continues rostrally along the floor of the orbit, sandwiched between the maxillary and mandibular trigeminal rami. A brief anastomosis occurs between the inner buccal and maxillary rami at a midorbital level, after which the inner buccal ramus divides into lateral and medial ramules. As the lateral ramule courses rostrally, it issues one or two small fiber bundles, as already noted, which anastomose with the outer buccal ramus to innervate the third neuromast within the infraorbital canal.

Both ramules of the inner buccal ramus continue rostrally and laterally toward the rostral orbital wall. At this level, the lateral ramule issues numerous small branches that ramify within the skin, and then a larger branch that innervates the fourth neuromast within the infraorbital canal. The smaller branches of the lateral ramule almost certainly innervate the ampullary organs as well as the taste buds adjacent to the rostral orbit, because the only other nerve within this region, the maxillary ramus, remains encapsulated in connective tissue and does not issue any ramules.

The lateral ramule of the inner buccal ramus continues rostrally to innervate the fifth and sixth neuromasts within the infraorbital canal (Figs. 1A, 6). The lateral ramule then continues lateral to the olfactory capsule where it again forms a brief anastomosis with the maxillary trigeminal ramus before repeatedly dividing into smaller branches that ramify within the skin on the lateral surface of the snout. The medial ramule of the inner buccal ramus has paralleled the lateral ramule but now passes ventral to the olfactory capsule and does not seem to innervate any structures until it turns dorsally around the rostral pole of the olfactory capsule. At this point, it divides into two major bundles, each of which innervates one of the two superficial neuromasts (numbers 7 and 8) of the infraorbital line (Figs. 1A, 6). Both bundles also give rise to numerous small fascicles that ramify within the dorsolateral surface of the snout just prior to innervating the superficial neuromasts.

Anteroventral lateral line nerve.

The ganglion of the anteroventral lateral line nerve is located ventral to the ganglion of the anterodorsal lateral line nerve and lateral to the ventral subdivision of the facial sensory ganglion (Figs. 10, 13A,B). The cells of the anteroventral lateral line ganglion have essentially the same diameter (x = 19.5 μm SE ± 0.27, N50), as the cells of the anterodorsal lateral line ganglion in the 39-cm-long catfish. In addition to being slightly larger, cells of both the lateral line ganglia have a paler cytoplasm than cells of the adjacent ganglia within the anterior ganglionic complex. The sensory root of the anteroventral lateral line nerve, like that of the anterodorsal nerve, passes caudally along the lateral surface of the ganglionic complex and fuses with the sensory root of the anterodorsal lateral line nerve before their respective fibers enter the medulla (Fig. 10).

The peripherally directed fibers of the anteroventral lateral line nerve initially form a distinctly recognizable fascicle (Fig. 12A), but as they join the overlying facial sensory and motor fibers within the anterior ganglionic complex to form the hyomandibular trunk, they can no longer be distinguished from the facial fibers. As the hyomandibular trunk separates from the more rostrally coursing superolateral strand, a small but distinct bundle of fibers emerges from the trigeminal sensory ganglion and runs along the medial surface of the hyomandibular trunk for a short distance (Fig. 10) before fusing with it. Thus, the hyomandibular trunk is apparently composed of facial sensory and motor fibers, lateral line fibers, and trigeminal sensory fibers. Application of HRP to the hyomandibular trunk confirms this interpretation, as retrogradely labeled cells occur in the anteroventral lateral line, facial, and trigeminal ganglia, as well as in the facial motor nucleus. A small number of labeled cells were seen in the trigeminal motor nucleus in two of the four cases and probably result from HRP being inadvertently taken up by trigeminal fibers that were transected along with a portion of the mandibular adductor muscle, which overlies the hyomandibular trunk. After exiting the neurocranium, the hyomandibular trunk courses ventrally (Fig. 6) and initially caudally as it passes along the medial surface of the hyomandibular bone. As the trunk approaches the distal end of the hyomandibular bone, it passes through a foramen and emerges onto the lateral surface of the bone. The hyomandibular trunk then passes along the lateral surface of the quadrate bone and continues to pass medial to the preoperculomandibular canal where a small ramule emerges from the trunk to innervate the first neuromast within this canal (Figs. 1A, 6). As the hyomandibular trunk continues ventrally it issues a variable number of small ramules that pass caudally over the surface of the opercular bone to ramify in the overlying skin (Fig. 6). These small ramules seem to innervate the ampullary organs located over and adjacent to the more caudal horizontal segment of the preoperculomandibular canal (Fig. 1B).

After issuing these small ramules, the hyomandibular trunk almost immediately divides into a ventrally directed hyoid ramus and a rostrally directed mandibular ramus (Fig. 5), which again divides into deep and superficial subdivisions (dmd and smd, Fig. 6). As the hyoid ramus continues ventrally and medially, it divides into three main ramules, the most caudal of which ramifies in the skin overlying the ventral half of the opercular region (ch, Fig. 6). The fibers of the caudal ramule seem to innervate a small number of ampullary organs (Fig. 1B) and taste buds located in the more rostroventral segment of the opercular region. The second ramule that arises from the hyoid ramus (mh, Fig. 6) courses medially over the hypobranchial region to ramify in the overlying skin. Again, this ramule seems to innervate ampullary organs (Fig. 6) and taste buds near the ventral midline of the middle segment of the hypobranchial region. The third ramule (rh, Fig. 6) of the hyoid ramus courses rostrally to innervate the hypobranchial muscles and overlying skin, which also contains ampullary organs (Fig. 1B) and taste buds.

Application of HRP to the proximal stump of the base of the hyoid ramus retrogradely labels sensory neurons in the anteroventral, facial and trigeminal rami, as well as in the facial motor nucleus. The centrally directed fibers of these sensory neurons could be traced into the lateral line lobe, facial lobe, and descending trigeminal tract, respectively. The presence of labeled cells within the trigeminal ganglion after application of HRP to the hyoid ramus suggests that this ramus also carries somatic sensory fibers that innervate the skin of the lower cheek and caudal hypobranchial region. This innervation may be provided by the trigeminal sensory bundle that initially joined the hyomandibular trunk near its base (Fig. 10).

The deep mandibular ramus of the anteroventral lateral line nerve contains only lateral line fibers, experimentally confirmed, that innervate the remaining neuromasts (two through eight, Fig. 1A) of the preoperculomandibular canal. This subdivision of the mandibular ramus is located ventral to the articular and dentary bones and runs along the medial surface of the preoperculomandibular canal. Before coming to lie along the lateral surface of the articular bone, the superficial subdivision of the mandibular ramus passes adjacent to the ventrolateral surface of the adductor mandibular muscle for some distance and issues sizable ramules that innervate the superficial neuromasts of the quadratojugal and mandibular pit lines (Figs. 1A, 6), as well as the ampullary organs and taste buds in the skin associated with the middle segment of the preoperculomandibular line. As the superficial subdivision of the combined anteroventral and facial ramus continues rostrally along the lateral surface of the dentary bone, it repeatedly issues ramules (not shown in Fig. 6) that innervate ampullary organs and taste buds in the skin overlying the rostral segment of the preoperculomandibular canal. In Bodian-stained transverse serial sections, it is possible to follow a single ramule as it branches and innervates both ampullary organs and taste buds.

As the superficial subdivision of the combined mandibular ramus continues rostrally, adjacent to the lateral surface of the dentary bone, a laterally and ventrally directed ramule of the mandibular trigeminal ramus fuses with it at approximately the level of the third infraorbital neuromast (Fig. 6). Nearly doubled in size by this fusion, the superficial subdivision of the combined mandibular ramus continues rostrally to the very tip of the lower jaw. As it approaches its termination, it divides into several ramules that innervate ampullary organs and taste buds. It is also possible that the superficial subdivision contains somatic sensory fibers of the trigeminal nerve. After transection and application of HRP to its proximal stump, a short distance from its separation from the deep subdivision, retrogradely labeled cells occur in the anteroventral, facial, and trigeminal ganglia. Thus, it seems that trigeminal fibers join the superficial subdivision of the combined mandibular ramus as it emerges from the hyomandibular trunk, as well as more rostrally where a ramule of the mandibular trigeminal ramus fuses with the superficial subdivision.

Otic lateral line nerve.

The otic is the only lateral line nerve in the channel catfish in which a distinct and separate sensory ganglion cannot be recognized. The nerve bundle termed the otic lateral line nerve is the most rostral bundle to arise from the dorsal surface of the anterodorsal ganglion (Figs. 6, 10). In adult catfish, a slight elevation formed by ganglionic neurons may represent the fusion of the otic ganglion with the anterodorsal lateral line ganglion (Fig. 10). We will treat the bundle of fibers that issues from this elevation as a separate lateral line nerve, based on the fact that an outgroup analysis of bony fishes (Northcutt and Bemis, 1993; Piotrowski and Northcutt, 1996) indicates that an otic lateral line nerve with a distinct and separate sensory ganglion represents the primitive condition for bony fishes. In any case, a single ramus, usually termed the otic ramus, can be traced dorsally and rostrally to exit the neurocranium by means of a separate foramen in the sphenotic bone. After entering the sphenotic bone, the otic ramus turns caudally to course immediately adjacent to the ventromedial wall of the otic lateral line canal (Figs. 1A, 6). It passes under the anterior ramule of the superficial ophthalmic ramus of the anterodorsal lateral line nerve and then continues caudally within the sphenotic, and a short distance rostral to the single neuromast in the otic canal (Fig. 1A) it divides into two ramules (Fig. 6). One innervates the neuromast of the otic canal, whereas the other continues caudally to enter the operculum where it ramifies throughout the dorsal half of the opercular flap. In Bodian preparations, it is possible to trace small bundles of otic fibers into the base of ampullary organs and taste buds. It is clear from these observations that visceral sensory fibers arising from the facial ganglion constitute one fiber population of the otic ramus. It is possible that trigeminal somatic sensory fibers also join the otic ramus as it ramifies over the upper half of the operculum. This region is also innervated by mandibular trigeminal fibers, as well as facial opercular fibers (ope, Figs. 5, 6). Unfortunately, we were not able to label any of these rami with HRP and could not confirm whether or not they include somatic sensory fibers.

Middle lateral line nerve.

The ganglion of the middle lateral line nerve consists of a small number of large cells (no more than 100) with pale cytoplasm. It lies within the neurocranium, ventral to the root of the posterior lateral line nerve (Figs. 11, 13C). The root of the middle lateral line nerve is very short, and its fibers almost immediately join the root of the posterior lateral line nerve (Fig. 11). The single ramus of the middle lateral line nerve exits the neurocranium through a foramen that also houses the root of the glossopharyngeal nerve and the trunk of the vagal nerve. The ramus of the middle lateral line nerve runs rostrally and laterally (Fig. 11), immediately beneath the floor of the otic capsule. As it reaches the lateral edge of the otic capsule, the ramus turns dorsally and divides into anterior and posterior ramules (Fig. 6). The anterior ramule turns dorsally and rostrally to course along the lateral surface of the temporal canal (Fig. 1A). As it approaches the single neuromast within the temporal canal, the anterior ramule divides again, with one branch innervating the neuromast within the temporal canal. The second branch continues a slight distance rostrally and dorsally (Fig. 6) to innervate the two neuromasts of the middle pit line (MP, Fig. 1A). The posterior ramule of the middle lateral line ramus turns caudally and ventrally to enter the most caudal segment of the opercular flap. Unfortunately, we were unable to isolate this nerve in the rather small juvenile catfishes that we used for tracing studies and could not experimentally determine whether this nerve also contains facial and trigeminal sensory fibers.

Posterior lateral line nerve.

The sensory ganglion of the posterior lateral line nerve lies outside the neurocranium, below the caudalmost portion of the otic capsule, which houses the posterior semicircular canal and dorsal to the lateral vagal ganglion (Figs. 11, 13D). Although the posterior lateral line and lateral vagal ganglia are in contact (Fig. 13D), each is distinct because the diameter of the cells of the lateral line ganglion (x = 30.6 μm SE ± 0.59, N50 in the 39-cm-long catfish) is much larger than that (x = 18.6 μm SE ± 0.53, N50) of the lateral vagal ganglion. The cells of the posterior lateral line ganglion also have an extensive pale cytoplasm, as do the cells of the other lateral line ganglia, unlike the scant, darkly staining cytoplasm of the ventral subdivision of the facial ganglion and the lateral vagal ganglion.

The root of the posterior lateral line nerve enters the neurocranium with the trunk of the vagal nerve through the vagal foramen. As the root of the posterior lateral line nerve passes along the ventral and medial surfaces of the crista of the posterior semicircular canal, it lies on the lateral surface of the vagal trunk and is semilunar in shape. As it continues dorsally and rostrally, it passes over the most distal segment of the posterior ramus of the octaval nerve and the ganglion of the middle lateral line nerve where it fuses with the very short root of the latter (Figs. 6, 11). Technically, the combined fibers of the two nerves should be described as the root of the postotic lateral line nerves (pr, Figs. 6, 10, 11). As this root continues rostrally, it passes slightly dorsal to the roots of the glossopharyngeal nerve and then enters the medulla immediately beneath the ventrolateral border of the lateral line lobes (Fig. 11).

The trunk of the posterior lateral line nerve arises from the distal end of the ganglion and runs caudally and dorsally (Figs. 5, 6, 11) to pass over the dorsal surface of the posttemporal bone. A small bundle of fibers emerges directly from the distal end of the ganglion or almost immediately from the trunk (Figs. 6, 11) of the posterior lateral line nerve. The bundle then courses dorsally and laterally to anastomose with the dorsal somatic ramus of the first spinal nerve, then turns rostrally to pass along the lateral surface of the otic capsule. After a short distance, the bundle divides into numerous twigs. One of the larger twigs innervates the first neuromast of the trunk canal (Fig. 1A). Other twigs seem to ramify in the skin, dorsal to the canal, and possibly innervate the first accessory neuromast (AN, Fig. 1A), as well as the ampullary organs in that region.

As the posterior lateral line trunk continues caudally, it passes ventral to the epibranchial trunk muscles, where a second bundle of fibers arises (Figs. 6, 11). This bundle also turns dorsally and rostrally and innervates the second neuromast in the trunk canal (Fig. 1A). The dorsal ramus of the posterior lateral line nerve (Figs. 5, 6, 11) arises from the trunk of the posterior lateral line nerve, approximately one segment more caudally. The dorsal ramus turns dorsally and caudally to innervate the second accessory trunk line, which consists of two neuromasts, the dorsal trunk line, which consists of three neuromasts (Figs. 2, 6) and the ampullary organs associated with these lines.

The posterior lateral line trunk divides into lateral and ventral rami (Figs. 5-8) approximately two body segments after the dorsal ramus arises from the trunk. The lateral ramus innervates the neuromasts of the accessory trunk lines, neuromasts within the trunk canal, neuromasts at the base of the caudal fin, and all of the trunk ampullary organs dorsal to the ventral trunk line. The ventral ramus innervates the neuromasts of the ventral trunk line and all of the trunk ampullary organs ventral to this line of neuromasts (Figs. 2, 3). All of the trunk lateral line organs are innervated by dorsal ramules, ventral ramules, or both, that arise from the lateral and ventral rami (Figs. 6-8). The relationship of these ramules to the spinal nerve rami and ramules of the facial recurrent ramus are very complex, however. Even though the nerves that innervate the trunk represent three different sensory systems (pain, temperature, and touch information by means of the spinal nerves; gustatory information by means of the recurrent facial ramus; and electrosensory and mechanosensory lateral line information by means of the posterior lateral line nerve) all three systems use many of the same ramules as their fibers approach the periphery (Fig. 8). Analysis of Sudan-Black stained trunk nerves in conjunction with Bodian-stained serial transverse sections of the trunk have allowed us to reconstruct the pathways used by each of the three systems.

Fibers of the facial recurrent ramus that innervate taste buds distributed over the trunk initially exit the recurrent ramus by means of segmentally repeated intervertebral ramules (Figs. 6, 8) whose fibers pass through the dorsal root ganglia of the spinal nerves to enter either their dorsal or ventral somatic rami. The gustatory fibers then run toward the periphery in association with the sensory fibers of the spinal nerves (Fig. 8). The combined somatic and gustatory sensory fibers in the somatic rami of the spinal nerves reach the skin surface in one of two ways. The dorsal somatic ramus of each spinal nerve courses toward the surface in the horizontal septum, which separates the epaxial and hypaxial trunk muscles. As the dorsal ramus approaches the medial surface of the lateral ramus of the posterior lateral line nerve, it divides into dorsal and ventral ramules (dra and vra, Fig. 8). Each of these ramules then turns to course within the myoseptum, and fibers of both ramules ramify among the muscle fibers of a single myotome. However, each ramule innervates the skin in a different manner. Fibers of the dorsal ramule continue within the myoseptum until they reach the inner surface and ramify. As the fibers of the dorsal ramule ramify, many seem to anastomose with the branching fibers of the dorsal ramule (dr) of the lateral ramus of the posterior lateral line nerve. The lateral ramus issues a single dorsal and ventral ramule per trunk segment, and these ramules course dorsally or ventrally, immediately beneath the skin, above the myotome. As each ramule moves away from the lateral ramus, it branches repeatedly, and a single ramule may span as many as four trunk segments. Although anastomoses occur only at the skin surface between the dorsal ramule of the lateral ramus and the dorsal ramule of the dorsal somatic ramus, the ventral ramule (vra) of the dorsal spinal nerve anastomoses almost immediately with the ventral ramule (vr) of the lateral ramus (Fig. 8) so that somatic sensory, lateral line, and gustatory fibers run in the same fiber bundles as they ramify just beneath the surface of the skin. A similar relationship seems to characterize the lateral ramule (le) of the ventral somatic ramus and the ventral ramule (vr) of the ventral ramus (v) of the posterior lateral line nerve.

As the lateral ramus of the posterior lateral line nerve reaches the caudal end of the trunk, it assumes a new set of relationships with the facial recurrent ramus and the spinal nerves as all these nerves enter the caudal fin (Fig. 9). As the spinal cord approaches the base of the caudal fin, it is deflected dorsally to form a neurohaemal organ, the urophysis. At this point of transition, a terminal dorsal somatic ramus (tds) and a terminal ventral somatic ramus (tvs) arise from the spinal cord (Fig. 9B). The terminal dorsal somatic ramus parallels the caudal continuation of the urophysis and is joined by the facial recurrent ramus at the point of origin of the terminal dorsal somatic ramus (Fig. 9A). In contrast, the terminal ventral somatic ramus continues caudally at the same dorsoventral level as it arose from the spinal cord (Fig. 9B). Both the terminal dorsal and terminal ventral somatic rami continue caudally to the bases of the fin rays (lepidotrichia). At this point, each ramus divides into dorsal and ventral ramules that form a dorsal arch and a ventral arch (Fig. 9B). The dorsal arch is formed by the ventral ramule of the terminal dorsal somatic ramus and the dorsal ramule of the terminal ventral somatic ramus, whereas the ventral arch is formed by the ventral ramule of the terminal ventral somatic ramus and by four more rostral ventral somatic rami (vsr, Fig. 9A). Neural elements arise from each arch and continue within the center of each fin ray. Each neural element is composed of somatic sensory fibers as well as special visceral (gustatory) sensory fibers. These sensory fibers also will be joined by the electroreceptive fibers of the lateral ramus of the posterior lateral line nerve (Fig. 9). As the lateral ramus approaches the base of the caudal fin, it divides into three or four ramules that continue caudally just beneath the skin. As these ramules fan out dorsoventrally, some of their fibers innervate the vertical line of neuromasts at the base of the caudal fin (VLN, Fig. 2B). The remaining fibers of the ramules of the lateral ramus of the posterior lateral line nerve continue caudally and medially to anastomose with the neural elements within the fin rays (Fig. 9A), and these lateral line fibers innervate the ampullary organs located on the caudal fin (Fig. 3B).