Biomedical subjects

T E Finger

Publications and source records attributed to T E Finger.

At least 37 records · Page 2Linked to original sources

PubMed1996

Visceral afferent and efferent columns in the spinal cord of the teleost, Ictalurus punctatus.

In tetrapod vertebrates, neural circuitries subserving visceral and somatic reflexes are each represented in distinct columns of cells within the gray area of the spinal cord. To determine the location of visceral elements of the spinal cord of a teleost fish, crystals of the carbocyanine dye 1,1'dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine (DiI), were placed on either the abdominal sympathetic (mesenteric) nerves, the coeliac ganglia, or on the rostral three somatic spinal nerves, in fixed specimens of the channel catfish, Ictalurus punctatus. In fish in which DiI had been placed on the mesenteric nerves, labeled fibers coursed along the lateral margin of the dorsal horn within the first and second spinal segments, and appeared to terminate in a region at the base of the dorsal horn. In contrast, when DiI crystals were placed on the somatic spinal nerves, labeled primary afferents terminated in the dorsalmost two thirds of the dorsal horn, as well as in ventral and ventromedial areas of the medial funicular nuclear complex. Labeled somata (motor neurons) were situated in the ventral horn. When DiI crystals were placed bilaterally on the coeliac ganglia, labeled piriform and fusiform preganglionic neurons occurred in intermediate positions adjacent to the central canal, corresponding to the paracentral nucleus of Herrick, and in the lateral funiculus. These results demonstrate that somatic and visceral afferent and efferent functional columns are distinct in a teleost fish as they are in amniote vertebrates.

Animals↗

PubMed1996

Secondary connections of the dorsal and ventral facial lobes in a teleost fish, the rockling (Ciliata mustela).

In the rockling, Ciliata mustela (Teleostei), a portion of the dorsal fin is a specialized chemosensory organ possessing solitary chemoreceptor cells innervated by a recurrent branch of the facial nerve. Previous studies have demonstrated that the specialized solitary chemoreceptor cell system is represented in the dorsal segment of the medullary facial lobe (DFL), whereas the taste buds in the remainder of the facial-nerve-innervated skin are represented in the ventral division of the lobe (VFL). The carbocyanine dye DiI was used to investigate the secondary and higher order brain connections of these two distinct subdivisions of the facial lobe. Both segments of the facial lobe sent fibers into the contralateral DFL via a dorsocaudal facial commissure and to the contralateral vagal lobes and VFL via fibers arching ventrally through the reticular formation. Ascending fibers from both facial lobe segments were traced into the secondary gustatory nucleus and into the lateral superficial facial nucleus, a small area in the dorsolateral brainstem laterally adjacent to the nucleus medialis of the octavolateral complex. Additionally, the VFL had reciprocal connections with a newly described nucleus adjacent to the incoming facial nerve root. Both DFL and VFL had descending fibers reaching two portions of the funicular nuclear complex, although the VFL contribution to this area is far more extensive than the DFL input. Thus, substantial overlap exists in the connections of the two facial subsystems; i.e., the solitary chemoreceptor information is not processed in nuclei distinct from those making up the usual gustatory lemniscus.

Animals↗

PubMed1996

Axonal projection patterns of neurons in the secondary gustatory nucleus of channel catfish.

The second gustatory nucleus of teleost fishes receives ascending fibers from the primary gustatory center in the medulla and sends efferent fibers to several nuclei in the inferior lobe of the diencephalon. Similar to the corresponding parabrachial nucleus in birds and mammals, the secondary gustatory nucleus of catfish consists of several cytoarchitectonically distinct subnuclei which receive input from different portions of the primary gustatory nuclei. However, it is unclear how the subnuclear organization relates to the processing of gustatory information in the hindbrain and the subsequent transmission of that information to the forebrain. To determine whether cells within different subnuclei of the secondary gustatory nucleus of channel catfish project to different diencephalic targets, single cells were intracellularly labeled with biocytin. Three subnuclei have been identified in the secondary gustatory nucleus: a medial subnucleus spanning most of the rostrocaudal extent of the nucleus, a central subnucleus and a dorsal subnucleus, the latter two located in the rostrolateral portion of the complex. Cells throughout the secondary gustatory nucleus typically possessed similar collateral projections to several nuclei in the inferior lobe, although four of the six cells filled in the medial subnucleus projected only to nucleus centralis. The only apparent subnucleus-specific projection pattern involved cells at the rostral edge of the secondary gustatory nucleus and in the secondary visceral nucleus. Axons of these cells terminated only in restricted portions of nucleus lobobulbaris. These results suggest that efferents from different subnuclei of the secondary gustatory nucleus of catfish, like those of the parabrachial nucleus of birds and mammals, do not possess simple, topographical projections to target nuclei in the diencephalon.

Animals↗

PubMed1995

Taste receptor cells arise from local epithelium, not neurogenic ectoderm.

Except for taste bud cells, all sensory receptor cells and neurons have been shown to originate from neurogenic ectoderm (i.e., neural tube, neural crest, or ectodermal placodes). Descriptive studies on taste buds indicate that they, however, may arise from local epithelium. To determine whether taste receptor cells originate from neurogenic ectoderm or from local epithelium, the tongues of X chromosome-inactivation mosaic mice were examined. Results of this analysis show that taste bud cells and their surrounding epithelium always match in terms of the mosaic marker. This suggests that taste cells and epithelial cells arise from a common progenitor and that taste receptor cells originate from local tissue elements. Since taste buds are widespread in the oropharynx, they lie in epithelium derived from both ectoderm and endoderm. Therefore, taste receptor cells can be induced in tissue from two different germ layers. Thus in terms of tissues of origin, taste receptor cells are unlike other cells with neuronal characteristics.

Animals↗

PubMed1995

Gustatory control of feeding behavior in goldfish.

The present study tests the effects of gustatory quality on feeding behaviors exhibited by goldfish. Goldfish were trained to feed on gelatin pellets, then tested with pellets containing potentially aversive taste stimuli (quinine or caffeine), or mixtures of aversive and appetitive components (amino acids or food). Goldfish feeding includes: initial intake of food particles, a sorting phase consisting of rinsing and backwashing, and a final phase terminating in either rejection or ingestion. Pellets flavored with quinine or caffeine were rejected at concentrations above 10(-5) and 10(-3) M, respectively. Pellets containing a mixture of quinine and ground food or quinine and either L-alanine or L-proline were ingested more frequently than pellets flavored only with quinine. These pellets with mixed chemical components also produced extended sorting behaviors, including prolonged periods of rinsing and backwashing. The prolonged sorting of quinine-food mixtures enabled the animals to separate mechanically the food from the gelatin matrix. These results show that goldfish use chemical cues to drive the intraoral sorting and rejection/ingestion behaviors.

Amino Acids↗

PubMed1994

Expression of GAP43 mRNA in normally developing and transplanted neurons from the rat ventral mesencephalon.

These experiments were designed to determine whether the neuronal growth-related protein GAP43 is expressed at high levels by neurons that collateralize extensively or have long periods of synaptogenesis. We also evaluated the effects of target availability on GAP43 expression. Dopaminergic neurons of the rat ventral mesencephalon (VM) were chosen for investigation because they undergo extensive collateralization and synaptogenesis during postnatal development. Double label in situ hybridization histochemistry (ISHH) and immunocytochemistry (ICC) were used to measure changes in GAP43 mRNA levels within tyrosine hydroxylase (TH)-immunoreactive and -nonimmunoreactive neurons of the VM during postnatal development (p5-adult). TH neurons show higher levels of GAP43 mRNA than do non-TH neurons throughout normal postnatal development and in the adult. This result may be due to more extensive axonal arborization and synaptic remodeling on the part of TH neurons as they innervate the striatum. To test the effects of target availability on GAP43 utilization, grafts of embryonic (e15) VM were placed within previously 6-hydroxydopamine (6-OHDA)-lesioned striata and allowed to develop for 10-28 days. Levels of GAP43 mRNA in grafted TH neurons were reduced at all time points. The short distance to target in the graft paradigm may shorten the overall axonal process length, resulting in lower requirements for growth-related proteins such as GAP43. However, grafted non-TH neurons had elevated levels of GAP43 mRNA, perhaps attributable to prolonged target seeking by neurons that have been isolated from their normal targets.

Animals↗

PubMed1994

Mosaic analysis of the embryonic origin of taste buds.

The embryonic origins of taste receptor cells have not been established experimentally. Although related receptor cells (e.g. hair cells of the inner ear, lateral line receptors) are known to arise from neurogenic ectoderm (e.g. neural crest or placodes), taste buds are described as arising from local epithelial cells. Also unknown is whether or not each taste bud is a clone of cells, i.e. arising from a single progenitor. To address these problems, mosaic and chimeric analyses of lingual epithelium and taste buds have been undertaken. This paper describes the theory of chimeric and mosaic cell lineage analyses, the advantages and disadvantages, and the preliminary results obtained from the examination of the taste buds and lingual epithelium of: 1) mosaic Xenopus, 2) chimeric mice and 3) X-inactivation mosaic mice.

Animals↗

PubMed1994

Differential distribution of the synapsins in the rat olfactory bulb.

The distribution of the different forms of synapsin in the rat olfactory bulb was investigated by biochemical and immunocytochemical methods. Western blots of tissue derived from microdissection of the surface and core regions of the olfactory bulb were performed using antibodies to synapsin I and synapsin II. The relative levels of the synapsins in the core region of the olfactory bulb were similar to the cerebral cortex. In contrast, the surface region of the olfactory bulb had significantly higher levels of synapsin IIa and significantly lower levels of synapsin I, relative to the cortex. Immunocytochemical localization of synapsin I and synapsin II revealed that synapsin I immunoreactivity was the most dense in the external plexiform layer and in the glomeruli; immunoreactivity was also present in the granule cell layer and the periglomerular regions. Synapsin II immunoreactivity was the most dense in the glomeruli. The external plexiform layer, internal plexiform layer, and granule cell layer exhibited much lower immunoreactivity. To determine the source of synapsin II immunoreactivity in the glomeruli, the olfactory epithelium was damaged to decrease the primary afferent input to the bulb. Three to four days later, olfactory bulb sections were double labeled with anti-olfactory marker protein (OMP) antibodies and anti-synapsin II antibodies. Following denervation, both OMP and synapsin II immunoreactivities were diminished, and continued to colocalize in regions retaining immunoreactivity. Individual puncta were immunoreactive for both OMP and synapsin II. Occasional puncta contained only synapsin II immunoreactivity. These results indicate that the distribution of the synapsins in the olfactory bulb differs from most other brain regions.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

GABAergic innervation of the Mauthner cell and other reticulospinal neurons in the goldfish.

The Mauthner cells are pair of identifiable hindbrain neurons that participate in the escape response of fishes. Membrane excitability in these cells is regulated by inhibitory neurons that use glycine as a transmitter. We examined the possibility that the inhibitory transmitter gamma-amino butyric acid (GABA) may also act on the Mauthner cells. We used immunocytochemical methods involving an antibody against glutamic acid decarboxylase (GAD), the synthesizing enzyme for GABA. Our study revealed dense GAD immunoreactive terminals surrounding the Mauthner cells. Puncta counts showed that the distribution of GAD immunoreactivity was densest at the distal lateral dendrite of the Mauthner cells; the distribution of puncta tapers gradually in regions closer to the soma. The axon cap was devoid of GABAergic immunoreactivity. We also performed unilateral lesions of the octaval nuclei to evaluate the origin of the GAD immunoreactive terminals. Following the lesions, we found marked decreases in GAD immunoreactive terminals on the proximal lateral dendrite, soma, and proximal ventral dendrite of both Mauthner cells. These results suggest that the octaval region contributes to bilateral inhibition of the Mauthner cells. The distal lateral dendrite of the ipsilateral Mauthner cell also showed a reduction in GAD immunoreactive terminals. This suggests that GABA mediates remote dendritic inhibition of this cell. GAD immunoreactive puncta also surrounded other large reticulospinal neurons, some of which are serially reiterated along the anterior-posterior axis of the hindbrain. Thus, GABA may also exert an influence not only on the Mauthner cells, but also on other reticulospinal neurons.

Afferent Pathways↗

PubMed1993

Immunolocalization of different forms of neural cell adhesion molecule (NCAM) in rat taste buds.

Taste buds consist of approximately 100 taste cells, including three morphological types of short receptor cells which synapse on the peripheral gustatory nerves. Some of the receptor cells produce neural cell adhesion molecule (NCAM), which may play a role in formation of specific connections in this system. Antibodies directed against different forms of NCAM were utilized in an attempt to define not only the distribution, but also the type of NCAM within taste buds. Within each taste bud approximately 10% of the taste cells exhibit abundant immunoreactivity for 180 kD (ld) or 140 kD (sd) forms of NCAM (i.e., those with an intracellular domain) along virtually the entire surface of the cell. Ultrastructural analysis reveals that these abundantly immunoreactive taste cells are of the intermediate morphological type, although not all of the intermediate taste cells within any bud are immunoreactive. In addition, the ultrastructural studies show that punctate (ld/sd) NCAM-immunoreactivity occurs on the membranes of taste cells and nerve fibers throughout each taste bud. The embryonic form of NCAM (E-NCAM), rich in polysialic acid residues, is present only in association with nerve fibers and other unidentified elongate, thin profiles of a few taste buds. The nerve plexus beneath the gustatory epithelium is also rich in NCAM-immunoreactivity. These fibers occasionally reveal immunoreactivity indicative of only the 120 kD (ssd) form of NCAM, typical of glial cells.

Animals↗

PubMed1993

Peripheral peptidergic fibers of the trigeminal nerve in the olfactory bulb of the rat.

Axons immunoreactive for calcitonin gene-related peptide (CGRP) and substance P are present in the olfactory nerve, although few, if any, olfactory receptor cells contain immunocytochemically detectable levels of these peptides. The possible trigeminal origin of these fibers was tested by performing unilateral stereotaxic lesions of the ophthalmic division of the trigeminal nerve, followed 2-25 days later by immunocytochemistry for CGRP and substance P. As reported previously, free nerve endings immunoreactive for both peptides were found transversing the nasal epithelium on the unlesioned side. Also on the unlesioned side, peptidergic axons, immunoreactive for both CGRP and substance P, could be traced from the olfactory nerve into the glomerular layer throughout the olfactory bulb, but especially into its rostral third. Ipsilateral to the trigeminal ganglion lesion, such peptide-immunoreactive fibers were absent or markedly reduced in the bulb, nerve, and epithelium. These results indicate that the peripheral branches of the ophthalmic branch of the trigeminal nerve enter the olfactory bulb along with the olfactory nerve and terminate in the glomerular layer along with the olfactory axons. Ultrastructural analysis of the CGRP-immunoreactive terminals in the glomeruli reveal vesicle-filled axonal processes terminating in the absence of obvious pre- or postsynaptic specializations. Whether the trigeminal fibers in the bulb are functional, e.g., convey information to the olfactory bulb via an axon reflex, or relay information from the olfactory bulb to the brainstem trigeminal nuclei is unclear.

Animals↗

PubMed1993

Spinal and facial innervation of the skin in the gadid fish Ciliata mustela (Teleostei).

The pattern of innervation of the skin of the rockling Ciliata mustela was investigated to sort out spinal from facial nerve innervation of cutaneous chemosensory and mechanosensory systems. This fish has a variety of appendages with different functional sensory specializations, i.e., the chin barbel, pelvic fin, anterior dorsal fin, and dorsal trunk skin. The carbocyanine dye, diI, was applied to nerve stumps in dissected aldehyde-fixed tissue. In the case of the chin barbel, the dye was applied to both the trigeminal and facial nerve components. In the other cases, the dye was applied either selectively to the spinal nerves, to the facial nerves, or to both components. In the chin barbel, diI labeled nerve fibers associated with taste buds (TBs) and solitary chemosensory cells (SCCs) as well as relatively blunt free nerve endings, which closely approach the epidermal surface. In the pelvic fin, anterior dorsal fin, and dorsal trunk skin, taste buds, solitary chemosensory cells, and their innervation were labeled only after diI was applied to the facial nerve stumps. Application of diI to spinal nerves labeled delicate, free nerve endings and nerve fibers associated with small cells deep in the epidermis with features characteristic of Merkel cells. Transmission electron microscopy supports these results; after denervation of the facial component of the anterior dorsal fin, synaptic contacts with Merkel cells remained intact, whereas the synapses with the SCCs vanished.

Animals↗

PubMed1993

What's so special about special visceral?

The brainstem is classically divided into functional columns including special and general subdivisions for somatic and visceral components. The term 'special visceral motor' is applied to branchiomotor nuclei, while 'special visceral sensory' refers to nuclei devoted to incoming gustatory and olfactory senses. The use of the term 'special visceral motor' is questioned in that the branchiomotor neurons function more like general somatic than general visceral motoneurons. The designation of taste and smell as 'special visceral sensory' systems seems inconsistent on several bases. First, taste and smell are not homologous systems: (1) the receptors are grossly dissimilar in morphology and relationship to other elements of the nervous system; (2) the two systems mediate very different behaviors and respond to different types of chemical stimuli, and (3) chemosensory systems are not 'special' (i.e. limited to cranial nerves) in that solitary chemoreceptor cells, which are distributed across the body surface, can be innervated by spinal or cranial nerves. Finally, taste is considered 'special' because it involves specialized chemosensory end organs; the visceral nerves also provide innervation to other specialized chemoreceptors (e.g. carotid body) which are considered part of the general visceral sensory system. Thus the term 'special visceral sensory' appears without solid foundation when applied to gustatory systems in contradistinction to nongustatory branchial and thoracic interoceptive systems. A reformulation of the functional columns of the brainstem is suggested in which six columns can be identified: (1) somatic motor; (2) branchial motor; (3) visceral motor; (4) visceral sensory; (5) somatic sensory, and (6) hair cell (dorsolateral placode) sensory.

Animals↗

PubMed1992

Expression of the dopaminergic phenotype in the olfactory bulb: neither calcitonin gene-related peptide nor olfactory input is necessary.

In the olfactory bulb, expression of tyrosine hydroxylase (TH) in juxtaglomerular neurons is dependent on innervation by the olfactory nerve. The presence of the neuropeptide calcitonin gene-related peptide (CGRP) within the olfactory nerve has led to the hypothesis that CGRP is responsible for regulation of TH expression in the bulbar neurons. On the other hand, other investigators claim that olfactory receptors never produce CGRP and that functional contact with olfactory axons regulates production of TH by bulbar neurons. Two different experimental procedures were used to test whether either CGRP or contact with the olfactory nerve is essential for production of TH by bulbar neurons in vivo. The peptidergic innervation of the olfactory bulb was eliminated either by neonatal capsaicin treatment, or by stereotaxic, electrolytic lesions of the ophthalmic division of the trigeminal nerve. Both of the treatments leave the olfactory innervation of the bulb intact while eliminating the CGRP-immunoreactive fibers in the olfactory nerve and glomeruli. Subsequent immunocytochemistry reveals a normal complement of bulbar TH-immunoreactive juxtaglomerular neurons in the absence of peptidergic innervation. In order to test whether olfactory nerve input is necessary for expression of TH in vivo, the anlage of the olfactory bulb was removed from embryonic (E16) rat pups and transplanted into the anterior chamber. These ectopic olfactory bulbs, although devoid of olfactory nerve input, contain numerous TH-immunoreactive neurons. Thus olfactory nerve input is not necessary for expression of TH in bulbar neurons.

Afferent Pathways↗

PubMed1992

Ascending general visceral pathways within the brainstems of two teleost fishes: Ictalurus punctatus and Carassius auratus.

The primary general visceral nucleus in goldfish (Carassius auratus) and catfish (Ictalurus punctatus) is located at the ventroposterior boundary of the vagal gustatory lobe and receives coelomic visceral, but not gustatory inputs. The neuronal tracer horseradish peroxidase (HRP) was employed to visualize sources of input to and ascending projections from the primary general visceral nucleus in these species. In addition, immunocytochemical techniques were utilized to define the cytological divisions within the pontine gustatory-visceral complex. The pontine secondary visceral nuclei in both catfish and goldfish contains numerous somata and fibers immunoreactive for calcitonin gene-related peptide (CGRP). In contrast, the secondary gustatory nuclei are devoid of fibers and cells immunoreactive for CGRP. In both the goldfish and the channel catfish, the primary general visceral nucleus receives input from the vagal gustatory lobe, as well as the medullary reticular formation. In the channel catfish, the primary general visceral nucleus projects bilaterally to the secondary visceral nucleus, which lies rostrolateral to the secondary gustatory nucleus in the dorsal pons. Fibers cross the midline via the rostral part of the isthmic commissure. Injection of HRP into the primary general visceral nucleus of a goldfish labels ascending fibers that project to a secondary visceral nucleus situated ventral, lateral, and rostral to the secondary gustatory complex. In general, the results indicate that general visceral systems ascend in parallel to gustatory systems within the brainstem, and that general visceral but not gustatory nuclei are immunoreactive for the peptide CGRP.

Afferent Pathways↗

PubMed1992

Functional organization of vagal reflex systems in the brain stem of the goldfish, Carassius auratus.

The coordination of secretory and motor responses to food within the alimentary canal requires well organized brain stem reflex systems. In the goldfish, Carassius auratus, three vagal reflex systems control three phases of ingestion and digestion. The orobranchial system sorts food from substrate, the pharyngeal chewing organ prepares items deemed to be food for digestion and absorption, and the abdominal system regulates the digestion of food. Each system is represented in the central nervous system by separate sensory and motor nuclei. The aim of the present study was to determine whether the interrelationships among the vagal sensory and motor nuclei reflect the peripheral organization. The sensory nucleus of each vagal system was injected with the neuronal tracer horseradish peroxidase (HRP), in separate cases. HRP injections into the vagal lobe sensory layers (orobranchial system) labeled fibers projecting topographically to the vagal lobe motor layer, but not at all to the pharyngeal or abdominal motor nuclei. Similarly, injections of HRP into the pharyngeal and abdominal sensory nuclei selectively labeled nerve fibers projecting to the pharyngeal and abdominal motor nuclei, respectively. All injections resulted in labeled fibers and/or cells in the lateral reticular formation, and in fibers ascending in the secondary gustatory-visceral tract. Gustatory information from the pharynx is apparently processed in the same brain stem system as pharyngeal general visceral information, suggesting that functional or regional characteristics of visceral sensory information may be more important for brain stem processing than the traditional "special" (gustatory) versus "general" visceral dichotomy. These results indicate that anatomically and functionally separate reflex systems exist within the goldfish vagal visceral nuclei.

Abdomen↗

PubMed1992

Evoked responses from an in vitro slice preparation of a primary gustatory nucleus: the vagal lobe of goldfish.

The vagal lobe of goldfish (Carassius auratus) is a laminated structure in which primary gustatory afferents terminate in a stereotypical pattern. Because the afferent fibers enter and distribute within the lobe in the transverse plane, the structure appeared suitable for in vitro slice electrophysiology. Slices were cut on a vibratome at 400-800 microns thickness and placed in a fresh water teleost Ringer's solution. Following a recovery period, clear population responses were recorded following electrical stimulation of the incoming fiber bundle. The later two components of this evoked waveform were eliminated by removal of the calcium from the bathing solution indicating the synaptic origin of these potentials. Further, the waveform was highly dependent on the position of the recording electrode, both in terms of laminar and tangential position. Evoked response maxima corresponded to the layers in which the primary afferent fibers terminate. In addition, the maximal evoked response was limited to a tangential distance of approximately 100 microns. The spatial restriction of the evoked waveform therefore corresponds well with the known anatomical organization of the primary gustatory afferent fibers. The evoked waveforms are sensitive to stimulus repetition rate, being facilitated by stimulus trains less than 50 ms in duration and fatigued by stimulus repetitions as slow as 1 Hz. This in vitro preparation should provide a means for investigating the physiological and pharmacological properties of primary gustatory fibers and nuclei.

Animals↗

PubMed1991

Postlarval growth of the peripheral gustatory system in the channel catfish, Ictalurus punctatus.

The phenomenon of postlarval cell addition to the peripheral nervous system of fish has been reported for some sensory systems, but has yet to be characterized for the gustatory system. Many fishes, such as catfish, possess taste buds scattered across their body surface, and presumably, the number of taste buds increases during growth of the animal. The present study was undertaken in order to examine the process of growth in the peripheral gustatory system and to determine whether the degree of convergence of receptors onto primary sensory afferents changes during growth. The recurrent facial nerve of channel catfish was used for these studies since this nerve contains no general cutaneous components and innervates taste buds along the fish's body surface. Electron micrographs were made of cross sections of this nerve taken from individuals ranging in size from 5.1 to 39.5 cm standard length. In addition, estimates were made of the number of taste buds innervated by this nerve by determining taste bud density along selected regions of the flank and fins in large and small fish. As catfish get larger, the number of both myelinated and unmyelinated axon profiles in the recurrent facial nerve increases, but at a slower rate than the number of taste buds innervated by this nerve. Thus, on average, the number of taste buds innervated by each fiber increases as the fish enlarges; on average there are 2 taste buds per axons profile in small fish and nearly 14 taste buds per axon profile in large fish. The rate of addition of new axon profiles to the nerve is estimated at roughly 70 per day over the range of sizes studied. Although generation of new ganglion cells and axons may contribute to this increase, several lines of evidence indicate that axonal branching occurs. In addition, the mean axon diameter for both myelinated and unmyelinated axons increases during postlarval growth. The finest myelinated fibers (0.2 micron) in small animals were significantly smaller than the finest myelinated fibers (0.7 micron) in larger animals.

Animals↗