OFF-alpha and OFF-beta ganglion cells in cat retina: II. Neural circuitry as revealed by electron microscopy of HRP stains

An OFF-center alpha and an OFF-center beta ganglion cell in cat retina, which had been recorded from and intracellularly stained with horseradish peroxidase (HRP) were examined by serial section electron microscopy. We counted synapses and identified presynaptic neurons to the HRP-stained cells in 20 microns radial slices through the centers of their dendritic trees. Presynaptic amacrine and bipolar cells were identified on cytological criteria known from previous studies. The OFF-beta cell with a 62 microns dendritic arbor, restricted to S1 and S2 (sublamina a) of the inner plexiform layer (IPL), received 38% bipolar and 62% amacrine cell synapses. The bipolar input was from both cb1 and cb2 cone bipolar types. Input from three distinct amacrine cell types occurred upon the dendrites, namely from: (1) AII amacrine lobular appendages, (2) large pale amacrine profiles (possibly A2 or A3 cells), and (3) small, dark amacrine types (possibly A8 cells). Large pale amacrine profiles (possibly A13) were found on the cell body and apical dendrite in sublamina b of the IPL. In addition, several amacrine profiles synapsed directly on the sides and base of the cell body in the ganglion cell layer. We estimate that the complete dendritic tree of this beta cell received about 1,000 synapses contributed by 12-14 bipolar cells, 7-10 AII amacrines and 28-41 other amacrine cells. The OFF-alpha cell had a dendritic tree size of 680 x 920 microns. A 250 microns length of two major dendrites stratifying narrowly in S2 of the IPL was reconstructed. Amacrine cells provided most of the synaptic input (80%). This input came from: (1) AII amacrine lobular appendages, (2) amacrines exhibiting large, pale synaptic profiles (possibly A2 or A3 cells), (3) pale amacrines with large mitochondria and a few neurotubules (unknown type), and (4) densely neurotubule-filled amacrine profiles (possibly A19 cells). A large pale amacrine cell type (possibly A13) provided synaptic input to the cell body as a serial synaptic intermediary with rod bipolar cells. Cone bipolar synapses were from only one type of cone bipolar, the cb2 type and formed 20% of the total synaptic input. We estimate that a minimum of 142 bipolar cells, 256 AII amacrine cells and 1,011 other amacrine cells, altogether providing 6,000-10,000 synapses, converged on the dendritic tree of this OFF-alpha cell.     

Interactions of inhibition and excitation in the light-evoked currents of X type retinal ganglion cells

The excitatory and inhibitory conductances driving the light-evoked currents (LECs) of cat and ferret ON- and OFF-center X ganglion cells were examined in sliced and isolated retina preparations using center spot stimulation in tetrodotoxin (TTX)-containing Ringer. ON-center X ganglion cells showed an increase in an excitatory conductance reversed positive to +20 mV during the spot stimulus. At spot offset, a transient inhibitory conductance was activated on many cells that reversed near ECl. OFF-center X ganglion cells showed increases in a sustained inhibitory conductance that reversed near ECl during spot stimulation. At spot offset, an excitatory conductance was activated that reversed positive to +20 mV. The light-evoked current kinetics of ON- and OFF-center X cells to spot stimulation did not significantly differ in form from their Y cell counterparts in TTX Ringer. When inhibition was blocked, current-voltage relations of the light-evoked excitatory postsynaptic currents (EPSCs) of both ON- and OFF-X cells were L-shaped and reversed near 0 mV. The EPSCs averaged between 300 and 500 pA at -80 mV. The metabotropic glutamate receptor agonist 2-amino-4-phosphonobutyric acid (APB), was used to block ON-center bipolar cell function. The LECs of ON-X ganglion cells were totally blocked in APB at all holding potentials. APB caused prominent reductions in the dark holding current and synaptic noise of ON-X cells. In contrast, the LECs of OFF-X ganglion cells remained in APB. An increase in the dark holding current was observed. The excitatory amino acid receptor antagonist combination of D-amino-5-phosphono-pentanoic acid (D-AP5) and 2, 3-dihydroxy-6-nitro-7-sulfamoyl-benzo-(F)-quinoxalinedione (NBQX) was used to block ionotropic glutamate receptor retinal neurotransmission. The LECs of all ON-X ganglion cells were totally blocked, and their holding currents were reduced similar to the actions of APB. For OFF-X ganglion cells, the antagonist combination always blocked the excitatory current at light-OFF; however, in many cells, the inhibitory current at light-ON remained. ON-center X ganglion cells receive active excitation during center illumination, and a transient inhibition at light-OFF. In contrast OFF-center X ganglion cells experience a sustained active inhibition during center illumination, and a shorter increase in excitation at light-offset. Cone bipolar cells provide a resting level of glutamate release on X ganglion cells on which their light-evoked currents are superimposed [corrected].     

Synaptology of physiologically identified ganglion cells in the cat retina: a comparison of retinal X- and Y-cells

It has long been known that a number of functionally different types of ganglion cells exist in the cat retina, and that each responds differently to visual stimulation. To determine whether the characteristic response properties of different retinal ganglion cell types might reflect differences in the number and distribution of their bipolar and amacrine cell inputs, we compared the percentages and distributions of the synaptic inputs from bipolar and amacrine cells to the entire dendritic arbors of physiologically characterized retinal X- and Y-cells. Sixty-two percent of the synaptic input to the Y-cell was from amacrine cell terminals, while the X-cells received approximately equal amounts of input from amacrine and bipolar cells. We found no significant difference in the distributions of bipolar or amacrine cell inputs to X- and Y-cells, or ON-center and OFF-center cells, either as a function of dendritic branch order or distance from the origin of the dendritic arbor. While, on the basis of these data, we cannot exclude the possibility that the difference in the proportion of bipolar and amacrine cell input contributes to the functional differences between X- and Y-cells, the magnitude of this difference, and the similarity in the distributions of the input from the two afferent cell types, suggest that mechanisms other than a simple predominance of input from amacrine or bipolar cells underlie the differences in their response properties. More likely, perhaps, is that the specific response features of X- and Y-cells originate in differences in the visual responses of the bipolar and amacrine cells that provide their input, or in the complex synaptic arrangements found among amacrine and bipolar cell terminals and the dendrites of specific types of retinal ganglion cells.     

Morphological and functional study of dwarf neurons in the rat striatum

Combination of morphological and electrophysiological techniques provided data, suggesting existence in the young rat striatum of a peculiar class of neurons, the neurogliaform or dwarf neurons. Striatal neurons (n = 92), intracellularly recorded from rat brain slices, were filled (one in each slice) with the intracellular marker biocytin, to compare physiological and morphological properties in the same cell. Moreover, some neurons (n = 7) were filled with biocytin plus the fluorescent calcium indicator fura-2, identifying cells during electrophysiological recording. Electrophysiological recording showed that striatal neurons had different firing patterns, suggestive in most cases (n = 80) of spiny neuron class and in others (n = 12) of interneuron class. Fura-2 injection clearly identified the body of six medium-sized cells and of one distinctive tiny cell. This small cell, however, showed a resting membrane potential and spontaneous and evoked firing pattern characteristic of striatal interneurons. Moreover, the fura-2 injected in such small neuron also completely filled the cell body of a near large neuron; the fura-2 fluorescence changed synchronously in the two paired neurons after electrical stimulation of the impaled small one. Accordingly, the biocytin staining identified the morphology of the small recorded neuron as a neurogliaform-like cell apposed to a dendrite of an aspiny neuron, suggesting that the dye injected in one neuron had diffused to the other of a different type. Furthermore, such heterologous dye coupling unexpectedly involved seven pairs of cells detected with biocytin staining (7.6% of the recorded neurons), invariably represented by a medium or large neuron on one side, and on the other side by a small (5.44 +/- 0.15 x 9.14 +/- 0.7 microns, mean +/- SD; n = 7) neurogliaform cell, roundish in shape with few slender and short processes, usually apposed to a dendrite of the companion neurons (six out of seven). In the other cases, the biocytin staining revealed in each slice either the morphology of single spiny or aspiny neurons (80.4% of recorded neurons), or of two-three medium-sized spiny neurons detected near to each other, suggesting that dye coupling had occurred typically between similar neurons (11.9% of the recorded neurons). These data suggest that some neurogliaform cells in the striatum of young rat can be identified as dwarf interneurons, that may be dye-coupled with neurons of different classes.     

A novel signaling pathway from rod photoreceptors to ganglion cells in mammalian retina

Current understanding suggests that mammalian rod photoreceptors connect only to an ON-type bipolar cell. This rod-specific bipolar cell excites the All amacrine cell, which makes connections to cone-specific bipolar cells of both ON and OFF type; these, in turn, synapse with ganglion cells. Recent work on rabbit retina has shown that rod signals can also reach ganglion cells without passing through the rod bipolar cell. This route was thought to be provided by electrical gap junctions, through which rods signal directly to cones and thence to cone bipolar cells. Here, we show that the mouse retina also provides a rod pathway bypassing the rod bipolar cell, suggesting that this is a common feature in mammals. However, this alternative pathway does not require cone photoreceptors; it is perfectly intact in a transgenic mouse whose retina lacks cones. Instead, the results can be explained if rods connect directly to OFF bipolar cells.     

Contrast enhancement and distributed encoding by bipolar cells in the retina

Responses of bipolar cells, cone photoreceptors, and horizontal cells were recorded intracellularly in superfused eyecup preparations of the tiger salamander (Ambystoma tigrinum). Contrast flashes of positive and negative polarity were applied at the center of the receptive field while the entire retina was light adapted to a background field of 20 cd/m2. For small contrasts, many bipolar cells showed remarkably high contrast gain: up to 15-20% of the bipolar response was evoked by a contrast step of 1%. There was considerable variation from cell to cell but, on average, no striking differences in contrast gain were found between the depolarizing (Bd) and hyperpolarizing (Bh) bipolar cells. Quantitative comparisons of contrast/response measurements for cone photoreceptors and cone-driven bipolars suggest that the high contrast gain of bipolars is the consequence of a 5-10 x amplification of small signals across the cone-->bipolar synapse. Bipolar cells had a very restricted linear range of response and tended to saturate at stimulus levels that were within the linear range of the cone response. The contrast/response of horizontal cells was similar to that of cones and differed markedly from that of Bh cells. For steps of equal contrast, the latency of the Bh cells was approximately 20 ms shorter than that of the Bd cells regardless of the contrast magnitude. For both bipolar cells and cones, the effect of contrast polarity on latency seems largely due to the absolute value of the light step, delta L. In the large signal domain, properties of the contrast responses of bipolar cells varied appreciably, both within and between the Bd and Bh classes. Cells of either class could be positive- or negative-contrast dominant. These and additional results show that in the light-adapted retina, the bipolar population is functionally diverse and has the potential to provide a rich substrate for distributed encoding of visual images.     

Post-receptoral mechanisms of colour vision in New World primates

Diurnal platyrrhines, both di- and trichromats, have magnocellular (M-) and parvocellular (P-) retinal ganglion cells which are morphologically very similar to those found in catarrhines. Catarrhine central P ganglion cells contact single midget bipolar cells, which contact single cones. Physiological recordings of retinal ganglion cells of dichromatic Cebus monkeys showed very similar cell properties to the catarrhine macaque, except that P ganglion cells lacked colour-opponency. We describe the presence of single-headed midget bipolar cells in the Cebus retina. These midget bipolar cells have axon terminal sizes in the same range as the dendritic tree sizes of P ganglion cells as far as 2 mm of retinal eccentricity. This result supports the view that, as in catarrhines, central P ganglion cells of platyrrhines receive input from single midget bipolar cells which in turn, receive input from single cones. This finding is consistent with the idea that a P pathway with one-to-one connectivity was present in the anthropoid ancestor before the divergence between catarrhines and platyrrhines.     

Horizontal cell connections with short-wavelength-sensitive cones in macaque monkey retina

This study describes the connectivity between horizontal cells and short-wavelength-sensitive (SWS) cones in macaque monkey retina. H1 and H2 horizontal cells were either labelled with the carbocyanine dye, DiI, or injected intracellular with Neurobiotin. The retinas were then processed with an antiserum against human SWS cone pigment, which usually stained the entire SWS cone. In these double-labelled retinas, the pattern of connectivity of H1 (n = 91) and H2 (n = 7) cells with SWS cones has been determined. About 85% of the H1 cells examined do not contact SWS cones. The dendritic terminal knobs of five H1 cells that do contact SWS cones were counted. They have, at most, 3% of their dendritic terminal knobs at SWS cones. All H2 cells examined make contact with SWS cones. The dendritic terminal knobs of one H2 cell were counted; about 11% of the dendritic terminal knobs are at the SWS cone. We conclude that horizontal cells in macaque monkey retina show specific patterns of connectivity to SWS cones.     

Light- and electron-microscopic study of electrophysiologically characterized neurons in the mediolateral part of the lateral spetum of the guinea-pig

In slices of guinea-pig brains, 36 neurons located in the mediolateral part of the lateral septum were stained intracellularly with horseradish peroxidase (n = 28) or biocytin (n = 8) after electrophysiological characterization. These neurons belonged to class A neurons (n = 23), which generated pronounced Ca(++)-dependent high-threshold spikes in control medium, or to class C neurons (n = 9), which were recognized by the occurrence of small-amplitude sodic spikes followed by slower larger calcic spikes. The present results demonstrate that, despite the variety of individual cell types, the major morphological population (30/36 cells) was composed of a homogeneous class of large-sized neurons that displayed thick primary dendrites and abundant dendritic appendages. The remaining 6 cells were small-sized, poorly-spiny neurons. Somatic spines were observed on 5 out of the 30 large cells and on one out of the six smaller cells. Labeled axons were mainly oriented to the anterior commissure. The axons of nine cells richly collateralized near the perikaryon. Ultrastructural examination of 3 horseradish peroxidase-injected cells showed indented nuclei, classic organelles and somatic spines. Terminal boutons established symmetric synapses with the injected cells. These results describe the morphological features of electrophysiologically identified neurons and indicate that class A and class C neurons are distributed among morphological populations differing in perikaryal size. This suggests that the different electrical properties of class A and class C neurons reflect recordings from different parts of the neuron rather than from neurons of different types. Furthermore, the present findings demonstrate that, in the guinea-pig, electrical and morphological characteristics of somatospiny neurons are comparable with those of non-somatospiny neurons. Somatospiny neurons have a recognized integrative role in the hippocampo-septo-hypothalamic complex.     

Tracer and electrical coupling of rat suprachiasmatic nucleus neurons

Whole-cell recording from single neurons of the suprachiasmatic nucleus with an electrode containing the tracer neurobiotin resulted in the staining of multiple neurons in 30% of the cases. Typically, one neuron was darkly stained with dendritic processes and an axon clearly visible while other neurons were lightly stained. The darkly-stained cells were identified as the recorded neuron and tracer-coupled to one to five lightly stained neurons. The resting membrane potential, input membrane conductance, membrane capacitance, the decay time constant and the maximum H-current amplitude of the recorded neurons with tracer-coupled cells were not significantly different from those of neurons not showing tracer coupling. Stimulation of the preoptic area activated an antidromic action potential or an all-or-none small slow inward current in some neurons when the synaptic transmission was blocked by a calcium-free/Mn2+ solution. The small slow inward current did not "collide" with an orthodromically activated action spike suggesting that the current represents the signal from an electrotonically-coupled neuron. In addition, the frequency of biphasic field currents from a neighbouring cell firing were increased by depolarization and decreased by hyperpolarization of the recorded cell. These data demonstrate a chemical and electrical low-resistance coupling of suprachiasmatic nucleus neurons, which could be important in synthesizing the suprachiasmatic nucleus circadian rhythm.     

Inputs from three brainstem sources to identified neurons of the mouse inferior colliculus slice

A total of 40 neurons from of the central nucleus of the mouse inferior colliculus (IC) were recorded intracellularly from brain slices to determine input properties by electrical stimulation of the ipsilateral lateral lemniscus (LL), commissure of Probst (CP), and commissure of the IC (CoIC) together with cellular morphology (in 25 neurons) by biocytin injection and staining. Nine neurons had oriented (bipolar), 16 neurons non-oriented (multipolar) dendritic trees of various sizes. Axon collaterals of a given neuron often ran in several directions to provide multiple input to adjacent isofrequency laminae, the lateral nucleus of the IC, the brachium of the IC, the LL, the CP, and the IC commissure. Neurons were classified by spike response patterns to depolarizing current injection into onset- and sustained-spiking cells. The former had significantly shorter membrane-time constants, significantly less frequently and smaller hyperpolarizations after spike occurrence, and more Ca2+-humps. These properties and their preferred position in the dorsolateral ICC suggest a participation in binaural temporal processing. Almost all oriented cells showed only excitatory post-synaptic potentials (EPSPs) after LL stimulation, while in non-oriented cells inhibitory post-synaptic potentials (IPSPs) after the EPSPs were significantly more frequent. Neurons with largest dendritic trees and many dorsalward projecting axon collaterals were found in the ventral IC. There, neurons had average 4 ms (two synapses) shorter response latencies to LL stimulation than dorsally located neurons. Thus, neurons in the central and dorsal IC may receive mono- and disynaptic input from ventrally located neurons.     

Variations in photoreceptor directionally across the central retina

Cones show a differential sensitivity to light coming from different portions of the pupil, typically being most sensitive to light from the center of the pupil. We measured the directional properties of the cones across the central 6 deg of the retina, using an optical imaging technique. We find that the cones in the center of the fovea have the broadest tuning. The width of the angular tuning changes rapidly from 0 deg to 1 deg retinal eccentricity, with cones at 1 deg being much more narrowly tuned that the cones in the center of the fovea. Directional tuning of the cones remains relatively constant from 1 deg to 3 deg retinal eccentricity. Receptoral disarray contributes minimally to the measured directional properties of the foveal cones, and there is no evidence of asymmetry between horizontal and vertical retinal locations. There are only small differences among the five subjects in the change in angular tuning of the cones with retinal location. We find that at the foveal center the directional tuning of the cones is limited by the diameter of the cone apertures.     

Kainate receptors mediate synaptic transmission between cones and 'Off' bipolar cells in a mammalian retina

Light produces a graded hyperpolarization in retinal photoreceptors that decreases their release of synaptic neurotransmitter. Cone photoreceptors use glutamate as a neurotransmitter with which to communicate with two types of bipolar cell. Activation of metabotropic glutamate receptors in 'On' bipolar cells initiates a second-messenger cascade that can amplify small synaptic inputs from cones. In contrast, it is not known how the ionotropic glutamate receptors that are activated in 'Off' bipolar cells are optimized for transmitting small, graded signals. Here we show, by recording from a cone and a synaptically connected 'Off' bipolar cell in slices of retina from the ground squirrel, that transmission is mediated by glutamate receptors of the kainate-preferring subtype. In the dark, a cone releases sufficient neurotransmitter to desensitize most postsynaptic kainate receptors. The small postsynaptic current that persists (<5% of maximum) is quickly modulated by changes in presynaptic voltage. Since recovery from desensitization is slow (the decay time constant is roughly 500 milliseconds), little recovery can occur during the brief (roughly 100-millisecond) hyperpolarization that is produced in cones by a flash of light. By limiting the postsynaptic current, receptor desensitization prevents saturation of the 'Off' bipolar cell's voltage response and allows the synapse to operate over the cone's entire physiological voltage range.     

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.     

Calcium and protein kinase C regulate the actin cytoskeleton in the synaptic terminal of retinal bipolar cells

The organization of filamentous actin (F-actin) in the synaptic pedicle of depolarizing bipolar cells from the goldfish retina was studied using fluorescently labeled phalloidin. The amount of F-actin in the synaptic pedicle relative to the cell body increased from a ratio of 1.6 +/- 0.1 in the dark to 2.1 +/- 0.1 after exposure to light. Light also caused the retraction of spinules and processes elaborated by the synaptic pedicle in the dark. Isolated bipolar cells were used to characterize the factors affecting the actin cytoskeleton. When the electrical effect of light was mimicked by depolarization in 50 mM K+, the actin network in the synaptic pedicle extended up to 2.5 micrometer from the plasma membrane. Formation of F-actin occurred on the time scale of minutes and required Ca2+ influx through L-type Ca2+ channels. Phorbol esters that activate protein kinase C (PKC) accelerated growth of F-actin. Agents that inhibit PKC hindered F-actin growth in response to Ca2+ influx and accelerated F-actin breakdown on removal of Ca2+. To test whether activity-dependent changes in the organization of F-actin might regulate exocytosis or endocytosis, vesicles were labeled with the fluorescent membrane marker FM1-43. Disruption of F-actin with cytochalasin D did not affect the continuous cycle of exocytosis and endocytosis that was stimulated by maintained depolarization, nor the spatial distribution of recycled vesicles within the synaptic terminal. We suggest that the actions of Ca2+ and PKC on the organization of F-actin regulate the morphology of the synaptic pedicle under varying light conditions.     

Functional study of the rat cortical microcircuitry with voltage-sensitive dye imaging of neocortical slices

The computations performed within cortex are likely to be determined by its internal dynamics in addition to its pattern of afferent input. As a step toward characterizing these dynamics, we have imaged electrical activity in slices from rat primary visual cortex stained with the voltage-sensitive dye di-4-ANEPPS. In response to electrical stimulation two fluorescence signals of similar maximum amplitude are elicited, (i) A fast signal that peaks in a few milliseconds, is dependent on membrane voltage, and has a significant presynaptic component. This signal can be used to image electrical activity ratiometrically. (ii) A slow signal that peaks a few seconds after stimulation, does not reflect voltage changes, and may originate from changes in scattering properties of the slice and from interactions of the dye with the cells. The spatial pattern of fast signals obtained in response to focal stimulation of coronal slices is consistent with known interlaminar projection patterns. In tangential slices, imaging of fast signals reveals clustered horizontal responses. Finally, imaging of fast signals during epileptiform activation of the disinhibited circuit reveals propagating responses, without evidence for modular activation.     

Two functional effects of decreased conductance EPSP's: synaptic augmentation and increased electrotonic coupling

Three electronically coupled motor neurons, which mediate inking behavior in Aplysia californica, receive both increased and decreased conductance excitatory postsynaptic potentials (EPSP's). The increased conductance EPSP's reduce electrical coupling among the cells, whereas the decreased conductance EPSP's increase electrical coupling. The decreased conductance EPSP's also augment the action of a previously ineffective sensory input and this augmentation is enhanced by the increase in electrical coupling. Both effects combine to trigger a stereotypic behavioral response.     

The enhanced S cone syndrome: an analysis of receptoral and post-receptoral changes

The purpose of the study was to test the hypothesis that the retinae of patients with enhanced S cone syndrome (ESCS) have more S cones than the normal retina and these cones have replaced some of the L and M cones. Standard and spectral full-field electroretinograms, measurements of L, M, and S cone system sensitivities and S cone acuity were obtained from three patients with ESCS. The results were qualitatively consistent with the presence of more S cones and more S cone ganglion cells. To test this hypothesis further, a model of the receptoral and post-receptoral components of the S cone system was used in conjunction with psychophysical measurements of S cone system sensitivity under flashed and steady-state adaptation conditions. Within the context of the model, the data were consistent with an increase in the number of S cones and S - (L + M) ganglion cells and with a decrease in the total L + M cone input to each S - (L + M) ganglion cell.     

Neural substrate for motor control of feeding in amphibians

Descending pathways to premotor/motor centers and their cell groups of origin were studied by means of retrograde biocytin tracing experiments in the frog Discoglossus pictus and the plethodontid salamander Plethodon jordani, which differ remarkably in the structure and function of their feeding apparatus and their feeding strategy. Labeled neurons were found in 30 major cell groups located in the telencephalon, diencephalon, synencephalon, mesencephalon and rhombencephalon. The number and distribution of nuclei are very similar in both species. Furthermore, the descending pathways of these groups of neurons take the same courses inside the medulla oblongata. Axons of most nuclei descend either in the ventromedial or ventrolateral medulla oblongata, and it is concluded that the spatial arrangement of pathways is identical in the species studied. Bilateral electrical stimulation of the optic tectum of the plethodontid salamander Hydromantes italicus elicited strong discharges of short latencies in the hypoglossal nerve. In most hypoglossal motor neurons, excitatory postsynaptic potentials (EPSPs) of short latencies followed paired shocks applied at intervals as short as 3 ms, but showed temporal and spatial facilitation, suggesting that the EPSPs include mono- as well as polysynaptic components. In the ventral white matter, orthodromic single units were found that are candidates for excitatory reticular interneurons. These properties of tectal descending pathways in salamanders strongly differ from those found in toads. Differences in feeding behavior and its control by the premotor/motor networks between the species investigated do not appear to result from anatomically altered input or from a different organization of descending pathways to these premotor/motor centers, but rather from differences in local properties of reticular premotor networks as well as from different effects of neuromodulatory systems.