A quantitative study of morphological reorganization following chronic optic deafferentation in the adult cat dorsal lateral geniculate nucleus

Neuronal and synaptic reorganization in the lateral geniculate nucleus (dLGN) of adult cats following chronic visual deafferentation has been investigated with the aid of GABA immunocytochemistry and quantitative electron microscopy. The main purpose of this study was to establish the morphological counterpart of the functional plasticity of dLGN relay cells after total visual deafferentation (Eysel: Brain Res. 166:259-271, '79). The results provide evidence that the regained excitability of relay cells is not the result of disinhibition (caused hypothetically by the selective loss of GABAergic cells) since the proportion of GABA-positive and GABA-negative cells as well as the inhibitory synaptic density did not change. The alternative suggestion that the enhanced excitability could be the result of compensatory axonal sprouting by corticothalamic fibers had also to be dropped: the absolute number of corticothalamic axons to the deafferented dLGN remains unchanged. Because of shrinkage of the dendritic trees of dLGN neurons, however, the density of cortical synaptic input at dLGN cells becomes elevated by almost 60%. It is suggested that the regained excitability of relay neurons is the consequence of the combined effects of adaptive (structural) reduction in size ("atrophy") of retinally denervated nerve cells, and, as a consequence, increase of input resistance, reduced shunting effects, and relative increase in density of the excitatory cortical input per neuron.     

Termination of the geniculocortical projection in the striate cortex of macaque monkey: a quantitative immunoelectron microscopic study

The goal of this present study was to derive a new estimate of the synaptic contribution of the dorsal lateral geniculate nucleus (dLGN) to the subdivisions of its main recipient layer, layer 4C, of striate cortex of macaque monkey. The projection from the dLGN and its terminal boutons within layer 4C were visualized by immunodetection of the calcium binding protein, parvalbumin (PV), which is expressed in relay cells of the dLGN. The proportion of asymmetric synapses formed by PV-positive boutons within the alpha and beta sublayers of 4C was estimated by using a nonbiased stereological counting method. The proportion of asymmetric synapses contributed by the PV-positive boutons to layer 4Calpha is 8.7%; to 4Cbeta is 6.9%. Assuming all the PV-positive asymmetric synapses derive from the dLGN relay cells, this gives a ratio of dLGN synapses per neuron of 192 in layer 4Calpha and 128 in layer 4Cbeta. Thus, the recurrent excitatory input from neighboring cortical neurons must play an important part in responses of the neurons lying at the input stage of the cortical circuit.     

猫视网膜神经节细胞显示对长时间光栅刺激的弱图形适应和明显的图形易化作用

图形适应被认为仅存在于视觉皮质水平，然而本实验室新近实验表明，光栅图形适应也存在于正常猫和去视皮质猫的外膝体神经元。本研究目的在于澄清外膝体的图形适应是否来源于视网膜。研究了79个X和Y型猫视网膜神经节细胞对长时间的光栅刺激反应。结果显示：①与外膝体中继细胞不同，71%的神经节细胞对长时间刺激反应不变，只有6%的细胞显示光栅适应，而有23%细胞显示易化作用；②上述对长时间的光栅刺激的效应与细胞类型（X和Y型或On-和Off-中心型）无关；③如同外膝体细胞，视网膜神经节细胞的光栅易化作用在刺激后30s内结束，平均反应幅度上升17%，其时程符合指数函数，平均时间常数为17.7s。本结果提示，图形适应可能基本上起源于外膝体，而图形易化则起源于视网膜，因此外膝体内机制可能导致中继神经元的图形适应。     

The Length – Response Properties of Cells in the Feline Perigeniculate Nucleus

Perigeniculate cells receive visual input from the dorsal lateral geniculate nucleus (dLGN) and from the visual cortex. In contrast to the extensive literature documenting dLGN and cortical cell responses, comparatively little quantitative data exists for perigeniculate nucleus cells, and very little is known about the role of the corticofugal input to the perigeniculate nucleus. We have previously shown that dLGN relay cells have sharply length-tuned receptive fields and that a significant component of this is dependent on the corticofugal system. In this report, we have explored the length – response properties of perigeniculate nucleus cells in the presence and absence of corticofugal feedback. The response profiles of most perigeniculate nucleus cells contrasted markedly with the sharply length-tuned fields of dLGN cells, but exhibited a notable resemblance to those exhibited by VI cells with short summation lengths, which have recently been shown to constitute a considerable proportion of the layer VI cell population. This might suggest that the responses of perigeniculate nucleus cells to long bars derive from their cortical input. However, our data failed to reveal a discernible change in their profiles after removal of the corticofugal drive. This surprising observation implies that their length-tuning profiles follow from subcortical circuitry. The ways in which this might occur are discussed.     

Regrowth of lesioned retinal axons associated with the transplantation of Schwann cells to the brachial region of the rat optic tract

Attempts were made to enhance the regrowth of retinal axons which had been lesioned in the brachial region of the rat optic tract. Pieces of nitrocellulose paper (Millipore) were placed into the lesioned area between the dorsal lateral geniculate nucleus (dLGN) and superior colliculus (SC) in 13- to 18-day-old Wistar rats. Five types of implant were used: (1) uncoated implants, (2) coated with Poly-l-lysine (PLL), (3) coated on one side with cortical astrocytes, (4) coated with tectal astrocytes and (5) coated with Schwann cells. About half the Schwann cell-covered implants were precoated with PLL. Schwann cell-coated implants (16-40 × 103 cells per implant) were placed with the cells lying on either the dorsal or ventral (inverted) surface of the paper. 5-7 weeks after surgery, eyes were injected with WGA-HRP, the animals were perfused and frozen or vibratome sections (40-50 μm) processed for TMB histochemistry. Selected sections containing retinal axons were osmicated and prepared for electron microscopic examination. 45 out of 86 implants were found attached to the caudal dLGN. A small number of retinal axons were found growing onto the rostral end of one uncoated implant, two PLL-coated implants and over the surface of 4 of the astrocyte-coated implants. The densest and most extensive growth was seen on the Schwann cell-coated implants. In 15 of the 30 animals with such implants attached to the dLGN, retinal axons were found regrowing for 50-1120 μm (mean 530 μum). In about half of these rats (8 out of 15), the regrowth involved relatively large numbers of optic axons which were sometimes densely packed together. In the subgroup of Schwann cell-coated implants where the cells were placed upwards, retinal axons regrew on the dorsal surface of the paper for more than 500 μm in 7 animals. Almost no growth was seen on the uncoated (ventral) surface of the implants. In the subgroup of inverted implants, where the Schwann cells were placed downwards, in 4 animals retinal axon regrowth was densest on the ventral surface and extended for 300-550 μm. In 3 cases, occasional axons were also seen on the dorsal implant surface. Myelin sheaths surrounding some of the regenerating axons had PNS characteristics, suggesting that they were formed by the implanted Schwann cells, but most of the myelin appeared to be of central origin. The data suggest that Schwann cells placed just caudal to the dLGN and adjacent to lesioned retinal axons can enhance and direct the regrowth of these axons in the rat optic tract.     

Specific neurotrophic interactions between cortical and subcortical visual structures in developing rat: in vivo studies

The specificity of trophic interactions in the rat visual system is investigated in vivo by using a combination of tissue culture and CNS transplantation methods. In a companion paper (Repka and Cunningham: '87) we showed that explants of embryonic day 14 (E14) occipital cortex are biased to contain different cortical cell populations depending on whether the explants develop in culture with diencephalon or with optic tectum. In this study we transplanted these precultured cortical explants into the cavity created by a lesion of the occipital cortex in newborn rats and then measured the neuron-occupied volume and the numbers of thymidine-labeled cells in the surviving ipsilateral dorsal lateral geniculate nucleus (dLGN) of the host rats. The results were compared to animals with lesions but no transplants, animals with transplants of E14 cortical tissue that had not been precultured, and animals with cerebellar transplants that had been similarly precultured either with other cerebellar tissue or with diencephalon. At 5 days postlesion, both the largest dLGN volume and the greatest number of labeled dLGN neurons survive in animals with cortical transplants precultured with diencephalon or other cortex. The surviving dLGN neurons that are rescued by these transplants are generated on E15 or E16, a period that corresponds to the latter part of geniculate neurogenesis. Relatively few cells generated on E14 survive in any group of animals. Furthermore, animals with all types of cortical transplants have significantly larger volumes of surviving dLGN than animals with either lesions only or cerebellar transplants.(ABSTRACT TRUNCATED AT 250 WORDS)     

Synaptic targets of cholinergic terminals in the pulvinar nucleus of the cat

We compared the cholinergic innervation of the pulvinar nucleus, a thalamic association nucleus, to previous studies of the cholinergic innervation of the dorsal lateral geniculate nucleus (dLGN), a thalamic relay nucleus. Both nuclei receive a dense innervation from cholinergic cells of the brainstem parabrachial region (PBR). In the dLGN, PBR terminals are located in close proximity to retinal terminals. Our goal was to determine whether PBR terminals in the pulvinar nucleus are located in close proximity to corticothalamic terminals. We identified PBR terminals with a monoclonal antibody directed against choline acetyltransferase (ChAT). Cholinergic terminals contacted dendrites (142 of 160, or 89%) or vesicle-filled profiles (18 of 160, or 11%). A subset of 55 terminals was stained for γ-aminobutyric acid (GABA) to determine whether profiles postsynaptic to cholinergic terminals originate from thalamocortical cells (GABA-) or interneurons (GABA+). The majority (44 of 55, or 80%) of postsynaptic profiles were GABA- dendrites. The minority (11 of 55, or 20%) were GABA+ dendrites with vesicles. This distribution of contacts is very similar to that seen in the dLGN. However, the most significant finding was that most cholinergic contacts (121 of 160, or 76%) were located within complex clusters identified as glomeruli. This is the primary site of contacts made by corticothalamic terminals originating from layer V cells. These results suggest that while the PBR enhances retinal signals in the dLGN, it may also enhance cortical signals in the pulvinar nucleus. Thus, activity in the PBR may stimulate both an increased flow of retinal information to visual cortex, as well as an increased flow of information between different visuomotor areas of cortex. J. Comp. Neurol. 387:266–278, 1997. © 1997 Wiley-Liss, Inc.     

Subcortical mechanisms in orientation sensitivity of cat visual cortical cells.

The orientation biases seen in the responses of neurones of the dorsal lateral geniculate nucleus (dLGN) can be reduced by the local application of the GABA antagonist, bicuculline methiodide. This fact was exploited to investigate whether these biases are important for cortical orientation selectivity by measuring the orientation sensitivity of cortical cells before and during iontophoretic administration of bicuculline in the topographically corresponding region of the dLGN. This procedure led to a significant reduction in the orientation sensitivity of the cortical cell. The results suggest that subcortical orientation biases are at least partly responsible for the orientation sensitivity seen at the level of the striate cortex.     

Principal cells in lateral geniculate: Effects of metrazol on capacity to after-discharge

The effect of metrazol on the capacity of dorsal lateral geniculate nucleus (dLGN) principal (P) cells to repetitively burst (after-discharge) following visual system stimulation was examined in P cells which differed in terms of response patterns to visual stimulation (latency of initial spike, extent of repetitive bursting, and on- or off-type responding cells). Twenty-seven P cells were examined in as many subjects. Metrazol augmented repetitive bursting irrespective of the type of P cell as long as repetitive bursting was present in the pre-metrazol period. P cells that displayed only a single initial burst to photic stimulation did not exhibit after-discharge bursting during the metrazol challenge. In all but one cell metrazol enhanced baseline firing rate. These results are discussed in terms of the putative nature of inhibition in the rat dLGN.     

Insular cortex projection to the nucleus of the solitary tract and brainstem visceromotor regions in the mouse

Anterograde transport of horseradish peroxidase (HRP) and HRP conjugated to wheat germ agglutinin (WGA-HRP) demonstrated a substantial, bilateral projection from insular cortex to the nucleus of the solitary tract (NTS) in the mouse. Injections that labeled the projection were restricted to the cortical sector homologous to taste-visceral cortex in the rat and label was antero- and retrogradely transported to several subcortical structures along ascending taste-visceral pathways. Multiple, small injections in this cortical region labeled fibers and terminals throughout the rostro-caudal extent of NTS. Small, single injection showed that the projection is topologically organized: Rostral points along the cortical strip project to rostral parts of NTS, intermediate points to the intermediate levels of NTS and caudal parts of the cortical field to caudal parts of NTS. In NTS the primary cranial nerve afferents distribute along a rostral to caudal gradient with the VII nerve rostral, the IX intermediate and the Xth caudal [2,6,37]. The present results indicate that the cortical sensory representation of these cranial nerve afferents reflects their topographic distribution in NTS. This suggests that there is an organized anatomical substrate by which the cerebral cortex may selectively influence the central processing of both gustatory and visceral afferent information in the primary CNS relay for these modalities. This insulofugal pathway also terminates in parts of NTS and additional medullary areas that contain preganglionic parasympathetic motoneurons. This appears to be the first anatomical demonstration of a projection from any part of the cerebral cortex to parasympathetic motor nuclei. The pathway provides a substantial direct channel by which higher cortical activity may modulate parasympathetic function.     

Auditory and non-auditory subcortical afferents to the inferior colliculus in the rat

Afferent projections to the rat inferior colliculus (IC) were studied by using the method of retrograde transport of horseradish peroxidase (HRP). Microinjection of HRP revealed an ordery arrangement of fiber projections between the cochlear and the central nucleus of IC; it entails a reversal of the dorso-ventral nucleotopic organization. An indistinct dorso-lateral nucleotopic projection was found between the lateral superior olivary nucleus and the central nucleus. Small number of neurons in some brainstem non-auditory structures were always labeled: the parabrachial region of the midbrain lateral tegmentum, the pars lateralis of substantia nigra, dorsal part of the central gray matter at a caudal 2/3 level of IC, and deep layers of the superior colliculus, ipsilaterally, and the spinal trigeminal and posterior column nuclei, contralaterally. Small injection restricted within the external and pericentral nuclei (cortical zone) of IC resulted in a higher distribution ratio of labeled neurons in the non-auditory structures as compared with those in the central nucleus. On the other hand, the ratio in the brainstem auditory nuclei decreased definitely after HRP-injection within the cortical zone, with an exception of the ipsilateral central nucleus of IC which contained many labeled cells following the injections in the cortical zone. The present results suggest a dual function of the inferior colliculus. The central nucleus acts as a relay station in the main auditory system, while the cortical zone, with its converging auditory, visual and somatic inputs, may act as a subcortical integration center for acoustico-motor behavior.     

Different populations of dorsal lateral geniculate nucleus neurons have concentration-specific requirements for a cortically derived neuron survival factor

A macromolecular fraction of conditioned culture medium (CM) derived from explant cocultures of embryonic rat posterior cortex and caudal thalamus is able to support the survival of neurons in the dorsal lateral geniculate nucleus (dLGN) of newborn rats following ablation of dLGN cortical target areas. In the present study we tested whether the survival-promoting activity of this target-derived neurotrophic agent was concentration dependent and whether different subpopulations of dLGN neurons were equally responsive. With the starting concentration of the CM fraction designated X, increasing concentration results in a progressive falloff in trophic activity so that at 200X overall dLGN survival is similar to that seen in unconditioned medium (UM) controls. In contrast, diluting the fraction produces an increase in activity until maximal survival is achieved at 0.2X. Further dilutions result in a decline in trophic activity until control values are reached at 0.001X. Two populations of neurons within the dLGN, defined by their time of origin, respond in a specific manner to the different concentrations. Neurons generated during the early stages of neurogenesis (E14) have maximal survival (25.8%) at 0.05X, whereas those neurons generated later (E15/16) are maximally supported (30.7% survival) at 10X, a 200-fold difference in concentration. While it is possible that separate neurotrophic and neurotoxic molecules exist for each of these populations of dLGN neurons, the most parsimonious interpretation of the data is that a single cortically derived neurotrophic factor exists whose production is strictly controlled during development to achieve maximal effect on different populations of thalamic neurons that may be functionally distinct.     

Neonatal enucleations reduce number, size, and acetylcholinesterase histochemical staining of neurons in the dorsal lateral geniculate nucleus of developing rats

Previous studies have demonstrated that transient patterns of acetylcholinesterase (AChE) activity are characteristic of geniculo-recipient regions of rat cortical area 17 during the second and third postnatal weeks of life. Neonatal enucleation results in a marked reduction of this transiently expressed cortical AChE. Parallel studies have demonstrated that the dorsal lateral geniculate nucleus (dLGN) also expresses AChE transiently during development. The present study examines neuronal number and size as well as AChE histochemical staining in the dLGN of normal and neonatally enucleated rat pups to determine whether changes in dLGN neurons could account for the decreased visual cortical AChE staining that results from neonatal enucleation. Changes in 4 parameters in dLGN were noted after neonatal enucleation. First, a 26-37% shrinkage in the volume of dLGN occurred contralateral to enucleation. Second, enucleation resulted in a loss of 16-30% of AChE-stained neuronal somata. Third, remaining AChE-positive neuronal somata appeared shrunken by approximately 40%. Fourth, intensity of AChE histochemical staining of individual dLGN neurons was reduced by approximately 24% following neonatal enucleation. These data suggest that loss of transient AChE activity in cortical area 17 consequent to neonatal enucleation is secondary to enucleation-induced alterations in the dLGN; these alterations include loss of neurons, shrinkage of neurons, and an apparent decrease in the ability of neurons to synthesize AChE. These data support the hypothesis that geniculocortical projection neurons express AChE transiently during development of geniculocortical connectivity and indicate that normal afferent connections and/or activity are important for the transient expression of AChE by these neurons.     

The organization of subcortical projections of the hamster's visual cortex

The subcortical projections of the hamster's visual cortex were determined by use of injections of tritiated proline and heat lesions placed in different cortical loci. The brains were processed for autoradiography and silver impregnation of degenerating axons. Striate cortex was shown to project ipsilaterally to the dorsocaudal region of the caudate nucleus, a dorsolateral area within the thalamic reticular nucleus (RT), a laterodorsal region of the nucleus lateralis anterior (LA), the rostral half of nucleus lateralis posterior (LP), the whole territory of the dorsal (dLGN) and ventral (vLGN) geniculate nuclei, the anterior (PA) and posterior (PP) pretectal nuclei, the superior colliculus (SC), and the precerebellar pontine nuclei. In addition, the medial visual area (18b) was shown to project to a medial band of LA and part of the caudal half of LP, while the adjoining parietal cortex was seen to terminate in a lateral part of the caudate, a ventral band of LA, and the ventral half of rostral LP. Segregation of different cortical inputs was clear in LA, LP, caudate, and pons. The projections to dLGN, vLGN, SC, LP, and PA were retinotopically organized. Clear evidence of some topography was found within RT, PP, and the pons, although a consisten map could not be derived from the data.     

Morphologically distinct classes of relay cells exhibit regional preferences in the dorsal lateral geniculate nucleus of the mouse

A fundamental feature of the mammalian visual system is the presence of separate channels that work in parallel to efficiently extract and analyze specific elements of a visual scene. Despite the extensive use of the mouse as a model system, it is not clear whether such parallel organization extends beyond the retina to subcortical structures, such as the dorsal lateral geniculate (dLGN) of thalamus. To begin to address this, we examined the morphology of biocytin-filled relay cells recorded in dLGN of mice. Based on a quantitative assessment of their dendritic architecture, we found that even at early postnatal ages relay cells could be readily classified as X-like (biconical), Y-like (symmetrical), or W-like (hemispheric) and that each cell type was regionally specified in dLGN. X-like cells were confined primarily to the monocular ventral region of dLGN. Y-like cells occupied a central core that also contained ipsilateral eye projections, whereas W-like cells were found along the perimeter of dLGN. Similar to cat, Y-like cells were more prevalent than X- and W-like cells, and X-like cells tended to be smaller than other cell types. However, the dendritic fields of X- and W-like cells did not exhibit an orientation bias with respect to optic tract or boundaries of dLGN. Although we found clear morphological differences among relay cells, an analysis of their electrophysiological properties did not reveal any additional distinguishing characteristics. Overall, these data coupled with recent observations in the retina suggest that the mouse has many of the hallmark features of a system-wide parallel organization.     

Distribution of cells of origin of the corticotrigeminal projections to the nucleus caudalis of the spinal trigeminal complex in the cat. A horseradish peroxidase (HRP) study

The cortical distribution of cells of origin of the corticotrigeminal projections to the nucleus caudalis of the cat was examined using the method of retrograde axonal transport of horseradish peroxidase (HRP). After injections of HRP into the nucleus caudalis, labeled cells were distributed densely in the anterior suprasylvian gyrus, the coronal gyrus, and the ventral part of the anterior sigmoid gyrus, and moderately in the rostral part of the anterior ectosylvian gyrus on the contralateral side. In the anterior suprasylvian gyrus, the distribution extended rostrocaudally from the lateral ansate sulcal level to about 4.0 mm caudal to this level and mediolaterally throughout the convex of the anterior suprasylvian gyrus. All cortical labeled cells were pyramidal cells of various sizes in layer V.     

Neurofilament protein: a selective marker for the architectonic parcellation of the visual cortex in adult cat brain.

In this immunocytochemical study, we examined the expression profile of neurofilament protein in the cat visual system. We have used SMI-32, a monoclonal antibody that recognizes a nonphosphorylated epitope on the medium- and high-molecular-weight subunits of neurofilament proteins. This antibody labels primarily the cell body and dendrites of pyramidal neurons in cortical layers III, V, and VI. Neurofilament protein-immunoreactive neurons were prominent in 20 visual cortical areas (areas 17, 18, 19, 20a, 20b, 21a, 21b, and 7; posteromedial lateral, posterolateral lateral, anteromedial lateral, anterolateral lateral, dorsal lateral, ventral lateral, and posterior suprasylvian areas; anterior ectosylvian, the splenial, the cingulate, and insular visual areas; and the anterolateral gyrus area). In addition, we have also found strong immunopositive cells in the A laminae of the dorsal part of the lateral geniculate nucleus (dLGN) and in the medial interlaminar nucleus, but no immunoreactive cells were present in the parvocellular C (1-3) laminae of the dLGN, in the ventral part of the LGN and in the perigeniculate nucleus. This SMI-32 antibody against neurofilament protein revealed a characteristic pattern of immunostaining in each visual area. The size, shape, intensity, and density of neurofilament protein-immunoreactive neurons and their dendritic arborization differed substantially across all visual areas. Moreover, it was also obvious that several visual areas showed differences in laminar distribution and that such profiles may be used to delineate various cortical areas. Therefore, the expression of neurofilament protein can be used as a specific marker to define areal patterns and topographic boundaries in the cat visual system.     

Induction of c-fos protein by patterned visual stimulation in central visual pathways of the rat

Localized patterned visual stimulation was used in rats to investigate the feasibility of stimulus-dependent induction of the immediate early gene c-fos in neurons of cortical and subcortical visual centers of this mammal. Moving and stationary visual patterns, consisting of gratings and arrays of dark dots, induced Fos-like immunoreactivity in populations of neurons in retinotopically corresponding stimulated regions of the dorsal and ventral lateral geniculate nucleus (dLGN, vLGN), stratum griseum superficiale of the superior colliculus, nucleus of the optic tract, and primary (striate) visual cortex. Only moving stimuli induced Fos-like immunoreactive (FLI) neurons in extrastriate visual areas, particularly in the anterolateral (AL) visual area. This suggests that area AL is equivalent to the motion sensitive areas MT and PMLS of the monkey and cat. Stimulus-induced FLI neurons in the striate cortex were predominantly distributed in layers 4 and 6, while few labeled neurons were present in layers 2–3, and almost none in layer 5. The laminar distribution of stimulus-induced FLI cells in the extrastriate cortical area AL was similar to that of the striate cortex, with the exception that more FLI cells were present in layer 5. Statistical comparison of somata size of the stimulus-induced FLI neurons in dLGN with that of Cresyl violet stained neurons in the same sections revealed that the population of geniculate FLI neurons is composed of relay cells and interneurons.     

Afferent projections to the orbicularis oculi motoneuronal cell group. An autoradiographical tracing study in the cat

The motoneurons innervating the orbicularis oculi muscle from a subgroup within the facial nucleus, called the intermediate facial subnucleus. This makes it possible to study afferents to these motoneurons by means of autoradiographical tracing techniques. Many different injections were made in the brainstem and diencephalon and the afferent projections to the intermediate facial subnucleus were studied. The results indicated that these afferents were derived from the following brainstem areas: the dorsal red nucleus and the mesencephalic tegmentum dorsal to it; the olivary pretectal nucleus and/or the nucleus of the optic tract; the dorsolateral pontine tegmentum (parabrachial nuclei and nucleus of K?lliker-Fuse) and principal trigeminal nucleus; the ventrolateral pontine tegmentum at the level of the motor trigeminal nucleus; the caudal medullary medial tegmentum; the lateral tegmentum at the level of the rostral pole of the hypoglossal nucleus and the ventral part of the trigeminal nucleus and the nucleus raphe pallidus and caudal raphe magnus including the adjoining medullary tegmentum. These latter projections probably belong to a general motoneuronal control system. The mesencephalic projections are mainly contralateral, the caudal pontine and upper medullary lateral tegmental projections are mainly ipsilateral and the caudal medullary projections are bilateral. It is suggested that the different afferent pathways subserve different functions of the orbicularis oculi motoneurons. Interneurons in the dorsolateral pontine and lateral medullary tegmentum may serve as relay for cortical and limbic influences on the orbicularis oculi musculature, while interneurons in the ventrolateral pontine and caudal medullary tegmentum may take part in the neuronal organization of the blink reflex.