Cholecystokinin-like immunoreactivity occurs in ocular sensory neurons and partially co-localizes with substance P

Based on immunohistochemical analysis of the trigeminal, superior cervical, ciliary and sphenopalatine ganglia and of the eye after sensory denervation and sympathectomy, cholecystokinin (CCK)-like immunoreactive nerves in the guinea pig eye derive from the trigeminal ganglion. Substance P (SP) also occurs in some ocular sensory neurons, suggesting the possible co-localization in this system of CCK- and SP-immunoreactivities. A double-labeling immunofluorescence technique stained 3 types of trigeminal cells and ocular nerve fibers: some immunoreactive for both peptides, some immunoreactive only for CCK and some immunoreactive only for SP.

Expansion of the ipsilateral retinal projection to the medial terminal nucleus of the accessory optic system in rats with one eye removed

Expansion of ipsilateral retinal inputs into terminal regions of the accessory optic system was assessed in rats with one eye removed either at birth or as adults and in one animal with a congenital absence of one eye. The increase in the ipsilateral projection to the medial terminal nucleus in animals receiving unilateral enucleation at birth was greater than after eye removal as an adult. The greatest increase was observed in the animal with the congenital loss of one eye. No ipsilateral projection to other nuclei of the accessory optic system was observed.

The binocular organization of complex cells in the cat's visual cortex

We have studied the manner by which inputs from the two eyes are combined in complex cells of the cat's visual cortex. The stimuli are drifting sinusoidal gratings presented dichoptically at optimal spatial frequency and orientation. The relative phase between the gratings for left and right eyes is varied over 360 degrees. Approximately 40% of complex cells show phase-specific binocular interaction where response amplitudes vary depending on the relative phase of the gratings shown to the two eyes. This interaction is similar to that observed for most simple cells. We devised a test to examine whether the phase-specific interaction in complex cells results from linear convergence of neural signals at subunits of the receptive fields. The data from this test are consistent with a linear combination model. The phase-specific binocular interaction data from complex cells imply that the optimal relative phase of the receptive field subunits is closely matched. Another type of complex cell, approximately 40% of the total, could be driven through either eye, but exhibited non-phase-specific responses to dichoptically presented gratings. This type of interaction is found only in complex cells. Binocularly non-phase-specific complex cells may have subunits whose optimal relative phases are random or monocular. The division of complex cells into these two major groups (binocularly phase specific and non-phase specific) is independent of whether they are standard or special complex-cell types. A small proportion (8%) of complex cells that appear monocular by alternate tests of each eye show a purely inhibitory influence from the silent eye. This inhibition is not generally dependent on the relative phase of the gratings. Unlike simple cells, complex cells are not a homogeneous group. However, nearly half of complex cells show phase-specific binocular interaction that is probably the result of linear convergence. Combined with the results from simple cells, the majority of binocular interaction in the striate cortex may be accounted for by linear summation of neural signals from each eye. This provides a simplified view of the nature of binocular interaction in the visual cortex.

The binocular organization of simple cells in the cat's visual cortex

We have studied the manner by which inputs from the two eyes are combined in simple cells of the cat's visual cortex. The stimuli for this study are drifting sinusoidal gratings, shown dichoptically at optimal spatial frequency and orientation. The relative spatial phase (disparity) between the gratings for left and right eyes is varied over 360 degrees. Most simple cells show phase-specific binocular interaction such that response amplitudes and phases vary depending on the relative spatial phase. At one phase, response is greater than either of the monocular responses and often greater than the sum of the two. At the phase 180 degrees away from the optimal, the cell's responses are strongly inhibited and often completely suppressed. Phase-specific binocular interaction disappears when the gratings presented to one eye are made orthogonal to the optimal orientation. The degree of binocular interaction does not depend critically on the ocular dominance of the cells. Simple cells that are nearly equally dominated by each eye always exhibit strong phase-specific interaction. The majority of cells that are strongly dominated by one eye, and even those that appear monocular, show phase-dependent changes in responses. We examined the extent of binocular interaction for cells with preferred orientations near vertical compared with those tuned to other optimal orientations. If these cells are conveying information about depth, one might expect a greater degree of binocular phase-specificity for units preferring nearly vertical orientations, which would then be processing horizontal disparities. We find no evidence for this. Predictions of simple-cell responses are derived from a linear model of binocular convergence in which light-evoked neural signals from each eye are summed linearly to determine cell responses. Data from cells generally follow the prediction of the model for both response amplitude and phase. Deviations from predictions of the linear model are found for a minority of cells. This deviation may be accounted for by a threshold mechanism that comes into play after the linear binocular summation. A small proportion of simple cells that appear monocular by alternate tests of each eye show a purely inhibitory influence from the silent eye. This inhibition is not generally dependent on the relative phase of the gratings. We conclude that most binocular interaction in striate simple cells may be accounted for by linear summation of neural signals from each eye.(ABSTRACT TRUNCATED AT 400 WORDS)

Long-term retention of Fast Blue in sympathetic neurones after axotomy and regeneration--demonstration of incorrect reconnections

Using the fluorescent dyes Fast Blue and Diamidino Yellow to trace neuronal connections, the regeneration of the superior cervical ganglionic neurones after axotomy has been examined. Fast Blue has the property of remaining within the neuronal perikaryon for many months after its retrograde axonal transport, even after transection of the axon. Thus, Fast Blue can be used to label neurones as to their original target and Diamidino Yellow subsequently used to demonstrate the specificity of reconnection. The results suggest that apparent return of appropriate function occurs in the presence of a large component of inappropriate reconnections.

Vasoactive intestinal polypeptide and the ocular innervation

The indirect immunofluorescence technique with antisera to vasoactive intestinal polypeptide (VIP) stains peripheral nerve fibers in both the anterior segment and the posterior segment of rat, guinea pig, cat, and rhesus monkey eyes. While immunoreactive corneal nerves are lacking, all four species have a prominent innervation of the superficial limbal blood vessels. The aqueous humor outflow apparatus of the rat, guinea pig, and cat, but not the monkey, contain VIP-like immunoreactive nerves. All four animals have immunoreactive iris nerve fibers, tending either to surround large blood vessels or to lie as free stromal nerves. Only in the cat are immunoreactive nerve fibers seen within the iris muscles. A modest number of VIP-like immunoreactive nerves are present in the ciliary body of all four animals; immunoreactive nerve fibers within the ciliary processes occur only in the rat and guinea pig. VIP-like immunoreactive nerves are found in the choroid of all four animals. An association of immunoreactive nerve fibers to uveal melanocytes also is apparent. The present findings expand several prior immunohistochemical studies of mammalian eyes in which the VIP-like immunoreactive nerves to the choroid was emphasized.

Veratridine-stimulated release of amine conjugates from centrifugal fibers in the Limulus peripheral visual system

Centrifugal fibers that originate in the brain and project to the Limulus peripheral visual system synthesize and store octopamine and conjugates of octopamine and tyramine. In a previous study we showed that depolarization, induced by elevating extracellular K+, stimulated a preferential release of octopamine from these fibers. Here we show that veratridine-induced depolarization stimulates a rapid, transient release of octopamine and a delayed, sustained release of amine conjugates. Veratridine-stimulated release of both octopamine and amine conjugates depends on the influx of extracellular Ca2+ and is blocked by tetrodotoxin or the absence of extracellular Na+. The depolarization-stimulated release of amine conjugates raises the possibility that these molecules serve as intercellular messengers in the Limulus peripheral visual system.

Fodrin degradation by calcium-activated neutral proteinase (CANP) in retinal ganglion cell neurons and optic glia: preferential localization of CANP activities in neurons

The activity of calcium-activated neutral proteinases (CANPs) toward endogenous substrates was measured in axons of retinal ganglion cell (RGC) neurons and separately in adjacent optic glia under in vitro conditions that preserved the ultrastructure and anatomic relationships between these cellular elements. RGC neurons and optic glia both expressed CANP activity. In contrast to RGC axons, which contained at least two CANP activities with calcium requirements in the millimolar (CANP A) and micromolar (CANP B) range (Nixon et al., 1985), CANP activity in optic glia was detectable only at millimolar calcium concentrations. When maximally activated, CANP(s) in optic glia exhibited a broad specificity for endogenous proteins but degraded larger proteins at a faster rate. The cytoskeletal protein fodrin (brain spectrin) was among the most susceptible endogenous substrates in RGC axons or glia. The similar properties of fodrin in neurons and glia, including its susceptibility to a purified millimolar calcium-sensitive brain CANP (mCANP), provided the basis for using this protein as a substrate to compare the relative activity of neuronal and glial CANPs in situ. Fodrin degradation mediated by CANPs proceeded at least 6 X more rapidly in intact RGC axons than in optic glia. Comparable differences in the relative degradation rates of the total neuronal and glial protein pools were also observed. These results indicate that the potential activity of CANPs is substantially greater in RGC neurons than in glia. The enormous potential activity and preferential localization of multiple CANP activities in RGC neurons support previously hypothesized roles for CANPs in the processing of axonally transported proteins and in the regulation of neuronal cytoskeletal dynamics and geometry.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuropeptide Y and the innervation of the human eye

Using the indirect immunohistochemical technique, neuropeptide Y as found in human ocular nerves. While limbal blood vessels were innervated and a few nerves were seen in the trabecular meshwork, the cornea proper contained no immunoreactive nerve fibers. Neuropeptide Y-like immunoreactive nerves supplied the iris dilator muscle and, to a lesser extent, the iris sphincter. Immunoreactive nerve fibers were present within the ciliary muscle and the ciliary processes. Neuropeptide Y-like immunoreactive choroidal nerve fibers were frequently associated with blood vessels. The apposition of immunoreactive nerve fibers and melanocytes occurred throughout the uvea. The distribution of neuropeptide Y-like immunoreactive nerves in the eye closely parallels the adrenergic innervation. Evidence linking neuropeptide Y to the ocular sympathetic innervation is discussed.

Neuropeptide Y and the ocular innervation of rat, guinea pig, cat and monkey

By the immunohistofluorescence technique, peripheral nerves of the rat, guinea pig, cat and monkey eye contain a neuropeptide Y-like immunoreactive peptide. A broad distribution of immunoreactive nerve fibers is present in all four animals, innervating tissues of the aqueous humor outflow apparatus, the limbal blood vessels, and uveal blood vessels. A dense plexus of neuropeptide Y-like immunoreactive nerve fibers is present to the ciliary processes. A rich innervation exists to the iris dilator muscle, but that to the iris sphincter is modest. Throughout all regions of the uvea, neuropeptide Y-like immunoreactive nerves are associated closely with melanocytes. When acid extracts of anterior uvea and choroid from rat and guinea pig are analyzed by radioimmunoassay and reverse phase high performance liquid chromatography, the immunoreactive ocular peptide occurs in a single molecular form indistinguishable from porcine neuropeptide Y. The present findings indicate that neuropeptide Y is present in ocular nerves of rat, guinea pig, cat, and monkey. Their distribution, with a few small exceptions, closely parallels that of ocular adrenergic nerves as revealed by histofluorometric techniques. While no ocular effects of neuropeptide Y have been reported to date, its other known biological effects imply potential functions in the eye.

Flying primates? Megabats have the advanced pathway from eye to midbrain

The pattern of connections between the retina and midbrain has been determined with electrophysiological and neuroanatomical methods in bats representing the two major subdivisions of the Chiroptera. Megachiropteran fruit bats (megabats), Pteropus spp., were found to have an advanced retinotectal pathway with a vertical hemidecussation of the kind previously found only in primates. In contrast, the microchiropteran bat Macroderma gigas has the "ancestral" or symplesiomorphous pattern of retinotectal connections so far found in all vertebrates except primates. In addition to linking primates and megachiropteran bats, these findings suggest that flight may have evolved twice among the mammals.

Studies on the circadian rhythm of IOP in rabbits: correlation with aqueous inflow and cAMP content

We investigated the relationship of aqueous humor inflow rate and cyclic AMP concentrations to the spontaneous and dramatic changes in IOP associated with onset of darkness in our previously described model of circadian rhythm of intraocular pressure. After onset of darkness, rabbits entrained in an environment with a daily alternating cycle of 12 hours light and 12 hours darkness (12L:12D) showed an 85% increase in outflow pressure, a nearly 60% increase in aqueous inflow rate and an 80% increase in aqueous cAMP. Animals desynchronized by an unpredictable light cycle showed no increase in IOP or inflow rate when measured at the same time intervals as were the entrained animals. Thus, the IOP, aqueous inflow rate and aqueous cAMP are all seen to change in the same direction in a pharmacologically unperturbed rabbit eye. Previous pharmacological studies in rabbits have correlated an increase in cAMP with a decrease in IOP and aqueous inflow.

Vasoactive intestinal polypeptide-like immunoreactive nerves to the human eye

Using immunohistochemical techniques, vasoactive intestinal polypeptide (VIP) is visualized in nerves distributed to the human eye. Immunoreactive nerve fibers occur about limbal blood vessels and within the trabecular meshwork. In the iris, free-running stromal nerves are the most common, but nerves to both dilator and sphincter muscles are present as well. Immunoreactive nerves are seen within the ciliary muscle and occasionally within a ciliary process. Innervation to choroidal blood vessels constitutes a prominent feature; innervation to more anterior uveal blood vessels is seen only irregularly. Immunoreactive to more anterior uveal blood vessels is seen only irregularly. Immunoreactive nerves are apposed to melanocytes throughout the uvea. The present findings extend prior reports in the human eye, indicating a potential role for VIP in ocular physiology. Additional neuroanatomical, biochemical and physiological studies are necessary to define fully the ocular function of VIP and to determine ultimely whether VIP has clinical and pharmacological implications.

The location of descending fibres to sympathetic neurons supplying the eye and sudomotor neurons supplying the head and neck

Evidence is given of the location in man of the fibres going to the sympathetic neurons of the lateral horn that supply the intrinsic and extrinsic muscles of the eye and the sweat glands of the head and neck. For the region of the pons and medulla, the evidence is abstracted from the literature. For the cervical spinal cord, the evidence is from our cases of anterolateral cordotomy. In the medulla, thrombosis of the artery of the fossette latérale destroys the fibres; this locates the fibres in the posterolateral retro-olivary area. But not all fibres to the sudomotor neurons lie there: some run elsewhere, though they probably remain ipsilateral. In the cervical cord, the fibres supplying the sympathetic neurons of the intrinsic and extrinsic muscles of the eye run near the posterior angle of the anterior horn. Most of the fibres supplying the sudomotor neurons lie in the same region, though some lie outside this area but on the same side of the cord.

Eye position signals in cat superior colliculus

Single unit activity was studied in the intermediate and deep layers of the superior colliculus in two trained cats. Eye movements were recorded with a magnetic search coil, the head being fixed. Discharge rates which varied as a function of eye position were consistently observed in 7 of 67 (about 10%) of the sample of eye movement-related units. These units showed similar changes in firing rate as a function of eye position in total darkness and during task related fixation of visual targets and thus appear to convey an "eye position" signal. Their activity may originate either from proprioception or from corollary discharge.

Response fields of the periodontal mechanosensitive units in the superior alveolar nerve of the cat

Response properties of periodontal mechanoreceptor primary afferent fibers recorded from the superior alveolar nerve were studied in the cat. The left maxillary canine tooth was stimulated manually in 8 directions and/or in 24 directions in the horizontal plane by a specially designed stimulator. The responses of 328 slowly adapting units observed were affected by the direction of stimulus. These units were classified into three groups according to the shape of the response field: a broad type (more than 180 degrees), a medium type (90 degrees to 180 degrees), and a narrow type (less than 90 degrees). The groups contained 27 units (8.2%), 284 units (86.6%), and 17 units (5.2%), respectively, and the remaining 10 units (10.0%) were unclassified. The shape of each response field was little changed by changes in the stimulus intensity. Every response field investigated showed a unimodal distribution. These results were different from those of Mei et al. (1975) who reported that the response fields of units recorded from a Gasserian ganglion had generally consisted of two parts.

Facial nerve distribution to the eye

Anatomical and physiological investigations indicate that facial parasympathetic (FP) innervation to blood vessels in the eye can dilate uveal vessels and raise the intraocular pressure. There is evidence that both acetylcholine and a vasoactive intestinal polypeptide (VIP) act as neurotransmitters in this system.

Visuotopic information conveyed by each eye to the opossum's superior colliculus

The uniocular visual field representations on the superior colliculus (SC), as estimated from multiunit response field centres about the horizontal meridian, were compared in midpontine pretrigeminal opossums (Didelphis marsupialis aurita Wied 1826). Recordings from the rostral pole (RP) and its caudal neighbour, the direct binocular region (DBR), as defined by Rocha-Miranda et al. (1978), were distinguished by the histological control. The results showed that while the hemifield contralateral to the recording site was well represented on the DBR by both eyes, the ipsilateral hemifield was generously represented at the RP only by the contralateral eye. At the RP the ipsilateral eye usually conveyed information about the vertical meridian, bringing about an expanded representation of the central visual space. Distinct patterns of representation were also recognized on graphs which relate recording sites along the AP axis of the SC with the azimuths of response field centres. The representation of the vertical reference meridian upon this axis on an oculocentric system was estimated from the DBR data and localized in the RP, at about 500 microns from the rostral end, for the ipsilateral eye (Vo') and in the DBR, at about 800 microns for the other eye (Vo). Similarly, plots of the magnification factor against the AP collicular axis indicated different strategies of representation for each eye. At the segment between 500 and 800 microns on this axis the magnification factors of the ipsilateral eye were usually much higher than those of the other eye. Furthermore, horizontal disparities between field centres were shown to have distinct distributions along the AP axis within the RP and DBR regions.(ABSTRACT TRUNCATED AT 250 WORDS)

Peripheral and central oculomotor organization in the goldfish, Carassius auratus

Peripheral and central oculomotor organization was studied in the goldfish. The sizes of the extraocular muscles were quantified by counting the fibers contained in a given muscle and by area measurements of the cross-sectional surfaces. All the muscles were of approximately similar size. Kinematics were determined by electrical stimulation of a given muscle. The macroscopic appearance and kinematics of the muscles had the characteristics of other lateral-eyed animals (e.g., rabbit). Locations of extraocular motor neurons were found by retrograde transport of horseradish peroxidase (HRP) following injections into individual extraocular muscles. The eye muscles were innervated by four ipsilateral (lateral rectus, medial rectus, inferior oblique, inferior rectus) and two contralateral (superior rectus, superior oblique) motor neuron pools. The oculomotor nucleus was found in the midbrain, at the level of the caudal zone of the inferior lobe of the hypothalamus. Inferior rectus motor neurons were located rostrally in the oculomotor nucleus, whereas medial rectus, superior rectus, and inferior oblique motor neurons were intermingled in its more caudal portions. All labelled cells were located dorsally and medially to the medial longitudinal fasciculus (MLF) in close proximity to either the floor of the ventricle or the midline region. Occasionally, motor neurons were interspersed within the fiber bundles of the MLF or the exiting fibers of the oculomotor nerve. The trochlear nucleus, containing superior oblique motor neurons, was found in the immediate lateral and caudal neighborhood of the oculomotor nucleus, where its rostral border overlapped with the caudal border of the latter. The abducens nucleus, containing lateral rectus motor neurons, was located in the posterior brainstem in the neighborhood of the vestibular nuclear complex. This nucleus was divided into a rostral and a caudal portion. The axons of ipsilaterally projecting motor neurons headed toward their respective nerve roots via the shortest possible route, as did the axons of superior rectus motor neurons, which crossed the midline without detour to enter the contralateral oculomotor nerve. In contrast, trochlear motor neuron axons arched around the dorsal aspect of the ventricle through the cerebellar commissure to reach the contralateral trochlear nerve. The morphology of individual motor neurons was visualized by intrasomatic injection of HRP. Cell somata had oblong shapes, and their large dendrites were oriented laterally and ventrally. The axons did not collateralize within the midbrain region or the oculomotor nerve as far as they could be traced.(ABSTRACT TRUNCATED AT 400 WORDS)

Spread of herpes simplex virus and distribution of latent infection after intraocular infection of the mouse

Intraocular inoculation of HSV 1 in the mouse results not only in uveitis, but also in the spread of virus via sensory, sympathetic and optic nerves. During the acute infection with HSV 1 strain SC 16 in both outbred and NIH (inbred) mice, virus reached the ipsilateral trigeminal ganglion, superior cervical ganglion, both sides of the brain stem and the contralateral (uninoculated) eye. With HSV 1 strain KOS in outbred mice the same tissues became infected but virus was also isolated from the ophthalmic part of the contralateral trigeminal ganglion. After resolution of the acute disease in outbred mice, latent infection with strain KOS was demonstrated in both trigeminal ganglia and in the ipsilateral superior cervical ganglion. With strain KOS, virus was sometimes isolated from eyes removed more than a month after inoculation and then cultured in vitro for 2-3 weeks. By electron microscopy infected cells were seen in the choroid and sclera of such eyes.

Formation of neuronal pathways in the imaginal discs of Drosophila melanogaster

We have followed the formation of neuronal pathways in different imaginal discs of Drosophila. The pattern is highly reproducible for a given disc type but distinct for each type of discs: in leg discs, several neurons are present before metamorphosis and provide two major pathways that are joined by later neurons; in the wing and haltere discs, a few pairs of neurons appear after the onset of metamorphosis and pioneer the major pathways; in antenna discs, no pioneers are detected before massive neuronal differentiation begins. The mechanisms used for axonal guidance seem common to all discs, and the differences between discs can be accounted for simply by differences in the arrangement and birth time of pioneer neurons. Different subsets of pioneer neurons are deleted by mutations such as scute and engrailed.

Anatomical circuitry of lateral inhibition in the eye of the horseshoe crab, Limulus polyphemus

Lengthy uninterrupted series of sections of the neural plexus in the compound eye of the horseshoe crab, Limulus polyphemus, have been used to reconstruct all the arborizations and their synaptic interconnections in a neuropil knot. This one microglomerulus contains the axons of 19 retinular cells, which pass by without contacts; 13 efferent fibres with 44 synapses to and from eccentric cell collaterals; and arborizations from 54 eccentric cells with 577 synapses. Eccentric cell axons are devoid of synaptic input. Their collaterals ramify in synaptic knots and subserve both pre- and postsynaptic functions simultaneously. Arborizations near the axon of origin have a highly branched pattern (up to 20 bifurcations), a high synaptic input: output ratio (up to about 9:1), and high synaptic density (a maximum of 12 per micrometre of neurite length). The opposite extreme is represented by sparsely branched eccentric cell collaterals distant from their axons of origin with very little synaptic input and sparse output. Spatially graded lateral inhibition is the apparent outcome of a radially decreasing distribution of inhibitory synapses on the arborizations of eccentric cell collaterals combined with possible decremental signal transmission in the plexus. The synaptic analysis has a bearing on most physiological aspects of lateral inhibition that have been studied in the Limulus eye. Implied in the results is the suggestion that synapse formation is an intrinsic property of the presynaptic element, but that the connectivity is governed by the electrical activity of target neurons.

Substance P-like immunoreactive nerves in the human eye

Using immunohistochemical methods, substance P is localized to nerves of the human eye. Immunoreactive nerve fibers occur in the cornea, about limbal blood vessels, and within the trabecular meshwork. Substance P-like immunoreactive nerve fibers surround uveal blood vessels, especially in the choroid and ciliary body. Immunoreactive nerves are seen in ciliary processes. The ciliary muscle is innervated, as are the iris dilator and sphincter muscles. Apposition of immunoreactive nerves to uveal melanocytes is apparent. The distribution of substance P-like immunoreactive nerves in the human eye parallels that found in other mammals. While substance P probably has important neurotransmitter or neuromodulator roles in the eye, further physiologic studies are required to define its ocular function.

Interneuronal and glial-neuronal gap junctions in the lamina ganglionaris of the compound eye of the housefly, Musca domestica

The cell-body layer of the lamina ganglionaris of the housefly, Musca domestica, contains the perikarya of five types of monopolar interneuron (L1-L5) along with their enveloping neuroglia (Strausfeld 1971). We confirm previous reports (Trujillo-Cenóz 1965; Boschek 1971) that monopolar cell bodies in the lamina form three structural classes: Class I, Class II, and midget monopolar cells. Class-I cells (L1 and L2) have large (8-15 microns) often crescent-shaped cell bodies, much perinuclear cytoplasm and deep glial invaginations. Class-II cells (L3 and L4) have smaller perikarya (4-8 microns) with little perinuclear cytoplasm and no glial invaginations. The 'midget' monopolar cell (L5) resides at the base of the cell-body layer and has a cub-shaped cell body. Though embedded within a reticulum of satellite glia, the L1-L4 monopolar perikarya and their immediately proximal neurites frequently oppose each other directly. Typical arthropod (beta-type) gap junctions are routinely observed at these interfaces. These junctions can span up to 0.8 micron with an intercellular space of 2-4 nm. The surrounding nonspecialized interspace is 12-20 nm. Freeze-fracture replicas of monopolar appositions confirm the presence of beta-type gap junctions, i.e., circular plaques (0.15-0.7 micron diam.) of large (10-15 nm) E-face particles. Gap junctions are present between Class I somata and their proximal neurites, between Class I and Class II somata and proximal neurites, and between Class II somata. Intercartridge coupling may exist between such monopolar somata. The cell body and proximal neurite of L5 were not examined. We also find that Class I and Class II somata are extensively linked to their satellite glia via gap junctions. The gap width and nonjunctional interspace between neuron and glia are the same as those found between neurons. The particular arrangement and morphology of lamina monopolar neurons suggest that coupling or low resistance pathways between functionally distinct neurons and between neuron and glia are probably related to the metabolic requirements of the "nuclear" layer and may play a role in wide field signal averaging and light adaptation.