Electrophysiology of the Insect Dorsal Ocellus : I. Origin of the components of the electroretinogram

Dorsal ocelli are small cup-like organs containing a layer of photoreceptor cells, the short axons of which synapse at the base of the cup with dendritic terminals of ocellar nerve fibers. The ocellar ERG of dragonflies, recorded from the surface of the receptor cell layer and from the long lateral ocellar nerve, contains four components. Component 1 is a depolarizing sensory generator potential which originates in the distal ends of the receptor cells and evokes component 2. Component 2 is believed to be a depolarizing response of the receptor axons. It evokes a hyperpolarizing postsynaptic potential, component 3, which originates in the dendritic terminals of the ocellar nerve fibers. Ocellar nerve fibers in dragonflies are spontaneously active, discharging afferent nerve impulses (component 4) in the dark-adapted state. Component 3 inhibits this discharge. The ERG of the cockroach ocellus is similar. The main differences are that component 3 is not as conspicuous as in the dragonflies and that in most cases ocellar nerve impulses appear only as a brief burst at "off." In one preparation a spontaneous discharge of nerve impulses was observed. As in the dragonflies, this was inhibited by illumination.     

Electrophysiology of the Insect Dorsal Ocellus : II. Mechanisms of generation and inhibition of impulses in the ocellar nerve of dragonflies

Large nerve fibers in the ocellar nerves of dragonflies are spontaneously active. In the absence of inhibitory influence the spontaneous activity is rhythmic. Inhibition occurs in the dark-adapted state and during illumination. Miniature inhibitory postsynaptic potentials occur in the dark-adapted state. These modulate by temporary suppression the otherwise rhythmic discharge of ocellar nerve impulses. The presence of random spontaneous receptor cell excitations is inferred from the presence of the miniature i.p.s.p.'s. Light stimulates many or all the receptor cells simultaneously, masking the random spontaneous activity of individual receptor cells. The result is a sustained hyperpolarizing i.p.s.p. and sustained inhibition of the nerve discharge. Preceding resumption of the spontaneous activity at "off" the i.p.s.p. may oscillate, overshoot the baseline as a negative after-potential, or do both. These phases of the off-effect may generate nerve impulses in an off-burst.     

Neural Organization of the Median Ocellus of the Dragonfly : I. Intracellular electrical activity

Intracellular responses from receptors and postsynaptic units have been recorded in the median ocellus of the dragonfly. The receptors respond to light with a graded, depolarizing potential and a single, tetrodotoxin-sensitive impulse at "on." The postsynaptic units (ocellar nerve dendrites) hyperpolarize during illumination and show a transient, depolarizing response at "off." The light-evoked slow potential responses of the postsynaptic units are not altered by the application of tetrodotoxin to the ocellus. It appears, therefore, that the graded receptor potential, which survives the application of tetrodotoxin, is responsible for mediating synaptic transmission in the ocellus. Comparison of pre- and postsynaptic slow potential activity shows (a) longer latencies in postsynaptic units by 5–20 msec, (b) enhanced photosensitivity in postsynaptic units by 1–2 log units, and (c) more transient responses in postsynaptic units. It is suggested that enhanced photosensitivity of postsynaptic activity is a result of summation of many receptors onto the postsynaptic elements, and that transients in the postsynaptic responses are related to the complex synaptic arrangements in the ocellar plexus to be described in the following paper.     

Componental analysis of the ocellar electroretinogram of the locust, Schistocerca gregaria

Ruck's componental analysis of the ocellar electroretinogram (ERG) has been reappraised using techniques of signal averaging and waveform subtraction. Components (1), (3), and (4) can readily be isolated in the locust ocellus but component (2) as recorded in the locust ocellus is probably an artefact. Component (1), produced by the receptor cells, only contributes significantly to the total ERG at higher light intensities and it is this contribution which changes most with the degree of light and dark adaptation employed in these experiments. Component (3), the response of the second-order neurones, indicates that the majority of second-order neurones hyperpolarize on illumination of the ocellus. Component (4), the afferent activity of the second-order cells, indicates that more than one afferent axon is involved in the production of off spikes in the locust ocellus.     

Studies on the Distribution of Factor I and Acetylcholine in Crustacean Peripheral Nerve

Extracts of whole nerve (chelipeds of Cancer magister) cause inhibition of impulse generation of the crayfish stretch receptor preparation, similar to that produced by gamma-aminobutyric acid (GABA). This is not found with extracts containing only sensory or sensory and motor fibers. Extracts of inhibitory fibers inhibit the stretch receptor discharge—indicating an inhibitory action equivalent to that of up to 30,000 micrograms of GABA per gm. wet weight of inhibitor fiber. This high value is taken as an indication that the inhibitory substance in crab inhibitory fibers is not identical with gamma-aminobutyric acid. Whole nerves were found to contain 1.7 to 6.7 µg. acetylcholine per gm. nerve tissue (clam ventricle and frog rectus abdominis muscle). No acetylcholine could be detected in extracts of motor and inhibitory fibers. The acetylcholine content of sensory fibers can account for the acetylcholine activity of whole nerve extract. It is concluded that the factor I of crustacean nerve is an exclusive property of the inhibitory fibers. The results support the assumption that factor I is the transmitter substance of inhibitory neurons in these animals. The absence of acetylcholine in motor fibers indicates that this substance does not function as a transmitter of motor impulses in Crustacea, and explains the previously observed failure of the substance to elicit motor responses in these animals. The function of acetylcholine in sensory fibers is not yet clarified.     

Neural Organization of the Median Ocellus of the Dragonfly : II. Synaptic structure

Two types of presumed synaptic contacts have been recognized by electron microscopy in the synaptic plexus of the median ocellus of the dragonfly. The first type is characterized by an electron-opaque, button-like organelle in the presynaptic cytoplasm, surrounded by a cluster of synaptic vesicles. Two postsynaptic elements are associated with these junctions, which we have termed button synapses. The second synaptic type is characterized by a dense cluster of synaptic vesicles adjacent to the presumed presynaptic membrane. One postsynaptic element is observed at these junctions. The overwhelming majority of synapses seen in the plexus are button synapses. They are found most commonly in the receptor cell axons where they synaptically contact ocellar nerve dendrites and adjacent receptor cell axons. Button synapses are also seen in the ocellar nerve dendrites where they appear to make synapses back onto receptor axon terminals as well as onto adjacent ocellar nerve dendrites. Reciprocal and serial synaptic arrangements between receptor cell axon terminals, and between receptor cell axon terminals and ocellar nerve dendrites are occasionally seen. It is suggested that the lateral and feedback synapses in the median ocellus of the dragonfly play a role in enhancing transients in the postsynaptic responses.     

Electroretinogram components of the ocellus of the adult cabbage looper moth Trichoplusia ni

The electroretinogram (ERG) of the adult cabbage looper (Trichoplusia ni) ocellus has been studied by extracellular recording methods. Using white light stimulation, the ERG was found to have four components, two of which differ from those of ocelli previously studied. Here component 3 is an excitatory post-synaptic potential (EPSP) and component 4 is an excitatory spike discharge from the ocellar second-order neurons. The excitatory nature of these components has been verified by two experiments. In a light adaptation experiment decreased stimulus intervals caused a reduction in the number of excitatory spikes. In an experiment with the anticholinesterase tetraethylpyrophosphate (TEPP), treatment of the preparation abolished the excitatory spike discharge and reduced the magnitude of the EPSP.     

Spectral sensitivity studies on the visual system of the praying mantis, Tenodera sinensis

In these studies a constant ERG response was used as a measure of visual sensitivity to different wavelengths of light. The dark-adapted compound eye of Tenodera sinensis is dominated by a single class of photoreceptors. with a major peak of sensitivity at about 510–520 nm, and with a minor peak of sensitivity in the near-ultraviolet region at about 370 nm. The dark-adapted dorsal ocellus does not contain a homogeneous population of sensory receptors. The sensitivity function of the dark-adapted ocellus to longer wavelength light (yellow and red) is determined by a single receptor with a major peak of sensitivity in the green at 510–520 nm with some sensitivity in the near-ultraviolet. Sensitivity at shorter wavelengths (near-ultraviolet and blue), however, involves the stimulation of both this and a near-ultraviolet-sensitive receptor with a maximum sensitivity at about 370 nm. Anatomically, the sensory cells of the dorsal ocellus of Tenodera were determined histologically to be grouped into two distinct regions, each group making its own separate contribution to the ocellar nerve. This may represent the separation of two different photoreceptor types in the ocellus of the mantis.     

Synaptic inhibition in an isolated nerve cell

Following the preceding studies on the mechanisms of excitation in stretch receptor cells of crayfish, this investigation analyzes inhibitory activity in the synapses formed by two neurons. The cell body of the receptor neuron is located in the periphery and sends dendrites into a fine muscle strand. The dendrites receive innervation through an accessory nerve fiber which has now been established to be inhibitory. There exists a direct peripheral inhibitory control mechanism which can modulate the activity of the stretch receptor. The receptor cell which can be studied in isolation was stimulated by stretch deformation of its dendrites or by antidromic excitation and the effect of inhibitory impulses on its activity was analyzed. Recording was done mainly with intracellular leads inserted into the cell body. 1. Stimulation of the relatively slowly conducting inhibitory nerve fiber either decreases the afferent discharge rate or stops impulses altogether in stretched receptor cells. The inhibitory action is confined to the dendrites and acts on the generator mechanism which is set up by stretch deformation. By restricting depolarization of the dendrites above a certain level, inhibition prevents the generator potential from attaining the "firing level" of the cell. 2. The same inhibitory impulse may set up a postsynaptic polarization or a depolarization, depending on the resting potential level of the cell. The membrane potential at which the inhibitory synaptic potential reverses its polarity, the equilibrium level, may vary in different preparations. The inhibitory potentials increase as the resting potential is displaced in any direction from the inhibitory equilibrium. 3. The inhibitory potentials usually rise to a peak in about 2 msec. and decay in about 30 msec. After repetitive inhibitory stimulation a delayed secondary polarization phase has frequently been seen, prolonging the inhibitory action. Repetitive inhibitory excitation may also be followed by a period of facilitation. Some examples of "direct" excitation by the depolarizing action of inhibitory impulses are described. 4. The interaction between antidromic and inhibitory impulses was studied. The results support previous conclusions (a) that during stretch the dendrites provide a persisting "drive" for the more central portions of the receptor cell, and (b) that antidromic all-or-none impulses do not penetrate into the distal portions of stretch-depolarized dendrites. The "after-potentials" of antidromic impulses are modified by inhibition. 5. Evidence is presented that inhibitory synaptic activity increases the conductance of the dendrites. This effect may occur in the absence of inhibitory potential changes.     

Fine structure of the dorsal ocellus of the worker honeybee

The three dorsal ocelli of worker honeybees have been studied by light and electron microscopy. Each ocellus has a single flattened spheroidal lens and about 800 elongated retinular cells. Retinular cells are paired and form a two-part plate-like rhabdom between their distal processes. Each rhabdomere comprises parallel microvilli projecting laterally from the apposed retinular cells. Primary receptor cell axons synapse within the ocellus with ocellar nerve fibers of two different calibers. Each ocellus has eight thick fibers ca 10 m?m in diameter and several thinner ones less than 3 m?m in diameter. Fine structural evidence suggests that retinular axons end presynaptically on both types of ocellar nerve fibers. Since all retinular cells apparently synapse repeatedly with the thick fibers this involves a convergence of about 100:1. Thick fibers always terminate postsynaptically within the ocellus while thin fibers terminate presynaptically on other thin fibers, thick fibers or retinular axons. Structural evidence for synaptic polarization indicates that retinular cells and thick fibers are afferent, thin fibers efferent. Thus complex processing of the ocellar visual input can occur before the secondary neurons of the three ocelli converge to form the single short ocellar nerve which runs to the posterior forebrain.     

STRUCTURE OF THE MACULA UTRICULI WITH SPECIAL REFERENCE TO DIRECTIONAL INTERPLAY OF SENSORY RESPONSES AS REVEALED BY MORPHOLOGICAL POLARIZATION

The anatomy of the labyrinth and the structure of the macula utriculi of the teleost fish (burbot) Lota vulgaris was studied by dissection, phase contrast, and electron microscopy. The innervating nerve fibers end at the bottom of the sensory cells where two types of nerve endings are found, granulated and non-granulated. The ultrastructure and organization of the sensory hair bundles are described, and the finding that the receptor cells are morphologically polarized by the presence of an asymmetrically located kinocilium in the sensory hair bundle is discussed in terms of directional sensitivity. The pattern of orientation of the hair cells in the macula utriculi was determined, revealing a complicated morphological polarization of the sensory epithelium. The findings suggest that the interplay of sensory responses is intimately related to the directional sensitivity of the receptor cells as revealed by their morphological polarization. The problem of efferent innervation is discussed, and it is concluded that the positional information signaled by the nerve fibers innervating the vestibular organs comprises an intricate pattern of interacting afferent and efferent impulses     

STUDIES OF EXCITABLE MEMBRANES : I. Macromolecular Specializations of the Neuromuscular Junction and the Nonjunctional Sarcolemma

The neuromuscular junctions and nonjunctional sarcolemmas of mammalian skeletal muscle fibers were studied by conventional thin-section electron microscopy and freeze-fracture techniques. A modified acetylcholinesterase staining procedure that is compatible with light microscopy, conventional thin-section electron microscopy, and freeze-fracture techniques is described. Freeze-fracture replicas were utilized to visualize the internal macromolecular architecture of the nerve terminal membrane, the chemically excitable neuromuscular junction postsynaptic folds, and the electrically excitable nonjunctional sarcolemma. The nerve terminal membrane is characterized by two parallel rows of 100–110-Å particles which may be associated with synpatic vesicle fusion and release. On the postsynpatic folds, irregular rows of densely packed 110–140-Å particles were observed and evidence is assembled which indicates that these large transmembrane macromolecules may represent the morphological correlate for functional acetylcholine receptor activity in mammalian motor endplates. Differences in the size and distribution of particles in mammalian as compared with amphibian and fish postsynaptic junctional membranes are correlated with current biochemical and electron micrograph autoradiographic data. Orthogonal arrays of 60-Å particles were observed in the split postsynaptic sarcolemmas of many diaphragm myofibers. On the basis of differences in the number and distribution of these "square" arrays within the sarcolemmas, two classes of fibers were identified in the diaphragm. Subsequent confirmation of the fiber types as fast- and slow-twitch fibers (Ellisman et al. 1974. J. Cell Biol. 63[2, Pt. 2]:93 a. [Abstr.]) may indicate a possible role for the square arrays in the electrogenic mechanism. Experiments in progress involving specific labeling techniques are expected to permit positive identification of many of these intriguing transmembrane macromolecules.     

Neuronal pathways from the dorsal acelli of the house cricket,Acheta domesticus

Each ocellar nerve in the house cricket Acheta domesticus contains giant nerve fibers of 10-15 μ diameter, characterized in Golgi Cox preparations by a single row of short collaterals which runs along nearly the entire length of a fiber. Numerous long collaterals are given off by thin fibers in the ocellar nerve; medium-size fibers give off relatively few collaterals. The lateral ocellar tracts extend posteriorly through the dorsal protocerebrum, crossing the protocerebral bridge dorsally. The smaller median ocellar tract runs more ventrally through the pars intercerebralis; posterior to the bridge its fibers turn out toward the lateral nerves. Golgi and cobalt preparations reveal branching of giant and mediu_-size ocellar fibers posterior to the bridge at two levels, forming bilateral regions of ocellar neuropile. No ocellar processes appear to be given off to the corpora pedunculata, centra! body, nervi corporis cardiaci, antenna! lobes, or circumesophageal connectives; it is uncertain whether ocellar collaterals extend into the protocerebral bridge or optic lobes. Cell bodies of giant and medium-sized fibers are located in the pars intercerebralis.     

Processes of excitation in the dendrites and in the soma of single isolated sensory nerve cells of the lobster and crayfish

The stretch receptor organs of Alexandrowicz in lobster and crayfish possess sensory neurons which have their cell bodies in the periphery. The cell bodies send dendrites into a fine nearby muscle strand and at the opposite pole they give rise to an axon running to the central nervous system. Mechanisms of excitation between dendrites, cell soma, and axon have been studied in completely isolated receptor structures with the cell components under visual observation. Two sensory neuron types were investigated, those which adapt rapidly to stretch, the fast cells, and those which adapt slowly, the slow cells. 1. Potentials recorded from the cell body of the neurons with intracellular leads gave resting potentials of 70 to 80 mv. and action potentials which in fresh preparations exceeded the resting potentials by about 10 to 20 mv. In some experiments chymotrypsin or trypsin was used to make cell impalement easier. They did not appreciably alter resting or action potentials. 2. It has been shown that normally excitation starts in the distal portion of dendrites which are depolarized by stretch deformation. The changed potential within the dendritic terminals can persist for the duration of stretch and is called the generator potential. Secondarily, by electrotonic spread, the generator potential reduces the resting potential of the nearby cell soma. This excitation spread between dendrites and soma is seen best during subthreshold excitation by relatively small stretches of normal cells. It is also seen during the whole range of receptor stretch in neurons in which nerve conduction has been blocked by an anesthetic. The electrotonic changes in the cells are graded, reflecting the magnitude and rate of rise of stretch, and presumably the changing levels of the generator potential. Thus in the present neurons the resting potential and the excitability level of the cell soma can be set and controlled over a wide range by local events within the dendrites. 3. Whenever stretch reduces the resting membrane potential, measured in the relaxed state in the cell body, by 8 to 12 mv. in slow cells and by 17 to 22 mv. in fast cells, conducted impulses are initiated. It is thought that in slow cells conducted impulses are initiated in the dendrites while in fast cells they arise in the cell body or near to it. In fresh preparations the speed of stretch does not appreciably influence the membrane threshold for discharges, while during developing fatigue the firing level is higher when extension is gradual. 4. Some of the specific neuron characteristics are: Fast receptor cells have a relatively high threshold to stretch. During prolonged stretch the depolarization of the cell soma is not well maintained, presumably due to a decline in the generator potential, resulting in cessation of discharges in less than a minute. This appears to be the basis of the relatively rapid adaptation. A residual subthreshold depolarization can persist for many minutes of stretch. Slow cells which resemble the sensory fibers of vertebrate spindles are excited by weak stretch. Their discharge rate remains remarkably constant for long periods. It is concluded that, once threshold excitation is reached, the generator potential within slow cell dendrites is well maintained for the duration of stretch. Possible reasons for differences in discharge properties between fast and slow cells are discussed. 5. If stretch of receptor cells is gradually continued above threshold, the discharge frequency first increases over a considerable range without an appreciable change in the firing level for discharges. Beyond that range the membrane threshold for conducted responses of the cell soma rises, the impulses become smaller, and partial conduction in the soma-axon boundary region occurs. At a critical depolarization level which may be maintained for many minutes, all conduction ceases. These overstretch phenomena are reversible and resemble cathodal block. 6. The following general scheme of excitation is proposed: stretch deformation of dendritic terminals → generator potential → electrotonic spread toward the cell soma (prepotential) → dendrite-soma impulse → axon impulse. 7. Following release of stretch a transient hyperpolarization of slow receptor cells was seen. This off effect is influenced by the speed of relaxation. 8. Membrane potential changes recorded in the cell bodies serve as very sensitive detectors of activity within the receptor muscle bundles, indicating the extent and time course of contractile events.     

Inhibition in the eye of Limulus

In the compound lateral eye of Limulus each ommatidium functions as a single receptor unit in the discharge of impulses in the optic nerve. Impulses originate in the eccentric cell of each ommatidium and are conducted in its axon, which runs without interruption through an extensive plexus of nerve fibers to become a fiber of the optic nerve. The plexus makes interconnections among the ommatidia, but its exact organization is not understood. The ability of an ommatidium to discharge impulses in the axon of its eccentric cell is reduced by illumination of other ommatidia in its neighborhood: the threshold to light is raised, the number of impulses discharged in response to a suprathreshold flash of light is diminished, and the frequency with which impulses are discharged during steady illumination is decreased. Also, the activity that can be elicited under certain conditions when an ommatidium is in darkness can be inhibited similarly. There is no evidence for the spread of excitatory influences in the eye of Limulus. The inhibitory influence exerted upon an ommatidium that is discharging impulses at a steady rate begins, shortly after the onset of the illumination on neighboring ommatidia, with a sudden deep minimum in the frequency of discharge. After partial recovery, the frequency is maintained at a depressed level until the illumination on the neighboring receptors is turned off, following which there is prompt, though not instantaneous recovery to the original frequency. The inhibition is exerted directly upon the sensitive structure within the ommatidium: it has been observed when the impulses were recorded by a microelectrode thrust into an ommatidium, as well as when they were recorded more proximally in single fibers dissected from the optic nerve. Receptor units of the eye often inhibit one another mutually. This has been observed by recording the activity of two optic nerve fibers simultaneously. The mediation of the inhibitory influence appears to depend upon the integrity of nervous interconnections in the plexus: cutting the lateral connections to an ommatidium abolishes the inhibition exerted upon it. The nature of the influence that is mediated by the plexus and the mechanism whereby it exerts its inhibitory action on the receptor units are not known. The depression of the frequency of the discharge of nerve impulses from an ommatidium increases approximately linearly with the logarithm of the intensity of illumination on receptors in its vicinity. Inhibition of the discharge from an ommatidium is greater the larger the area of the eye illuminated in its vicinity. However, equal increments of area become less effective as the total area is increased. The response of an ommatidium is most effectively inhibited by the illumination of ommatidia that are close to it; the effectiveness diminishes with increasing distance, but may extend for several millimeters. Illumination of a fixed region of the eye at constant intensity produces a depression of the frequency of discharge of impulses from a nearby ommatidium that is approximately constant, irrespective of the level of excitation of the ommatidium. The inhibitory interaction in the eye of Limulus is an integrative process that is important in determining the patterns of nervous activity in the visual system. It is analogous to the inhibitory component of the interaction that takes place in the vertebrate retina. Inhibitory interaction results in the exaggeration of differences in sensory activity from different regions of the eye illuminated at different intensities, thus enhancing visual contrast.     

Lateral ocellar nerve projections in the dragonfly brain

Summary The central projections of the lateral ocellar neurons of the dragonfly were examined using whole nerve cobalt iontophoresis, supplemented by sectioning of the nerve and brain for inspection in the light and electron microscopes. At E.M. level the presence of cobalt in filled axon profiles and cell bodies was confirmed by analysis of X-ray energy spectra in the microscope.The pathways, cell body sites and terminal arborizations of four large (7–25 m diameter) lateral ocellar neurons are described. Two of these fibers arborize in the ipsilateral posterior neuropil of the protocerebrum and two cross the brain and arborize in the contralateral posterior neuropil. Within each half of the posterior neuropil, two spatially separated regions of ocellar input have been identified. These regions receive median ocellar input plus input from either the ipsi- or contralateral ocellus, but not both. The arborizations of the contralateral fibers are more extensive than those of the ipsilateral fibers.One of the contralateral neurons crosses the brain in the region of the protocerebral bridge giving off a collateral in that region before descending to the posterior neuropil. This collateral arborizes almost immediately in a region receiving input from arborizations of a number of small ocellar neurons (those less than 5 m in diameter) from the ipsilateral ocellar nerve, together with small neurons from the median ocellar nerve, forming a region in each half of the brain which receives input from all three ocelli. The small lateral ocellar neurons associated with these arborizations have cell bodies adjacent to the lateral ocellar tracts.This work was supported in part by National Institute of Health Grants 2 RO1 EY-00777 and 1 KO4 EY-00040     

Interneuronal contacts and propagation mechanism in paired receptors of the propo-dactylopodite organ of the crayfish Astacus leptodactylis]

In recordings of biopotentials from the propo-dactylopodite-organ (the PD-organ) nerve, repetitive pairs of impulses are frequently observed, which presumably reflect paired structure of receptor elements where two sensory neurons are morphologically linked by non-polarized interdendritic ephapse. It is suggested that propagation via the ephapse out to be bilateral and that each of the postephaptic fibers transmits impulses from both of the cilia of the paired receptor. However, identical impulses from different cilia reach the central nervous system at different time, this mechanisms being presumably employed for differentiation of information on each of the cilia. One of the postephaptic fibers passes, seemingly, to the right, whereas the other one - to the left half of the first thoracic ganglion.     

Effects of excitatory amino acid antagonists on synaptic transmission in the ampullae of Lorenzini of the skate Raja clavata

Summary The spectral sensitivity of the ocellus in the cucumber looper moth, Anadevidia peponis, was investigated by recording electroretinograms (ERGs). The peak sensitivities were observed at 340 nm in the ultraviolet and at 520–540 nm in the green. Selective spectral adaptation revealed the existence of at least two receptor types in the ocellar retina. The ratio of green to ultraviolet sensitivities for an ocellus whose ocellar nerve was cut was higher than that for an intact ocellus. It is suggested that efferent signals which control the spectral sensitivity of the ocellus are present in the ocellar nerve.Abbreviations ERG electroretinogram - GR/UV green to ultraviolet sensitivities - ON ocellar nerve     

Two types of binocular convergence of visual information in the cat superior colliculus

During binocular stimulation of different sectors of the retina the amplitude of the two first postsynaptic components of the evoked potential in the superior colliculus to the second stimulus varies with the time delay between the testing and conditioning stimuli. Correlation is shown between the form of the evoked potential arising in response to the conditioning stimulus and the character of convergence of visual impluses in the superior colliculus. Qualitative differences are found in binocular interaction between sensory impulses depending on the way in which the conditioning impulses reach the region of the superior colliculus tested. An attempt is made to assess interaction between sensory volleys in the superior colliculus quantitatively.Institute of the Brain, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 5, No. 2, pp. 133–137, March–April, 1973.     

Physiology of Photoreceptor Neurons in the Abdominal Nerve Cord of the Crayfish

Nerve fibers which respond to illumination of the sixth abdominal ganglion were isolated by fine dissection from connectives at different levels in the abdominal nerve cord of the crayfish. Only a single photosensitive neuron is found in each connective; its morphological position and pattern of peripheral connections are quite constant from preparation to preparation. These cells are "primary" photoreceptor elements by the following criteria: (1) production of a graded depolarization upon illumination and (2) resetting of the sensory rhythm by interpolated antidromic impulses. They are also secondary interneurons integrating mechanical stimuli which originate from appendages of the tail. Volleys in ipsilateral afferent nerves produce short-latency graded excitatory postsynaptic potentials which initiate discharge of one or two impulses; there is also a higher threshold inhibitory pathway of longer latency and duration. Contralateral afferents mediate only inhibition. Both inhibitory pathways are effective against both spontaneous and evoked discharges. In the dark, spontaneous impulses arise at frequencies between 5 and 15 per second with fairly constant intervals if afferent roots are cut. Since this discharge rhythm is reset by antidromic or orthodromic impulses, it is concluded that an endogenous pacemaker potential is involved. It is postulated that the increase in discharge frequency caused by illumination increases the probability that an inhibitory signal of peripheral origin will be detected.