Patterns of functional synaptic connections in organized cultures of cerebellum

Organized cultures of mouse cerebellum with separated regions containing cortical, deep nuclear neurons and brain stem neurons from the peduncular zone were used for electrophysiological studies of axonal projections and synaptic interactions. Responses of single neurons of each of the regions were recorded extracellularly and intracellularly during localized electrical stimulation of other parts of the explant, and indicated extensive synaptic interactions. Cortical stimulation inhibited deep nuclear neurons, apparently monosynaptically, and frequently caused antidromic activation of these cells. Synaptic responses of brain stem neurons to cortical stimulation were usually excitatory, but these were often succeeded by inhibitory potentials. Since brain stem cells were often antidromically activated, the excitatory synaptic responses may be mediated by collaterals of antidromically stimulated brain stem axons. Stimulation of the deep nuclear region could evoke inhibitory or excitatory potentials in cortical neurons, the most frequent response being an excitatory postsynaptic potential which was followed in about 2 ms by an inhibitory potential. Most excitatory and some inhibitory postsynaptic potentials followed high frequency stimulation with constant latencies.The results indicate that within these cultures there are rich synaptic interconnections, many of which follow patterns resembling those seen in the intact brain. The monosynaptic inhibitory projection from the cortex to the deep nuclei and collateral inhibition by Purkinje cell axons appear to be features of cerebellar function that are reproduced in this culture model. In addition, a projection from the deep nuclei to the cortex recently described in the intact cerebellum is also present in the cultures and gives postsynaptic potential responses typical of excitatory afferents to the cerebellar cortex. Such cultures appear useful as an experimental model for the study of synaptic mechanisms or the effects of drugs in the mammalian CNS.     

An electrophysiological study of the accessory olfactory bulb in the rabbit--II. Input-output relations as assessed from analysis of intra- and extracellular unit recordings

The input-output relations of the rabbit accessory olfactory bulb were studied by intra- and extracellular single unit recordings following electrical stimulation of the vomeronasal nerves, the lateral olfactory tract and the corticomedial amygdala. Cellular activity of accessory bulb mitral cells evoked by stimulation of the vomeronasal nerves consisted of a brief excitation with a latency of 16 ms. This initial response was followed by a period of reduced firing probability which was due to an inhibitory postsynaptic potential. In many cases this secondary response was followed by a second excitatory postsynaptic potential on which action potentials were generated at higher stimulus intensities. Deeper cells in the granule cell layer responded with a long latency, long duration, excitation, often consisting of bursts of 2-3 spikes. The majority of mitral cells were antidromically invaded by amygdala stimulation. The latencies of the antidromic spikes showed a wide range of variation (12-80 ms). Due to this great variation in antidromic latency the inhibitory postsynaptic potential following the antidromic action potential was rather modest but prolonged in duration. In many cases the onset of the inhibitory postsynaptic potential preceded the antidromic response. The majority of cells did not respond to lateral olfactory tract stimulation. Only 10% of the tested cells were invaded antidromically by stimulation at this site. These neurons were also driven antidromically by amygdala stimulation. We conclude that, although the physiological characteristics of mitral cells of the main and accessory olfactory bulb are very similar, there are important differences. The efferent fibres of the accessory bulb conduct at very slow and variable rates and project directly to the corticomedial amygdala.     

Synaptic actions of the superior colliculus on medial rectus motoneurons in the cat

Synaptic effects of superior colliculus stimulation on medial rectus motoneurons were studied in encéphale isolé cats. Excitatory postsynaptic potentials were observed in all medial rectus motoneurons located on the side of stimulation, whereas contralateral motoneurons received mainly inhibition. The latencies of stimulus-locked excitatory and inhibitory postsynaptic potentials were in the ranges of 1.3–2.6 and 2.0–3.5 ms. respectively, i.e. on the average longer than in abducens motoneurons. Acute lesions of paramedian structures at bulbar levels did not affect the excitatory responses. Pontine transection at the level of the abducens nucleus reduced the mass response of medial rectus motoneurons, but failed to abolish short latency excitatory potentials in motoneurons studied intracellularly.The present data suggest that the shortest excitatory pathway from the superior colliculus to medial rectus motoneurons is disynaptic. The inhibitory pathway appears to contain at least one additional interneuron. The reciprocal pattern of synaptic action on antagonistic (left and right) medial rectus motoneurons indicates that collicular stimulation activated connections responsible for conjugate contraversive eye movements. According to the results of transection experiments. bulbar structures cannot be regarded as the main relay site of tectofugal effects on ocular motoneurons. Although the exact location of relay neurons could not he at present established. the observed timing of synaptic events is not inconsistent with the idea that tectal influences on medial rectus and abducens motoneurons are mediated by common internuncial cells in the parabducens region.     

Synaptic linkage of masseter muscle afferents to masseteric motoneurones in the cat

The synaptic linkage of masseter muscle afferents to masseteric motoneurones was investigated under blockage of soma-dendritic invasion of antidromic spikes by passing constant inward current across the cell membrane. A monosynaptic latency for excitatory postsynaptic potentials produced by group Ia afferents was measured as 1.3 ms and no group lb component was obtained. Inhibitory postsynaptic potentials with latencies of 5.5 ms were produced at a stimulus strength of 4.5 times the threshold of group Ia fibers. On the basis of stimulus strength, muscle afferents activated at 4.5 times the threshold and producing inhibitory postsynaptic potentials in masseteric motoneurones are probably group II afferents. The same reversal point was obtained in both the lingually induced and the group II IPSPs, indicating that the group II inhibitory postsynaptic potential is dependent on an increased permeability to Cl ions. The inhibitory postsynaptic potentials produced by stimulation of the high threshold muscle afferents were the composite of a strychnine-sensitive and strychnine-insensitive inhibitory postsynaptic potential. The latency of the inhibitory postsynaptic potentials caused by the high threshold muscle afferents was about 10 ms.     

Depolarization of cortical glial cells in response to electrical stimulation of the cortical surface

The responses of 155 neurones and 91 glial cells to the electrical stimulation of the cortex were recorded in the suprasylvian gyrus of 20 cats under pentobarbital anaesthesia. Glial cells were identified by electrophysiological criteria: absence of action potentials and postsynaptic potentials; high membrane potential; slow depolarization during the electrical stimulation of the cortex. 50 glial cells showed membrane potentials between 80 and 100 mV. Stimuli of low intensity which evoked only excitatory postsynaptic potentials of apical dendrites, the so-called dendritic potentials, failed to evoke glial depolarization. However, glial depolarization could be elicited at high-frequency stimulation. Stimuli, which evoked not only the dendritic potential but also subsequent slow negativity, could usually bring about glial depolarization too. The amplitude of glial depolarization in response to one stimulus did not exceed 2 mV, the latency being 3–5 ms. A phenomenon of decrementai summation of glial depolarization was observed. The stronger and more frequent the stimulation, the larger was glial depolarization. However, at frequencies over 50/s glial depolarization decay was observed already during the stimulation and in some cases, membrane potential was drastically reduced to zero. After cessation of stimulation, glial depolarization decayed exponentially in 3–4 s; in some cases the decay was prolonged up to 10s and slow irregular fluctuations of the membrane potential were recorded; at the same time, spikes of the neighbouring neurone could be recorded from the glial cell. With a decrease of the membrane potential glial depolarization was attenuated, but it could be elicited even at membrane potential below 20 mV.The results are interpreted in relation to the potassium ion hypothesis. It is suggested that glial depolarization is determined by release of K⁺, which is associated with excitation of non-myelinated fibres and with excitatory postsynaptic potentials generated in the cortical neuropile. Significant increases in the concentration of extracellular potassium ions could provoke actual movement of glial cells. It is supposed that glial depolarization of small magnitude which is recorded occasionally at the membrane potential below 30 mV is the result of electronic spread of glial depolarization from the neighbouring glial cells.     

Ultrastructure of synaptic connections of a bimodal pacemaker giant neuron in the central nervous system of Helix pomatia L.

Following intracellular labelling with horseradish peroxidase, the arborization and synaptic connections of the bimodal pacemaker giant neuron (RPal) of Helix pomatia were investigated in the right parietal and visceral ganglia. The RPal neuron possesses extensive axonal branching, the elements of which could be observed and traced within the entire neuropil region of both ganglia. The main axonal branches showed further arborization. The thin axon processes enter the synaptic neuropil, where they receive numerous synapses. At least six ultra-structurally different terminals form synaptic contacts on peroxidase-labelled axon processes of the cell. On the basis of their vesicle and granule content, they are likely to contain different neurotransmitters. Some intraganglionic efferent contacts of the RPal neuron were also observed.It is suggested that, besides its peripheral efferent connections, this cell might also serve as an interneuron.     

Effects of ammonium salts on synaptic transmission to hippocampal CA1 and CA3 pyramidal cells in vivo

The effects of ammonium acetate or chloride, perfused through the lateral ventricle, were studied on the hippocampal formation of the rat. During perfusion with ammonia, the population spikes, evoked by stimuli delivered to the fimbria, were first increased and then reduced. On the other hand, the late positive wave gradually decreased throughout the application of ammonia. The inhibition, studied by the paired-pulse test, was found to be reduced when the population spike was transiently enhanced, indicating that disinhibition could be responsible for the enhancement of synaptically evoked responses. Neither antidromically evoked population spikes nor the typical effects of iontophoretically applied glutamate, aspartate or gamma-aminobutyrate were changed by ammonia. These findings can be accounted for by a single action of ammonia, a depression of excitatory synaptic transmission, the excitatory synapses on inhibitory interneurons being more readily depressed than those on the pyramidal cells. Both effects, early hyperexcitability and late depression, are probably due to a reduction in the release of the excitatory neurotransmitter, glutamate and/or aspartate. We tentatively suggest that these mechanisms are responsible for some of the symptoms observed during the development of hyperammonemic encephalopathies.     

Convergence of specific visual and commissural impulses upon inhibitory interneurones in cat's visual cortex

Interaction between inhibition converging from the specific visual and commissural pathways to the efferent cells of cat's visual cortex was studied by paired stimulation of the lateral geniculate body and the corpus callosum. When stimulation of the lateral geniculate body was preceded by relatively weak stimulation of the corpus callosum, there was facilitation of the test inhibitory postsynaptic potentials (IPSPS) at an interval of about 1 msec and depression at longer intervals. With conditioning by stronger stimulation of the corpus callosum, the facilitation disappeared leaving the depression starting at 1 msec and continuing for more than 100 msec. Double shock stimulation of the lateral geniculate body or the corpus callosum alone also revealed similar depression of the test IPSPs. The IPSPs evoked by stimulation of the cortical surface close to the penetrated cell also exhibited similar depression after stimulation of the lateral geniculate body or corpus callosum.These findings suggest that the specific visual and commissural pathways share an inhibitory interneurone in the final common pathway to the efferent cells in the visual cortex.     

The lateral vestibular nucleus of the toadfish Opsanus tau: Ultrastructural and electrophysiological observations with special reference to electrotonic transmission

The lateral vestibular nucleus of the toadfish Opsanus tau was localized by means of axonal iontophoresis of Procion Yellow. The ultrastructure of the lateral vestibular nucleus neurons was then correlated with their electrophysiological properties. The lateral vestibular nucleus consists of neurons of various sizes which are distributed in small clusters over a heavily myelinated neuropil. The perikarya and main dendrites of the large and the small neurons are surrounded by a synaptic bed, which is separated from the neighboring neuropil by a layer of thin astrocytic processes. The synaptic bed contains three main classes of axon terminals, club endings, large and small terminals, the first being quite infrequent. All the large terminals as well as the occasionally observed club endings contain a pure population of rounded synaptic vesicles. In some of the small axon terminals there are also rounded vesicles; however, the majority contain flattened vesicles or a pleomorphic population. These data indicate that the small terminals originate from different afferent sources. The synaptic interfaces of the large boutons and of the club endings bear three types of junctional complexes: attachment plates, gap junctions and active zones. Those showing both gap junctions and active zones were designated as morphologically ‘mixed synapses’. Gap junctions, although in large number, have only been observed at the synaptic interfaces between terminals with rounded vesicles and the perikarya or the dendrite of the lateral vestibular nucleus neurons. Therefore electrotonic coupling would only be possible by way of presynaptic fibers. Some axons observed in the neuropil were found to establish gap junctional complexes with two different dendritec profiles and this observation is in favour of electrotonic coupling by way of presynaptic terminals.Field and intracellular potentials were recorded in the lateral vestibular nucleus. The field potential evoked by stimulation of the vestibular nerve consisted of an early positive-negative wave followed by a slow negativity, and that evoked by spinal cord stimulation was composed of an antidromic potential followed by a slow negative wave. Vestibulo-spinal neurons were identified by their antidromic spikes. In these cells, stimulation of the ipsilateral vestibular nerve evoked an excitatory postsynaptic potential with two components. The short delay of the first component of this excitatory postsynaptic potential and its ability to follow paired stimulation at close intervals without reduction of the second response suggest that it is transmitted electrotonically from primary vestibular afferent fibers. By contrast the latency of the second peak of the vestibular evoked excitatory postsynaptic potential and its sensitivity to high stimulus frequencies are compatible with monosynaptic chemically mediated transmission from primary vestibular afferents. Spinal stimulation evoked graded antidromic depolarizations in vestibulo-spinal neurons. The latency of these potentials was too short to allow for chemical transmission through afferents or recurrent collaterals and suggests electrotonic spread of antidromic activity from neighboring neurons. An important finding is that the graded antidromic depolarizations can initiate spikes; thus coupling between neurons in the lateral vestibular nucleus is sufficiently close that a cell can be excited by activity spread from neighboring cells. Similar graded depolarizations were recorded in identified primary vestibular afferents; their latencies and time course indicate that they were brought about by electrotonic spread of postsynaptic potentials and spikes to the impaled presynaptic fibers; this confirms the morphological evidence that coupling between lateral vestibular nucleus neurons occurs, at least in part, by way of presynaptic vestibular axons. As the spinal stimulus strength was increased, these graded depolarizations became large enough to initiate spikes which presumably propagate to the vestibular receptors. Thus antidromic invasion of the presynaptic terminals may provide negative feedback by preventing their re-excitation at short intervals after a synchronous discharge of an adequate number of postsynaptic cells. Excitatory inputs to the neurons of the lateral vestibular nucleus were identified from the spinal cord and from the contralateral vestibular nerve. Long latency excitatory postsynaptic potentials large enough to excite the cells were recorded following spinal stimulation; the threshold intensity for evoking them was consistently higher than that adequate to generate the graded antidromic depolarizations. Field potentials recorded after stimulation of the contra lateral vestibular nerve consisted of an initial positive negative wave followed by a slow negative wave. the stimulus intensity for evoking these potentials was the same or slightly above the threshold for those evoked in the lateral vestibular nucleus on the stimulated side. Also lateral vestibular nucleus neurons exhibited excitatory postsynaptic potentials large enough to excite the cells following stimulation of the contralateral vestibular nerve. but no inhibitory postsynaptic potentials were detected. This lack of commissural inhibition indicates a qualitative difference between the central organization of these cells in the toadfish and in mammals.The presence of neurons in the lateral vestibular nucleus which send their axons to the labyrinth was confirmed by their heavy staining with Procion Yellow following axonal iontophoresis. In a number of vestibular neurons. abruptly rising spikes were evoked at short latencies after adequate stimulation of the ipsilateral vestibular nerve. Graded stimuli applied to the vestibular nerve evoked graded short latency depolarizations as well as long latency excitatory postsynaptic potentials in these presumed efferent neurons to the labyrinth; the former could indicate electrotonic coupling of the efferent cells or electrotonic transmission from primary afferents, resulting in a short latency feedback loop.From these studies, the synaptic organization of the lateral vestibular nucleus neurons is compared with that of the Mauthner cells of teleosts, and the possibility of a dual mode of transmission, electrical and chemical, by primary vestibular afferents is discussed.     

Electrotonic and dye coupling in hippocampal CA1 pyramidal cells in vitro

The presence of electrotonic and dye coupling in region CA1 of the guinea-pig hippocampus was investigated in the in vitro hippocampal slice preparation. No electrotonic coupling potentials were observed in simultaneous recordings from 101 pairs of pyramidal cells. Also, no electrotonically-coupled short latency depolarizations were observed in more than 75 pyramidal cells in response to antidromic activation of the pyramidal cell population, either in normal bathing medium or in medium with lowered Ca²⁺ concentration and added Mn²⁺. When the fluorescent dye Lucifer Yellow was injected into pyramidal cell somas, spread of the dye to other cells (dye coupling) was often observed. Injection of Lucifer Yellow into the dendrites of these neurons resulted in many fewer cases of dye coupling.The failure to find electrophysiological evidence of electrotonic coupling among CA1 pyramidal cells suggests that such coupling is not a functionally important feature of this area of the CNS. The lack of electrophysiological evidence of coupling combined with the observation that the site of Lucifer Yellow injection influences the extent of dye coupling further suggests that at least part of the observed dye coupling may be artifactual. Electrotonic coupling may exist in a small percentage of hippocampal pyramidal cells. However, it is not clear that this small amount of coupling is either necessary or sufficient for the synchronization of neural activity as has been hypothesized to occur during epileptogenesis.     

Penicillin-induced epileptiform activity in the hippocampal slice: A model of synchronization of CA3 pyramidal cell bursting

One of the most interesting aspects of penicillin-induced epileptiform activity in the hippocampal slice preparation is that CA₃ pyramidal cells periodically burst in synchrony. Penicillin is now known to block inhibitory postsynaptic potentials and permit excitatory postsynaptic potentials to trigger bursting in single neurons. The phenomenon of synchronous bursting is readily simulated with a simple but realistic quantitative model, assuming that CA₃ pyramidal cells are mutually excitatory. The degree of coherence in the population burst depends on the connectivity of the cells with each other and on the amount of coupling between them. If coherence is extremely high, then a given cell must be able to excite a number of other cells in its neighborhood, and conversely, any one cell can be triggered to burst by bursting in only one of its neighbors. Such predictions are in principle experimentally testable. This model does not predict the experimentally observed relation between the periods of spontaneous single cell bursting and synchronous bursting of the population (single cells burst more rapidly in the resting slice than does the population in the penicillin-treated slice). We offer plausible testable explanations for this discrepancy.     

Types of nerves in the enteric nervous system

The enteric nervous system is one of the three divisions of the autonomic nervous system, the others being the sympathetic and parasympathetic. In contrast to the other divisions, it can perform many functions independently of the central nervous system. It consists of ganglionated plexuses, their connections with each other, and nerve fibres which arise from the plexuses and supply the muscle, blood vessels and mucosa of the gastrointestinal tract. The enteric nervous system contains a large number of neurons, approximately 10⁷ to 10⁸. About ten or more distinct types of enteric neurons have been distinguished on electrical, pharmacological, histochemical, biochemical and ultrastructural grounds as well as on the basis of their modes of action. Both excitatory and inhibitory nerves supply the muscle and there are inhibitory and excitatory interneurons within the enteric plexuses. There are also enteric nerves which supply intestinal glands and blood vessels, but these receive less emphasis in this commentary.Correlations between groups of neurons defined on different criteria are poor and in many cases the physiological roles of the nerves are not known. The functions of noradrenergic nerves which are of extrinsic origin are reasonably well understood, but cholinergic nerves in the intestine are the only intrinsic nerves for which both the transmitter and to some extent the functions are known. In the case of non-cholinergic, non-noradrenergic enteric inhibitory nerves, the functions are understood but the transmitter is yet to be determined, both adenosine 5′-triphosphate and vasoactive intestinal polypeptide having been proposed. Other nerves have been defined pharmacologically (non-cholinergic excitatory nerves to neurons and muscle, intrinsic inhibitory inputs to neurons, and enteric, non-cholinergic vasodilator nerves) and histochemically (intrinsic amine-handling neurons and separate neurons containing peptides: substance P, somatostatin, enkephalins, vasoactive intestinal polypeptide, gastrin cholecystokinin tetrapeptide, bombesin, neurotensin and probably other peptides). Little is known of the functions of these nerves, although a number of proposals which have been made are discussed.     

Dendritic and axonic fields of purkinje cells in developing and X-irradiated rat cerebellum. a comparative study using intracellular staining with horseradish peroxidase

Intracellular staining of cerebellar Purkinje cells with horseradish peroxidase was achieved in normal developing rats (8–13 days old), in normal adult rats and in adult rats in which the cerebellum had been degranulated by X-ray treatment. The mono- and multiple innervation of Purkinje cells by climbing fibres was electrophysiologically determined and correlated with their dendritic pattern and axonal field.In immature rats, considerable variations in dendritic arborization were observed between cells at the same age, according to their position in the vermis. In adult X-irradiated animals, a large variety of dendritic shapes was found, confirming previous anatomical data, but no obvious correlation was found between the morphology of the dendrites of Purkinje cells and their synaptic investment by climbing fibres.As regards the axonal field, the adult branching pattern of recurrent axon collaterals was almost established by postnatal day 8, except for some cells which exhibited richer recurrent collaterals. On the other hand, in X-irradiated animals, profuse plexuses were the rule and they originated either from one collateral stem, or from several collaterals, also independently of the number of afferent climbing fibres. The existence of these enlarged recurrent collateral plexuses can be explained by the persistence of an immature stage, and certainly also by collateral sprouting following the largely impaired innervation of the terminal field during development.These results emphasize the role of the cellular interactions that occur during Purkinje cell growth in the formation of both its axonal and dendritic fields.     

An identifiable class of statoacoustic interneurons with bilateral projections in the goldfish medulla

Previous studies have demonstrated that some goldfish medullary neurons, inhibitory to the Mauthner cell, can be identified by a passive hyperpolarizing potential coincident with the antidromic impulse of the latter. We describe in this report a group of these interneurons that were further distinguished by their short latency responses to stimulation of the eighth nerve. Specifically, they exhibited graded short latency depolarizations following weak stimulation of the ipsilateral eighth nerve. Short latency depolarizations were followed at times by mono- or polysynaptic postsynaptic potentials. Our evidence indicated that short latency depolarizations were due to electrotonic transmission from eighth nerve afferents. Stronger stimuli evoked a shorter latency impulse which arose abruptly from the baseline. Collision tests and membrane hyperpolarizations did not reveal synaptic potentials underlying the impulses. These physiological results, therefore, suggested that the short latency impulse was a propagated response generated at an electrotonically remote site, possibly an efferent process in the eighth nerve. However, no such projection was found in morphological studies of 77 dye-injected neurons. The morphology rather indicated that these cells were statoacoustic interneurons. Thus, the short latency impulse may be due to remote, electrotonic synaptic inputs. These interneurons had their somata clustered dorsolateral and posterior to the soma of the Mauthner cell. The apparent axon projected contralaterally within the acoustic commissure and could be traced into a caudally directed tract which was lateral to the sensory division of the facial nerve. The axon ramified bilaterally and terminated in part on other statoacoustic neurons, reticular neurons and the Mauthner cells.Selective activation of these interneurons evoked unitary, inhibitory postsynaptic potentials in the Mauthner cell. Their projections suggested a widespread ipsi- and contralateral inhibitory action on other medullary areas as well. These results indicate that a re-evaluation of criteria for efferent identification and of present models for efferent function are required.     

The ionic mechanism of postsynaptic inhibition in motoneurones of the frog spinal cord

The ionic mechanism of postsynaptic inhibition in frog spinal motoneurones was studied with conventional and with ion-sensitive microelectrodes. In these neurones the inhibitory postsynaptic potential was depolarizing, its reversal potential being 15 mV less negative than the resting membrane potential. During the inhibitory postsynaptic potential the input resistance of the motoneurones was reduced to 20% of the resting value, indicating a strong increase of membrane conductance. The Cl- equilibrium potential calculated from intra- and extracellular Cl- activity measurements coincided with the reversal potential of the inhibitory postsynaptic potential to within a few millivolts. During repetitive inhibitory postsynaptic activity the intracellular Cl- activity decreased markedly, while the extracellular Cl- activity increased slightly. These changes of intra- and extracellular Cl- activities were no longer found after suppression of the inhibitory postsynaptic potential by strychnine. Blockade of an active, inward-going Cl- transport system in motoneurones by NH+4 led to a shift of the Cl- equilibrium potential and the reversal potential of the inhibitory postsynaptic potential towards the resting membrane potential. After prolonged action of NH+4, the Cl- equilibrium potential approached the membrane potential to within 5 mV, while the reversal potential of the inhibitory postsynaptic potential and resting membrane potential coincided. The difference between Cl- equilibrium potential and membrane potential after blockade of the Cl- pump is traced back to interfering intracellular ions, such as HCO-3 or SO42-, leading to an overestimation of intracellular Cl- activity and to the calculation of an erroneous Cl- equilibrium potential. Inhibitory amino acids like gamma-aminobutyrate or beta-alanine evoked depolarizations with reversal potentials similar to that of the inhibitory postsynaptic potential. These depolarizations were associated with a marked decrease of neuronal input resistance during inhibition. During the actions of these compounds a decrease of intracellular and a small increase of extracellular Cl- activity were found. The activities of other ions (K+, Ca2+ and Na+) did not change significantly, with the exception of extracellular K+ activity, which was slightly increased. Evidence is presented that the inhibitory postsynaptic potential, as well as the depolarizing action of inhibitory amino acids in motoneurones, is the result of an increase in membrane Cl- permeability and an efflux of Cl- from these cells, while other ions do not seem to be involved.     

Light and electron microscopic immunocytochemical localization of vasoactive intestinal polypeptide VIP-like activity in the rat small intestine

Vasoactive intestinal polypeptide nerve processes and cell bodies were identified by electron microscopic immunocytochemistry in the rat small intestine. Labeled nerve processes were numerous in the inner circular smooth muscle coat and mainly in the mucosa, but were absent in the longitudinal muscle layer. Submucosal blood vessels were often surrounded by immunoreactive vasoactive intestinal polypeptide positive nerves, in close associations (distance less than 40 mn) to blood vessel basement membranes and to smooth muscle cells. In the ganglia of the myenteric and submucous plexuses, labeled fibers surrounded unstained neural cell bodies. The synaptic vesicles of vasoactive intestinal polypeptide positive terminals were 35-40 nm in diameter and some dense core vesicles (80-120 nm in diameter) were also observed in the same profiles. These observations suggest that vasoactive intestinal polypeptide nerves may participate in regulating smooth muscle activity and local blood flow in the small intestine.     

A reanalysis of the ventrolateral input in slow and fast pyramidal tract neurons of the cat motor cortex

In deeply anesthetized cats the temporal characteristics of ventro-lateral (thalamic) excitatory postsynaptic potentials (EPSPs) induced in pyramidal tract cells were studied with an averaging technique. Stimulation of the ventrolateral thalamus induced EPSPs in all pyramidal tract neurons at latencies of 1–5 ms. It was found that there was a positive relationship between the latency and rise time of stimulation-induced EPSPs and the latency of antidromic invasions of pyramidal tract neurons. In response to two closely spaced shocks the second EPSP had the same latency and amplitude as the first one in both slow and fast pyramidal tract neurons. Moreover, the span of antidromic latencies of ventrolateral thalamic relay cells to motor cortex stimulation showed that these thalamic neurons had the necessary conduction velocities to account for the distribution of EPSP latencies recorded in pyramidal tract neurons. From these electrophysiological results, it has been concluded that slow and fast pyramidal tract neurons receive a monosynaptic input from neurons in the ventrolateral thalamus. We also report morphological evidence, obtained at the electron-microscopic level, supporting this conclusion. Terminal degeneration induced by a lesion in the ventrolateral thalamus was found on the apical dendrite of a slow pyramidal tract neuron that had been injected with horseradish peroxidase.It is proposed that the matching between the latencies of EPSPs evoked from the ventrolateral thalamus and the latencies of antidromic invasions of pyramidal tract neurons may reflect a matching between the conduction velocity of thalamocortical and cortico-spinal neurons and/or it may be due to the electrotonic properties of the apical dendrites.     

Modelling the postsynaptic location and magnitude of tonic conductance changes resulting from neurotransmitters or drugs

Tonic conductance changes, synaptically or drug-mediated, can occur near the soma, in one or more distal dendrites, or diffusely. Using simple analogue passive neuronal models, means of localizing tonic conductance changes from intrasomatic recordings were explored. The input resistance measured from the soma was more influenced by perisomatic than distal conductance changes. The membrane time constant, τ₀, was quite sensitive to shunt magnitude, but not to shunt location. Semilogarithmic plots of the voltage response to a short constant current pulse showed that proximal shunts caused a faster earlier decay than distal shunts. Combining these data it is often possible to estimate the location of a tonic conductance change in a neuron from intrasomatic recordings.     

Involvement of cortical neurones in conditioned and unconditioned startle reflex

Postsynaptic potentials in the sensorimotor cortex of unanaesthetized rabbits were recorded simultaneously with electromyogram of the unconditioned startle reflex or the ‘local conditioned startle reflex’. The startle reflex was produced by a loud click in naive animals. The ‘local conditioned startle reflex’ was evoked by a click of a moderate intensity after a conditioning procedure (pairing of the formerly neutral click with direct cortical and hypothalamic stimulation). The latency of the startle reflex and the ‘local conditioned startle reflex’ was from 12 to 17 ms. Postsynaptic potentials or spike discharges after less than 7 ms latency were found in about 20% of neurones in the sensorimotor cortex during both the startle reflex and the ‘local conditioned startle reflex’. Stimulation of subcortical auditory structures evoked EMG responses after 4–8 ms latency. About 25% of sensorimotor cortical neurones responded with postsynaptic potentials and spike discharges within 4 ms after the stimulation of the colliculus inferior.The data support an idea of multiple level organization of the startle reflex and suggest that a pathway for the startle reflex and the ‘local conditioned startle reflex’ may pass through the sensorimotor cortex.