Localized responses in the midsuprasylvian gyrus of the cat following stimulation of the central lateral nucleus in thalamus

Summary Evoked responses were mapped in the cerebral cortex following low intensity electrical stimulation in serial penetrations of the medial and intralaminar nuclei of the thalamus of the cat. A projection was found from one of the intralaminar nuclei, the central lateral nucleus (CL) to the midsuprasylvian gyrus, mainly areas 5 and 7. The projection is suggested to be direct, since the evoked responses had a short latency initial positivity. The most characteristic type of response consisted of this early positivity followed by two successive negativities. The earlier, so called first negativity followed high frequency stimulation and was recorded in a smaller area of the cortex than the later, so called second negativity. The first negativity is suggested to depend on monosynaptic depolarization and activation of cortical cells. The second negativity failed at frequencies higher than 10 Hz and was strongly depressed by the administration of barbiturates; it is suggested to depend on polysynaptic depolarization and cellular activity. In electrode penetrations of the cortex both negativities reversed at the border between cortical layers II and III, indicating a superficial termination of thalamic afferents in the cortex. The cortical evoked response to CL stimulation was facilitated by light mechanical and low intensity electrical stimulation of the periphery, as well as by electrical stimulation of the tooth pulp. The possible significance and function of this projection is discussed in relation to arousal, attention and pain.Abbreviations AV anteroventral nucleus - CM centre médian nucleus - CL central lateral nucleus - GL lateral geniculate nucleus - LD lateral dorsal nucleus - LG lateral gyrus - LP lateral posterior complex - MD mediodorsal nucleus - MSSG midsuprasylvian gyrus - OT optic tract - PAC paracentral nucleus - PF parafascicular nucleus - PP pes pedunculi - VA ventroanterior nucleus - VL ventrolateral complex - VMB basal ventromedial nucleus - VMH ventromedial hypothalamic nucleus - VPL ventroposterolateral nucleus - VPM ventroposteromedial nucleus     

Properties of single neurons in the cat midsuprasylvian gyrus

Summary Responses of cells in the midsuprasylvian gyrus (MSSG) of cats were investigated following electrical stimulation of the central lateral nucleus (CL) of the thalamus and tooth pulp, low-threshold cutaneous or visual afferents. Electrical stimulation in CL induced excitation in many cells located in cortical areas 5 and 7. Cells in these areas also received input from somato-sensory and visual afferents. Cells in MSSG showed a wide convergence from tooth pulp, low-threshold cutaneous afferents and from the CL. The majority of wide convergent cells in area 5 were found in layers IV and V, while cells excited by CL and tooth pulp were found in layers II and III. Similarities were found between CL and tooth pulp evoked responses with regard to the excitation-inhibition pattern. The excitation evoked from CL and tooth pulp was less often followed by a hyperpolarizing potential compared to that seen after low-threshold lip, paw and visual afferent stimulation. Stimulation sites in the lateral parts of CL-evoked responses with the shortest latencies in area 5. In this part of the cortex, short latency synaptic potentials were found in cells in superficial layers. In the same area, synaptic potentials of short latency were also evoked by electrical stimulation of tooth pulp, lip and paw. Light-flash stimulation evoked responses with the shortest latencies in area 7. The results of this study demonstrate that putative nociceptive information reaches the parietal association cortex and that part of this input may be relayed via CL. We suggest that the excitatory influences of nociceptive and CL stimulation is related to behavioral arousal and attention mechanisms.Abbreviations AV anteroventral nucleus - CL central lateral nucleus - CM centre median nucleus - GL lateral geniculate nucleus - LD lateral dorsal nucleus - LP lateral posterior complex - MD mediodorsal nucleus - MSSG midsuprasylvian gyrus - OT optic tract - PAC paracentral nucleus - PF parafascicular nucleus - Po posterior thalamic nuclei - PP pes pedunculi - STT spinothalamic tract - VB ventrobasal complex - VA ventroanterior nucleus - VL ventrolateral complex - VMB basal ventromedial nucleus - VMH ventromedial hypothalamic nucleus - VPL ventroposterolateral nucleus - VPM ventroposteromedial nucleus - C.Max contralateral maxillary canine tooth - I.Max ipsilateral maxillary canine tooth - C.Mand contralateral mandibular canine tooth - I.Mand ipsilateral mandibular canine tooth - C.Lip contralateral upper lip - I.Lip ipsilateral upper lip - C.F.Paw contralateral forepaw - I.F.Paw ipsilateral forepaw - C.H.Paw contralateral hindpaw - I.H.Paw ipsilateral hindpaw - AP anteroposterior plane (in mm anterior to the interauricular plane) - ML mediolateral plane (in mm lateral to the midline)     

Two modes of cerebellar input to the parietal cortex in the cat

Summary The characteristics of cerebellar input to the parietal cortex through the ventroanterior-ventrolateral (VA-VL) complex of the thalamus were investigated in the adult cat by using combined electrophysiological and anatomical methods. Two distinct parietal regions were activated by stimulation of the cerebellar nuclei (CN). In the first region located in the depth of the bank of the ansate sulcus, stimulation of the CN induced early surface positive-deep negative potentials and late surface negative-deep positive potentials. In this cortical area, potentials of similar shape and time course were evoked at a shorter latency by stimulation of the ventrolateral part of the VA-VL complex where large negative field potentials were evoked by stimulation of the CN. After injection of the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) in this part of the VA-VL complex, axon terminals of thalamocortical (TC) fibers were found in layers I, III and IV in the depth of the bank of the ansate sulcus and layers I and III in the motor cortex. In the second region located in the suprasylvian gyrus, late surface negative-deep positive potentials were evoked by stimulation of the CN and similar potentials were evoked at a shorter latency from the dorsomedial part of the VA-VL complex where large cerebellar-evoked potentials could be recorded. PHA-L injection in this thalamic region stained TC fibers and their terminals in layer I of the suprasylvian gyrus, and in layers I and III of the motor cortex. The laminar distribution of TC axon terminals in two different regions of the parietal cortex could account for the depth profiles of the cerebellar- and the thalamic-evoked potentials in each region. These results show that cerebellar information is conveyed to two separate areas in the parietal cortex by two different TC pathways.     

Substantia nigra stimulation evoked antidromic responses in rat neostriatum

Summary Electrical stimulation of the substantia nigra of rats elicits a burst of small amplitude waves with a latency of 4–6 ms that may last for 10–15 ms throughout much of the neostriatum. Frontal cortex stimulation also elicits a burst response, which can occlude the substantia nigra response. The substantia nigra evoked burst response was still present after chronic neocortical ablation or thalamic transection or both treatments combined. The response corresponds to the first sharp negative wave of the substantia nigra evoked neostriatal field potential. Single substantia nigra evoked action potentials were recorded in neostriatum with a mean latency of 9.8 ms, ranging from 4–22 ms. These action potentials were considered to be antidromic because 1) they were occluded during appropriate collision intervals by orthodromic action potentials elicited by frontal cortex stimulation. Subthreshold frontal cortex conditioning stimulation did not alter the threshold for activation from substantia nigra. The refractory period for the axon was at least as long as that for the soma and ranged between 0.8–2.0 ms. The antidromic responses failed to follow low frequency stimulation (< 40 Hz for 3000 ms). This failure occurred in the axon between substantia nigra and globus pallidus. The burst response and first sharp negative wave of the field potential probably represent the antidromic activation of the ubiquitous and densely packed medium spiny neostriatal projection neurons. These responses 1) occur at the same latency, 2) respond in the same manner to twin pulse and repetitive stimulation and 3) are occluded by frontal cortex stimulation in the same manner as antidromic action potentials.     

Effect of chronic protein malnutrition on non-specific thalamo-cortical evoked potentials in the rat

Cortical incremental responses to repetitive thalamic stimulation were studied in normal and protein malnourished rats. The rate of cortical incrementation ipsilateral to the thalamic site of stimulation varied as a function of stimulus repetition rate but was unaffected by dietary treatment. The waveform of the ipsilateral response likewise was unaffected by protein malnutrition and a characteristic positive-negative-positive response was observed. In the cortex contralateral to thalamic stimulation, seven of eight malnourished animals exhibited a surface positive peak, in some cases quite prominent, at a latency at which seven of eight normals exhibited a prominent surface negativity. These results are discussed in relation to reported effects of malnutrition on sensory evoked potentials.     

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.     

A study of neuronal circuitry mediating the saccadic suppression in the rabbit

Summary Stimulation of the deep layers of the superior colliculus (SC) evoked an IPSP in the relay cells of the lateral geniculate nucleus (LGN). The latency of the JPSP ranged from 3.3 to 4.7 ms with an average of 3.87±0.56 ms (S.D.). The IPSP from SC stimulation was proposed to be mediated by the recurrent inhibitory circuit to LGN, since the recurrent inhibitory interneurones in the thalamic reticular nucleus (R) responded to the same stimulation with a latency of 2.14±0.43 ms, which was 1.73 ms shorter than the latency of the IPSP in LGN relay cells. This was in good agreement with our previous observation that the recurrent interneurones always fired about 1.8 ms prior to the onset of the recurrent IPSP in LGN (Lo and Xie 1987b). The recurrent inhibitory interneurones could also be excited by stimulation of the central lateral nucleus (CL) with a very short latency (0.57±0.15 ms), suggesting a monosynaptic connection between the central lateral nucleus and the reticular recurrent interneurones. This suggestion was supported by the fact that CL neurones, which projected to the striate cortex (Cx), were antidromically excited by stimulation of the caudal part of R where the recurrent inhibitory interneurones were situated. CL neurone's response to stimulation of the deep layers of SC (SC-CL response) has a latency of 1.68±0.56 ms, which was comparable with the difference between the latency of SC-R response and that of CL-R response, just as expected from the notion that the saccadic suppression is mediated by a circuit of SC (deep layers) -CL-R-LGN.     

The projection of the posterior knee joint nerve to the cerebellar cortex

1. In decerebrate cats, electrical stimulation of the posterior knee joint nerve evokes in the depth of the cerebellar cortex field potentials which have been identified as due to both mossy and climbing fibre inputs.2. The potentials evoked through the mossy fibres have a latency of 15-17 msec and are more widespread. Those evoked through the climbing fibres have a latency of 26-33 msec and are more restricted.3. Stimulation of the larger fibres originating from the Golgi receptors gives only a very restricted projection. When the fibres originating from the Ruffini receptors are also activated, the effect on the cerebellar cortex is more widespread.4. The results have been confirmed by recording unitary activity.     

Electrophysiological studies on the cerebellocerebral projections in monkeys

1. Responses evoked by stimulation of the cerebellar and thalamic nuclei were recorded by microelectrodes introduced at various depths in the cerebral cortex of monkeys (Macaca mulatta) under light Nembutal anaesthesia. 2. Stimulation of the medial (fastigial) cerebellar nucleus produced, at a latency of 4-5 msec, deep thalamo-cortical (T-C) responses (surface positive-deep negative potentials) mainly in the medial part of the precentral gyrus (area 4, "motor area for hindlimb") and in the superior parietal gyrus (area 5) on both contralateral and ipsilateral sides to the nucleus stimulated. 3. Stimulation of the lateral (dentate) cerebellar nucleus elicited, at a latency of about 3 msec, superficial T-C responses (surface negative-deep positive potentials) predominately in the lateral part of the precentral gyrus (area 4, "motor area for forelimb and face") and in the rostromedial part of the gyrus (area 6, premotor area) on the contralateral side. 4. Stimulation of the interpositus cerebellar nucleus set up superficial T-C responses chiefly in the motor area between those influenced by the medial and the lateral cerebellar nucleus stimulation and also in the premotor area on the contralateral side. 5. The respective areas responsive to the medial, interpositus and lateral nucleus stimulation overlapped considerably each other in the motor cortex. 6. Comparison of the responses in the cortex induced by stimulation of the cerebellar and thalamic nuclei indicated different relay portions in and around the VA-VL region of the thalamus for the superficial and the deep T-C responses respectively. 7. Functional implications of the results were discussed in referring to the cerebellocerebral projections in cats.     

Synaptic organization in teleost spinal motoneurons.

Glass microelectrodes were inserted into motoneurons innervating pectoral fin muscles to record action and synaptic potentials, evoked by electrical stimulation of ventral and dorsal roots, and the medulla oblongata. Ventral root stimulation evoked a small depolarizing response which had properties compatible with those of the EPSP; its amplitude changes were graded, being increased by membrane hyperpolarization and decreased by high frequency repetitive stimulation. The latency of the response was sufficiently longer than that of the antidromic spike to allow for a monosynaptic delay. Stimulation of the dorsal root produced EPSPs with relatively long latencies, suggesting mediation by a polysynaptic pathway. EPSPs with short latencies were evoked by stimulation of the medulla oblongata, indicating the presence of a monosynaptic excitatory connection. Action potentials, recorded from peripheral nerve after stimulation of the medulla oblongata, were facilitated by conditioning volleys via ventral roots. This facilitation was blocked by dihydro-beta-erythroidine hydrobromide and atropine sulphate, indicating the cholinergic nature of the EPSP of ventral root origin. The conduction velocities of motor axons and of the ventral roots fibers responsible for production of EPSPs were about the same. The EPSP of ventral root origin had a slower rising time course and lesser sensitivity to shifts of membrane potential than the EPSP of medulla oblongata origin, suggesting that the sites of generation of the former EPSP were on the peripheral dendrites. From the above results, it was concluded that the EPSP of ventral root origin was mediated by recurrent axon collaterals of motoneurons.     

Responses of neurones in the brainstem raphe nuclei to stimulation of the cerebellar fastigial nuclei in the cat

Stimulation of the fastigial nuclei of the cerebellum was found to excite units located throughout the raphe nuclei of the brainstem. Raphe spinal units with axons conducting at velocities below 20 m/s were either unaffected by fastigial nucleus stimulation or inconsistently excited at long latency. In contrast units with faster conducting spinal axons throughout the raphe and more dorsally in the reticular formation were excited by stimulation of the fastigial nuclei. Many of these units responded at short latency and followed high rates of repetitive stimulation.     

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.     

Inputs from the olfactory bulb and olfactory cortex to the entorhinal cortex in the cat

Summary Field potentials and unit activity elicited by electrical stimulation of the olfactory bulb (OB) and anterior and posterior prepiriform cortex (PPCa and PPCp) were measured extracellularly in the entorhinal cortex (EC) of the cat. Different topographic distributions of the amplitude and peak latency of average evoked potentials (AEPs) were obtained depending on the stimulated area. The maximal evoked activity in the EC showed a gradient in a latero-medial direction with the extremes corresponding to the stimulation of OB and PPCp respectively. Analysis of firing patterns of units in the EC in response to stimulation of the OB, PPCa and PPCp showed that an appreciable number of units responded to stimulation of different areas, mainly PPC- a and PPCp. It was found that the pathways being stimulated differed in conduction velocities with the PPCp-EC being the slowest. Most responding units lay in layer I and II of the EC. The AEPs to PPCaand PPCp-stimulation presented different types of depth profiles. Stimulation of the PPCa evoked an initial surface-negative depth-positive potential whereas the PPCp evoked a different type of AEP with an initial positive component at the surface and negative in depth. It is assumed that the stimulated fibres have their active synapses at different levels within the superficial layers of the EC. The possibility of direct influence of olfactory inputs on the hippocampus mediated by one synapse in the EC is discussed.     

Signal Averaging the Human Somatosensory Evoked Response: A Focal Evoked Response?

The aim of this study was to compare stimulus-time-locked electrical activity obtained from two widely separated cranial sites. Simultaneous records were signal-averaged from human scalp over postcentral gyrus and from three different orbital derivations, during repetitive, controlled mechanical indentation of the contralateral finger pad. Precautions were taken to exclude known sources of extra-cranial stimulus-time-locked artifacts. Reliable somatosensory evoked responses (SERs) appeared in the parietal derivation; one of the orbital derivations showed more variable, but repeatable, activity, having always a broad negativity similar to that of the SER, and sometimes an early positive of shorter latency than the first positive of the SER. Such orbital activity may originate from the corneo-retinal potential or from the frontal eye fields of the cortex, and is not necessarily stimulus-time-locked, since it appeared in the absence of programmed stimulation, when no SER was recorded. Data derived by other workers directly from the middle frontal gyrus are cited.     

Brain stem afferents to the rat medial septum.

1. Activity of neurones in the medial septal nucleus and the diagonal band was recorded from urethane anaesthesized rats. Responses of the cells to electrical stimulation of the raphe nuclei and nucleus locus coeruleus (LC) were measured. 2. LC stimulation caused a long latency, 30-100 msec, and long duration, 100-300 msec cessation of spontaneous activity of most recorded neurones. When bursting-type neurones were recorded, the stimulation occasionally caused a synchronized repetitive bursting firing pattern. 3. Pre-treatment with drugs which interfere with catecholamine neurotransmission, i.e. reserpine and 6OHDA, prevented the appearance of cellular responses to LC stimulation. 4. Stimulation of the dorsal or the median raphe nuclei generated more complex and less clear-cut responses. These included several types of long (20-50 msec) and short (2-5 msec) latency responses. These responses were also accompanied in some cells by synchronized repetitive bursting. 5. Interference with serotonin neurotransmission with PPCA or reserpine reduced the detection of long latency responses. 6. Short latency responses accompanied by evoked field potentials were recorded also after stimulation of dorsal tegmental nucleus. 7. Rates of spontaneous firing cells were augmented after monoamine neurotransmission interruption whereas after fornix lesion, when there is supposedly an increased monoamine innervation of the septum, cells fire at lower rates than normal. 8. It is suggested that noradrenaline and serotonin may serve as neurotransmitters in the medial-septum-diagonal band areas.     

Corticofugal effects on the formation of feeding motivation

Influences from various parts of the cortex on the development of feeding motivation giving rise to goal-directed behavior were established in chronic experiments on rabbits. Electrical stimulation of the frontal and anterior parietal regions of the cortex inhibited the formation of feeding responses evoked by stimulation of the hypothalimic “food center.” Inhibitory influences of the frontal region were stronger. Stimulation of the posterior parietal and occipital regions was accompanied by lowering of the threshold of the evoked feeding response or even by its appearance in satiated animals.     

The central release of acetylcholine during stimulation of the visual pathway

1. In rabbits anaesthetized with Dial ACh has been collected from the surface of the cerebral cortex during stimulation of the visual pathways.2. The spontaneous release of ACh from the visual and non-visual areas of the cortex was found to be similar.3. Stimulation of the retinae by diffuse light produced a large increase in ACh release from the primary visual receiving areas (4.3 times the spontaneous release) and a smaller increase (1.9 times the spontaneous release) from other parts of the cortex.4. Direct unilateral electrical stimulation of the lateral geniculate body evoked a large increase in ACh release (3.4 times the spontaneous release) from the ipsilateral visual cortex and a smaller increase (1.7 times the spontaneous release) from the contralateral visual area and other regions of the cerebral cortex. The evoked increase from the contralateral cortex was not mediated by transcallosal pathways.5. The increase in ACh release evoked from the visual cortex by stimulation of the ipsilateral lateral geniculate body was dependent on the frequency of stimulation. The evoked release was smallest at low stimulus frequencies and increased to a maximum at 20 stimuli/sec.The evoked ACh release from other areas of the cortex was independent of the frequency at which the lateral geniculate body was stimulated.6. The possible central nervous pathways associated with the spontaneous release of ACh and the release evoked by stimulation of the eyes by light and by direct stimulation of the lateral geniculate body are discussed.7. It is concluded that two ascending cholinergic systems may be involved; the non-specific reticulo-cortical pathways responsible for the e.e.g arousal response, and the more specific thalamo-cortical pathways associated with augmenting and repetitive after-discharge responses. The first system is thought to be concerned with the small but widespread increase in ACh release from the cortex following stimulation of the visual pathway while the second system could give rise to the larger increases evoked from the primary receiving areas of cortex. The spontaneous release of ACh from the surface of the brain may be the result of contributions from both systems.     

Interaction of cortical evoked potentials in the rat

1. Unitary and mass potentials were recorded with glass micropipettes at different depths in and around the primary somatosensory area of the cortex in rats anaesthetized with urethane; in addition, surface mass potentials were recorded with chlorided silver ball electrodes. Potentials were evoked by stimuli to the contralateral forepaw and the contralateral cortex. Observations were confined to potentials evoked within 20 msec of stimulation.2. If forepaw stimuli were applied at times when the cortex was not showing spontaneous activity, one component of the evoked response was a widespread depth negativity. This was accompanied by a smaller surface positivity which had the same time course. It could be recorded over the same area of cortex. Unitary activity was found with the same latent period and spatial distribution as the depth negativity. These components of the evoked mass and unitary responses to forepaw stimulation were absent if times of spontaneous cortical activity were chosen for delivering the stimuli.3. Contralateral cortical stimuli evoked mass activity similar to the component of the mass response to forepaw stimulation described in 2. Stimuli given simultaneously to the two sites elicited less unitary activity than the sum of the unitary activity evoked by the two stimuli separately. The mass activity exhibited the interactions which would be expected on the hypothesis that it is generated by the unitary activity. The time course of the interaction is described.4. The interaction was confined to the components of the evoked potentials which were only present if stimuli were applied when the cortex was quiescent. Stimuli applied when the cortex was active evoked mass and unitary potentials which showed no interaction.     

Optical recording of epileptiform voltage changes in the neocortical slice

Summary Voltage sensitive probes were used to monitor the development, distribution, and spread of epileptiform potentials with a photodiode array in neocortical slices of guinea pigs. Epileptiform activity was induced by bath application of bicuculline-methiodide or 3,4-diaminopyridine and electrical stimulation of white matter or cortical layer I. Stimulation evoked a primary or early potential which was followed by a delayed epileptiform potential with a larger spatial extent. Shape, duration and amplitude of the delayed epileptiform potential varied strongly among slices and across the recording area and could reach largest amplitudes at a distance from the stimulation point. At a specific recording site, however, with repeated stimulation, potentials were generated in a stereotyped way. Intracellularly recorded delayed epileptiform potentials corresponded very closely at least to the early part of the optical response. Epileptiform activity appeared in layer III as soon as the primary potential reached sufficient amplitude there. Apart from this relationship, the distribution and spread of maximal amplitudes of delayed epileptiform potentials were segregated from those of early potentials. Early potentials reached maximal amplitudes close to the stimulation site. In contrast, the largest amplitudes of delayed epileptiform potentials were always found in layer III. A second maximum occasionally occurred in layer V. The horizontal amplitude distribution of epileptiform potentials was asymmetric, i.e. amplitudes increased to one side and decreased to the other. Early potential maxima spread from deeper to upper layers when initiated by white matter stimulation and from upper to deeper layers when initiated by layer I stimulation. In contrast, delayed epileptiform potentials always spread from layer III to lower layers and to the sides. Velocity of spread of early potentials and delayed epileptiform potentials differed systematically along the vertical and horizontal axis. The distribution of maximal amplitudes, shape, and pattern of spread of epileptiform potentials was the same whether white matter or layer I was stimulated. The independence of delayed epileptiform potential characteristics from the point of stimulation and from early potential characteristics suggests that epileptiform activity is determined by intrinsic properties of the cortex and not by afferent activation.     

Hippocampal evoked field potentials and interictal spikes in hippocampally kindled cats.

Kindling-related changes of the hippocampal evoked field potentials and patterns of the spontaneous interictal spikes were investigated in 10 hippocampally kindled cats. A complex potential waveform was recorded by macroelectrodes placed in the CA3 region of the hippocampal gyrus and hilus of the gyrus dentatus, close to the granule cell layer, after stimulation of the entorhinal cortex. After high intensity repetitive (10/s) stimulation a late component could be recorded with the latency of about 30-40 ms, in addition to the early response originating in the gyrus dentatus. Probably this component developed during kindling into a delayed, high amplitude spike. After application of the double shock test, post-stimulus facilitation of the spike response was observed within time limits of 20-100 ms. Another observation was a widespread, ipsilateral and bilateral long-term enhancement of the amplitudes of field potentials evoked by entorhinal and intrahippocampal stimulation. It was the most common effect observed during kindling. Widespread synchronized discharges of hippocampal spikes and localized clusters of brief irregular spikes were the most significant features of spontaneous interictal spikes. The paroxysmal discharges of spikes could be evoked by ipsi or by contralateral stimulation of the afferent pathways projecting to the kindled hippocampus, rather than by direct electrical stimulation of the kindled hippocampal gyrus.