Participation of feed-forward and feed-backward inhibition in the "priming" effect in hippocampal sections

Focal potentials (FP) elicited by stimulation of collateral-commissural fibers were recorded in the radial and pyramidal layers of the CA1 area in surviving mouse hippocampal slices. The influence of conditioning stimulation on responses in the tested neuronal pathway (the "priming" effect) at 50–1000 msec intervals between the conditioning and test stimuli and variable stimulation strengths was investigated. The relationship of the duration of the FP of the radial layer to the strength of the test stimulation at a 200 msec interval between the conditioning and test stimuli was studied in the first series of experiments. Three different regions of like relationship were distinguished. In region I (weak stimulation) the duration of the FP did not depend on the stimulus strength or on conditioning (i.e., the "priming" effect was not observed). In region II the duration of the FP in the control was shorter as compared with that observed in region I, which is associated with the triggering of the process of feed-forward inhibition. Conditioning led to the partial restoration of the duration of the FP (the "priming" effect, which evidently develops as the result of the suppression of feed-forward inhibition). In region III, by contrast with region II, the stimulation strength was sufficient for the suprathreshold excitation of the pyramidal neurons, which conditioned the development not only of feed-forward, but of feed-backward inhibition as well. The form of the FP in the radial layer is distorted in the process, and their duration cannot serve as an index of "priming." The influence of conditioning on the effect of paired-pulse depression of population peaks in the pyramidal layer was studied in the second series of experiments in order to identify possible changes in feed-backward inhibition. The principal effect consisted in a decrease in paired-pulse depression in the circuits tested; from this the conclusion was drawn of a suppression of feed-backward inhibition. It was concluded that both feed-forward and feed-backward inhibition are suppressed in "priming."Institute of the Brain, Russian Academy of Medical Sciences, Moscow. Translated from Neirofiziologiya, Vol. 24, No. 2, pp. 178–185, March–April, 1992.     

Excitatory postsynaptic potentials of hippocampal neurons and their extinction during repeated stimulation

A method of detecting "minimal" excitatory postsynaptic potentials (EPSP) in neurons of hippocampal area CA₃ of the unanesthetized rabbit during stimulation of the septo-fimbrial region and the dentate fascia is described. The method consists of presenting a strong (a current of up to 1 mA) conditioning stimulus, inducing a distinct inhibitory postsynaptic potential (IPSP), before a near-threshold (current of 0.03–0.35 mA) testing stimulus. The response to the testing stimulus, develoing after the previous conditioning IPSP, in most cases was purely depolarizing and, judging from the change in the latent period in some cases and the absence of correlation between its amplitude and that of the IPSP, it is a pure EPSP. If the testing stimuli are presented at low enough frequency (intervals of not less than 1 sec) the amplitude of the EPSP evoked by them gradually falls. This decrease exhibits some of the characteristic properties of extinction of behavioral responses (recovery after an interruption, a more rapid decrease during repeated series of stimuli, a slower decrease in amplitude during less frequent stimulation). The amplitude of the IPSP also fell or showed no significant change. The results are evidence in support of the hypothesis that extinction is based on a mechanism of homosynaptic depression.Brain Institute, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 10, No. 1, pp. 3–12, January–February, 1978.     

Characteristics of cortical inhibitory neurons

Spikes were recorded extracellularly and IPSPs intracellularly from auditory cortical neurons of cats immobilized with D-tubocurarine in response to stimulation of geniculo-cortical fibers. Fibers whose stimulation induces IPSPs in auditory cortical neurons mainly have low thresholds. When two stimuli, each of which separately evoked an IPSP of maximal amplitude, were applied to them the shortest interval at which the second stimulus evoked an effect was 2.5–3 msec. This effect consisted of an increase in the duration of the integral IPSP, the amplitude of which either remained unchanged or increased under these circumstances by only 5–10%. The interval at which a separate IPSP appeared in response to the second stimulus depended on the duration of the ascending phase of the IPSP and varied from 4 to 22 msec for different neurons. The amplitude of the second IPSP in this case depended on the interval between stimuli. Under moderately deep pentobarbital anesthesia the number of neurons responding to stimulation of the geniculo-cortical fibers by spikes fell sharply but the number of neurons responding by primary IPSPs remained almost unchanged. Under very deep pentobarbital anesthesia, when spike responses of the cortical neurons completely disappeared, the IPSPs also were completely suppressed. It is concluded that inhibitory neurons of the auditory cortex are excited by thick low-threshold fibers, they have a short refractory period, and they are resistant to the narcotic action of pentobarbital.     

Corticofugal influences on the motoneurons of the trigeminal nucleus

We studied the postsynaptic potentials evoked from 76 trigeminal motoneurons by stimulation of the motor (MI) and somatosensory (SI) cortex in the ipsilateral and contralateral hemispheres of the cat. Stimulation of these cortical regions evoked primarily inhibitory postsynaptic potentials (PSP) in the motoneuron of the masseter muscle, but we also observed excitatory PSP and mixed reactions of the EPSP/IPSP type. The average IPSP latent period for the motoneurons of the masseter on stimulation of the ipsilateral cortex was 6.1±0.3 msec, while that on stimulation of the contralateral cortex was 5.2±0.4 msec; the corresponding figures for the EPSP were 7.6±0.5 and 4.5±0.3 msec respectively. Corticofugal impulses evoked only EPSP and action potentials in the motoneurons of the digastric muscle (m. digastricus). The latent period of the EPSP was 7.6 msec when evoked by afferent impulses from the ipsilateral cortex and 5.4 msec when evoked by pulses from the contralateral cortex. The duration of the PSP ranged from 25 to 30 msec. Postsynaptic potentials developed in the motoneurons studied when the cortex was stimulated with a single stimulus. An increase in the number of stimuli in the series led to a rise in the PSP amplitude and a reduction in the latent periods. When the cortex was stimulated with a series of pulses (lasting 1.0 msec), the IPSP were prolonged by appearance of a late slow component. We have hypothesized that activation of the trigeminal motoneurons by corticofugal impulsation is effected through a polysynaptic pathway; each functional group of motoneurons is activated in the same manner by the ipsilateral and contralateral cortex. The excitation of the digastric motoneurons and inhibition of the masseter motoneurons indicates reciprocal cortical control of their activity.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 3, No. 5, pp. 512–519, September–October, 1971.     

Rubrospinal synaptic influences on the lumbar motoneurons of the rat

Intracellular recording was employed in experiments on rats with the nervous system intact and after acute pyramidotomy to study the postsynaptic effects produced in the lumbar motoneurons on stimulation of the nucleus ruber. Stimulation of this nucleus with single stimuli and with a short series of stimuli caused excitatory and inhibitory postsynaptic potentials (EPSP and IPSP) to develop in the motoneurons. Most of the EPSP recorded were disynaptic, but response development involved a monosynaptic segmental delay in five of the 124 cells that exhibited EPSP. A capacity for high-frequency potentiation was a characteristic feature of the disynaptic excitatory and inhibitory effects. Transmembrane polarization of the motoneurons had a marked influence on the amplitude of the disynaptic EPSP and IPSP. The properties of the disynaptic rubrospinal influences were similar to those described for the cat.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 3, No. 3, pp. 266–273, May–June, 1971.     

Inhibitory synaptic potentials of the smooth muscle cells of guinea pig taenia coli

A single submaximal intramural application of rectangular stimuli (duration 0.2–0.5 msec) to an atropine-treated taenia coli muscle band evoked inhibitory postsynaptic potentials (IPSP) and a marked relaxation of the muscle band in the vast majority of muscle cells. The latency period of the IPSP was 122±16 msec; the times for a rise and fall of amplitude were 96±8 and 370±60 msec, respectively. The mean latency period of muscle relaxation was 800 msec. The latency period, and especially the amplitude of the IPSP depended on the intensity of the intramural stimulation. This indicates that one muscle cell is inhibited by several nerve fibers. IPSP evoked by threshold stimuli displayed a tendency toward summation, while the amplitude of the second and of subsequent IPSP evoked by low-frequency maximal stimuli was always less than that of the first IPSP. After periodic stimulation (frequency 10–60 impulses/min) was discontinued, a posttetanic decrease in IPSP amplitude was observed. Anodic polarization of the muscle band with a direct current raised the effectiveness of synaptic transmission, as was evidenced by the considerable increase in IPSP amplitude. When the muscle membrane was hyperpolarized with noradrenaline, IPSP inhibition was reversible. This is evidence that the unknown mediator and noradrenaline have a common ionic inhibitory mechanism.A. A. Bogomol'ts Institute of Physiology of the Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 2, No. 5, pp. 544–551, September–October, 1970.     

Mechanism of inhibition following reflex discharge in spinal cord interneurons

Using the method of microelectrode (intracellular and extracellular) recording, the mechanism of inhibition following reflex discharge in interneurons of the lumbosacral section of the spinal cord of cats on activation of cutaneous and high-threshold muscle afferents was studied. It was shown that the postdischarge depression of the reflex responses 10–20 msec after the moment of activation of the neuron is due to afterprocesses in the same neuron and presynaptic pathways. The depression of spike potentials from the 20th to the 100th msec is produced by inhibitory postsynaptic potentials (IPSP). During the development of IPSP the inhibition of spike potentials can be due to both a decrease of the depolarization of the postsynaptic membrane below the critical threshold and a decrease of sensitivity of the cell membrane to the depolarizing action of the excitatory postsynaptic potential (EPSP). At intervals between the stimuli of 30–100 msec the duration of EPSP after the first stimulus does not differ from that after the second stimulus. Hence, it is suggested that the presynaptic mechanisms do not play an essential part in this type of inhibition of interneurons. The inhibition following the excitation favors the formation of a discrete message to the neurons of higher orders.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 2, No. 1, pp. 3–9, January–February, 1970.     

Synaptic potentials of digastric-muscle motoneurons

We studied the antidromic and synaptic potentials evoked from 32 digastric-muscle motoneurons by stimulation of the motor nerve to this muscle, different branches of the trigeminal nerve, and the mesencephalic trigeminal nucleus. Antidromic potentials appeared after 1.1 msec and lasted about 2.0 msec. Stimulation of the infraorbital, lingual, and inferior alveolar nerves led to development of excitatory postsynaptic potentials (EPSP) and action potentials in the motoneurons. The antidromically and synaptically evoked action potentials of the digastric-nerve motoneurons were characterized by weak after-effects. We were able to record EPSP and action potentials in two of the motoneurons investigated in response to stimulation of the mesencephalic trigeminal nucleus, the latent period being 1.3 msec. This indicates the existence of a polysynaptic connection between the mesencephalic-nucleus neurons and the digastric-muscle motoneurons. Eight digastric-muscle motoneurons exhibited inhibitory postsynaptic potentials (IPSP), which were evoked by activation of the afferent fibers of the antagonistic muscle (m. masseter). The data obtained indicate the presence of reciprocal relationships between the motoneurons of the antagonistic muscles that participate in the act of mastication.A. A. Bogomol'ts Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 3, No. 1, pp. 52–57, January–February, 1971.     

Responses of bulbar neurons to stimulation of locomotor and inhibitory sites in the cat brainstem

Bulbar locomotor and inhibitory sites were located in the pons of mesencephalic decerebellate cats. Rhythmic stimulation of locomotor sites through microelectrodes at the rate of 60 Hz elicited stepping movements in the forelimbs which were halted when the inhibitory sites were rhythmically stimulated. Neuronal response was elicited by single or paired stimulation of locomotor sites at the rate of 1.5 Hz or by applying a series of 2–4 stimuli spaced 2 msec apart to the inhibitory site. Medial neurons generated synaptic responses (postsynaptic potentials or action potentials) to stimulation of the inhibitory site twice as frequently as when the locomotor site was stimulated. Responses in lateral neurons, however, occurred twice as frequently to stimulation of the locomotor site, while IPSP were only observed half as often as EPSP in neurons of both groups. In neurons excited by stimulation of the locomotor site, stimulation of the inhibitory site did not normally produce IPSP. Possible mechanisms underlying the halt of locomotion occurring in response to stimulation of the inhibitory site are discussed.Information Transmission Institute, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 18, No. 4, pp. 525–533, July–August, 1986.     

Aspects of excitation and inhibition produced in parietal association cortex neurons by stimulating thalamic association nuclei

Neuronal response to single stimuli applied to the thalamic dorsolateral and posterolateral nuclei (DLN and PLN resepctively) was investigated in the parietal association cortex. Primary IPSP following DLN and PLN stimulation was noted in 62.5% and 79.6% of instances respectively. Latencies of EPSP and IPSP when stimulating the two nuclei were longer for the DLN. The amplitude of EPSP evoked by stimulating association nuclei rose and declined smoothly, while that of IPSP showed a fast rise and a more steady decline. The EPSP appearing during the evolution of IPSP were of higher amplitude than control level of resting potential. Both amplitude and duration of IPSP induced in a single unit by stimulating different association nuclei were extremely similar, thus confirming the involvement in this operation of the same inhibitory cortical interneurons. Duration of IPSP was shorter than that of inhibitory background spike activity. It is postulated that the discrepancy in duration can largely be ascribed to properties of the neurons themselves.State University, Odessa. Translated from Neirofiziologiya, Vol. 23, No. 5, pp. 529–536, September–October, 1991.     

Unit responses of the second auditory area to acoustic stimulation

Responses of 116 neurons of the second auditory area to clicks were recorded extracellularly in experiments on unanesthetized cats immobilized with D-tubocurarine. Neurons with and without (54.6%) took part in the response to clicks. The unit response to a click consisted of 1 or 2 spikes or a short volley. Different neurons responded to clicks at different times. The latent period of 25.8% of all neurons recorded was 6.5–13 msec, of 70% it was 14–25 msec, and of 4.2% it was over 25 msec. Long-latency responses to clicks (40, 50, and 100 msec) also were recorded. The responding neurons were found throughout the thickness of the cortex, but more frequently in layers III and IV. No relationship was found between the depth of the neuron and its latest period. Responses consisting of EPSP, EPSP-spike, EPSP-spike-IPSP, EPSP-IPSP, and primary IPSP were recorded intracellularly from the neurons of this area. It was concluded from the results that neurons of the second auditory area can be activated by the arrival of an afferent volley along the geniculo-cortical pathway and also by the arrival of impulses from the first auditory area.     

Corticofugal postsynaptic influences on red nucleus neurons

The responses of red nucleus neurons to stimulation of the sensorimotor cortex was studied on nembutal-anesthetized cats. Most of the rubrospinal neurons were identified according to their antidromic activation. Stimulation of the sensorimotor cortex was shown to evoke in the red nucleus neurons monosynaptic excitatory potentials with a latency of 1.85 msec, polysynaptic excitatory potentials (EPSP), and inhibitory postsynaptic potentials (IPSP) with a latency of 9–24 msec. The EPSP often produced spikes. The probability of generation of spreading excitation is greater with motor cortex stimulation. The monosynaptic EPSP are assumed to arise under the influence of the impulses arriving over the corticorubral neurons as a result of excitation of axodendritic synapses. The radial type of branching of red nucleus neurons facilitates the transition from electrotonically spreading local depolarization to an action potential triggered by the initial axonal segment. Polysynaptic EPSP and IPSP seem to be a result of activation of fast pyramidal neurons whose axon collaterals are connected via interneurons with the soma of the red nucleus neurons.L. A. Orbeli Institute of Physiology of the Academy of Sciences of the Armenian SSR, Erevan. Translated from Neirofiziologiya, Vol. 3, No. 1, pp. 43–51, January–February, 1971.     

Post-tetanic changes in intracellular response to direct cortical stimulation

Changes in responses to single stimulations of the cortical surface after tetanization (frequency 50/sec, duration 1–10 sec) were studied in sensorimotor cortical neurons of an unanesthetized rabbit on intracellular and "quasiintracellular" recordings. After tetanization insufficient to generate epileptiform after-discharges, an increase was observed in the amplitude and duration of exciting postsynaptic potentials (EPSP) induced by a single test stimulus. This increase is considered as post-tetanic potentiation (PTP). Its duration did not exceed 1 min. The amplitude of inhibitory postsynaptic potentials (IPSP) showed a considerably smaller increase or did not change or even decreased. The PTP increased with an increase in the strength and duration of the tetanization, reaching especially high values during tetanization sufficiently intensive to evoke epileptiform after-discharges. In this case the response to a single test stimulus was identical to an epileptiform intracellular discharge. The data obtained confirm the important role of PTP of the exciting synapses in the generation of epileptiform after-discharges. A simple model of a neuron network with exciting and inhibiting feedbacks which accounts for the generation of epileptiform activity is examined.Institute of the Brain, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 2, No. 6, pp. 601–610, November–December, 1970.     

Synaptic effects of contralateral somatic afferents on spinal motoneurons

Electrical activity of flexor and extensor alpha-motoneurons of the lumbar segments of cat's spinal cord as recorded intracellularly during electric stimulation of afferents of the contralateral posterior limb. Contralateral postsynaptic potentials (PSP) were shown to be evoked by activation of cutaneous and high-threshold muscle afferents. The high-threshold afferents of various muscle nerves participate to varying degrees in the generation of contralateral PSP. Contralateral inhibitory postsynaptic potentials (IPSP) were recorded in both flexor and extensor motoneurons along with contralateral excitatory postsynaptic potentials (EPSP). There are no fundamental differences in their distribution between flexor and extensor neurons. Inhibitory influences as a rule are predominant in both during the first 20 msec, and EPSP are predominant in the interval between 20 and 100 msec. The balance of excitatory and inhibitory pathway activity was found to be not as stable as that of the homolateral pathways.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 3, No. 4, pp. 418–425, July–August, 1971.     

Activation of various areas of the ventral horn of lumbar segments during stimulation of the motor cortex

Focal potentials (FP) in segments L₆–L₇ of the ventral horn, evoked by stimulation of the motor cortex with series of stimuli of threshold magnitude for the flexor nerve response, were studied in acute experiments on cats. Appreciable differences were found to exist between the FP arising in the medial zone (layer VIII of Rexed) and those in the inner and outer parts of the lateral zone (layer IX). The FP of the medial zone appear earlier than in other zones (with a latency of 5–12 msec); they are multiphasic, negative components predominating over the positive ones. The FP from the inner part of layer IX possess the largest amplitude (up to 500 µV), a latency of 7–13 msec, a large first negative phase, and marked late positivity. Positive — negative FP (latency 9–15 msec) of small amplitude are recorded from the outermost portion of the ventral horn. The FP of the three zones mentioned above differ also with respect to other functional criteria. The FP of the medial zone are assumed to reflect the realization at the segmental level of the extrapyramidal component of descending cortical activity, the FP of both lateral zones reflecting reciprocal interrelations between postsynaptic processes in the motoneurons of flexor and extensor nuclei during implementation of a cortical motor reaction.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 3, No. 2, pp. 175–184, March–April, 1971.     

Postsynaptic potentials of neurons of the cat auditory cortex

The electrical reactions of 294 neurons of the auditory cortex to a click were recorded in experiments on cats immobilized with tubocurarine (174 intra- and 120 extracellularly). The value of the membrane potential varied from 30 to 70 mV with intracellular leads. The following types of reactions were obtained (the number of neurons is given in parentheses): a peak without slow oscillations in the membrane potential (4), EPSP (3), EPSP-peak (6), EPSP-peak-IPSP (17), EPSP-IPSP (9), primary IPSP (114, including 23 with an after-discharge). Twenty one neurons did not react to a click. The amplitude of the sub-threshold EPSP was 1–1.5 mV, the duration of the ascending part was about 10 and of the descending part 20–30 msec. The peak potential on the ascending part of the EPSP developed at the critical level of 3–4 mV. The amplitude of the peaks varied from several millivolts to 50–60. In 17 neurons prolonged hyperpolarization having all the properties of an IPSP, developed after the peak. The amplitude of these IPSP varied in different neurons from 1 to 10 mV and the duration varied from 20 to 80 msec. IPSP without preceding excitation of the given neuron were the predominant types of reaction. The latent period of these primary IPSP varied from 7 to 20 msec and the amplitude from 1 to 15 msec with a duration of 30–200, more frequently 80–100 msec. It is suggested that two types of inhibition develop in neurons of the auditory cortex in response to a click: recurrent and afferent. The functional significance of the first consists in limiting the duration of the discharge in the reacting neurons, the second prevents the development of excitation in adjacent neurons, thereby limiting the area of neuronal activity.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSSR, Kiev. Translated from Neirofiziologiya, Vol. 3, No. 4, pp. 339–349, July–August, 1971.     

Suppression of associative long-term potentiation in the hippocampus after tetanization of reinforcing afferents

Field potentials (FP) induced in area C1 by gentle orthodromic stimulation were recorded in murine hippocampal slices and associative long-term potentiation (ALTP) produced by C2 tetanization associated with intensive tetanization of another group of fibers (C1) was investigated. A comparison was made between the effects of additional C1 tetanization produced at 50–300 msec before and after combined tetanization of both afferents. Where these intervals measured 50–200 msec, preliminary tetanization of C1 suppressed ALTP (rise in FP amplitude: 10.4±5.2%) in comparison with the regimen whereby additional C1 tetanization came later (giving a rise of 32.4±5.3%); no significant difference was noted at an interval of 300 msec. The three possible reasons for ALTP suppression are discussed, namely: inactivation of "fast" calcium channels, post-activation hyperpolarization of postsynaptic neurons, and synaptic inhibition. The ALTP suppression mechanism is thought to resemble that underlying the relative inefficacy of "reversible" combinations in the shaping of behavioral conditioned reflexes.Institute for Brain Research, Academy of Medical Sciences of the USSR, Moscow. Institute of Chemical Physics, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 21, No. 5, pp. 636–643, September–October, 1989.     

Pharmacological characterization of postsynaptic potentials evoked in the bimodal pacemaker neuron of Helix pomatia L

Stimulation of various peripheral nerve trunks evokes very similar compound postsynaptic potentials (PSP) composed of one or more excitatory postsynaptic potentials (EPSP) followed by fast and slow inhibitory postsynaptic potentials (IPSP) on the identified RPal neuron of Helix pomatia L. Evoked EPSPs were reduced or blocked by nicotine, atropine and d-tubocurarine. The two components of IPSP were different in their pharmacological sensitivity. Slow IPSP was partly or totally eliminated by ergometrine and chlorpromazine and was reduced by atropine, nicotine as well as by propranolol. Fast IPSP was reduced only in the presence of ergometrine and could not be blocked by either of the applied drugs. Participation of cholinergic transmission seems to be essential in the evoked EPSP but its partial involvement in the slow IPSP can also be supposed. A dopaminergic mechanism may take part in the generation of both components of IPSP but the receptors responsible for the slow IPSP were sensitive to other catecholamine antagonists as well, referring to a more complex origin, or to the involvement of an unknown transmitter. Comparison of PSPs evoked by stimulation of different nerves shows that presynaptic areas belonging to various peripheral sources are overlapped on the RPal neuron, and they probably act by similar transmitter substances.     

Effect of pentobarbital,chloralose, and urethane on inhibitory postsynaptic potentials of cortical neurons

The effect of pentobarbital, chloralose, and urethane on IPSPs arising in auditory cortical neurons in response to electrical stimulation of geniculocortical fibers was studied in experiments on cats immobilized with D-tubocurarine. Pentobarbital (60–80 mg/kg body weight, intraperitoneally) sharply reduced the number of neurons responding by spikes to geniculocortical stimulation. Only short-latency responses remained. The number of neurons responding with IPSPs was unchanged. Pentobarbital increased the duration of the IPSPs by 1.5–2 times and shortened their latent periods. Under the influence of chloralose (60 mg/kg, intraperitoneally) the number of responses of EPSP—spike—IPSP type was increased and the duration of the IPSPs also was increased by 3–4 times. The latent period of the primary IPSPs was shortened. Unlike pentobarbital and chloralose, urethane (1000 mg/kg, intravenously) reduced the duration of the IPSPs to 30 msec. About 2% of IPSPs recorded before anesthesia had a latent period of 1.0–1.5 msec. Under the influence of anesthesia the relative number of these IPSPs increased to 5.7%. It it postulated that they are monosynaptic. The mechanism of action of general anesthetics on the cortical inhibitory system is discussed.     

Effects of stimulating the superior colliculus on motoneurons of the neck muscle in the cat

Postsynaptic potentials produced by stimulating three sites of the midbrain superior colliculus were examined in motoneurons innervating the sternocleidomastoid, the trapezius, and the platysma cervical muscles in anesthetized cats. Stimulating the ipsilateral colliculus produced EPSP in the motoneurons as well as action potentials with a latency of 1.5–3.5 msec, averaging 2.6 ± 0.1 msec. Stimulation of the contralateral colliculus evoked EPSP with a latency of 1.5–3.2 msec and averaging 2.1 ± 0.1 msec together with IPSP with latency ranging from 2.6 to 5.0 msec. It is postulated that these postsynaptic responses are both monosynpatic and bisynaptic in nature. This type of synaptic action is assumed to be one of the mechanisms responsible for coordinated head movements produced by tectofugal impulses.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 18, No. 2, pp. 197–202, March–April, 1986.