Action of habenular efferents on ventral tegmental area neurons studied in vitro.

Action of habenular efferents on neurons of the ventral tegmental area (VTA) was studied with a slice preparation that preserved the habenula (Hb) and the VTA together with the interconnecting fiber bundle, the fasciculus retroflexus (FR). In the VTA, two types of neurons, presumably corresponding to the dopaminergic and nondopaminergic neurons, were discerned on the basis of the electrophysiological properties. Of 52 VTA neurons sampled, 42 [with the mean resting membrane potential of 56 +/- 7 mV (mean +/- SD)] responded with excitatory postsynaptic potentials (EPSPs) to FR stimulation. The EPSPs were monosynaptic in nature and rather weak in effect in the sense that they rarely triggered spikes. No significant differences in latency, duration, and time to peak were noted between the EPSPs generated in different types of neurons. FR stimulation evoked inhibitory postsynaptic potentials (IPSPs) in only six neurons, their resting membrane potential being 51 +/- 4 mV. The IPSPs frequently showed a fluctuation in latency. FR stimulation also produced antidromic responses in a few VTA neurons, but their long latencies precluded the possibility that the VTA-Hb projections contributed to the FR-evoked orthodromic responses in the VTA. EPSPs evoked by FR stimulation could be suppressed by kynurenic acid (1 mM). The findings indicate that the efferents of the Hb primarily have an excitatory effect on VTA neurons of any type and that the excitation may be mediated by amino acid receptors.     

Neuronal and synaptic mechanisms of cortical inhibition

Data are presented concerning the latencies, amplitude and durations of IPSPs evoked in neurons of different cortical regions by peripheral and thalamocortical fibre stimulations and intracortical microstimulation in cat. The duration of IPSPs developing due to single afferent stimulus ranged between 20 and 250 ms (60-80 ms as a rule). In response to intracortical microstimulation monosynaptic IPSPs of 5-10 ms duration arose parallel with IPSPs of 20-100 ms duration. Barbiturates and chloralose increased the duration of IPSP up to 300-500 ms. The latencies of 73% of IPSPs evoked in the auditory cortex neurons by thalamocortical fibres stimulation exceed the latencies of monosynaptic EPSPs evoked by the same kind of stimuli not more than by 1.2 ms. Conclusion is made that inhibition developing in neurons of the cortical projection areas in response to afferent volley is a direct one and is produced by cortical inhibitory interneurons. Some cortical neurons were inhibited by a recurrent mechanism. Only 2% of IPSPs developed monosynaptically. The synaptic delay of IPSP evoked by intracortical microstimulation ranged between 0.3-0.4 ms. The length of the inhibitory neuron axons in layer IV of the auditory cortex reached 1.5 mm. The excitation conduction velocity in these axons was calculated to be 1.6-2.8 m/s (2.2 m/s on the average).     

An intracellular analysis of the entopeduncular inputs on the centrum medianum-parafascicular nuclear complex in cats

Intracellular recording from the CM-PF neurons was performed by stimulation of the EN, the caudate nucleus (Cd), the cerebellar nuclei (CN) and the motor cortex in the cat under Nembutal anesthesia. Twenty-seven neurons in the CM-PF nuclear complex and two neurons near the habenular nucleus received monosynaptic inhibitory postsynaptic potentials (IPSPs; latency of 1.0–4.0 ms, mean 2.3 ms) by EN stimulation. Cd stimulation evoked excitatory postsynaptic potentials (EPSPs) followed by long hyperpolarizations in most of the CM-PF neurons and produced antidromic activation in 7 neurons. Six neurons received EPSPs (latencies of 4–7 ms) by cortical stimulation. CN stimulation affected only two neurons in the present study. Intracellular HRP staining revealed that some CM-PF neurons have polygonal or spindle-shaped somata with fine, long and sparsely spinous dendrites.     

Baclofen-induced disinhibition in area CA1 of rat hippocampus is resistant to extracellular Ba

The mechanism of disinhibition produced by (±)-baclofen was studied using intracellular recording in area CA1 of rat hippocampal slices. Baclofen reversibly depressed monosynaptic IPSPs evoked by direct activation of interneurons in the presence of the excitatory amino acid receptor antagonists 6,7-dinitroquinoxaline-2,3-dione (DNQX) andd,l-2amino-5-phosphonovalerate (APV). Ba²⁺ prevented baclofen-induced hyperpolarization of pyramidal neurons but not depression of monosynaptic IPSPs by baclofen. Baclofen reversibly depressed monosynaptic IPSPs when applied close to the recording site, but was ineffective when applied close to the stimulating site in stratum radiatum. These results suggest that baclofen disinhibits pyramidal neurons in area CA1 of the rat hippocampus by activating receptors on the terminals of inhibitory neurons that are coupled to a Ba²⁺-insensitive effector mechanism.     

Strychnine-induced changes of the membrane and postsynaptic potentials in neocortical neurons]

Intracellular responses of neurons of the suprasylvian gyrus to the intracortical stimulation (ICS) before and after superficial application of strychnine were investigated in experiments on immobilized and unanaesthetized cats. The normal cortex neurons reacted to ICS by monosynaptic EPSPs followed by IPSPs. Strychnine application triggered the epileptiform activity and appearance in neurons of paroxysmal depolarization shifts of the membrane potential (MP) which were replaced by hyperpolarization potentials. An increase and summation of the latter elicited the MP enlargement and either reduction or suppression of background spike activity. Intracellular injections of EGTA blocking the membrane calcium-dependent potassium conductivity (gK(Ca)) have eliminated the hyperpolarization potentials. Development of epileptiform activity was accompanied by depression of IPSPs and increase of the monosynaptic EPSPs. The contribution of gK(Ca) and of postsynaptic inhibition to the epileptogenesis is discussed.     

Pallidal inputs to subthalamus: Intracellular analysis

Neuronal responses of the subthalamic nucleus (STH) to stimulation of the globus pallidus (GP) and the substantia nigra (SN) were studied by intracellular recording in the decorticated rat. (1) GP and SN stimulation evoked antidromic spikes in STH neurons with a mean latency of 1.2 ms and 1.1 ms, respectively. Based on the above latencies, the mean conduction velocity of the STH neurons projecting toward GP was estimated to be 2.5 m/s, and that toward SN was 1.4 m/s. Many STH neurons could be activated following stimulation of both GP and SN, indicating that single STH neurons project to two diversely distant areas. In spite of differences in conduction distance of GP and SN from STH, differences in the conduction velocities of bifurcating axons make it possible for a simultaneous arrival of impulses in the target areas to which these STH neurons project. (2) GP stimulation evoked short duration (5-24 ms) hyperpolarizing potentials which were usually followed by depolarizing potentials with durations of 10-20 ms. These potentials were tested by intracellular current applications and intracellular injections of chloride ions. The results indicated that the hyper- and depolarizing potentials were IPSPs and EPSPs respectively. These IPSPs were considered to be monosynaptic in nature since changes in the stimulus intensities of GP did not alter the latency of IPSPs. The mean latency of the IPSPs was 1.3 ms. Based on the above mean latency the mean conduction velocity of GP axons projecting to STH was estimated to be 3.8 m/s. (3) Analysis of electrical properties of STH neurons indicated that: (i) input resistance estimated by a current-voltage relationship ranged from 9 to 28 M omega; (ii) the membrane showed rectification in the hyperpolarizing direction; (iii) direct stimulation of neurons by depolarizing current pulses produced repetitive firings with frequencies up to 500 Hz. (4) Morphology of the recorded STH neurons was identified by intracellular labeling of neurons with horseradish peroxidase. Light microscopic analysis indicated that the recorded neurons were Golgi type I neurons with bifurcating axons projecting toward GP and SN.     

Postsynaptic potentials of tectal neurons evoked by electrical stimulation of the pretectal nuclei in bullfrogs (Rana catesbeiana)

Postsynaptic responses of the tectal cells to electrical stimulation of pretectal (Lpd/P) nuclei were intracellularly recorded in the bullfrog (Rana catesbeiana). The pretectal stimulation elicited mainly two types of responses in the ipsilateral tectum: an EPSP followed by an IPSP and a pure IPSP. The latter predominates in the tectal cells responding to ipsilateral pretectal stimulation. In a few cells, biphasic hyperpolarization appeared under stronger stimulus intensities. Only one type of response was found in the contralateral tectum, a pure IPSP. The antidromically invaded tecto-pretectal projecting cells were recorded in both tecta, which revealed reciprocal connections between the tectum and particular pretectal nuclei. This paper demonstrates the synaptic nature underlying pretectotectal information transfer. EPSPs with short latencies were concluded to be monosynaptic. Most IPSPs were generated through polysynaptic paths, but monosynaptic IPSPs were also recorded in both optic tecta. Nearly 98% of impaled tectal cells (except for intra-axonally recorded and antidromically invaded cells) showed inhibitory responses to pretectal stimulation. The results provide strong evidence that pretectal cells broadly inhibit tectal neurons as suggested by behavioral and extracellular recording studies.     

Tectal afferents monosynaptically activate neurons in the pigeon isthmo-optic nucleus

Postsynaptic responses of 105 neurons in brain slices were intracellularly recorded from the isthmo-optic nucleus (ION) in pigeons, and 18 of these neurons were labeled with Lucifer yellow. Excitatory postsynaptic potentials (EPSPs) or spikes were produced in 93 cells, inhibitory postsynaptic potentials (IPSPs) in 10 cells, and EPSPs followed by IPSPs in two cells following electrical stimulation of the tecto-isthmooptic tract. The EPSPs occurred in an all-or-none fashion, with short latencies (1.3 +/- 0.6 ms). Repetitive stimulation increased their amplitude and duration, demonstrating that temporal summation was involved. Neurons producing excitatory responses were distributed throughout cellular layers of the nucleus. Pure IPSPs had a latency of 3.9 +/- 2.3 ms, and cells that responded in this manner were only distributed in the rostral portion of the nucleus. In the remaining two cells with EPSP-IPSP responses, the latency of excitatory responses was 1.5 ms in one cell and 1.4 ms in the other, and that of inhibitory responses was, respectively, 5.1 and 4.1 ms. Thus, it appeared that excitation was monosynaptic, whereas inhibition may be polysynaptic. Four single injections resulted in dye-coupled labeling, and two pairs of closely apposed cells fired spikes, probably resulting from spatial summation of their excitatory responses. The present study suggests that tectal cells directly activate ION neurons and that tectal fibers contact isthmo-optic neurons in a one-to-one fashion. Taken together with previous studies, it appears that the entire tecto-ION-retinal pathway is excitatory.     

Physiological studies of brainstem reticular connectivity. I. Responses of mPRF neurons to stimulation of bulbar reticular formation

The connectivity between medial pontine reticular formation (mPRF) and bulbar reticular formation (BRF) was studied by intracellular recordings of mPRF neuronal responses to microstimulation of BRF in unanesthetized, undrugged cats. There was a very high percentage (75-90%) of monosynaptic latency postsynaptic potentials (PSPs) in mPRF neurons in response to microstimulation of 3 BRF areas: the magnocellular tegmental field (FTM), the bulbar gigantocellular tegmental field (BFTG), and bulbar lateral tegmental field (BFTL). The type of initial orthodromic response produced in mPRF neurons by BRF stimulation was predominantly (75-95%) a monosynaptic excitatory PSP (EPSP) which was characterized by a rapid rise time, a nearly constant latency, and often led to spike potential generation. In contrast, the percentage of initial monosynaptic inhibitory PSPs (IPSPs) was much lower for FTM (12.3%), for BFTG (12.5%) and was zero for BFTL. While microstimulation techniques alone cannot differentiate between excitation of fibers of passage and neuronal somata, the very high percentage of initial EPSPs in our data and the anatomical evidence for dense BRF to mPRF neuronal projections as compared with less dense projections from fibers passing through BRF to mPRF suggest that excitatory BRF-mPRF connections are predominant. The high degree of connectivity between BRF and mPRF may furnish an important substrate for functional interaction. Comparison of the mPRF neuronal population that was not antidromically activated by FTM microstimulation vs the mPRF neuronal population that was antidromically activated from FTM and also studied for orthodromic responsiveness showed no statistically significant differences between these populations on the parameters of percentage of monosynaptic input, monosynaptic initial EPSPs, monosynaptic initial IPSPs and presence of a PSP with a latency of less than 5 ms. For BRF connectivity this suggests an identity of mPRF input and output neurons with respect to synaptic response properties.     

Electrical membrane properties of rat subthalamic neurons in an in vitro slice preparation

The electrical membrane properties of subthalamic (STH) neurons and their response characteristics to stimulation of the internal capsule (IC) were studied in an in vitro slice preparation. Most STH neurons recorded exhibited spontaneous repetitive firing. The input resistance of STH neurons was 146 +/- 48 M omega and showed both an anomalous and a delayed rectification when the membrane was hyperpolarized or depolarized by current injections. In neurons with the membrane potential less negative than 65 mV, depolarizing current pulses generated repetitive firing with the maximum frequency of up to 500 Hz. Two types of tetrodotoxin (TTX)-resistant cobalt-sensitive potentials, slow depolarizing potential and slow action potential, were observed in STH neurons. The slow depolarizing potential had a long duration (over 500 ms in some cases) and was able to trigger repetitive firing. The slow action potential had a duration of about 30 ms and triggered a burst of firing. The slow action potential was seen only when the neurons were hyperpolarized to more negative than 65 mV by a current injection. Electrical stimulation of IC evoked monosynaptic inhibitory postsynaptic potentials (IPSPs) in most of the neurons examined. The polarity of IPSPs was reversed in the depolarizing direction by intracellular injection of Cl-. Bath application of bicuculline markedly suppressed IPSPs and unmasked monosynaptic excitatory postsynaptic potentials (EPSPs). The EPSP was able to trigger a slow depolarization with repetitive firing or a slow action potential with burst of firing when the neuron was hyperpolarized by a continuous current injection. The results demonstrated that STH neurons in an in vitro preparation have spontaneous discharges, high input resistance, capability to generate high-frequency firing, and Ca potentials. The pattern of responses of STH neurons to synaptic inputs is dependent on their membrane potentials.     

Responses of medullary neurons to stimulation of locomotor and inhibitory loci in the brain stem of the cat

The medullary locomotor point (L) and the pontine inhibitory point (I) were found in mesencephalic decerebellate cats. Repetitive (60/s) microstimulation of L elicited stepping of forelimbs which terminated during repetitive microstimulation of I. Responses of neurons were evoked applying 1.5 s-1 single or paired stimuli to L or a train of 2-4 stimuli to I, the interstimulus interval being 2 ms. Medial neurons (N-301) gave PSPs or action potentials of 1 stimulation three times as often as to L stimulation. Contrary, lateral neurons (N-166) responded two times as often as to L stimulation. IPSPs were recorded in both groups of neurons two times as seldom as EPSPs. In medial neurons IPSPs were produced mainly by I stimulation. I stimulation did not evoke usually IPSPs in neurons excited from L. Possible mechanisms of termination of stepping due to I stimulation are discussed.     

青蛙前视盖与视顶盖间神经传导的细胞内电生理研究(英文)

目的已有许多研究报告了青蛙的前视盖对视顶盖起抑制作用,但关于此神经活动的特性尚不清楚。本研究探讨了这种复杂的神经活动的机理。方法用细胞内记录方法,通过电刺激前视盖的神经细胞核来记录视顶盖细胞的神经活动。结果前视盖的电刺激在同侧视顶盖主要唤起了两种神经反应:一种是兴奋性(excitatory postsynaptic potential,EPSP)和抑制性突触后电位(an inhibitory postsynaptic potential,IPSP)同时出现,另一种是单纯的IPSP,后者在本记录中占主导地位。另外我们也记录到了某些投射到前视盖的视盖投射细胞的神经电位。它揭示了视顶盖和前视盖之间存在着交叉性的相互作用。短潜时的EPSP可能是通过单突触进行传导的,而大多数的IPSP是通过多突触方式进行神经信息传递的。几乎98%被记录的视盖细胞对前视盖的刺激显示出了抑制性反应。结论前视盖的神经细胞对视顶盖的神经活动发挥了强烈的抑制性作用。     

Reciprocal connections between the lateral hypothalamus and the frontal complex in the rat: electrophysiological and anatomical observations

Inputs to rat lateral hypothalamus (LHA) from prefrontal cortex (FC), and vice versa, were studied by intracellular recording, and by retrograde horseradish peroxidase (HRP) method. Stimulation of the FC evoked 3 types of responses: a polysynaptic EPSP-IPSP sequence, IPSPs alone, or antidromic response in LHA neurons. Forty-five per cent of IPSPs were considered to be monosynaptic since the latencies were constant when stimulus intensity was changed. The neurons labeled in the FC following electrophoretic injections of HRP into LHA were located in the medial and sulcal FC. In these cortical areas, not only pyramidal neurons in layer V, but also non-pyramidal neurons in layer VI were labeled. Stimulation of the LHA evoked an EPSP-IPSP sequence, or antidromic response in FC neurons. Some of the fast EPSPs were considered to be monosynaptic. The neurons labeled in the LHA following HRP injection into the FC were either relatively large spherical neurons or small ovoid-shaped neurons. These were distributed diffusely throughout the LHA.     

青蛙前视盖与视顶盖间神经传导的细胞内电生理研究

目的已有许多研究报告了青蛙的前视盖对视顶盖起抑制作用,但关于此神经活动的特性尚不清楚。本研究探讨了这种复杂的神经活动的机理。方法用细胞内记录方法,通过电刺激前视盖的神经细胞核来记录视顶盖细胞的神经活动。结果前视盖的电刺激在同侧视顶盖主要唤起了两种神经反应：一种是兴奋性（excitator ypostsynaptic potential,EPSP）和抑制性突触后电位（an inhibitory postsynaptic potential,IPSP）同时出现,另一种是单纯的IPSP,后者在本记录中占主导地位。另外我们也记录到了某些投射到前视盖的视盖投射细胞的神经电位。它揭示了视顶盖和前视盖之间存在着交叉性的相互作用。短潜时的EPSP可能是通过单突触进行传导的,而大多数的IPSP是通过多突触方式进行神经信息传递的。几乎98％被记录的视盖细胞对前视盖的刺激显示出了抑制性反应。结论前视盖的神经细胞对视顶盖的神经活动发挥了强烈的抑制性作用。     

Caudate evoked synaptic activities in nigral neurons

Intracellular recordings from neurons in the substantia nigra have revealed three varieties of monosynaptic PSPs in response to stimulation of the ipsilateral head of the caudate nucleus: short (3–5 msec) latency EPSPs; short (3–5 msec) latency IPSPs; and long (15–20 msec) latency IPSPs. The data indicate that at least three efferent fiber systems link the caudate to the nigra: two fast conducting axonal systems of comparable diameters mediate the short-latency PSPs whereas a slow conducting axonal system mediates the long-latency PSPs. The caudate evoked long-latency IPSPs in nigral neurons were preceded by antidromic and orthodromic potentials in the motor cortex; these cortical potentials are regarded as epiphenomena. The dual, facilitatory-inhibitory control of caudate on nigral neurons is consonant with the proposal that the caudate self-regulates its input from the nigra. The caudate-evoked EPSPs in nigral neurons are sine qua non for the operation of the caudato-nigrothalamic projection system.     

Lateral hypothalamus and local stimulation induced postsynaptic responses in zona incerta neurons in an in vitro slice preparation of the rat.

Postsynaptic potentials evoked in the zona incerta (ZI) neurons were studied in in vitro slice preparations. Lateral hypothalamus (LH) and local stimulation evoked fast IPSPs, fast EPSPs, and slow EPSPs. The amplitude of the slow EPSPs increased when the neuron was hyperpolarized by a low intensity current injection but was blocked when it was hyperpolarized with a strong current. The slow EPSPs were reversibly suppressed by an application of 50 microM DL-2-amino-5-phosphonovaleric acid (APV) and 20 microM 3-[(+/-)-2-carboxypiperazine-4-yl-]-propyl-1-phosphonic acid (CPP). The slow EPSPs were augmented in Mg-free medium and by train pulse stimulation. Pressure application of NMDA induced a depolarization similar to the slow EPSP. On the other hand, the fast EPSPs showed a conventional voltage dependency and were antagonized by kynurenic acid but not by APV or CPP. The fast IPSPs were completely blocked by 10 microM bicuculline methiodide. The results indicate that LH and local stimulation evoked monosynaptic fast EPSPs and slow EPSPs mediated by N-methyl-D-aspartate (NMDA) and non-NMDA receptors, respectively. The IPSPs appear to be mediated by GABAA receptors and regulate the expression of NMDA receptor-mediated slow EPSPs.     

Intracellular study of rat entopeduncular nucleus neurons in an in vitro slice preparation: response to subthalamic stimulation

Responses of rat entopeduncular nucleus (EP) neurons after stimulation of the subthalamic nucleus (STh) and the morphology of the EP neurons were studied using brain slice preparations. EP neurons were classified into two types based on their electrophysiological properties as reported previously. Of 87 EP neurons, 72 were Type I and the rest were Type II. Synaptic responses to STh stimulation were different in these two cell types. STh stimulation evoked excitatory postsynaptic potentials (EPSPs) followed by strong inhibitory postsynaptic potentials (IPSPs) in Type I neurons and EPSPs without strong IPSPs in Type II neurons. The EPSPs were considered to be monosynaptic because no large change in the latency (1.7 ± 0.5ms) resulted by alteration of stimulus intensity. The EPSPs were reversibly suppressed by kynurenic acid in a dose-dependent manner. Bath application of (+)-tubocurarine (10–50 μM) had no effect on EPSPs or IPSPs. Bath application of bicuculline methiodide (50–100 μM) markedly suppressed IPSPs evoked by STh stimulation and at the same time increased the amplitude and duration of EPSPs without affecting the latency. In the presence of bicuculline methiodide, EPSPs could induce plateau potentials and slow action potentials. Some Type I and Type II neurons were intracellularly labeled by biocytin. Type I neurons were located throughout the EP but Type II neurons were located mainly in the dorsal portion of the EP. Medium sized somata of both Type I and Type II neurons were spine-free and fusiform or round in shape. They had 3–4 thick primary dendrites with diameters of 2–5 μm that branched into thin secondary dendrites. The secondary and tertiary dendrites of Type I neurons were sparsely covered with spines. Dendritic terminals of some Type I neurons had complex arborizations with abundant spines and appendages. The dendrites of Type II neurons were generally smooth and had no complex arborizations at their terminals.     

Nigral inhibitory termination on efferent neurons of the superior colliculus: an intracellular horseradish peroxidase study in the cat

Intracellularly recorded responses of deeper tectal neurons to stimulation of the substantia nigra and the cerebral peduncle were obtained to demonstrate the monosynaptic inhibitory nature of the nigrotectal pathway in the cat. We also employed antidromic stimulation (contralateral predorsal bundle and superior colliculus) and intracellular labeling with HRP to demonstrate which types of tectal efferent neurons are nigrorecipient. The response to nigral stimulation in 61% of the cells studied was a monosynaptic IPSP of short duration. Recovered HRP-labeled nigrorecipient neurons include X1 (large multipolar radiating), X2 (tufted), X4 (medium-size vertical), X5 (medium-size horizontal), T1 (medium-size trapezoid radiating), T2 (small ovoid vertical), I (small sparsely ramified), and A (small horizontal) neurons. Nigrorecipient cells participate in all four of the major efferent axonal systems of the deeper tectal layers: crossed descending (X and T neurons), ipsilateral descending (I and T neurons), ascending (A, X, and T neurons), and commissural (T neurons). EPSPs accompanied by long-lasting hyperpolarizing potentials were recorded from the remaining tectal neurons in response to stimulation of the substantia nigra, cerebral peduncle, and pericruciate cortex. Collision experiments indicate that at least part of the excitatory responses of tectal neurons to nigral and penduncular stimulation are mediated by corticotectal fibers traversing the cerebral peduncle and the substantia nigra. Excitatory effects of nigral, peduncular, and cortical stimulation were disclosed in X neurons including the non-nigrorecipient large vertical neurons of the X3 subgroup. Cortical excitatory and nigral inhibitory inputs converge only on X neurons (X1, X2, X4, X5). In this case, nigrally evoked IPSPs were preceded by a brief EPSP. Collectively, these results demonstrate the inhibitory termination of the nigrotectal pathway on a wide variety of deeper tectal efferent neurons. Such findings imply the versatility of the nigral involvement in tectal mechanisms of gaze control. We suggest that the substantia nigra pars reticulata contacts tectal neurons differing as to their response properties and shapes the signal carried by all the major tectofugal bundles.     

Two different mechanisms control inhibition of spike discharge in neurons of cat motor cortex after stimulation of the pyramidal tract

Intracellular recordings were made from neurons of the motor cortex of awake cats while the pyramidal tract (PT) was stimulated at the level of the facial nucleus. In some neurons IPSPs of 35-120 ms peak latency were recorded that diminished in size or reversed with hyperpolarizing current. During these IPSPs a decrease in input resistance reflective of a conductance increase was measured. More often, however, PT stimulation produced IPSPs with comparable latencies that increased in size with hyperpolarizing current. These IPSPs diminished with depolarizing current, and in some instances they appeared to reverse with strong depolarization. During these IPSPs an increased input resistance reflective of a decreased conductance was measured. The results indicate that two different mechanisms control rapid inhibition of spike discharge in neurons of the motor cortex after PT stimulation.     

Synaptic connections of buccal mechanosensory neurons in the opisthobranch mollusc, Navanax inermis

Mechanical stimulation of various areas of the pharyngeal wall and lips can produce EPSPs and IPSPs, as well as abruptly rising impulses, in primary sensory cells. IPSP fields are generally larger than EPSP fields and these fields are distributed without obvious order around fields from which afferent spikes are evoked. Apparently monosynaptic excitatory and inhibitory contacts are formed between primary sensory neurons. These synapses are blocked by high Mg2+ indicating chemical transmission. IPSPs are inverted by Cl- injection. Excitatory inputs can be electrically far from the soma. Sensory cells form apparently monosynsptic excitatory or inhibitory contacts on motoneurons mediating pharyngeal expansion. Brief sensory excitation can initiate sustained firing within this neuronal population and sustained synaptic activity in motoneurons. Interactions of sensory neurons may be important in information processing and in generating motor paterns. These neurons serve both primary sensory and interneuronal functions.