The effect of pyramidal stimulation upon tail muscle motoneurons in the decerebrate cat

This study aimed to determine the effects of the corticospinal tract (CST) on the motoneurons innervating the tail muscles in cats. The stimulation of the pyramidal tract predominantly evoked excitatory postsynaptic potentials (EPSPs; 48/90 motoneurons: 53%). Single-pulse stimulation produced EPSPs in 18 of 48 motoneurons, but double shocks evoked postsynaptic potentials in most of the remaining cells (26/48). Monosynaptic excitatory connections between pyramidal tract fibers and tail motoneurons were confirmed in 4 motoneurons. Inhibitory postsynaptic potentials (IPSPs) were recorded from motoneurons innervating long tendinous tail muscles (7/90: 8%) and the shortest neuronal pathways of IPSPs were shown to be disynaptic pathways. Interactions between the CST and reflex pathways from low-threshold muscle and cutaneous afferents innervating the tail and hindlimbs were observed.     

A survey of spinal collateral actions of feline ventral spinocerebellar tract neurons

The aim of this study was to identify spinal target cells of spinocerebellar neurons, in particular the ventral spinocerebellar tract (VSCT) neurons, giving off axon collaterals terminating within the lumbosacral enlargement. Axons of spinocerebellar neurons were stimulated within the cerebellum while searching for most direct synaptic actions on intracellularly recorded hindlimb motoneurons and interneurons. In motoneurons the dominating effects were inhibitory [inhibitory postsynaptic potentials (IPSPs) in 67% and excitatory postsynaptic potentials (EPSPs) in 17% of motoneurons]. Latencies of most IPSPs indicated that they were evoked disynaptically and mutual facilitation between these IPSPs and disynaptic IPSPs evoked by group Ia afferents from antagonist muscles and group Ib and II afferents from synergists indicated that they were relayed by premotor interneurons in reflex pathways from muscle afferents. Monosynaptic EPSPs from the cerebellum were accordingly found in Ia inhibitory interneurons and intermediate zone interneurons with input from group I and II afferents but only oligosynaptic EPSPs in motoneurons. Monosynaptic EPSPs following cerebellar stimulation were also found in some VSCT neurons, indicating coupling between various spinocerebellar neurons. The results are in keeping with the previously demonstrated projections of VSCT neurons to the contralateral ventral horn, showing that VSCT neurons might contribute to motor control at a spinal level. They might thus play a role in modulating spinal activity in advance of any control exerted via the cerebellar loop.     

Glossopharyngeal and tectal influences on tongue-muscle motoneurons in the Japanese toad

Anuran tongue-muscle motoneurons receive excitatory inputs both from the glossopharyngeal nerve afferents and from the optic tectum. As a step toward elucidating the neural bases for controlling the tongue movements, we searched intracellularly for the neuronal pathways from the glossopharyngeal afferents to the tongue-muscle motoneurons in paralyzed Japanese toads. The electrical stimuli applied to the glossopharyngeal nerve (ipsilateral lingual branch) evoked polysynaptic excotatory postsynaptic potentials (EPSPs) in tongue-protractor motoneurons and mixed mono- and polysynaptic EPSPs in tongue-retractor motoneurons. Furthermore, we investigated the mode of the convergency of the excitatory inputs from the glossopharyngeal nerve and those from the optic tectum. A spatial facilitation was observed between the tectal EPSPs and the glossopharyngeal EPSPs in some motoneurons tested. These results suggest the existence of common excitatory premotor interneurons, on which the tectal descending volleys and the glossopharyngeal afferent volleys converge.     

Long C3-C5 propriospinal neurones in the cat

Intracellular recording was made in the C3-C5 segments of cats from cells identified as long propriospinal neurones (PNs) by antidromic activation from the lower thoracic segments. The cell bodies were in laminae VII and VIII and their ventrally located axons were either uncrossed or crossed. Stimulation of higher motor centres revealed monosynaptic excitatory postsynaptic potentials (EPSPs) from cortico-, rubro-, tecto-, reticulo-, interstitio-, fastigio- and trigeminospinal fibres. Monosynaptic inhibitory postsynaptic potentials (IPSPs) were evoked from reticulospinal fibres. These PSPs were in addition to the separately described effects from the vestibular nuclei. Monosynaptic EPSPs were also evoked in some cells from neck or forelimb afferents and disynaptic EPSPs or IPSPs from forelimb afferents.     

Organization of hindlimb muscle afferent projections to lumbosacral motoneurons in the chick embryo.

We have examined the organization of muscle afferent projections to motoneurons in the lumbosacral spinal cord of chick embryos between stage 37, when muscle afferents first reach the motor nucleus, and stage 44, which is just before hatching. Connectivity between afferents and motoneurons was assessed by stimulating individual muscle nerves and recording the resulting motoneuron synaptic potentials intracellularly or electrotonically from other muscle nerves. Most of the recordings were made in the presence of DL-2-amino-5-phosphonovaleric acid (APV), picrotoxin, and strychnine to block long-latency excitatory and inhibitory pathways. Activation of muscle afferents evoked slow, positive potentials in muscle nerves but not in cutaneous nerves. These potentials were abolished in 0 mM Ca2+, 2mM Mn2+ solutions, indicating that they were generated by the action of chemical synapses. The muscle nerve recordings revealed a wide-spread pattern of excitatory connections between afferents and motoneurons innervating six different thigh muscles, which were not organized according to synergist-antagonist relationships. This pattern of connectivity was confirmed using intracellular recording from identified motoneurons, which allowed the latency of the responses to be determined. Short-latency potentials in motoneurons were produced by activation of homonymous afferents and the heteronymous afferents innervating the hip flexors sartorius and anterior iliotibialis. Stimulation of anterior iliotibialis afferents also resulted in some short-latency excitatory postsynaptic potentials (EPSPs) in motoneurons innervating the knee extensor femorotibialis, though other connections were of longer latency. Afferents from the adductor, a hip extensor, did not evoke short-latency EPSPs in any of these three types of motoneurons. Short-latency, but not long-latency EPSPs, persisted during repetitive stimulation at 5 Hz, suggesting that they were mediated monosynaptically. Long-latency, fatigue-sensitive potentials were maintained in the presence of APV, picrotoxin, and strychnine, suggesting that polysynaptic pathways utilize non-NMDA receptors as well as NMDA receptors. We found no difference in the pattern of inputs to femorotibialis motoneurons between stage 37-39 and near hatching at stage 44, suggesting muscle afferent projections to these motoneurons are correct at stage 37, when the afferents first reach the lateral motor column in substantial numbers.     

Postsynaptic potentials of motor neurons in the nucleus of the hypoglossal nerve in cats, evoked by rubrofugal impulses

The effects of ipsi- and contralateral red nuclei stimulation on the hypoglossal motoneurons were studied in the cats under chloralose-nembutal anesthesia. Repetitive ipsi- and contralateral rubrofugal volleys evoked PSPs in 35 (69%) from 51 investigated motoneurons (3-5 stimuli of threshold intensity at frequency 500-600 per/s were used). EPSPs appeared in 33 motoneurons with latencies from 3.5 to 14.0 ms (mean value 5.7 +/- 0.75 ms for ipsilateral and 6.8 +/- 0.8 ms for contralateral rubral stimulations). In two motoneurons IPSPs were observed with latency 6.2 ms. Stimulation of the lingual nerve evoked EPSPs and action potentials in 31 motoneurons and IPSPs in 4 motoneurons. 16 motoneurons which could not be activated by rubral stimulation also responded by IPSPs to lingual nerve stimulation. The presented data testify to the existence of two rubrobulbar systems connected by polysynaptic pathways mainly with motoneurons of tongue retractor muscle.     

An Organotypic Spinal Cord - Dorsal Root Ganglion - Skeletal Muscle Coculture of Embryonic Rat. I. The Morphological Correlates of the Spinal Reflex Arc

The cytoarchitecture of a spinal cord - dorsal root ganglion - skeletal muscle tissue coculture system was investigated at the level of the light microscope using a number of different staining techniques. In these cultures central synapses between dorsal root ganglion (DRG) cells and interneurons in the ventral spinal cord and between DRG cells and motoneurons were visualized by parvalbumin immunostaining and by intracellular horseradish peroxidase (HRP) filling of DRG cells. Skeletal muscle fibres regenerated in vitro first into multinucleated myotubes, and around day 8 in vitro into well differentiated muscle fibres with regular cross-striation. At the same time newly formed motor endplates could be visualized using acetylcholinesterase staining. The axons of motoneurons could be traced retrogradely by local application of HRP to the regenerated muscle fibres. The motor axons sometimes gave off collaterals reminiscent of Renshaw collaterals at about 300 microm from the axon hillock. Intracellular filling to motoneurons with HRP revealed that only a minority of the motoneurons within a culture had reached their appropriate target. Comparing the dendrograms of the motoneurons which had innervated muscles to those which had not suggested that motoneurons innervating muscle tissue had more complex dendritic trees and larger somata than those which did not innervate muscle tissue. Peripheral neurites of parvalbumin-immunoreactive DRG cells coiling around regenerated muscle fibres could be demonstrated in these cultures. These probably correspond to that part of the sensory muscle spindle apparatus which developed in vivo. However, only a few of the several hundred DRG cells found in every culture were parvalbumin-immunoreactive, suggesting that the actual number of Ia and II afferents within the population of DRG cells in culture is very small. This study demonstrates that all the neural elements necessary for the segmental spinal reflexes develop and can be maintained for several weeks in vitro.     

Synaptic inhibition of accessory motoneurons evoked by stimulation of the trigeminal nerve in the cat.

Stimulation of the trigeminal nerve produced polysynaptic inhibitory postsynaptic potentials (IPSPs) in accessory motoneurons of the cat. This contrasts with the observation that dorsal cervical motoneurons responded with EPSPs to trigeminal stimulus. Stimulation of the rostral part of spinal trigeminal nucleus elicited di- or polysynaptic IPSPs in accessory motoneurons. Transection of the anterior funiculus at the upper cervical cord selectively abolished the IPSPs. The IPSPs were antagonized by systematically administrated strychnine but not bicuculline.     

Morphological analysis of cat masseteric motoneurons after intracellular staining with horseradish peroxidase

Intracellular injection of horseradish peroxidase (HRP) into 58 masseteric motoneurons identified by antidromic activation was performed in cats under pentobarbital anesthesia. Monosynaptic EPSPs were evoked by masseteric nerve stimuli in 52 cells, and were absent in the remaining six cells. The antidromic nature of the evoked spikes was confirmed by IS-SD separation observed at high frequency (50 Hz) stimulation. Motoneurons with monosynaptic excitation from masseter afferents showed IPSPs following stimulation of lingual and inferior alveolar nerves. Motoneurons which did not show monosynaptic excitation from masseter afferents showed no IPSPs from the above nerves. There were no differences in cell size or the number of stem dendrites between motoneurons with and without monosynaptic EPSPs. No recurrent collaterals were observed in any motor axons. Motoneurons with monosynaptic EPSPs were located at all rostrocaudal levels throughout the trigeminal motor nucleus, whereas motoneurons without such EPSPs were encountered only at the middle level. Dendrites of motoneurons with monosynaptic EPSPs did not extend into the medial portion of the nucleus where motoneurons innervating the anterior belly of the digastric muscle were located. In contrast, motoneurons without monosynaptic EPSPs had dendrite branches extending well into the medial part. The results show that there are two subpopulations of masseteric motoneurons that differ in peripheral inputs as well as dendritic morphology.     

Polysynaptic neuronal pathways from group I and group II afferents innervating tail muscles to hindlimb motoneurons in the cat

Postsynaptic potentials evoked in motoneurons innervating m. posterior biceps and semitendinosus (PBSt) and m. triceps surae (GS) by low threshold afferents from various tail muscles located at the level of the second–third caudal vertebrae were investigated in the non-anesthetized and spinalized cat. Afferent inputs from tail muscles on both sides predominantly evoked depolarizing potential in PBSt motoneurons and hyperpolarizing potential in GS motoneurons. The findings suggest that in general, tail muscle afferents facilitate flexor and inhibit extensor hindlimb motoneurons through polysynaptic pathways, so that the pelvic girdle is kept in a low position to maintain the stability of the body irrespective of different movements or posture of the tail.     

Stimulus time-locked responses of motoneurons during forelimb fictive locomotion evoked by repetitive stimulation of the lateral funiculus

In cat forelimb fictive locomotion evoked by repetitive stimulation of the upper cervical lateral funiculus, locomotor discharges consisted of activities time-locked to each stimulus, which were rhythmically modulated. The stimulus time-locked activities were investigated by intracellular recording from motoneurons. In both elbow flexor and extensor motoneurons, there observed stimulus time-locked disynaptic EPSPs, trisynaptic IPSPs and polysynaptic EPSPs, all of which were rhythmically modulated with specific patterns. The disynaptic EPSPs of flexor motoneurons were facilitated in the flexor phase of locomotion, whereas those of extensor motoneurons were facilitated from the flexor phase to the flexor-to-extensor transition phase. Modulation depth was larger in flexor motoneurons. Trisynaptic IPSPs changed in amplitude in parallel with the disynaptic EPSPs of the antagonistic motoneurons. Late, polysynaptic EPSPs of both flexor and extensor motoneurons increased in amplitude along with corresponding nerve discharges. After lesions of the lateral funiculus at C6/C7, both the disynaptic EPSPs and trisynaptic IPSPs were abolished in the motoneurons located caudally to the lesions. However, only trisynaptic IPSPs were lost in the rostrally located motoneurons. Furthermore, the lesions disclosed that extensor motoneurons received another kind of stimulus time-locked EPSPs, trisynaptic EPSPs, which were transmitted through the ventral part of the spinal cord, and rhythmically facilitated in the extensor phase. Stimulus time-locked PSPs observed in this study may at least in part be evoked by last-order interneurons of the central pattern generator, which may be reciprocally organized.     

Changes in the postsynaptic responses of spinal cord motor neurons against a background of rhythmic stimulation of the locus coeruleus

Influences of locus coeruleus rhythmical stimulation on the postsynaptic reactions of spinal motoneurons were studied in chloralose anesthetized cats. IPSPs evoked by flexor reflex afferent (FRA) stimulation were inhibited, but EPSPs evoked by FRA and PSPs evoked by low-threshold muscle afferents stimulation were unchanged under these conditions. The inhibition of IPSPs disappeared in reserpine-pretreated animals. Both IPSPs and EPSPs evoked by FRA stimulation were diminished also during rhythmical stimulation of nucleus reticularis pontis oralis. However this effect was not of monoaminergic nature because it was present in reserpine-pretreated animals.     

Presynaptic mechanisms of the change in segmental responses accompanying the formation of the spinal locomotor generator in the cat

It is shown on unanesthetized immobilized decorticated cats that spinalization of the animal leads to depolarization of the central afferent terminals, decrease of early polysynaptic responses in motoneurons and dorsal root potentials (DRP) evoked by stimulation of the low-threshold cutaneous and muscle afferents, increase of early polysynaptic responses and DRP evoked by stimulation of high-threshold muscle afferents, reduction in the activity of intermediate nucleus interneurons mono- and polysynaptically connected with primary afferents, rise of the activity of interneurons di- and oligosynaptically connected with primary afferents. Injection of DOPA into spinal animals leads to opposite changes. Dependence between changes in the state of segmental neuronal apparatus and the level of spinal locomotor generator activity are discussed on the basis of the data obtained.     

Tongue-muscle-controlling motoneurons in the Japanese toad: neural inputs from the thalamus

The anuran tongue is an effector organ specialized for snapping up prey during visually guided prey-catching behavior. As a step toward elucidating the control mechanisms of the tongue movement and overall organization of visually guided behavior, properties of neural inputs from the thalamus (of which electrical stimulation elicited a behavior very similar to the visually guided predator-avoidance behavior under freely behaving conditions) were investigated in paralyzed Japanese toads. Tongue-muscle-controlling motoneurons (tongue-protractor motoneurons (PMNs) and tongue-retractor motoneurons (RMNs)) were identified antidromically, and synaptic inputs in response to electrical stimuli applied to various points in the thalamus (mainly the posterocentral thalamic nucleus) were examined. Hyperpolarizing potentials were evoked in both PMNs and RMNs in response to single electrical stimuli applied to the thalamus contralateral or ipsilateral to the recording side. Since these potentials reversed to depolarizing ones after injecting Cl- ions into the cell interior, these hyperpolarizing potentials were concluded to be the usual fast type of inhibitory postsynaptic potentials (IPSPs). On the other hand, depolarizing potentials which were superimposed on the underlying IPSPs were evoked when repetitive electrical stimuli were applied to the thalamus. The amplitude of these depolarizing potentials was decreased when depolarizing currents were injected intracellularly, while it was increased when hyperpolarizing currents were injected, indicating that these depolarizing potentials are excitatory postsynaptic potentials (EPSPs).(ABSTRACT TRUNCATED AT 250 WORDS)     

The pharmacology of synapses formed by identified corticocollicular neurons in primary cultures of rat visual cortex

Primary cultures of neurons from the visual cortex of 7-10-d-old Long Evans rats were used to study the pharmacology of synaptic transmission. Dissociated cells were grown either in mass cultures, which contained 8000-10,000 neurons, or in miniature island cultures of 50-100 cells. Prior to dissociation, cells in layer V of visual cortex that project to the superior colliculus were labeled in vivo by retrograde transport of fluorescent latex microspheres-a permanent fluorescent marker. After 2 d to 8 weeks in culture, labeled neurons were identified by epifluorescent illumination, and electrophysiological recordings were obtained from a labeled cell and, simultaneously, from a nearby unlabeled neuron in the same field of view. The 2 neurons were stimulated sequentially by current injection and the pharmacology of evoked postsynaptic potentials (PSPs) was investigated. In mass cultures, relatively few pairs of neurons from which we recorded were synaptically connected, although nearly every cell exhibited abundant spontaneous EPSPs and IPSPs. Neurons grown on island cultures generally did not exhibit spontaneous synaptic activity; however, stimulation of one of the cells in a pair frequently elicited a short-latency PSP in the follower neuron. Retrogradely labeled corticocollicular neurons produced only excitatory PSPs in follower cells, while unlabeled neurons were either excitatory or inhibitory. Three antagonists of excitatory amino acid receptors, kynurenic acid, piperidine dicarboxylic acid, and gamma-D-glutamylglycine, completely blocked EPSPs produced by labeled corticocollicular neurons, as well as EPSPs produced by nearly all of the unlabeled excitatory cells. We have previously shown that these compounds block both N-methyl-D-aspartate (NMDA)-type and non-NMDA receptors on cultured cortical neurons (Huettner and Baughman, 1986). The specific NMDA receptor antagonist 2-amino-5-phosphonovaleric acid (APV) did not alter short-latency EPSPs recorded in 1 mM Mg2+, but did reduce longer-latency EPSPs polysynaptic activity. Since responses mediated by the NMDA receptor are known to be antagonized by Mg2+ (Mayer and Westbrook, 1985), we perfused cultures with Mg2+-free medium and found that the falling phase of some monosynaptic EPSPs was prolonged. Addition of APV to Mg2+-free medium reduced the duration of the falling phase of EPSPs such that they returned to the time course obtained in 1 mM Mg2+.(ABSTRACT TRUNCATED AT 400 WORDS)     

Specific monosynaptic sensory-motor connections form in the absence of patterned neural activity and motoneuronal cell death

The importance of neural activity and motoneuronal cell death in the formation of specific synaptic connections between muscle afferents and motoneurons was studied in chick embryos. Patterned neural activity was blocked by applying d-tubocurarine (dtc) chronically to embryos during the period when sensory-motor connections are formed [stages (St) 28-42]. Dtc blocks neurogenic muscle contractions, thereby abolishing any temporal correlation between neural activity in motoneurons and stretch-sensitive afferents. The normal pattern of motoneuronal bursting is also blocked (Landmesser and Szente, 1986), as is motoneuronal cell death (Pittman and Oppenheim, 1979). Dtc applications were started more than 1 d before muscle sensory afferent collaterals make anatomical contact with motoneuronal dendrites and continued until St 38-42, when the pattern of synaptic connectivity was examined by recording synaptic potentials intracellularly from identified lumbosacral motoneurons upon stimulation of identified populations of muscle afferents. In both normal and dtc-treated animals, large monosynaptic excitatory potentials were evoked in homonymous motoneurons (those that supply the same muscle as the sensory afferents) and were often observed in motoneurons that supplied synergistic muscles. Monosynaptic potentials were uncommon in motoneurons supplying antagonistic muscles. The overt patterns of sensory-motor connections in normal and dtc-treated embryos were essentially identical. However, the amplitudes of the composite EPSPs recorded in dtc-treated animals were consistently about twice as large as normal. These observations suggest that neither normal patterns of neuronal activity nor motoneuronal cell death play a large role in determining the specificity of connections between the sensory and motor neurons involved in the stretch reflex.     

Synaptic inputs to medullary respiratory neurons from superior laryngeal afferents in the cat.

Synaptic inputs from afferents in the superior laryngeal nerve (SLN) to medullary respiratory neurons (n = 154) in the dorsal respiratory group (DRG), ventral respiratory group (VRG) and the region of the B?tzinger complex (BOT) were studied in anesthetized cats. Single pulse stimulation of the SLN-evoked monosynaptic EPSPs in most inspiratory bulbospinal (I-BS) neurons in the DRG, and disynaptic or oligosynaptic chloride-dependent IPSPs in other I-BS neurons in the DRG and VRG. Stimulation of laryngeal afferents also inhibited oligosynaptically expiratory bulbospinal neurons in the VRG, and all types of respiratory neurons recorded in the BOT region. Oligosynaptic potentials (usually EPSPs) were recorded in inspiratory and expiratory laryngeal motoneurons. These results provide evidence of a processing of SLN-evoked synaptic responses by all tested groups of medullary respiratory neurons. The pathways mediating these synaptic responses are discussed.     

Reduction of GABAB inhibitory postsynaptic potentials by serotonin via pre- and postsynaptic mechanisms in CA3 pyramidal cells of rat hippocampus in vitro.

The action of serotonin (5-HT) on GABAergic synaptic transmission was investigated with intracellular recordings in CA3 pyramidal cells of rat hippocampal slices. Local application of 5-HT (500 microM) hyperpolarized CA3 pyramidal cells, decreased cellular input resistance, and reduced slow afterhyperpolarizations. Serotonin attenuated the late (GABAB) component of polysynaptic inhibitory postsynaptic potentials (IPSPs; 47% of control) without affecting the early (GABAA) component. During bath application of the excitatory amino acid antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) (20 microM) and 2-amino-5-phosphonovalerate (AP-5) (40 microM), 5-HT similarly decreased the amplitude of the late (GABAB) component (17% of control) of monosynaptic IPSPs but did not affect the early (GABAA) component. The mean reversal potentials of poly- and monosynaptic IPSPs were unaffected by 5-HT. The conductance increases associated with the late component of poly- and monosynaptic IPSPs were reduced by 5-HT. Hyperpolarizing responses evoked in CA3 pyramidal cells by somatic applications of gamma-aminobutyric acid (GABA) were unaffected by 5-HT. During bath application of bicuculline (20-50 microM), hyperpolarizing responses elicited by dendritic GABA application were reduced by 5-HT (71% of control). The effect of 5-HT on these direct GABAB hyperpolarizations (29% decrease in response) does not appear sufficient to fully account for the effect of 5-HT on late GABAB IPSPs (53-83% decrease in response). Therefore, 5-HT may reduce GABAB IPSPs in CA3 pyramidal cells 1) by a postsynaptic action on pyramidal cells and 2) by a selective presynaptic action on GABAergic interneurons mediating the GABAB IPSP. This presynaptic action of 5-HT does not appear to involve excitatory afferents onto inhibitory interneurons.     

EPSP's of the motor neurons of cat masticatory muscles induced by stimulation of low-threshold infraorbital nerve fibers]

The effects of the infraorbital Aalpha afferents stimulation on the masseter motoneurons were investigated in cats under chloralose-nembutal anesthesia. It is found that stimuli 1.4-2.5 times threshold (T) evoke complex EPSPs in 69% of the investigated motoneurons, their latency being 2.1 +/- 0.2 ms (1.5-3.0 ms, n = 36), amplitude up to 30 mV and duration 9-15 ms. These EPSPs consist of two simpler responses produced by activation of two separate groups of afferent fibres. The short-latent EPSPs arise after activation of the infraorbital nerve by 1.1-1.5 T stimuli and have the same latency as the complex EPSP (1.5-3.0 ms) but a smaller amplitude (up to 2.0 mV) and shorter duration (up to 6 ms). The stability of these EPSPs during high frequency stimulation (120/c) and the development of facilitation and inhibition similar to those which appear in monosynaptic EPSPs in masseter motoneurons during stimulation of the 1a muscle afferents give reason to suggest that these EPSPs are monosynaptic. The slow rising rate indicates that they appear on distal dendrites of the motoneurons. The long-latent EPSPs arise with latency of 7-9 ms after activation of the infraorbital nerve by 1.1-1.5 T stimuli. Their amplitude reaches 1.5-2.0 mV and duration 7-9 ms. The long latency of these EPSPs in combination with low ability to repeat high frequency stimulation are consistent with their polysynaptic origin. It is suggested that the excitatory input from the lowest threshold Aalpha afferents of the infraorbital nerve to masseter motoneurons creates conditions for the development of a transient jaw closing reflex in response to light tactile stimulation of the cat's perioral region.     

An intracellular recording study of stimulus-specific response properties in cat area 17

Stimulus-specific response properties, such as direction or orientation selectivity, were studied intracellularly in cells recorded from area 17 of the cat. In all 5 direction selective complex cells and one orientation selective simple cell successfully studied, visually evoked excitatory postsynaptic potentials (EPSPs) were tuned to the preferred direction or orientation. Visually evoked inhibitory postsynaptic potentials (IPSPs) were also tuned to the preferred direction/orientation of stimulus. IPSPs evoked by the non-preferred stimulus when present were smaller than those evoked by the preferred stimulus. IPSPs were undetected in two of the 5 cells tested. These results suggest that directionally/orientationally tuned EPSPs make a major contribution to stimulus specificity in visual cortical neurons but IPSPs evoked by a stimulus with null-direction/orientation may sharpen the stimulus specificity.