Somatosensory cortical unit responses during avoidance conditioning to sound

Unit responses of the primary somatosensory cortical-projection area were recorded in cats with established conditioned avoidance to sound during the conditioned response. Five types of changes in unit activity were distinguished during the conditioned reflex. The responses were 400–1000 msec in duration; their time of onset coincided with that of the expected reinforcing stimulus. As a rule the change in unit activity was preceded by changes in the electromyogram of the corresponding limb muscles. Among neurons responding to the conditioning stimulus, the proportion with a short latent period of response to electrical stimulation of the skin was less than during investigation of ordinary unit responses to the same stimulus.     

Somatosensory cortical unit responses in the waking cat to afferent stimuli

Extracellular and intracellular responses of 183 neurons in the primary projection area of the somatosensory cortex to electrical and tactile stimulation of the skin on the contralateral fore limb and to stimulation of the ventro-posterolateral thalamic nucleus of the ipsilateral hemisphere were studied in chronic experiments on cats. Spike responses to afferent stimuli are subdivided into three types: initial with a latent period of under 60 msec; initial followed by late responses with a latent period of over 60 msec; late with a latent period of over 60 msec. In addition another group of neurons responding to peripheral stimuli in the interval between the initial and the late response was identified. In nearly all cases the initial responses to peripheral stimulation had the form of a series of spikes, unlike responses to thalamic stimulation. It is concluded from the durations of the latent periods of these responses that about 70% of neurons in the primary projection area are activated mono- and disynaptically in response to peripheral stimulation; consequently, the intracortical spread of excitation in this zone is restricted.     

Visual cortical unit responses to photic stimulation of different zones of the receptive fields in cats

In acute experiments on unanesthetized curarized cats the intensity functions, response thresholds, inhibition thresholds, and differential sensitivity of 96 neurons in the primary visual projection cortex were investigated by extracellular recording of unit activity during central and peripheral stimulation of their receptive fields. In darkness the neurons had wide threshold and above-threshold reliefs (3–30°). The threshold reliefs of the receptive fields of some cells were found to be V-shaped, whereas others were marked by alternation of zones of increased and reduced excitability. Sensitivity of both excitatory and inhibitory inputs of the receptive field as a rule was greatest in the center. Inhibitory inputs of different cortical neurons were much more standard and less sensitive to light, and they were mainly activated within the intermediate (mesoptic) range of brightnesses. During light adaptation the threshold contour of the receptive field narrows sharply, mainly because of the fall in sensitivity of its peripheral inputs. Compared with the lateral geniculate body and retina, the relative number of low-threshold elements, sensitivity in the system of inhibitory elements, and differential brightness sensitivity are greater in the cortex. The mechanisms of formation of receptive fields of cortical neurons and their modification during changes in the level of adaptation, and also the role of excitatory and inhibitory inputs of the cell in these effects are discussed.Institute of Higher Nervous Activity and Neurophysiology, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 11, No. 3, pp. 227–235, May–June, 1979.     

Responses of primary somatosensory cortical neurons to paired stimulation of the infraorbital nerve and vibrissae in cats

Extra- and intracellular responses of 128 neurons to paired stimulation of the infraorbital nerve and vibrissae, recorded in the projection zone of the vibrissae in cortical area SI, were studied in adult cats immobilized with tubocurarine. Conditioning stimulation completely suppressed the ability of different neurons to respond for periods of between 10 and 120 msec. The duration of the period of total suppression of test responses was shown to depend on the location of the stimulated vibrissa in the peripheral receptive field of the neurons studied. Excitatory and inhibitory responses of maximal intensity arose in the neurons to stimulation of receptive field centers. The functional role of the decrease in intensity of excitatory responses during stimulation of vibrissae located at different distances from centers of the receptive fields of cortical neurons is discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 13, No. 2, pp. 117–124, April, 1981.     

Conditions of appearance of off responses of sensomotor cortical neurons to photic stimulation in cats

In acute experiments on cats under chloralose anesthesia (70 mg/kg) unit activity was recorded extra- and intracellularly in the sensomotor cortex (areas 4 and 6) during prolonged (up to 1000 msec) photic stimulation. Responses of on-off type were generated by 100% of neurons tested to photic stimuli whose duration corresponded to the recovery cycle of functional changes after a single flash, determined by the paired stimulation method. Cutaneous stimulation affected the appearance of the photic off response if it led to a spike discharge of the neuron before the off response. It is suggested that IPSPs of cortical neurons largely determine both the duration of the cycle of functional recovery after a single flash and also differences in the pattern of generation of the off response and its interaction with responses to cutaneous stimulation.A. A. Zhdanov Leningrad State University. Translated from Neirofiziologiya, Vol. 9, No. 4, pp. 355–360, July–August, 1977.     

Effect of intracortical chemical stimulation on spontaneous sensomotor cortical unit activity in cats

The effect of microelectrophoresis of glutamate on spontaneous activity of sensomotor cortical neurons located 80–250 µ from the point of application of glutamate was studied in cats anesthetized with pentobarbital. If glutamate was applied at distances of under 100 µ from the neurons the predominant response was one of excitation, evidently due to the direct action of the excitatory mediator. With more distant application inhibition of spontaneous activity predominated: at distances from 100 to 200 µ it was observed in 57%, and between 200 and 250 µ, in 70% of cases. Application of picrotoxin close to the neuron weakened inhibition induced by microelectrophoresis of glutamate through a neighboring microelectrode.A. A. Ukhtomskii Physiological Research Institute, A. A. Zhdanov Leningrad State University. Translated from Neirofiziologiya, Vol. 14, No. 4, pp. 347–352, July–August, 1982.     

Unit responses of the reticular and ventral anterior thalamic nuclei of cats to orbitofrontal cortical stimulation

Responses of 98 neurons of the reticular (R) and 72 neurons of the ventral anterior (VA) thalamic nuclei to stimulation of various zones of the orbitofrontal cortex were investigated in acute experiments on cats immobilized with D-tubocurarine. Not all zones of this cortex were found to be connected equally closely with R and VA. Most of the R (82.7%) and VA (66.7%) neurons responded to stimulation of the proreal gyrus, and fewest (37.3 and 48.9%, respectively) to stimulation of the posterior orbital gyrus. Among the responding neurons, 85.2–86.3% of R cells and 78.2–81.2% of VA cells were excited by cortical stimulation and the rest were inhibited. Excitation was expressed as the appearance of a single spike or of discharges of varied duration in response to each stimulus. The latent period of the spike responses varied from 0.5 to 55.0 msec and the minimal latent period of the discharges was 0.8 msec and its maximal value over 500 msec. The spike frequency in the discharge was 120–250/sec. Unit responses consisting of spikes with a latent period of under 1.3 msec and, it is assumed, some of the responses with a latent period of under 4.0 msec were antidromic. The axons of some R and VA neurons were shown to form branches terminating in different zones of the orbitofrontal cortex.     

Dynamics of auditory cortical unit responses of the waking cat during defensive conditioning to acoustic stimulation

In chronic experiments on cats unit responses of the primary auditory cortex (area 50) were studied by microelectrode recording during defensive conditioning to sound. During formation of the reflex biphasic responses with relatively short-latency (50–100 msec) and longer-latency (400–500 and 800–900 msec) activation predominated. Neighboring neurons, whose activity was recorded by the same microelectrode, also were involved more intensively in activity. Application of a differential stimulus in 70% of cases produced definite changes in unit activity, among which responses of activation type predominated. Analysis of the course of spike responses of the same neuron during the period of action of a large number of combinations and its comparison with the formation of the conditioned-reflex motor response revealed no direct correlation between these events.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 11, No. 2, pp. 99–108, March–April, 1979.     

Habituation of direct cortical responses in cats

Gradual diminution of direct responses of various cortical areas was studied in acute experiments on cats under barbiturate anesthesia. During repetitive stimulation of the surface of cortical association areas these responses underwent all changes characteristic of the habituation phenomenon. Direct cortical responses in primary projection areas were distinguished by great stability, and habituation was absent in this case. After neuronal isolation of an island of projection cortex, habituation of responses began to develop there just as in the cortical association areas. The possible causes of differences between changes in direct cortical responses in different areas are discussed. It is suggested that projection areas receive strong activating influences from deep brain structures, which exert a sensitizing effect on the direct responses.I. S. Beritashvili Institute of Physiology, Academy of Sciences of the Georgian SSR, Tbilisi. Translated from Neirofiziologiya, Vol. 13, No. 3, pp. 241–246, May–June, 1981.     

Abnormal cortical responses to somatosensory stimulation in medication-overuse headache

Background

Medication-overuse headache (MOH) is a frequent, disabling disorder. Despite a controversial pathophysiology convincing evidence attributes a pivotal role to central sensitization. Most patients with MOH initially have episodic migraine without aura (MOA) characterized interictally by an absent amplitude decrease in cortical evoked potentials to repetitive stimuli (habituation deficit), despite a normal initial amplitude (lack of sensitization). Whether central sensitization alters this electrophysiological profile is unknown. We therefore sought differences in somatosensory evoked potential (SEP) sensitization and habituation in patients with MOH and episodic MOA.

Methods

We recorded median-nerve SEPs (3 blocks of 100 sweeps) in 29 patients with MOH, 64 with MOA and 42 controls. Episodic migraineurs were studied during and between attacks. We measured N20-P25 amplitudes from 3 blocks of 100 sweeps, and assessed sensitization from block 1 amplitude, and habituation from amplitude changes between the 3 sequential blocks.

Results

In episodic migraineurs, interictal SEP amplitudes were normal in block 1, but thereafter failed to habituate. Ictal SEP amplitudes increased in block 1, then habituated normally. Patients with MOH had larger-amplitude block 1 SEPs than controls, and also lacked SEP habituation. SEP amplitudes were smaller in triptan overusers than in patients overusing nonsteroidal anti-inflammatory drugs (NSAIDs) or both medications combined, lowest in patients with the longest migraine history, and highest in those with the longest-lasting headache chronification.

Conclusions

In patients with MOH, especially those overusing NSAIDs, the somatosensory cortex becomes increasingly sensitized. Sensory sensitization might add to the behavioral sensitization that favors compulsive drug intake, and may reflect drug-induced changes in central serotoninergic transmission.

     

Dynamics of cortical unit responses during defensive conditioning to sound

Several phases were distinguished in single-unit responses in areas 3 and 4 during defensive conditioning to acoustic stimulation: an initial response, short inhibition of the spike discharge, early and late after-discharges, and changes arising after the end of acoustic stimulation. The initial spike response appeared or intensified (if present already) in the first period of defensive conditioning parallel with an increase in spontaneous unit activity. After-discharges appeared later. The conditioned-reflex movement usually began 100–400 msec after stimulation began. This latent period of the first movement was the same whether for a real conditioned reflex or an after-discharge. Comparison of the latent periods of conditioned movements with the phases of the unit responses showed that the conditioned responses of the cortical neuron were primarily modified after-discharges of neurons evoked by a conditioned stimulus. Differential unit responses to acoustic stimulation, also based on after-discharges, were formed just as actively as positive. The basic role of reinforcement during conditioning is not to increase the excitability of the neurons, which is important in connection with their acquisition of polysensory properties, but to modify the after-discharges of the neurons.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 10, No. 4, pp. 339–347, July–August, 1978.     

Effects of repetitive stimulation of the locus coeruleus on spinal inhibitory responses to suprasegmental stimulation in cats

Effects of repetitive stimulation of the locus coeruleus on spinal responses to activation of cortico-, reticulo-, and vestibulospinal tracts were studied in decerebellate cats anesthetized with chloralose. Descending influences of these structures were assessed from changes in amplitude of extensor and flexor monosynaptic discharges or from the magnitude of postsynaptic potentials recorded from the corresponding motoneurons. Stimulation of the motor cortex or modullary reticular formation as a rule evoked two-component inhibitory responses in extensor motoneurons and excitatory-inhibitory responses in flexor motoneurons. Stimulation of locus coeruleus effectively depressed the amplitude of the late component and, to a lesser degree, that of the early component of inhibition arising after stimulation of the cerebral cortex or reticular formation. During stimulation of the locus coeruleus no marked changes were found in inhibitory responses evoked by vestibulospinal influences in flexor motoneurons, and also in excitatory responses arising after stimulation of the above-mentioned descending pathways in both groups of motoneurons.     

Reorganization of segmental responses to peripheral stimulation during fictitious scratching in cats

Segmental reflex responses of the lumbosacral region of the spinal cord to stimulation of peripheral afferents were studied in immobilized decerebrate cats before and after application of D-tubocurarine or bicuculline to the superior cervical segments, potentiating the scratch reflex, and also during fictitious scratching evoked by mechanical stimulation of the ear. Application of these substances led to inhibition of the N₁-component of the dorsal cord potential, the dorsal root potential, and polysynaptic responses in efferent nerves. The appearance of fictitious scratching was accompanied by additional tonic inhibition of these responses, against the background of which modulation of the amplitudes of the responses was observed depending on the phase of fictitious scratching. Modulation of amplitudes of monosynaptic reflexes also developed during fictitious scratching. Against the background of these results the mechanisms and physiological role of reorganization of segmental responses during activation of the spinal scratching generator are discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 13, No. 2, pp. 196–203, March–April, 1981.     

Retuning of segmental responses to peripheral stimulation in cats during fictitious locomotion

Segmental responses of the lumbosacral region of the spinal cord to peripheral afferent stimulation were studied in decorticated, immobilized cats before and during fictitious locomotion. The appearance of fictitious locomotion was accompanied by a tonic increase in the N₁-component of the dorsal cord potential and dorsal root potential. Against the background of this tonic increase, modulation of these responses depending on the phase of fictitious locomotion was observed. When the N₁-component and dorsal root potential were evoked at the end of the "extension" phase and at the beginning of the "flexion" phase their amplitude was greater, but when they were evoked at the end of the "flexion" phase and the beginning of the "extension" phase it was smaller. Polysynaptic and monosynaptic reflex response of motoneurons exhibited the same phase dependence during fictitious locomotion. The mechanisms and physiological importance of this retuning of segmental responses are discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 13, No. 3, pp. 283–291, May–June, 1981.     

Unit responses of the clare-bishop cortical association area in the cat to photic stimulation

Electrical activity of single unit in the Clare-Bishop visual association area of the cortex was studied in acute experiments on cats immobilized with Flaxedil and after pretrigeminal sections. The method of extracellular recording of action potentials of single units was used. The experimental results showed that 95.5% of cells responding to visual stimulation responded to movement of a spot of light in the receptive field of the neurons, and 55% of the cells responded selectively to the direction of movement. Some neurons responded to movement of a stimulus only when it entered and left the receptive field. About 85.3% of cells responded to a flashing spot of light, and also to a general change in the intensity of illumination of the receptive field. The receptive field of neurons of the Clare-Bishop area in most cases were in the form of stripes with their long axis horizontal. The results point to the important role of this cortical association area in the central analysis of visual information.L. A. Orbeli Institute of Physiology, Academy of Sciences of the Armenian SSSR, Erevan. Translated from Neirofiziologiya, Vol. 10, No. 1, pp. 22–29, January–February, 1978.     

Responses of secondary sensomotor cortical neurons to electrodermal and acoustic stimulation in waking cats

Unit responses in the second somatosensory cortical projection area (SII) to clicks and electric shocks applied to the contralateral limb were investigated in chronic experiments on cats. In response to specific stimulation for the cortical region studied the discharge frequency of 75% of neurons increased, spontaneous activity of 18% was reduced in frequency or the discharges ceased altogether, and 25% of cells did not respond. In response to "nonspecific" stimulation (clicks) 30% of neurons were activated; the discharge of 25% of cells was inhibited and 45% did not respond. The results of investigation of intersensory convergence of stimuli from different sensory systems showed that a high proportion (55%) of SII neurons give bimodal responses. Another 18% of neurons give a specific response to both adequate and inadequate stimulation. It is suggested that the presence of polysensory convergence of SII neurons and of short pathways for the conduction of sensory information, and also the ability of neurons to acquire polysensory properties during stimulus presentation are evidence of the important role of this cortical region in conditioning.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 9, No. 5, pp. 453–459, September–October, 1977.