Responses of neurons of the first and second somatosensory zones of the cortex to peripheral, thalamic and cortical stimulation

Experiments were performed on cats immobilized with d-tubocurarine or myorelaxin. Neuronal responses were studied in the first somatosensory cortex (SI) to the second somatosensory cortex (SII), ventroposterior nucleus (VP) and contralateral forepaw stimulation. Besides, neuronal responses in SII to SI, VP and contralateral forepaw stimulation were also studied. It was shown that in SII the percentage of neurons excited by afferent volley with two or more synaptic change-overs in the cerebral cortex was larger than in SI. Neurons of SI and SII responded to cortical stimulation ortho- and antidromically, thus confirming the existence of bilateral cortico-cortical connections. Both in SI and SII, PSPs to cortical stimulation were similar in character to PSPs in the same neurons to VP stimulation. In 50.0% of SI neurons and 37.1 of SII neurons the difference in latencies of orthodromic spike potentials to VP and cortical stimulation was less than 1.0 ms. In 19.6% of SI neurons and 41.4% of SII neurons the latency of the response to cortical stimulation was 1.6-4.7 ms shorter than that of the response in the same neuron to VP stimulation. It is supposed that impulses from SI participate significantly in afferent activation of SII neurons.     

Visceral pain: responses of the reticular formation neurons to gallbladder distension

Visceral projection (gallbladder distension) to the gigantocellular nucleus of the reticular formation of the cat was tested in neurons classified as pain (P), nonpain-pain (NP-P) and nonpain (NP) units, according to their responses to noxious and/or innocuous natural stimuli from the somatic areas. 96% of P neurons (23 out of 24) responded to gallbladder distension. Quantitative criteria showed comparable effectiveness of the somatic and visceral inputs. NP-P neurons reacted to the gallbladder stimulation in 71% of cases (22 out of 31); NP neurons were activated less effectively. Stimulation of either the central tegmental field or "nonspecific" thalamic nuclei evoked direct responses in 38% of P and 26% of NP-P units, which, in most of the P neurons were followed by excitatory and inhibitory phases. The duration of the latter was approximately one second and it greatly affected the responses of the units to somatic as well as to visceral inputs. A large proportion of P neurons responding to a visceral input documents the important role of the reticular formation in the mechanisms of visceral pain. Findings concerning comparable modifying influences upon reactions of P units both in the case of visceral and painful somatic afferentation indicated that similar control mechanisms could be involved.     

Sensorimotor integration in human primary and secondary somatosensory cortices

We measured somatosensory evoked fields (SEFs) to electric median nerve stimuli from eight healthy subjects with a whole-scalp 122-channel neuromagnetometer in two different conditions: (i) 'rest', with stimuli producing clear tactile sensation without any motor movement, and (ii) 'contraction' with exactly the same stimuli as in 'rest', but with the subjects maintaining sub-maximal isometric contraction in thenar muscles of the stimulated hand. The aim was to study the role of the primary (SI) and secondary somatosensory (SII) cortices in sensorimotor integration. The amplitude of the SI response N20m did not change with coincident isometric contraction, whereas P35m was significantly reduced. On the contrary, activation of contra- and ipsilateral SII cortices was significantly enhanced during the contraction. We suggest that isometric contraction facilitates activation of SII cortices to tactile stimuli, possibly by decreasing inhibition from the SI cortex. The enhanced SII activation may be related to tuning of SII neurons towards relevant tactile input arising from the region of the body where the muscle activation occurs.     

Attention modulates both primary and second somatosensory cortical activities in humans: a magnetoencephalographic study

To clarify the role of primary and second somatosensory cortex (SI and SII) in somatosensory discrimination, we recorded somatosensory evoked magnetic fields during a stimulus strength discrimination task. The temporal pattern of cortical activation was analyzed by dipole source model coregistered with magnetic resonance image. Stimulus intensity was represented in SI as early as 20 ms after the stimulus presentation. The later components of SI response (latency 37.7 and 67.9 ms) were enhanced by rarely presented stimuli (stimulus deviancy) during passive and active attention. This supports an early haptic memory mechanism in human primary sensory cortex. Contra- and ipsilateral SII responses followed the SI responses (latency 124.6 and 138.3 ms, respectively) and were enhanced by attention more prominently than the SI responses. Active attention increased SII but not SI activity. These results are consistent with the concept of ventral somatosensory pathway that SI and SII are hierarchically organized for passive and active detection of discrete stimuli.     

Somatic sensory cortex of llama (Lama glama)

The somatic sensory cortex (SI and SII) was mapped in llamas using microelectrode mapping methods developed earlier in a study of SI of the slow loris. Projections to SI from the llama's prehensile browsing lips were differentially enlarged when compared to those reported for sheep. In llama, SII was reversed in its mediolatreal pattern from that reported for SII in most other mammals. Fissural landmarks reliably demarcated different projections within SI, between SI and SII and between SI or SII and other surrounding nonsensory areas. The use of microelectrode mapping methods in different mammals to determine gyral and fissural homologies is discussed.     

Isoflurane anesthesia blunts cerebral responses to noxious and innocuous stimuli: a fMRI study

We used functional magnetic resonance imaging to determine how isoflurane affected cerebral neuronal activation resulting from noxious and innocuous stimuli. Five male volunteers were subjected to mild electrical shock and tactile stimuli applied to the hand. During low (0.7%) and moderate (1.3%) isoflurane anesthesia the stimuli were repeated and a supramaximal electrical shock was also applied. Tactile stimulation activated bilateral SI and SII, but resulted in no significant activation at low or moderate anesthesia. Electrical shock activated contralateral SI and bilateral SII; low anesthesia completely abolished this response. The supramaximal stimulus activated the caudate nucleus and bilateral thalamus at low anesthesia; these responses were diminished at moderate anesthesia. Isoflurane anesthesia blunts cerebral responses to somatosensory stimuli, and the absence of cortical activation during supramaximal stimulation suggests that noxious-induced movement is generated in lower CNS structures.     

Visual, somatosensory, auditory and nociceptive modality properties in the feline suprageniculate nucleus

Response properties of 252 single-units to visual, auditory, somatosensory and noxious stimulation were recorded by means of extracellular microelectrodes in the suprageniculate nucleus of anaesthetized, immobilized cats. Of the 141 units tested for modality properties the majority (n=113, 80.1%) was found unimodal in the sense that stimuli of exclusively one sensory modality were able to elicit an activation of the unit. Twenty-four (17.0%) cells were bimodal and four (2.8%) were trimodal (visual, somatosensory and auditory). The visual modality dominated the unimodal cells (n=74, 65.5%), while cells responsive to somatic stimulation (n=20, 17.6%), auditory stimulation (n=16, 14.1%) or noxious stimulation of the tooth pulp (n=3, 2.6%) were less frequently encountered. Visual sensitivity dominated the multisensory cells, too. The visually responsive units were characterized by having a sensitivity to stimuli moving in a rather large, uniform receptive field that covered the contralateral lower quadrant, and encompassed a flanking area of about 20 degree width in both the upper contralateral and lower ipsilateral visual fields. Many cells (n=52, 47%) were sensitive to the direction of the stimulation and reacted to stimuli moving at a high velocity (20-200 deg/s). Most cells responded differently to stimuli of a variety of sizes. Somatosensory units reacted to stimuli presented over a wide area on the contralateral side of the body, thus showing no sign of somatotopic organization. The auditory sensitivity fell within a wide range of acoustic stimuli in extremely large auditory receptive fields. The physiological properties of suprageniculate nucleus cells strongly resemble the sensory properties of cells found along the ventral bank of the anterior ectosylvian sulcus and the deeper layers of the superior colliculus. Our results provide further support for the notion of a separate tecto-suprageniculate-anterior ectosylvian sulcus/insular pathway that takes part in the processing of multimodal signals important for various types of sensory related behaviours.     

Intracortical arborizations and receptive fields of identified ventrobasal thalamocortical afferents to the primary somatic sensory cortex in the cat

The intracortical arborizations of neurons from the ventroposterolateral thalamic nucleus (VPL) in the cat were studied by intraaxonal injections of horseradish peroxidase (HRP) following identification of their receptive fields. In the primary somatic sensory cortex (SI) VPL cells terminated in different cytoarchitectonic areas according to their receptive field modality. Fibers excited by deep tissue or joint rotation arborized preferentially in area 3a. Those responding tonically to cutaneous stimuli were located in the anterior part of area 3b; hairdriven cells terminated in area 3b and in the rostral pole of area 1. All fibers had a similar laminar distribution within SI. Axons terminated mostly in layers VI, iV, and the lower part of layer III. None terminated in layers I and II. Most terminal arbors were oriented along the mediolateral axis of the brain. The main arborization of a single VPL cell formed a bush of about 500 micrometers in diameter. some fibers generated two such bushes with an uninvaded region of about 300 micrometer between them. It is proposed that this patchy organization underlies in part the columnar organization of areas SI. Many VPL cells had secondary projection sites in SI. These were issued from smaller-sized collaterals and were located in a different cytoarchitectonic area than that of the main terminal plexuses. A significant number of these collaterals projected to area 4, Insufficient filling of the collaterals by HRP prevented a more complete characterization of the secondary arbors.     

Efferent and afferent connections of the nucleus lateralis posterior thalami ("pulvinar") in the albino rats (author's transl)

The efferent and afferent connections of the lateral posterior nucleus (LP) of the albino rat were investigated light microscopically with the silver-degeneration-methods and the HRP-methods as well. The results are: 1. The main projection region of the LP is the area of 18a of the peristriate visual cortex. Most degenerating axons terminate in layer IV. A few fibers pass layers III and II and terminate in layer I. It is not sure if there are also terminating fibers in layer IV. We could not find a topistic relation between LP and area 18 a. 2. We observed a small number of degenerating fibers in area 17, too. 3. A part of the degenerating fibers runs to the temporal cortex end enters area 20. 4. There is no evidence for a projection of the LP to both the subcortical regions and to the superior colliculus. 5. The majority of the LP's afferent fibers originates - on the subcortical level - from the superior colliculus. Especially the lamina III (Str. opticum) of the ipsilateral and of the contralateral side is here the source of fibers terminating in the LP. 6. Other subcortical sources of fibers terminating in the LP are: the pretectal region, the ventral part of the LGN, the Zona incerta, the thalamic reticular formation, and the dorsal raphe nucleus. 7. There exists a fiber projection of the area 17 to the LP. The axons originate mainly from pyramidal cells in layer V. It is discussed whether the area-17-fibers terminating in the LP are collaterals of the fibers terminating in the superior colliculus. The projection of the area 18a to the LP is of greater importance. The axons of this area originate mainly from cells of the layer VI. It becomes obvious that the thalamic relay-station of the second visual pathway seems to project nearly exclusively to the neocortex. In contrast to the dorsal LGN, however, the LP is not only a simple relay-station for visual information as also non-visual information arrives here. The morphological basis for these inputs has not yet been clarified completely. We have to take into consideration as well as the connections with the superior colliculus and the pretectal region and the cortical connections. It is remarkable that there exists also a projection of LP-fibers to a region outside the classical visual cortex. In mammals of higher evolution that kind of projection extends increasingly. It is discussed if - under comparative-anatomical aspect - the morphological changes in the pulvinar region are an expression of the neocorticalization, whereas the morphological changes in the dorsal LGN reflect mainly the functional specialization of the visual system.     

Organization of the visual reticular thalamic nucleus of the rat

The visual sector of the reticular thalamic nucleus has come under some intense scrutiny over recent years, principally because of the key role that the nucleus plays in the processing of visual information. Despite this scrutiny, we know very little of how the connections between the reticular nucleus and the different areas of visual cortex and the different visual dorsal thalamic nuclei are organized. This study examines the patterns of reticular connections with the visual cortex and the dorsal thalamus in the rat, a species where the visual pathways have been well documented. Biotinylated dextran, an anterograde and retrograde tracer, was injected into different visual cortical areas [17; rostral 18a: presumed area AL: (anterolateral); caudal 18a: presumed area LM (lateromedial); rostral 18b: presumed area AM (anteromedial); caudal 18b: presumed area PM (posteromedial)] and into different visual dorsal thalamic nuclei (posterior thalamic, lateral geniculate nuclei), and the patterns of anterograde and retrograde labelling in the reticular nucleus were examined. From the cortical injections, we find that the visual sector of the reticular nucleus is divided into subsectors that each receive an input from a distinct visual cortical area, with little or no overlap. Further, the resulting pattern of cortical terminations in the reticular nucleus reflects largely the patterns of termination in the dorsal thalamus. That is, each cortical area projects to a largely distinct subsector of the reticular nucleus, as it does to a largely distinct dorsal thalamic nucleus. As with each of the visual cortical areas, each of the visual dorsal thalamic (lateral geniculate, lateral posterior, posterior thalamic) nuclei relate to a separate territory of the reticular nucleus, with little or no overlap. Each of these dorsal thalamic territories within the reticular nucleus receives inputs from one or more of the visual cortical areas. For instance, the region to the reticular nucleus that is labelled after an injection into the lateral geniculate nucleus encompasses the reticular regions which receive afferents from cortical areas 17, rostral 18b and caudal 18b. These results suggest that individual cortical areas may influence the activity of different dorsal thalamic nuclei through their reticular connections.     

Recurrent inhibitory interneurons of the Rabbit's lateral posterior-pulvinar complex

We recorded from 118 neurons in the visual sector of the thalamic reticular nucleus (TRN) in anesthetized rabbits. Cells were identified by their location and characteristic burst responses to stimulation of the primary visual cortex (Cx) and optic chiasm (OX) and were classified into two groups. Type I cells had relatively short latencies from both OX and Cx stimulation, and the latency from OX was always longer than from Cx. In contrast, type II cells had much longer latencies after OX and Cx stimulation, and the latency from OX was always shorter than from Cx. Type I cells were located in the dorsal part of TRN, whereas type II cells were located in the ventral part of TRN. The physiological properties and location of type I TRN cells indicate that they are recurrent inhibitory interneurons of the dorsal lateral geniculate nucleus (LGN). Type II TRN cells most likely function as recurrent inhibitory interneurons for the lateral posterior nucleus-pulvinar complex (LP) because they could be activated antidromically by LP stimulation and orthodromically activated via axonal collaterals of LP cells. Type II TRN cells exhibited a prolonged depression after Cx or OX stimulation. Intracellular recordings showed that a prolonged inhibitory postsynaptic potential was evoked by Cx or OX stimulation. Therefore, these recurrent interneurons of LP, type II cells form mutual inhibitory connections just like those recurrent interneurons of LGN, type I cells. Our data suggest that the geniculocortical and extrageniculate visual pathways have similar recurrent inhibitory circuits.     

Physiology of nociception

Nociception is related to the mechanisms elicited by stimuli threatening the integrity of the organism. At the peripheral level, unmyelinated C fibres (C polymodal nociceptores) or fine myelinated A delta fibres are excited by noxious stimulation, directly or indirectly by inflammatory processes. Nociceptive afferent fibres terminate in the superficial laminae of the dorsal horn of the spinal cord where informations are integrated and controlled. These first synapses are modulated by excitatory amino acids (glutamate and aspartate) and many peptides (substance P, CGRP, CCK, endogenous opiods). The majority of ascending pathways involved in nociception are located in the ventrolateral controlateral quadrant of the cord (spinorelicular and spinothalamic tracts). Many supraspinal sites are activated following nociceptive stimuli, with relays in the reticular formation of the brain stem (including the subnucleus reticularis dorsalis), the ponto-mesencephalic regions (periaqueducal gray matter and parabrachial area) and thalamic sites. Amygdala and hypothamic targets could be involved in motivational reactions and neuroendocrine adaptations to a noxious event. The cingular, insular and somatosensory cortices also receive nociceptive informations. Nociceptive signals are modulated at all levels of their transmission; the more extensively studied controls are located at the spinal level. Segmental controls are inhibitory effects produced by non-noxious mechanical stimuli. Spinal signals can also be inhibited following activation of bulbopinal descending inhibitor pathways and release of serotonin, norepinephrine and, indirectly, endogenous opiods. Inhibitory controls triggered by noxious stimuli could facilitate the extraction of the nociceptive tone of informations having priority over other stimuli.     

Effect of electrical stimulation of the central amygdaloid nucleus on the nociceptive neuron of the cortex (SI) in the cat

The effect of conditioning stimulation of the central amygdaloid nucleus (ACE) on the response of tooth pulp-driven (TPD) neurons in the first somatosensory cortex (SI) was investigated in cats anesthetized with N2O-O2 (2:1) and 0.5% halothane. The tooth pulp test stimulus was a single 30-450 microA rectangular pulse, and the conditioning stimuli of the ACE were trains of 33 pulses (300 microA) delivered at 330 Hz. The ACE conditioning stimulation markedly suppressed the response of the slow-type neurons with latencies of more than 20 ms without any effect on the discharges of fast-type TPD neurons and spontaneous discharges. This inhibition was 68.9 +/- 24.7% (mean +/- SD) of the control. These findings suggest that there are at least two pathways for the ascending pulpal (nociceptive) information to the SI, and that the ACE modulates the transmission of impulses in one of the pathways.     

Neuronal control of mammalian vocalization, with special reference to the squirrel monkey

Squirrel monkey vocalization can be considered as a suitable model for the study in humans of the neurobiological basis of nonverbal emotional vocal utterances, such as laughing, crying, and groaning. Evaluation of electrical and chemical brain stimulation data, lesioning studies, single-neurone recordings, and neuroanatomical tracing work leads to the following conclusions: The periaqueductal gray and laterally bordering tegmentum of the midbrain represent a crucial area for the production of vocalization. This area collects the various vocalization-triggering stimuli, such as auditory, visual, and somatosensory input from diverse sensory-processing structures, motivation-controlling input from some limbic structures, and volitional impulses from the anterior cingulate cortex. Destruction of this area causes mutism. It is still under dispute whether the periaqueductal region harbors the vocal pattern generator or merely couples vocalization-triggering information to motor-coordinating structures further downward in the brainstem. The periaqueductal region is connected with the phonatory motoneuron pools indirectly via one or several interneurons. The nucleus retroambiguus represents a crucial relay station for the laryngeal and expiratory component of vocalization. The articulatory component reaches the orofacial motoneuron pools via the parvocellular reticular formation. Essential proprioceptive feedback from the larynx and lungs enter the vocal-controlling network via the solitary tract nucleus.     

The functional organization of the posterior lateral nucleus of the rat hypothalamus

It has been demonstrated that the posterior lateral thalamic nucleus of the rat receives information from the visual, somatic and auditory systems. Some of the neurons (63%) have a convergent input from these systems, although these neurons exhibit functional specificity with respect to the predominant inhibitory influence of the background activity of one of the sensory systems investigated. The other part of the neurons (37%) receives information only from the visual or somatic system, these neurons exhibiting excitatory phasic reaction to sensory stimuli.     

Fusion prevents the redundant signals effect: Evidence from stereoscopically presented stimuli.

In a simple reaction time (RT) experiment, visual stimuli were stereoscopically presented either to one eye (single stimulation) or to both eyes (redundant stimulation), with brightness matched for single and redundant stimulations. Redundant stimulation resulted in two separate percepts when noncorresponding retinal areas were stimulated, whereas it resulted in a single fused percept when corresponding areas were stimulated. With stimulation of noncorresponding areas, mean RT was shorter to redundant than to single stimulation, replicating the redundant signals effect (RSE) commonly found with visual stimuli. With stimulation of corresponding areas, however, no RSE was observed. This suggests that the RSE is driven by the number of percepts rather than by the number of stimulated receptors or sensory organs. These results are consistent with previous findings in the auditory modality and have implications for models of the RSE. (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

Neural activity in SII modifies sensory evoked potentials in SI in awake rats

The function of the projection from the secondary somatosensory cortex (SII) to the primary somatosensory cortex (SI) in rats was investigated by recording sensory evoked potentials (SEP) in SI during glutamate activation and lidocaine blockade of SII. In anesthetized animals, glutamate stimulation of SII decreased SEP latency and increased SEP amplitude, whereas no changes were evident during lidocaine blockade of SII. In awake animals, a second, later component of the SEP appeared. This second component was almost completely eliminated during lidocaine blockade of SII. We conclude that the projection from SII to SI in rats slightly facilitates the SEP response in anesthetized animals and is responsible for a major portion of the late component of the SEP in awake animals.     

Anatomical organization of the limb premotor network in the turtle (Chrysemys picta) revealed by in vitro transport of biocytin and neurobiotin

The in vitro turtle brainstem-cerebellum preparation has been a valuable tool in the study of central motor programs. In the present study, we investigate the anatomical organization of the turtle rubrocerebellar limb premotor network and its sensory connections in vitro by combining the rapid anterograde and retrograde transport of neurobiotin and biocytin with the extended viability of the isolated turtle brainstem-cerebellum. These compounds retrogradely labeled soma, dendrites, and axons, and orthogradely labeled axons and, to a lesser extent, terminals. The chelonian red nucleus receives a dense input from the contralateral lateral cerebellar nucleus and projects heavily to the contralateral spinal cord. Rubral axons sparsely innervate the lateral cerebellar nucleus and project heavily to the lateral reticular nucleus. Lateral reticular axons heavily innervate the lateral cerebellar nucleus before terminating in the pars lateralis of the cerebellar cortex as mossy fibers. These prominent, recurrent loops among the lateral cerebellar nucleus, red nucleus, and lateral reticular nucleus constitute the turtle rubrocerebellar limb premotor network. Sensory inputs to the red nucleus originate in the contralateral dorsal column nuclei, the principal trigeminal nucleus, and the spinothalamic system. These sites project bilaterally to the lateral reticular nucleus. The lateral cerebellar nucleus receives a contralateral input from the dorsal column nuclei. The red nucleus projects sparsely to the dorsal column nuclei. The red nucleus also receives an ipsilateral descending projection from the suprapeduncular nucleus, located in the diencephalon, and an ascending input from the rostral rhombencephalic reticular formation. An ipsilateral descending pathway originating in the red nucleus is likely to be the rubro-olivary tract.     

A new pressure ulcer risk assessment scale for individuals with spinal cord injury

The topography of somatosensory evoked magnetic fields (SEFs) following stimulation of the upper and lower lips was investigated in 6 normal subjects. When the lateral side of the upper lip was stimulated, P20m and its counterpart, N20m, were identified in the hemisphere contralateral to the stimulated side. The equivalent current dipoles (ECDs) of N20m-P20m were considered to be located in lip area of the primary sensory cortex (SI). Middle latency deflections (N40m-P40m, N60m-P60m, and N80m-P80m) were identified in bilateral hemispheres. Their ECDs were located in the SI in both hemispheres. Long latency deflections (P110m-N110m) were recognized in both hemispheres, and their ECDs were located inferior to the SI, in an area considered to be the secondary sensory cortex (SII). When the midline of the lip was stimulated, similar short and middle latency deflections was also identified, but SII deflections (P110m-N110m) were decreased in amplitude. When the lower lip was stimulated, the ECDs of short and middle latency deflections were located at a site in the SI inferior to or near those elicited by upper lip stimulation. The ECDs of P110m-N110m were located in an area of the SII similar to that upon stimulation of the upper lip, but their orientations were different.