Reevaluation of area 3b in the cat based on architectonic and electrophysiological studies: regional variability with functional and anatomical consistencies.

Most anatomical and electrophysiological studies of the cat primary somatosensory cortex rely on Hassler and Muhs-Clement's (J. Hirnforsch. 6:377-420, 1964) cyto- and myeloarchitectonic description distinguishing area 3a from area 3b; however, discrepancies in the delineation of these areas in published studies suggest that many workers have found it difficult to apply those criteria systematically. We examined the cytoarchitecture of area 3b in Nissl stained sagittal sections from which electrophysiological data had been obtained prior to sacrifice. Rostrocaudal rows of electrode penetrations placed at different mediolateral positions in the gyrus located regions responsive to stimulation of either cutaneous or deep structures. Small electrolytic lesions allowed these data to be related to the cytoarchitecture. A systematic study throughout the trunk and limb representations found cutaneous responses in cortical regions characterized by a thick and cell-dense granular layer IV, however these same regions had a variable population of medium-sized and/or large pyramidal cells in layer V. Pyramidal cells were practically absent from the forelimb representation, but were present to varying degrees in the trunk and hindlimb representations. Moreover, the relative thickness and cell-density in layer IV were greater in the forelimb than in the hindlimb representations. Deep responses were found in cortex characterized by a thinner layer IV. Since the characteristics of layer V in area 3a were variable, it was less useful for identification of the border between areas 3a and 3b. Clear changes in the intensity and laminar distribution of acetylcholinesterase staining occurred between areas 3a and 3b, making this a useful adjunct to the Nissl stain.

In vivo intracellular recording and labeling of neurons in the forepaw barrel subfield (FBS) of rat somatosensory cortex: possible physiological and morphological substrates for reorganization.

We examined the physiological properties and morphology of neurons in the forepaw barrel subfield (FBS) in somatosensory cortex (SI) of adult rats using in vivo intracellular recording and biocytin labeling techniques. Our results show that both pyramidal and non-pyramidal type spiny neurons can be activated with short latency by peripheral stimulation. FBS neurons within individual barrels receive both suprathreshold and subthreshold convergent input from one or more forepaw digits and pads. We hypothesize that some of these subthreshold inputs may be elevated to firing level by some, as yet unknown, mechanism(s) following peripheral deafferentation. Examination of the relationship between the dendritic pattern of labeled neurons and individual barrels within the FBS suggests that neurons with dendrites extending into neighboring barrel bands may serve as a possible morphological substrate for immediate reorganization.

Ulnar nerve innervation of paw and SI cortex of cat: substrate for reorganization.

We studied the distribution of the peripheral nerves innervating the distal forepaw by recording receptive fields from fascicles of the ulnar, radial, and median nerves and compared this result with the peripheral nerve representation in primary somatosensory (SI) cortex of cat. Our findings suggest that SI cortex receives input, in large part, from multiple peripheral nerves even when those nerves do not show a strong overlapping pattern in the periphery. This overlap pattern observed in SI cortex may be responsible, in part, for the immediate reorganization which is known to follow peripheral nerve deafferentation.

Electrical stimulation of a forepaw digit increases the physiological representation of that digit in layer IV of SI cortex in rat.

We studied the physiological representation of digit three (D3) in rat somatosensory cortex (SI) before and immediately after electrical stimulation (1.5x threshold for 2 h) of the glabrous tip of D3 in anesthetized animals (n = 6). Measurements of D3 representation were also made in anesthetized non-stimulated control animals (n = 2). The post-stimulation areal measurements of D3 representation in experimental animals were statistically significantly larger than both pre-stimulation measurements in experimental animals and post-stimulation measurements in control animals. Our results suggest that short-term electrical stimulation is sufficient to expand the D3 representation in each of the experimental animals, while the maps in non-stimulated controls showed little variation. The fact that these studies were carried out in anesthetized animals suggests that the results are independent of the state of the animal. The present findings emphasize the importance of afferent input in modulating cortical organization.

Relationship between representation of hindpaw and hindpaw barrel subfield (HBS) in layer IV of rat somatosensory cortex.

We describe the organization of the hindpaw barrel subfield (HBS) in layer IV of rat somatosensory cortex (SI) and relate this organization to the representation of the hindpaw. The ovoid-shaped, HBS is oriented anterior to posterior and comprises barrels and barrel-like structures, the most prominent of which consist of at least five anteriorly-located elongated barrel bands. Posterior to these elongated bands is a cluster of four barrels. Two additional barrels are found, one lateral, the other medial. The lateral border is formed by a nearly continuous band that overlaps portions of the anterior elongated bands and posterior barrels. The HBS shows considerable variability in size and shape; nevertheless, the overall pattern reflects a common plan of organization. Electrophysiological mapping confirmed that hindpaw representation is somatotopically organized. The glabrous toes are represented anteriorly, the pads posteriorly, and the dorsal hairy skin of the toes and hindpaw laterally. By aligning physiological and morphological (HBS) maps according to lesion sites, our data suggest that the elongated anteriorly-located barrel bands represent the hindpaw toes, the four toe pads are represented immediately posterior followed by barrels representing the plantar pads. The representations of dorsal hairy skin of toe and dorsal hindpaw form the lateral border; the heel and ankle are represented most posterior. We interpret our findings as support that individual barrels in the HBS are associated with discrete regions of the hindpaw; however, the precise relationship of structure and function reported between the vibrissae and posteromedial barrel subfield (PMBSF) and between the forepaw and the forepaw barrel subfield (FBS) were not observed.

Caudal cuneate nucleus projection to the direct thalamic relay to the motor cortex: an electrophysiological study.

We previously reported that injection of horseradish peroxidase (HRP) into a physiologically identified region of the thalamus between the ventrolateral nucleus and ventroposterolateral nucleus, VL-VPL, in cat results in labeled cell bodies in the caudal cuneate nucleus (CCN) of the dorsal column nuclei (DCN). We recognized, however, that the spread of HRP to localized regions of less than 1.0 mm distance from the injection site and subsequent uptake by neighboring fibers might have accounted for the resulting label in CCN. In the present study, therefore, we reexamined the DCN input to VL-VPL using a more sensitive physiological method. First, we used the microstimulation technique and corroborated the previous result. In 5 additional preparations, a modified collision procedure was used to ascertain that the same VL-VPL neuron which projects to the motor cortex also receives input from CCN. We report, for the first time, evidence of a lemniscal input to neurons in VL-VPL which are physiologically identified as projecting to the motor cortex.

Physiological properties and patterns of projection in the cortico-cortical connections from the second somatosensory cortex to the motor cortex, area 4 gamma, in the cat.

The physiological properties of neurons in the second somatosensory cortex (SII), and the pattern of projection of these neurons to area 4 gamma of the motor cortex in cat were studied by using single unit recording and collision techniques. Antidromically activated neurons were recorded along the anterior and posterior regions of the lateral bank of the anterior suprasylvian sulcus (ASSS) and from the middle part of the anterior ectosylvian gyrus (AESG) following weak intracortical microstimulation (ICMS) to area 4 gamma. Stimulation of the region around the activated neurons failed to produce muscle contraction or movement with currents of 30 microA or less. The majority of antidromically identified neurons received somatotopically organized afferent inputs from the skin on the contralateral side of the body. A small number of SII neurons received bilateral input. In 91% of the cases receptive field information was available for both the antidromically activated SII neuron and for neurons around the stimulating electrode in area 4 gamma. In 71% of these cases, both cortical sites were activated by sensory input from the same or adjacent peripheral area of the body. Neurons in the rostrocaudal region of the lateral bank of ASSS and the upper part of AESG (forelimb area) projected to the lateral cruciate gyrus of the motor cortex (forelimb area), while neurons in the ventrocaudal region of the medial part of AESG (hindlimb area) projected to the medial part of the postcruciate subregion of the motor cortex (hindlimb area). Antidromically activated SII neurons were typically found in layer III. These results suggest a topographically organized pattern of projection to the motor cortex from SII.

Caudal cuneate nucleus projection to the direct thalamic relay to motor cortex in cat: an electrophysiological and anatomical study.

The input to the border region between the ventrolateral nucleus (VL) and ventroposterolateral nucleus (VPL) of the thalamus, VL-VPL, was studied in cats using a combined electrophysiological and anatomical technique. Neurons within this border region receive somatic afferent input and project to a region of the motor cortex having similar receptive fields. In this study we asked the question whether neurons in the VL-VPL border receive input from the dorsal column nuclei (DCN). To answer this question we delivered intra-cortical microstimulation (ICMS) to the motor cortex while a second electrode inserted into the VL-VPL border, filled with a 20% solution of HRP dissolved in KCl, was used to record antidromically activated neurons. When an antidromically activated neuron was encountered and the neuron responded to natural peripheral stimulation, HRP was iontophoretically injected through the recording electrode. After a 48-72 h survival time, cats were sacrificed, and the brain tissue processed according to the method of Hardy and Heimer. Labeled cell bodies were found in the caudal cuneate nucleus (CCN) in all injected animals. These results suggested that neurons in CCN project to cells in VL-VPL which in turn project to the motor cortex.

Anatomical and physiological properties of the projection from the sensory cortex to the motor cortex in normal cats: the difference between corticocortical and thalamocortical projections.

Details of the distribution of terminal sites of the projection fibers from area 2 of the sensory cortex to the motor cortex were studied and compared with the distribution of terminals from the ventrolateral (VL) nucleus of the thalamus to the motor cortex. The results obtained were as follows: Intracortical microstimulation (ICMS) in area 2 produced measurable short-latency EPSPs only in neurons located in layers II and III of the motor cortex, whereas VL stimulation produced short-latency EPSPs in neurons throughout the depths of the motor cortex. The time from the beginning to the peak of the EPSPs was not significantly different for area 2- and VL-elicited EPSPs suggesting that there was no systematic difference between effective terminal sites for both inputs. However, there was a difference when a given neuron received both inputs suggesting that there was a segregation between the two inputs within a given cell. The majority of area 2-elicited EPSPs were smooth and monophasic, but some (40%) of them showed double peaks indicating that some neurons received mono- and disynaptic inputs from area 2. Intracellular injections of HRP suggested that neurons receiving input from area 2 were predominantly multipolar non-pyramidal neurons in layers II and III whereas neurons receiving thalamic input were pyramidal as well as non-pyramidal cells. Field potentials in the motor cortex evoked by area 2 stimulation did not change polarity in the depths of the cortex and therefore, differed from the VL-evoked potentials suggesting differences in the mechanisms of generating the electrical fields. It is concluded that association fibers effective for producing EPSPs terminate primarily on non-pyramidal cells in layer II and III whereas VL fibers terminate not only on pyramidal but also on non-pyramidal cells in layers III and V. This study provided a basis for examining the modifiability of association fibers after elimination of VL input to the motor cortex which is reported in the following paper.

Contributions of low-threshold calcium current and anomalous rectifier (Ih) to slow depolarizations underlying burst firing in human neocortical neurons in vitro.

The slow depolarization that underlies the voltage-dependent burst-firing behavior of human neocortical neurons is mediated by a low-threshold calcium conductance in concert with the anomalous rectifier current, Ih. The slow depolarization could be elicited by depolarization from negative membrane potentials or as a rebound following hyperpolarization. The rebound depolarization was time- and voltage-dependent. Most of the slow depolarization was blocked by inorganic calcium blockers. The remainder of the depolarization and the 'sag' in the hyperpolarizing voltage responses were blocked by extracellular Cs+.

Organization of the mouse motor cortex studied by retrograde tracing and intracortical microstimulation (ICMS) mapping.

The motor representation of the body musculature was studied in 11 adult mice by using ICMS. The motor responses elicited from both granular and agranular cortical fields showed that the mouse motor cortex is topographically organized; however, within the representation of individual body-parts the movements are multiply represented. In addition, several sites were encountered where more than one movement was elicited at the same stimulus threshold. The locations of pyramidal cells contributing axons to the pyramidal tract were examined by means of retrograde tracing with HRP injected into the cervical enlargement. This procedure labeled neurons only in lamina V in granular and agranular cortical fields. The similarities between the organization of motor cortex demonstrated in this study and the organization in the rat suggest that the rat and mouse share a common plan of rodent motor cortical organization.

Absence of responses to microstimulation at the hand-face border in baboon primary motor cortex.

Intracortical microstimulation (ICMS) was used to map the primary motor cortex of four adult female baboons, anesthetized with a mixture of halothane and nitrous oxide and supplemented with sodium pentobarbital. The sequence of observed muscle contractions in response to ICMS provided evidence of an orderly mototopic representation of the tongue, face, hand, forearm and upper body. A zone of cortex unresponsive to microstimulation was consistently observed at the border between the face and hand representation of the mototopic map. This zone was observed in all four animals and was consistent over time. Repeated confirmations of the unresponsive nature of these regions were obtained both early and late in the same experiment. No motor-unit responses or muscle contractions were detected by electromyographic (EMG) recording during stimulation of the unresponsive zones. The absence of both visually observed and EMG-recorded contractions and the fact that muscle contractions could be elicited from adjacent regions of cortex with ICMS as low as 1-5 microA provide compelling evidence that the finding reflects a true physiological condition rather than an experimental artifact.

Large unresponsive zones appear in cat somatosensory cortex immediately after ulnar nerve cut.

The organization of the primary somatosensory cortex innervated by the ulnar nerve was studied before and immediately after ulnar nerve transection in 11 cats electrophysiologically mapped under Nembutal or Ketamine anesthesia. The cortex was reexamined a second time beginning 42 hr after nerve transection in four cats anesthetized with Nembutal. One additional sham-operated control was also mapped. The region of cortex formerly served by the ulnar nerve remained largely unresponsive to somatic stimulation independent of the type of anesthetic used during recording. Nonetheless, animals anesthetized with Ketamine had more new responsive sites in deafferented cortex following nerve cut than cats anesthetized with Nembutal. New responses, when observed, were evoked by stimulation of a region of skin adjacent to the region served by the ulnar nerve. These findings suggest that the immediate response to deafferentation of somatosensory cortex is a limited acquisition of novel responses restricted to a region immediately adjacent to cortex containing normal afferent input.

EDTA chelation effects on urinary losses of cadmium, calcium, chromium, cobalt, copper, lead, magnesium, and zinc.

The efficacy of a chelating agent in binding a given metal in a biological system depends on the binding constants of the chelator for the particular metals in the system, the concentration of the metals, and the presence and concentrations of other ligands competing for the metals in question. In this study, we make a comparison of the in vitro binding constants for the chelator, ethylenediaminetetraacetic acid, with the quantitative urinary excretion of the metals measured before and after EDTA infusion in 16 patients. There were significant increases in lead, zinc, cadmium, and calcium, and these increases roughly corresponded to the expected relative increases predicted by the EDTA-metal-binding constants as measured in vitro. There were no significant increases in urinary cobalt, chromium, or copper as a result of EDTA infusion. The actual increase in cobalt could be entirely attributed to the cobalt content of the cyanocobalamin that was added to the infusion. Although copper did increase in the post-EDTA specimens, the increase was not statistically significant. In the case of magnesium, there was a net retention of approximately 85% following chelation. These data demonstrate that EDTA chelation therapy results in significantly increased urinary losses of lead, zinc, cadmium, and calcium following EDTA chelation therapy. There were no significant changes in cobalt, chromium, or copper and a retention of magnesium. These effects are likely to have significant effects on nutrient concentrations and interactions and partially explain the clinical improvements seen in patients undergoing EDTA chelation therapy.

Location and distribution of Fos protein expression in rat hippocampus following acute moderate aerobic exercise.

Hippocampal neurons are activated during endurance exercise; however, little attention has been given to the location and spatial distribution of these neurons. We have, therefore, used Fos protein expression to identify the location and distribution of hippocampal neurons that become activated during acute moderate aerobic exercise. Adult rats were assigned into trained running (TR), trained nonrunning (TNR), untrained nonrunning (UNR), and cage-bound (CB) groups. Rats in the TR and TNR groups were trained to run, for three 20-min running periods separated by 3 min rest, on a treadmill. Rats in the UNR group spent identical time on a nonactivated treadmill, while rats in the CB group remained in their home cages throughout the training and experimentation. After training to criterion performance for both TR and TNR groups, both groups were rested for 1 day. Rats in the TR were then run on the treadmill to criterion level, while those in TNR and UNR groups spent equivalent time on the nonactivated treadmill. Animals in all groups were then killed and their brains removed, sectioned, and processed for Fos protein immunocytochemistry. Fos-like immunoreactive (FLI) neurons were counted in the dentate and CA1-3 fields of the hippocampus. The total numbers of hippocampal FLI neurons, as well as FLI neurons in each hippocampal region, were compared among groups. The total numbers of FLI neurons in the hippocampus, as well as in individual regions, were significantly greater in the TR group compared with the other three groups. Similarly, significant differences were found between the TNR group when compared with UNR and CB groups. Conversely, a significant difference existed between UNR and CB only in the CA1 field, which may account for the significant difference in the total number of hippocampal FLI neurons between these two groups. These results show that Fos induction occurs in the hippocampus during moderate physical exercise. Furthermore, the importance of the incorporation of adequate controls to account for possible differences in expression of immediate early gene expression due to trained performing, trained nonperforming, and untrained groups is discussed. The results indicate that adequate control for nonexercise stimuli is necessary for studies of the effect of exercise on the brain when expression of immediate early genes such as c-fos is used as an outcome measure.

Movement of facial muscles following intra-cortical microstimulation (ICMS) along the lateral branch of the posterior bank of the ansate sulcus, areas 5a and 5b, in the cat.

The lateral branch of the posterior bank of the ansate sulcus within the parietal cortex, areas 5a and 5b, was mapped using intracortical microstimulation (ICMS). Motor effective sites for contraction of facial muscles were identified using ICMS currents of less than 30 microA. Of the 177 effective sites, 78% were activated with threshold currents of less than 20 microA, and of these, 33% responded to stimulus strengths of less than 10 microA. Since neurons in this portion of the ansate region send projections to the face region of motor cortex, area 4 gamma, the possibility existed that the motor effects might be mediated through area 4 gamma. To examine this possibility we ablated both the anterior sigmoid gyrus (ASG) and the second somatosensory cortex, SII, including area 2pri, a region shown to contain a low threshold motor component, and report that the motor effect recorded from the ansate region is independent of both the ASG and area 2pri.

Thalamocortical arbors extend beyond single cortical barrels: an in vivo intracellular tracing study in rat.

Neurons in layer IV of rat somatosensory (SI) barrel cortex receive punctate somatic input from well-defined regions of the periphery. Following peripheral deafferentation, SI neurons in deafferented cortex respond to new input from neighboring regions of the skin surface. The precise mechanism(s) through which this occurs is unknown, although corticocortical and barreloid to barrel connections have been suggested as possible substrates. Because layer-IV barrels receive a strong afferent input from ventroposterior (VP) thalamic projection neurons, any divergence in the thalamocortical (TC) projection to multiple cortical barrels could also provide an anatomical substrate for rapid cortical reorganization. We used in-vivo intracellular recording methods to record and physiologically identify neurons in rat VP and to label those neurons with an intracellular tracer. Thalamic neurons (n=117) were impaled with sharp intracellular electrodes, and the receptive field(s) and firing pattern were measured. Cells were then injected with biocytin or biotinylated dextran amine (BDA). A total of 38 labeled TC neurons were quantitatively analyzed for soma size and dendritic arborization size; quantitative analysis of TC-axon arborizations in layer IV of barrel cortex was carried out in a total of 13 TC neurons. Two different axon-arborization patterns were identified in SI cortex: direct-projecting axons (n=6) were observed to project to and arborize within a single cortical barrel as well as extend their fibers into adjacent barrels; bifurcating-type axons (n=7) were seen to bifurcate in the subcortical white matter or in layer VI and then project to multiple barrel columns, where they arborized in layer IV. Axon fibers were always observed in three or more cortical barrels (mean=5, range=3-7). The mean mediolateral extent of arborizations in layer IV for the direct-projecting and bifurcating type axons were 458 microm and 1,302 microm, respectively, and these were significantly different (t=3.78, P<0.01). Axon-fiber length within cortical laminae was measured for each arborization pattern in relationship to the total fiber length within a cortical column. Direct-projecting axons always had greater than 50% of their fiber length within layer IV. Bifurcating-type axons were differentially distributed within multiple columns and always had less than 50% of their total column fiber length in layer IV. Morphological analysis of TC somata and dendrites revealed no correlation between local neuron morphology and axonal-arborization patterns. All intracellularly recorded TC neurons had similar adapting firing patterns when injected with a long-duration pulse. Our results showed that TC neurons project to multiple cortical barrels with one barrel receiving the principal input. This divergent TC projection pattern in SI cortex may provide an anatomical substrate for cortical plasticity and must be considered in any mechanism of rapid cortical reorganization.

Early development of the representation of the body surface in SI cortex barrel field in neonatal rats as demonstrated with peanut agglutinin binding: evidence for differential development within the rattunculus.

Physiological studies have demonstrated a highly organized somatotopic representation of the body surface in SI cortex of rat. This representation is correlated morphologically with the presence of barrel-shaped structures in layer IV. Conventional staining techniques reveal barrels in the latter part of the first postnatal week. Recently, the peroxidase conjugates of lectins, which recognize glycosylated molecules, have been used to study barrel field formation. Con A, for example, has been shown to bind primarily to prospective barrel sides and septa as early as postnatal day 3 (PND-3) in mouse. To date, investigations of SI cortex using the lectin (Arachis hypogaea) peanut agglutinin (PNA) have been confined to the study of the barrel field representation of the face and mystacial vibrissae in the mouse. In the present study we extend these findings to the development of the representation of the entire body surface called the rattunculus. Rats ranging from PND-1 (first 24 h after birth) to PND-12 were anesthetized with Nembutal and perfused with 4% paraformaldehyde and 2% glutaraldehyde in 0.2 M sodium cacodylate buffer. Brains were removed, flattened tangentially, and sectioned on a vibratome at 30-120 microns. Sections were blocked in TRIS-buffered saline (TBS) plus 2% bovine serum albumin and incubated in peanut lectin at 4 degrees C. Following incubation, sections were washed with TBS and processed using peroxidase histochemistry. Lectin binding in the prospective forelimb representation was apparent by PND-5 whereas lectin binding to the prospective face-mystacial vibrissae representation occurred before PND-4. These results suggest that body part representations show individual variations during early pattern formation.(ABSTRACT TRUNCATED AT 250 WORDS)